A P\.I8UCAT1ON OF ntEDEPARTIIENT Of CONSERVATIONDMSION Of'.-.ES AND GEOlOGY

CAUFORNIA

GEOLOGY

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PETE Wll:.oN.....­DOUGLAS P WHEELERSIIct'e1ary for Rf1$OUrr;»$

EDWARD G. HEIDIGD,rector

In This Issue IMINERAL EDUCATION CONFERENCE 2DETACHMENT FAULTS 3TEACHER FEATURE 18FAULTED WAVE·CUT TERRACE NEAR POINT ARENA 20OPEN·FILE REPORT RELEASES 24PUBLICATIONS REOUEST FORM 25SUBSCRIPTION FORM 26BOOK REVIEWS 27

JAMES F DAIIISS'al.~

CAliFORNIA GEOLOGY

Aslisanl TechnocaJ Ed tor'Assi5WII EdllOtGt~andOaog.,

Ccwer photo: Detachment lauh: Itllhe Whipple Mountains <Itsoulheaslem Califomia The ftaI-¥ng canr.ct IIIt1e detactmellilaull wtuch separates the recIdIsI'l-l:lr twlging WIll rocks tramthe underiying, lIght-eoklf8d Iootw8I rocb. The hBngIng WIllCOf'IS$I$ of mid-Terbary YObniC and , fOCb end lielQoIwaII COI\SlStS 01 aIlerfId PrecamtwI8n end ' II-*: crys-taline~ Pho«J by C L PrdnoI..

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JANUARY/FEBRUARY 1992JVoh,flTle 45 Numbef ICGECA 45 (l}l-28 (1992)

Mineral Education Conference

The California MlI)(!ral Education Foundation will host its second MineralEducation Conference on August 12-14. 1992 al Califomia Stale University.Sacramento. The conference will present minerals and mining infonnation.activities. and workshops which can be used in the classroom.

The program INill include the history of mining and mining techniques inCalifomia. environmental and redamation issues. geology. mineral uses. andgeo-political and socioeconomk. issues.

The refundable registration fee of $10_00 inddes a tour of an active ITIIn­

ing operatIOn. gold panning demonstrations. and a barlJecue. Registration isopen to California teachers of aD grade Ievets. For more infOllTlation contact:

Barbara Stewart(209) m-0658

0'

Brenda KressCalilorrna Minerai EducatIOn Foundabon

9647 Folsom Blvd.&ollte 148

sacramento. CA 95827(916) 362·9305

, CAUFORNIA GEOLOGY J.4.NUARY"FEBAUARY 1992

Detachment FaultsCalifornia's Extended Past

CYNTHIA l. PRIDMOAE, GeologistDivision of Mines and Geology

andERIC G. FROST. GeologistSan Diego Stale University

Photo 1. Detachment fault exposed In the south-central Whipple Mountains. The prominent rnlcrobl"eccla ledge along the faultseparates the tilted hanging wall rocks from lhe underl~mg brecciated and altered mylonitic gneisses. Photos by CL Pndmoreunless otherwise noted.

INTRODUCTION

DUring mid-Tertiary time. the earth'scrusl. in what is now the southwesternUnited States, was being stretched andpulled apart. leaving a profound imprinton the rocks in the desert areas of Cali­fornia. This regional extension was ac­commodated by the widespread develop­ment of normal faults. Detachment faults.newly recognized features of this exten­sion. are normal faults that are very gen­tly dipping or subhorizontaJ (cover photo

and Photo I). Major structural features inmany mountain ranges in the desertregions of California. Nevada. and Ari­zona are now Identified as detachmentfaults.

High-angle normal faults have beenrecognized for years in the Basin andRange province of the Southwest. Theyare typically the range-front faults lhathave given the province its distinctive

geomorphic character. Detachment faults,in addition to high-angle normal faults. arenow known to pervade the Basin andRange province. as "''ell as other portionsof California (Agure l). Because so muchof the information regarding detachmentfaults has been generated at the researchlevel. these ideas are just being introducedin undergraduate geology courses. Simi­larly. the economic importance of detach­ment faulting and crustal extension with

CALIFORNIA GEOLOGY JANUARY FEBRUARY 1992 3

10. Tierra Blanca Mountains11. Pinyon Mountains

12. Borrego Springs13. Santa Rosa Mountains14. Yaqui Ridge

15. Orocopia Mountains

16. Palm Desert17. Old Woman Mountains18. Chemehuevi Mountains

19. Sacramento Mountains

20. Dead Mountains

21. Homer Mountains

22. Castle Mountains

23. Clark Mountains24. Kingston Range

25. Halloran Hills

26. Bullion Mountains

27. Newberry Mountains

N

1. Whipple Mountains2. Riverside Mountains

3. Big Mana Mountains4. Midway Mountains5. Chocolate Mountains6. Picacho Peak

7. Cargo MuchachoMountains

8. Coyote Mountains9. Fish Creek Mountains

oIo

50I

28. Waterman Hills!Hinkley Hills/MitchelRange

29. Harper Lake

30. Sierra Pelona31. Lockwood Valley

32. Edison33. Slate Range34. Wingate Wash35. Black Mountains35. Panamint Range37. Funeral Mountains38. Northern Panamint

Range39. Grapevine Moun,

tains40. Cottonwood Moun·

tains41. Cuesta Ridge42. Northern Diablo

Range43. Clear Lake44. Paskenta45. Klamath Mountains

100 150 MILES

il t I150 225 KILOMETERS

Figure 1, Locations ollow·angle normal faults in California. BR .. Basin and Range.

precious metal mineralization and withoil-bearing basins of southern California isjust now being investigated.

EARLY WORK

Although detachment faults are cur­rently recognized in several areas of Cali·fornia, many VJere originally interpretedas thrust faults, Thrust faults are prooucedby compressive forces thrusting one rockmass over another along a 10000angiefault. Many gently inclined faults wereinitially interpreted to be thrust faultsbased on their dip. rather than on therelationships of the rocks they displaced,

In the 1950s. Peter Misch and hisshldents VJere among the first to questionthe origin of some of these low-anglelaults. In eastern Nevada they recognizedthat many of these faults could not becorrelated with older Mesozoic thrusting

events (Misch. 1960). RichardAnnstrong's (1972) work in this regionled to a pivotal paper in which hereinterpreted earlier mapping and usedgeochronological relationships to show amuch younger Tertiary age for the 10lN­angle faulting throughout much of east­central Nevada.

In 1971 Ernest Anderson proposedthat the high-angle faults that tilted theTertiary rocks in portions of southernNevada flattened into low-angle normalfaults at depth. The existence of theselistric (CUlVed) faults indicated that muchmore extension had occurred than waspreviously thought.

In Death Valley. detailed mapping byLauren Wright and Bennie Troxel (1969,1973; Wright and others, 1974) indicatedthat normal faults made up the complexfault zone previously mapped as the

Amargosa thrust (Nobel. 1941). Sirni­larly. regional, subhorizontal normalfaults in southeastern California were firstrecognized by Terry Shackelford and hisadvisor. Greg Davis (Shackelford. 1976).[n southeastern Arizona. George Davisand his students were documenting thewidespread existence of this same type ofregional. gently inclined nonnal faulting(Davis. 1975).

Geologists who had been working inareas from southern Canada to north­western Mexico began to appreciate theregional occurrence of similar relation­ships. Many of these workers attendedthe Geological Society of AmericaPenrose Conference in 1977 and subse­quently published GSA Memoir 153(Crittenden and others, 1980). This land­mark volume brought together theirstudies which covered an immense areaaffected by detachment faults.

, CALIFORNIA GEOLOGY JANUARY/FEBRUARY 1992

Photo 2. Detachmentlault exposed in the Waterman Hills. near the microwave tow­ers north of Barstow. The view is to the southeast. The hanging wall is composedof brecciated and altered rhyolite. The footwall IS composed 01 mylonitiC granodIOr­ite thai has been shattered and chlontized. This location is a Geological Societyot America field trip stop described In Glazner and others (1988, p. 235. stop 2).

By the early 19805. geologists acceptedthat much of westem Nonh America hadundergone a major period of extensionalfaulting. Many low-angle faults of the Basinand Range. Mojave. and Sonoran desertareas are now recognized as detachmentfaults (Photo 2).

Metamorphic Core Complexes

In the 19805 many geologists beganusing the term metamorphic core complex(Coney, 1980a. 198Ob) to describe areasIAlith detachment faults. Initially, a metamor­phic core complex indicated the associationof strongly metamorphosed footwall rocksand lDlmetamorphosed hanging wall rocksalong a dome-shaped detachment fault. Asstudies of detachment faults became morewidespread. it became apparent that not alldetachment faults are associated with meta­morphic contrasts. nor are they all dome­shaped. MetamorphiC core complexes arell()\,V considered to be areas where the deep­est rocks affected by mid-Tertiary crustalextension are exp:>se(l. HoVJeVer. manyextended areas expose shallower and lessdeformed pans of the system and thereforediffer from the c1assk core complex. It maybe more useful to replace metamorphic corecomplex with the term "highly extendedterrane" because of the recognition thatcrustal extension affects many crustal levels.

DETACHMENT FAULTS

The spectacular characteristics of detach­ment faults capture the enthusiastic allentionof geologists and geology students. Many areinspired by discoveries of major structuralfeatures that others have overlooked or mis+interpreted. New exposures of detachmentfaults are continually being found. thus ex­panding the areas known to be affected bysuch faulting.

Detachment faults cut deeply into thecrust so they often juxtapose deeper crustalrocks in the footwall with shallower crustalrocks in the hanging wall (F'tgure 2). Acrossthe fault. the contrast in rock types is usuallyconspicuous. so detachment faults are oftendiscovered by examining the contacts be­tween strongly varying rock types.

Along many of the detachment faults inthe Colorado River region of southeasternCalifornia and southern Arizona. the faultsurfaces are resistant ledges composed ofhard. dark orange to reddish brown. flintymicrobreccia (Photos 3 and 4). This very

fine-gralned lithified fault gouge can beup to a foot (30 em) thick. Spectacularexamples of microbreccia along weD­exposed detachment faults can be seenin the Whipple Mountains. theChemehuevi Mountains, and otherareas in the Colorado River region(Frost and Martin, 1982a. 1982b:Davis and Anderson. 1991).

Detachment faults typically have abroad zone of fractured rock called thechlorite breccia zone. 1be rocks in thiszone have been altered by f1ukls that havemoved through the fractures. Chlorite. aproduct of the alteration, gives the brec­cia a greenish hue. In some areas. thisalteration has been accompanied by pre­cious metal mineralization, explaining

CALIfORNIA GEOLOGY JANUARY,fEBRUARY 1992 5

-'~WO" 6'"

Incipient hanging wall normallaulls,

~T~~--------Bnttle - Incipient brecciation ® r...~~

DUClile 1ranSI~on zone and chloritlc alteration Incipient ~~::=:--.---......along detachment fault Mylonization -

Prolfett (1977) provided some of themost highly regarded evidence from hisextensive mapping and logging of morethan 100.000 feet (30.500 m) of drillcores at Yerington. Nevada. His studyshowed that high-angle normal faultsbecame very gently indined by the tilting01 successive generations 01 nonnalfaults (Agure 4).

Normal faults in the hanging wallprobably occur as a combination oftilted planar faults and listric faults(Photo 6). Rather than being rigid like atilted book. most fault blocks are inter­nally deformed. As the block moves, italso changes shape by internal faulting,

®-= .....

Brecciation and Mylo"'t1zation~~ ~-:::.-chlontic alterallOn ~-overprinting mylorll~c fabriC

alte,atio!!.----

Hall-graben basins\

"pp'o. J M'

Mid-Tertiary Basins

Figure 2. Sequential model of a rooted detachment fault. The gently dipping faultprojects downward across the brittle-ductile transition zone. Movement along theductile portion 01 the laull produces the mid-Tertiary mylorliles. With further move­ment. rocks that were once deep in the crust (a) are juxtaposed against uppercrustal rocks (b). Brian Wernicke's (1981) preeminent interpretation 01 detachmentfaults as rooted shear zones' provided the foundation lor understanding exten·sional processes in the soulhwestern United States and other parts 01 the world.Modified from Spencer and Reynolds. 1989.

why many old prospects and diggingsoccur along the lault zones. In manyareas the chlorite breccia zone gradesdownward into more coherent footwallrocks. which are not appreciably altered.

Hanging Wall Structure

The hanging wall rocks of detachmentlaults are broken up by normal faultsaccompanied by abundant fracturing andbrecciation (photo 5). Within a givenregion or structural domain. the laultsoften have a consistent orientation. Insoutheastern Calilornia and southwesternArizona. the normallaults strike north­west indicating the crust in this regionwas extended in a northeast-southwestdirection (perpendicular to the strike ofthe faults).

These types ollaull5 were studied indetail in the early part of the century inthe Bullfrog district of southern Nevada.just east of Death Valley. Emmons (1907)and Emmons and Garrey (l91O) used theanalogy of a set of books tipped to oneside to describe the area's structural lea­tures. The faults become more gentlyinclined by tilting as the rocks betweenthem till (Agure 3A).

To explain more complex tilting andfaulting. Anderson (1971) used listricfaults to resolw the relationships he ob­served in southern Nevada. He noted thatwhere faults had steep dips, the stratadipped gently. and where they had gentledips. the strata dipped steeply (Figure3BI.

Whether extension took place alongpredominantly planar or curved faults hasbeen an ongoing controversy. John

Sedimentary basins fonned in thehaJf-graoons created by the tilting 01crustal blocks (Figure 5). As coarse­grained sediments were shed into thebasins. faulting continued to lUt thestrata. Angular unconfonnities andshallowing dips indicate that faultingcontinued during deposition. Becausethese sediments were deposited andfaulted during regional extension. therocks they fonned provide a record ofthe progressive uplift and erosion of thebasement rocks (Photo 7 and backcover photo) (Pridmore and Craig.1982; Teel and Frost. 1982; Pridmore.1983; Miller and John. 1988; Nielsonand Beratan. 1990; Travis and others.1990; Beratan. 1991). Age determina-

• Terms in boldface type are in lhe glossary onpage 15.

Photo 3. Close-upof microbreccia andunderlying chloritebreccia along theWhipple detach­ment fault in south·eastern California.The presence ofoxidi:l:ed iron in themicrobreccia givesit a typical reddish­brown color. In con·trast. the underlyingbreccia is typicallygreen due to thepresence of chloriteand epidote.

6 CALIFORNIA GEOLOGY JANUARY/FEBRUARY 1992

tions of rocks that lNere tilted during thisexlension indicale thai most of this typeof faulting in southeastern Californialook place during Iale Oligocene or earlyMiocene time.

MYLONITIC ROCKS

Myk:lnitic rocks. deformed rocks thaioften~ both a foliation and a lin­eation. are characteristic of deep faultwoes (Photo 8). Where these rocksoccur ad}acent to a detachment fault,they are often incorporated in thick chlo­rile breccia zones. The chloritic alteratk>nand the brecciation are believed to havetaken place after the mylonites werebrought closer to the surface along de­tachment faults.

Pholo 4. ExhumedfTllCfobreccia sur­face. Striae onfault surface helpIndicate lhe direc­uon 01 movement.

Photo 5. Normal tautls in hanging wall of Picacho detachment tauIt. southeastern California.Faulls are sleep until they f1anen near the detachment fault. The geolOgists are standing on'fne 6etactmlent tault sur/ace thattorms Ihe lloor of the wash.

It is controversial whether themylonitic deformation is directly associ­ated with detachment faulting or withother defonnationaJ events. notably olderMesozoic thrust faulting. Isotopic studiesindicate that some of the mylonitic fabncdid form at approximately the same limeas detachmenl fa~ and thereforewithin the same tectonic environment!Wright and o<h=. 1986, B<yan. andWooden. 1989). The presence of these

mylonitic rocks now exposed adjacent todetachment faults provides strong evi­dence for crustal thinning during regionalextension. Studies show that rnyk>nitesform at depths of 510 9 miles (810 15kmllAnde=n. 1988, And<=>n andother.i. 1988) so for these rocks 10 nowreside at the surface. aU of the overlyingrock must have been removed.

ANTIFOAMS

Broad antifonns also charactensticaUyoccur with detachmenl faults (Davis.1980, Rehng and R_. 1980,Cameron and Frost. 1981; John. 1984.1987). In some regions there are two setsof antifOllT1S thai lrend roughly perpen­dicular to each other. \.Vhere this occurs.the detachmenl faults and the rTIOlI1tainranges thaI contain them are dome­"",pOO.

One mOOeI for the development offolds parallel to the Irend of the hanging­wall faults is referred to as roll-over fold­ing (Gibbs. 1983. 1984) or reverse dragfolding (Hamblin. 1965). As a hangingwall is dragged downward over a curvedor downward flattening fault. it will fonna fold to accommodate volume changes.This folding may be accompanied byantithetic faulling. A detachment faultfokled in this manner indkates that thefaull itself lies in the hanging waD of an­other slructurally deeper fault (FlgUl"e 6).

From her work in the ChemehueviMountams. Samar.. John (1984. 1987)considers the undulations that lreOO paral­lel to the direction of extensKln 10 haveformed nol as folds. but as large grooYeSor corrugalions.

In southeastern California. these fea­tures have wavelengths of 100 feet(30 mllo lens of miles. The larger undu­lations could have formed as trough-likefeatures when large crustal blocks of rock

CALIFORNIA GEOLOGY JANUARY/FEBRUARY 1992

As extension progresses,lhe fault·bounded blocksas well as the taulls rotale.To accommodate lurthermoyemenl, new high·anglefaults and Iractures areInitiated.

Figure 4. This lault modelillustrates rotallon of planarnormal faults.

Photo 6. An example 01 the Inlenslty and compleXity 01 faulting seen W1thm the hangmg walrocks above the Wtupple Mounlalns detachment lault. SuallgraphlC sequence has been reopealed several bmes by a senes 01 subparallellaults and smaller antithetic faults. Nole thepresence of both planar and cuMld laults.

\

'", ~

-~---'------

. 0 C Q,:::)

\

'",=::_-- ..... -

........ _._._._._._._._._._.- _ _.

A. Plal'\3r normal tauhs

Figure 3. Planar and kstnc normal faults.A) Parallel planar normal fau!tJng IS oltenreferred to as ttlled domIt'IO"s!yle faulting.The fault blocks as well as the faults areuniformly lilted. B) l.J$tnc normallaults !lat­len dOwnward and merge with a detach­ment tault. thiS lault model explains howadjacenl blocks are tilted independently.NOle lhatln the direction of downthrow,the tilts are successfully sleeper.

IT'\r'\~

~

.... _ _ -._._ •••• _ •••• _._ •••• _._ H _·· ••• ._._ _ _ _ ................... _._ •••• _._ •••••• _ H _ __._ _._ _._ _._ .

f-.

An overalllislric shape ISproduced as !he older faultsegments are rOlated Intoa subhonzonlal onenlalJon.The result is a seoes 01intersecting planar faultsthat acconvnodate elClen­$IOrl and rotallon of thehangIng wan rocks

Photo 7. Coarse-grained clastic rock Inter·preted to have been deposlted by a combina­tIOn of avalanche and debns flow processes.thIS unsorted, angular deposit was probablydenved from nearby upilted and faulted base­ment rocks. Ayalanche. alluvial fan. braidedstream. and playa depoSItional enwonmenlSare charaetenSlIC of detachmen\ f~\ed~sedlmentabon.

• CAUFORNlA GEOlOGY JANUARY FEBRUARY , 992

\.Wre pulled apart along Iow-angle faults (Figure 7). Therocks that once were structurally adjacent to many ofthe ranges along the Colorado River region in Califor­nia may now lie in western Arizona and southern Ne­vOOa

DETACHMENT FAULT MODELS

Since detachment faults have been recognized asregional normal faults. many models have been pro­posed to explain how they work. One of the most com­mon questions is whether these faults originate as low,angle faults or whether they are rotated into moregentle inclinations with ongoing extension.

Wernicke (1981) proposed that detachment faultsoriginate as shalJow.dipping faults that extend deep intothe crust (Figure 2). The rooled detachment model isused to explain features in the Mojave Desert (Dokka.1986.1989: Dokka and others. 1988. 1991) such asthe \.WlI-exposed Harper Lake. Newberry Mountains.and Mitchel Range detachment faults. Based on theobservations and field data of many workers in detach·ment faulted regions. the moclel interprets myloniticrocks to be the OOumward ductile continuation of de­tachrnent faults (Wernkke. 1985).

Detachment fault formation can also be thought of asa system of large normal faults that tilt blocks of crust(Figure 8). Tilting of individual blocks results in tilting ofthe ma;or faults as \.WII. This is similar to the tilting thattakes place in the hanging wall rocks. but on a largerscale. The high'angle faults become Iow·angle faultswith progressive motion within the fault system. Reklevidence from eastern Nevada indicates that some of

Photo 8. Miocene mylonitic gneiss in the lootwall of the Chamehuevidetachmentlault. The block in the cenler of the photograph has beenrotated along low-angle faulls. Underlying layers have arched 10 fill inthe void. This outcrop helps show how the large-scale development 01delachmenllaults can ttlt crustal slabs.

Figure 5. Block diagram of hall-graben basins. The progres­sive tilling 01 hanging wall blocks formed half-graben baSins.Sedimentation dUring this motion left a record of the uplitland erosion oltha basement terrane. Early formed stratawere tilled progressively steeper.

these faults penetrated to deplhs of 6 miles (10 kml prior torotation (Gans and Miller. 1983).

Progressive offsci and tilting brought the mylonitic rocks outof the ductile middle crust. Uplift of the middle crust to Ihesurface was probably accompanied by Tertiary intrusions in thelower crust ((ians and others. 1989).

CRUSTAL·SCALE PROFILING

By adding lhe dimension of depth. seismic profiles haveenhanced our understanding of how crustal extension takesplace. Within the past few years. two groups have been effec­tive in providing cruslal-scale perspectives on extensional pro-­cesses. COCORP (ConsortilUTI for Continental Reflection Pro­ming) has provided deep seismic reflection data across largeportions of the Basin and Range (Cheadle and others. 1986:Allmendinger and others. 1987: Serpa and others. 1988:Serpa. 1990). These stlX!ies have helped identify extensionalslructures in the subsurface and have provided a picture 01 theseismic character 01 the crust on a regional basis.

CALCRUST (California Consortium for Crustal Studies) hasfocused on the problems associated with detachment faullingwithin southeastern California (Henyey and others. 1987:Aueh and Okaya. 1989: Wang and others. 1989). The study­ing and reprocessing of industry seismic lines in the regiondemonstrated that detachment faulls. hanging waU normalfaults. and mylonitic rocks in this largely crystalline terrane areall imageable (F"Jgure 9). Additional seismic profiling byCALCRUST in the Whipple Mountains area provided theopportunity to match surface geology with seismic reflections.Interpretations of the data (Frost and Okaya. 1986: Okaya andFrost. 19800. 1986b: Lucazeau and Okaya. 1991) indicatethat a zone of mid-cruslal reflectors extends westward from theWhipple MOlUltains and may be conlinuous with the myloniticrocks exposed in the central Mojave detachment terrane stud­ied by Dokka (1989). Qazner and others (1989), and Walkerand olhers (1990).

CALIFORNIA GEOLOGY JANUARY/FEBRUARY 1992 9

Figure 7. The large undulatIOns that are parallel to the di­rection ot movement are considered 10 be accentuations ofthe onglnal taul! surlaces. Rocks that were adjacent to oroverlying mountains along the Colorado RlVer in southeast­ern California may be found in ranges 10 the nonheast inArizona.

La,ll" c:ort\IlPloQflS ar" 1I'oOl.II'Uon·1ll8troughs ;ond &l8$IS

.......-NE

An early-formed detach­ment fault..

As the hanging wallmOl/as away from thesecond fault it folds Intothe fault to fill the void.5egment "a" of the IIrstdetachment fault definespari of the fall-over fold.Funher steepening ofsegment "b" may be dueto steepening 01 the sec­ond delachmentlaull atdepth.

becomes pari of thehanging wall as anotherdetachment faulliorms1.11 depth.

Uplift and erosion ell­poses the folded detach·ment fault. Rocks that aretopographic hlQhs at thecenter ollhese dome­shaped ranges wefeongmally the very deepparts of the detachmentfault system. Figure mod/­fied from Gibbs (1984).

Detachmenl faull

" a" 'b-

....~Jln&'pI!Inl second........ detachment laul1

..._-----

\\\~\"\\\\\"~"" \,\~\~\'-~,,"'''''\'- " " " ", '.

Figure 6. 5equential model 01 how a detachment fault can be foldedinto an antiform.

Non-mylonitic crystalline hanging wall rocks

Zone or tlfllllederormatlon

1Cuctlle zone 01mylonitIC rock lormaloon

_---::=""0',... Normal fa ultswithin hanging wall

Tilted hangmg wall blocksSedimentary BaSin

Non·mylOnltic crystallinetootwall rocks

~~~~~~~opproximo!ely I mIle .....-::..- /­

/,/'/ ,­/ ",.... /' -~ ././ /',..-

Detachment / TUted crustal bl0;t- -~-~- - Faults '" .. ~ /' /",.... ",....- -~ .... ///'/.-/ /------ Uplifted mylonillc rocks.-- /'" /' ",.............../

/" ....-- --- - - - y"':..:=-.,. ~ .. -"",:=""'--:;;~S'",~:-:==""1--- - ~...."./....--~...<:;... =-~ .....::::::::- ==-...::::---:::-

-..=::;---::::::::; :::=:::-.-:......:-- ------ - ---.::::--=-~- ---:::::-- - -opproximo!ely 9 miles .L_--====~ "---= ---'

Figure B. Tilled slab model for detachmenllaull systems. The low-angle character of major normal faults is a function 01 the tilting Of moder­ately inclined faults. Multiple faults desceocl into the ductile zone and ollset earty·formed mylonitic rocks. Although some mylonites mayhave formed aJong the roots of discrete detachment faulls, thiS model suppons the hypotheSIS Ihat a regional zone of subhorizontat myloniticrock was developed during crustaJ extension. This model Is based on held studies and seismic rellecllon data in southeastern California.Courtesy of E.G. Frost, R.G. 8Jom. and R.E. Cnppen.

" CALlFQflNIA GEOLOGY JANUARY/FEBRUARY 1992

Because detachment faulting is still notwell understood. developing an appropri­ate mexlel for crustal extension continuesto be the focus of many researchers.Extension in the southVJeSlem UnitedStates and other areas of the \A/Orld canbest be understood through the study ofthese fault systerns on a crustal scale.

DETACHMENT FAULTING INOTHER PARTS OF CALIFORNIA

DetachlTlcrlt faulting has been widelyrecognized and well studied in southeast­ern California. southern Arizona. andNevada because the effects of it are wellexposed. As our understanding of exten-

sienal processes improves. we need toreview the geology of other areas of Cali­fornia where similar features may bepresent

In southern California the extensionalcharacteristics of the Diligencia Basin.preserved in the Orocopia Mountains.

swWard Valley Turtle mountains

NEChemehuevi Valley 1

-I.e.

;:" C

..... . - -~', ;j..( ..~.:.",'. ... -~"~'."."

.r.~F'_ _ .,,_.,~ .'J: ',,;' ."<..

_",._....'-;;.~~,p.:-:j'c,: '"'~ ""':.\:_ .."~

c' ",'0 'f<,.:' :''''''-r- :"."., ~."t>; ..,".« '£<; il\,,~~,.. ,,-''h'1><~~·!'!IQ"'Y.{?l,Ro<\i' "'r 4,';:..., ',.:'>.'- ":"<~~"G~:::":' !:-:4,..:::-.'':-;: ',;'-~';-'--" ..."'~ §" -~,.<.. ).;~_ '~~. 0• _":.;.~..- ,..-..-.,: :-;'!'....1';-;.:~~.-.-:~ .....~.,2.{.:::;.~;,_~:v.~"-.·.~~z~~t~;j~~._~_: ~~?~i_;;j·%.:.':fo kilometers~~~:£~~~r~~~:tf~;~~1f~1\?'

Figure 9. seismic retlectlon data across the valleys to the west 01 the ChemehueviMountains. Northeast end of the hne begins a few thousand feet from the exposureor Chemehuevi detachment fault (COF). This raul! is projected onto the right side ofthe profile; the southwest dip IS interpreted to be part 01 a rollover fold resulfmgfrom movement along the underlying northeast-dipping fault. Note the tilted laultblocks and mid-Tertiary baSins. The short discontinuous reflections in lower hat! ofthe prolife are interpreted to be the seismic e:w::pression of mylonilJc rocks. Originaldata provided to CALCAUST by Compagnie Generale de Geophysique (CGG). In­terpretation and reprocessed data courtesy 01 D.L. Okaya and E.G. Frost

CALIFORNIA GEOlOGY JA.NUARY/FE8RUARY 1992 "

Pnoto 9. Detachmentlault in the Orocopia Mountains. Note the conspicuous green (tootwalt) and red (hanging wall) COnlacl. Geologists arewalking along the trace ot the tault. Movement along the detachmentlault is Interpreted to have brought up the chlontlcally altered looTWallrocks from deeper levels along the fault zone and placed them against the structurally higher, oxidized hanging walt rocks. The presence 01this fault near the east side 01 the san Andreas lault implies that the westward conllnuation of detachment faulting is probably 0"5etto thenorthwest.

are tied to detachment fault tectonics(Photos 9 and 10) (Robinson and Frost.1989. 1991). Because these rocks lie sonear the San Andreas fault. other mid­Tertiary basins located farther to thenorth and to the VJeSt of the San Andreasfault may also be related to detachmenttectonics. Although the origin of many ofCalifomia's southern and coastal basins oflate Oligocene to early Miocene age havetraditionally been tied to strike-slip tecton­ics. Tennyson (1989) indicates there is alack of conclusive evidence for strike-slip­faulted basins of this age.

More likely. the origin of late Oligo­cene and early Miocene .sedimentationand deformation may be related to thesame extension that affected areas in thedesert regions to the east. Seismic reflec­tion profiles seem to image components

of detachment systems across pans ofthe Los Angeles basin. Cuyama basin.and continental borderland area. eventhough these areas are overprinted byfolding and faulting associated with !aterSan Andreas deformation (Baker andothers, 1991; Buckner and others.1991; Fanahipourand Frost. 1991).

Another exciting application ofdetachment fault models is the reinter­pretation of portions of the Coast RangeThrust Wayko and others, 1987). Thin­ning of the crust by detachment fault-ing anSI.VeTS the question of how theFranciscan Complex rocks. which haveresided at depths of 15 to 18 miles (25 to30 km). are now Juxtaposed with shallow­burial rocks of the Coast Range Ophioliteand Great Valley Sequence. The loss of9to 12 miles (15 to 20 km)of rock is

attributed to extensional faulting (Harmsand others. 1987; Jayko and others.1987; Krueger and Jones. 1987;Krueger and others. 1987: Krueger andJones. 1989).

Geological relationships in the Kla­math Mountains in nonhem California.described by Schweickert and [rwin(1989). are similar to those identifiedwith detachment faults. In this region.tilled Tertiary conglomerates rest on thegently dipping mylonitic surface of theLa Grange fault. This. along with theabsence of several thousand feet of stnK:­turalthickness along other ponions ofthe fault. suggests the fault may be adetachment fault that has accommodatedsubstantial crusta! extension during Ter­tiary lime.

" CALIfORNIA GEOLOGY JANUARY/FEBRUARY 1992

Photo 10, Palm Desen detachment lault in the northern Santa Rosa Mountains. View IS to the southeasllrom vista point along StaleHighway 74. The Coachella Valley and the Orocopla MountainS are in the distance. Much at the delachment fault has been eroded,exposing the prominent footwall ramp which dips to the lell in lhe photo. beneath the crystalline hanging wall rocks of Sheep Mountain.Because this tault and many others like it OCC\Jr along the eastern side 01 the Peninsular Ranges, just to lhe west of lhe San Andreasfaull, their presence raises the questions as 10 how far this extenSional deformatIOn continued to the west and how large a role if playedin the development of many of the basins along coastal and offshore California.

CONCLUSION

Although detachment faults have been widely recognized inmuch of the California desert. it appears that other areas of theState may have experienced extension during mid-Tertiary time.Comparisons of well-studied detachment systems INith similarfeatures in other portions of the Stale may help link eastern

California geology with that of the northern and coastal regions.Continued study of geological relationships in the desert areaswhere these fealures are well-exposed will help refine the overallunderstanding of these phenomena.

-CALIFQANIA GEOLOGY JANUARY/FEBRUARY 1992 "

MINERAUZATION ALONGDETACHMENT FAULTS

VIeW of the Mesqlllte ITW'l8 on Ihe southern Chocolate MountaInS ... 1989 PicachoPeak and the C8tgo Muchac:ho Mountalns are.., the background. PhoIo by ChnsHwns

QuJing the 19805 the recogr\lOOnof the association of gold mll"lefaliza·tion and detachment faulting Shffill­

Iated exploration for large-tonnage,\ow-grade gold deposits in southeast­em California. southwestern Arizona.and Nevada,

The intensive brecciation of largevolumes of rock along detachmentfaults appears to be a strong controlfor favorable c1eposllIon of the oremU'l€fals (Spencer and Welty, 1986.1989). Hydrothennal circulation inthe thick brettia zones alters the rockand probdbly mobilizes some of theb.be and precious metaI5 (SpencerMId Welty. 1986). These me1a~arelhcn dqxlsited in breccia zones di

"""" owrly;ng !he detadunm' fduhand in open spaces aJong norrnaIf"'" " !he hangmg wall iWdkln> andHeidrid<. 1982). Some of !he depo<lIed mineral!t are calcite. hemallle.pyrite. chalcopyrite. manganese oxides. quartZ. barite. and fluorite(K.l>ith. 1978; Ridenour and other's.1982; Wilkins and Heidrick. 1982;Kohler and Bezore. 1988).

Most 01 the base and preciousmetal deposits in the Whipple Moun­tains are associated VJith hydrothermal processes and detachment faulting (Ridenour and others. 1982;Kohler and Bezore. 1988) Gold."ilver. and copper generally occurwithlJl altered fractures.~. andshear 7.OIleS, The majority of thesedeposits are high-srade but typicallyof limited extent both ...erticaJIy andhon=>laIy II<ohIo< and ae""e.1988)

At the P\cacho mine in so..nhe:cbt­em California. brecciation and hydro­thermal alteration associated WIthdetachment faulting are interpretedto have provided the environment forthe deposition of disseminated gold(Drobeck and others. 1986; Frost

and others. 1986; Liebler. 1986. 1988).The ore grade!> seem to increase in pro-­portion to the degree of shattering 01 thehost rock (Frost and WatO\l.lich, 1987).

The worId-class MesqUite mine. ap­proximately 15 miles (25 kill) northwestof the Picacho mine, Is another areawhere gold mineralization~ been re­lated to detachment lclU1tmg (Frost andWatOUlich. 1987; ShafiqUllah arxl others.1990). The orebody was disccM:'red onthe southwestern side 01 the OloooIateMoun""",. kugeIy buried by"""" Thebuial can be attributed to the highly frae·tured nature 01 the tn.t rock thdt~nally enabled the mineIalized fb:Is to Rowthrough the rock and deposIl the gold,The shattered Ildture of the rock then led10 rapid erosaon. forTT\dtion of pb::er golddeposib. and buIidl of the ore deposit

The origln of the fuicb at the M~temine~ been tied to Tertiary plutonic

bodieslD.M Fr~t. 19901. Fludsgen·erated from the plutonism associatedwith the Tertiary extcnsK:ln pass up­ward along the brittle fault zones andprecipUate along the structurally high·est components of the detachmentsystem,

The key to targeting these typeS ofgold deposits may lie in dentifying thestructurally highest portions of a de­lachment fault system in a region ofTertiary pkltonism. Although much ofsoutheastern California is affected bydetachment faulttng, not. aD of thear~ have the me,.Dsrte source rocks 10

have """""'" !he """"""'" flwd>Further~ of the st:rueturalprocesses and chemical envirorunenbi!lSSClC'Iated WIth detachment fau/ting If'\

known gold producing areas is funda·mental to mineral exploration in otherextended terranes.

.. CALIFORNIA GEOLOGY JANUARY FEBRUARY 1992

GLOSSARY

angular unconfonnity: A break in a depositional sequence in which youngersediments rest upon the eroded surface of tilted or folded older rocks.

antithetic faults: Minor faults that dip in the direction opposite to the major faultwith which they are associated.

foliation: 10e planar arrangement of textural or structural features in a rock.

lineation: A structure in or on a rock that fonns lines (for example. flow Jines orstretched minerals).

seismic reDection study: An exploration method that sends vibrations into theground and times their return from surfaces that bounce the energy back. The two­way traveltime, the time il lakes for the energy to come back to the surface. indi­cates the depth of a reflecting surface. 10e reflection profile is a two-dimensionaldisplay of this data which is used to delineate geologic structures and changes inrock type.

shear zone: A tabular zone of rock that has been crushed and brecciated bymany parallel faults.

Cindy Pridmore Is 0 geolo­gist with the Regional Ge0­logic Mapping Project at theDivision of Mines ond Geol­ogy. Her ongoing interests inthe Soulhwest ore profes­slonol as well as recreationol.

Eric Frost Is an associateprofessor wilh the Deparl­ment of Geological &Iences01 Son Diego Slate Unluersity.He teaches structural geology.leetonles. remote sensing,seismic Interpretotlon. ondfield srudles. His currentresearch Interest is the three­dimensional restoration 01extensional terranes.

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Ridenour, J., Moyle, P.R.. and Witlet, S.L..1982. Mineral occurrences in the WhIppleMount31ns Wildemess Study Area, sanBernardino CoonI}', Califorma, in Frost,E.G.. and Martin. DL, editors, Mesozoic­cenozOIC tectOl"llC evoIutJon 01 the Colo­fado RIY9f legoo, Ca~lomaa,Anzona,

and Nevada: San Diego. CordilleranPublishers. p. 69-75.

Robinson. K.l.. and Frost. E.G.. 1989,Orocopla Mountains detachment system:progressive ductlle to OOttle developmentof a tilted crustal slab dUring regIOnalextemoon: GeologICal SocIety of AmerICaAbstracts WIth Programs. v 21, no, 5,'35.

Robinson. K.L, and Frost, E.G" 1991, Ter­tiary extenSlOl'laI and basin developmentII'l southern Caldorma The temporal Slmi­ianI}'. style of delormatlOn and cruslalgeometry In the OrOCXJPla Mountams andthe san joaquIn H~ts: GeologICal Soaety01 Amenca Abstracts Wl1h Programs,v. 23, no. 5, p. AI32-A133.

SChweder!, R.A, and lrwm, W p. 1989.ExteOSlOflal faultltlg In southern KlamathMountaJns, CaJifOffila; Tectonlcs, v 8,no. 1. p. 135-149

Serpa, L., 1990, Structural styles across anext8llSlOl131 orogen; Results trom lheCOCORP Motave and Death Valley setS­mIC transects, In Werrucke, BP, editor,BaSIn and Range extensooal tectonICSnear the latltude 01 las Vegas, NevadaGeologICal Soaety of Amenca Memolr176, p. 335-344

Serpa, L., DeVoogd, B., Wnghl. L, Willemln.J., OlIver. J.. Hauser, e.. and Troxel, B.,1988, Structure of the central Death Val­ley pull-apart baSin and VlClnll}', fromCOCORP profiles in the southern GreatBasin: GeologICal SoCIety 01 AmencaBulletin, v 100, p. 1437-1450.

Shackelford, T.J., 1976, Structural geology01 the Rawhide Mountaitls, MohaveCounty. Arizona: unpublished Ph.D.dissenalion, University 01 SouthernCalifornia, 175 p.

Shafiqullah, M., Frosl. G" Frost. D.L., andDamon, P.. 1990, Regional extensionand gold mineralization in the southernChocolate Mountains. southeastern Cali­fornia- KAr constraints Irom lault rocks:Geological Society of America Abstractswith Programs, v. 22. no. 3, p. 83.

Spencer. J.E., and ReynoldS. S.J., 1989.Middle Tertiary tectonics 01 Arizona andadjacen1 areas in Jenney, J.P., andReynolds. S.J.. Geologic evolution 01Arizona: Arizona Geological SocietyDlQest. no. 17. p. 539-574.

Spencer. J.E., and Welty, J.W., 1986, Pos­sible controls 01 base-and preCIOUs-metalmineraUzatlOn assoCIated With Tertiarydetachment faults In the lower ColoradoRiver trough. Anzona and Calilornia:Geology, v. 14. p. 195-198

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Mountains. san Bernardino County, Call­lorma. in Frost. E.G.. and Martin, D.L..editors. Mesozoic-Cenozoic tectonicevolution 01 the Colorado RlVer region,California, Anzona, and Nevada: Cordille­ran Publishers, san 01890, p. 275-285.

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WernICke, B., 1981, Low-angle normal fault­Ing in lhe Basin and Range provinceNappe tectonICS In an extending orogen:Nature. v. 291. p. 645-648.

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the Whipple Mountains, California andBuckskin Mountains. Arizona. in Frost.E.G., and Martin, D.L., editors, Mesozoic­Cenozoic tectonic evolution 01 the Colo­rado River region. California, Arizona.and Nevada: Cordilleran Publishers. SanDiego, p. 182-203.

Wright, J.E.. Anderson, J.L.. and Davis,G.A" 1986. Timing of plutonism.mylotlllizalJon, and decompression In ametamorphic core complex. WhippleMountains. CA: Geological Society ofAmerica Abstracts with Programs. v. 18.no. 2, p. 201.

Wright, L.A.. OIIon. J.K., and Troxel. BW..1974, Turtleback Slrudures 01 DeathValley viewed as phenomena 01 exten·sionallectonics: Geology. v. 2. p. 53-54.

Wright, LA., and Troxel, B.W., 1969, Chaosstructure and Basin and Range normalfaults: Evidence for a genetlC relaoonshlp:Geological Soael}' 01 America AbstractsWIth Programs, part 7, p. 242.

Wnght, LA.. and Troxel. BW.. 1973. Shal­low fault Inlerpretatton 01 8asln andRange structure, southwestern GreatBaSIn, in De Jong. KA.. and Scholten. R..editors, Gr3VJty and Tectorics: JohnWiley and Sons, New York, p. 397-407. yo

CAlIFORNIA GEOlOGY JANUARY FEBRUARY 1992 "

ByCynthIa L Pridmore

CALIFORNIA HASITS FAULTS...

The San Andreas laull system and lIS pnoclpal branches are identified by blue~nes Some other laults are shown In black. ModIfied from Geology of calilomla.Noms and Webb. 1990

A fault is a fracture along which thereis~1. Some faults are aetuaIIycomposed of several fractures called faultbranches. Collectively the branches are afault zone (see map).

CalifonUa's diYerse landscape arxlcomplex geology can be attributed 10

faullmg Many of the Stale's valleys,momtain ranges. and desert aTeaS showthe effects of faulting. Faults create Wlder·ground traps in which vakJable reservoirsof petroleum lonn. and spaces in whichunderground walers deposit valuable mel­aIs in the fann of veins and masses of ore.

Faults are distirq.Jished by abrupIchanges in rock struetuTe or composition.SometJmes a fault can be recognized bythe displacement of a partia..dar featuresuch as a bed or a vein.

TIle best places to observe faults areusually In roadcuts. quarries. and sea cliffexposures.

FAULT ClASSIFICATION

Faults and fault zones are dassiflEld byhow the rocks on each side of the fauh orfault zone move past each other. 1hereare two main types of movement alongfautts: 1) a sideways movement calledstrike slip. and 2) an up or doYJn Il"IClVe­

menl called dip slip.

Strike-Slip Faults

II

Sacramento'o

N

\• ~ '00 UO "'IUS, ,, '00 -""""'..., , ,

"

lbe movement along a strike-slip fault b approximatelyparallel to the strike of the fault. meaning the rocks movepast each other horizontally.

The San Andreas is a strike-slip faub mal has displacedrocks htn:Ireds of miles. As a resUl of horizontdl mo"emenlalong the fault. rocks of~ different age and compositionhave been pIac:ed side by side. The San Andreas fault ~ afault zone rather than a sangle fault and movement may oc­cur along any of the many fault surfaces in the zone, Thesurface effects of the San Andreas fault zone can be observedlor over 600 miles (1.000 km).

CALIFORNIA GEOlOGY

STRIKE·SlIP FAULT

..w«JAAY FEBRUAAY 1992

NORMAL FAULT

(lootwan)

Rever-se faults· are dip-slip faults in \lJhich the har.glngwaD moves up relative 10 the footwall. Reverse faults are theresui of compressk)n (forces that push rocks toqetherl_

The Sierra Madre fauk zone of southern California Ii anexample of ~fault rT'lCM!'IllCfV.. There the rocks of theSan Gabriel Mountains are being pushed up and over ttwrocks of the San Fernando and San Gabriel valleys. Move­menl on the Sierra Madre fault zone is part of the processthat created the San Gabriel MOl.ll'tains_

(footwall),

Dip-slip faults are faults on \lJhich the movement Is para!k!l tD the dip of the f.mll surf«e. Nonnotl hlulb· Me dip­slIP faults on which the hanging "air· (the rocks above thefid: swface) move oo.m re01M! to the f()()lU.ra8·· (the rocksbelow the fau.t surface). Normal faults are the resuh of exten­sion (forces thai pull rocks apart),

~\ hangl wall)

Dip-Slip Faults

Where the dip of a nonnaI fauk's surface is sleep. it iscalled a hlgh-angte normal fault:. or Simply a normal faullThe Owens Valley and ihe Sierra Nevada fault zones are ex·ample; of rug,-angIo nonnaJ fault> Togeth..-. they~.~ bkrl that forms I~ o.wns Valley. Tbs typeof fault-bounded valley is caBed a graben A fauh--bomdedndge ts caIIed a horst

REVERSE FAULT

A thrust fault is a reverse fauk with a gmtly-dppir.gfault surface. Thrust faults are very common in the KIaInaIhMou-D-.sd """"'"' Calfaroa.

.",bon

THRUST FAULT

HORST AND GRABEN

Where the dtp of a nonnaI faulfs surface Is very gentle oraImo!.t fiat. it is referred 10 as a detachment fault or~nonnaI fault. Detachmenl faults are common in lhe desert areasof Dlifomla

"The terms -normar and ....evelS8· were first used by EngUsh coal mirl8fS 10describe laults. When wotIlIng a flat coat bed whefell 'IlI8$ etisloeal!CI by anormal fauI:. !he Inr*S contlflUed !he WOltangs 81tt. upward or cbM lWardon tt'I8 ButI SUflace in tt'I8 same. or ·normal: dkeQion.. The WClfkJn95 in.56IIm dsloCined by. reverseliluh __ al50 oontJnU8d upwIrd or dorwnwatdon the laull. but in ll'le oppo$IIe, or ·r8Vef5e,· dif8Ct1Ofl (o,akangas 1991 J.'

NORMAl REVERSE

DETACHMENT FAULT

."The terms "hangng walr and "tootWalr are also old mining lerms. Theseterms ...ere ooomaty used in inclined underground PMSllo-"'Y5 10 r11llJf 10the rock "haJ'lOlng" oyefhead (Itle hanging wal) IIIld lhe IIoor beneiI/tI rhe1TUIl8f'S'leet (me IoolWall) (Otakanoas, 1991).'

,~ A W . liIll. Scta.ofoIllIUllinIo GIl 1>10o')' _ ~GII-.etrwygoo<IlIlIogy~ Hill. Inc _ v....... 2lIoI "

CALIFORNIA GEOLOGY JANUARY/FEBRUARY 1992 "

FAULTED WAVE-CUT TERRACENEAR POINT ARENA

Mendocino County, CaliforniaA Photo Essay

ERIK E. OLSBORG, Engineering Geologist

Photos by Erik E. Olsoorg

The tide is about elevation minus 2 feel(60 em) in this photograph taken from thebluff. south across the wi.lve-i:ulterrace.This terrace is inundated up to the sandybeach 1.11 the bluff toe during high tide.The area is known locally as &>wling LaneBeach, and Bowling Ball Beach, becauseof its appearance. The ocean waves in thedistance are breaking over Saunder's Reef.

The terrace is composed of theGallaway-Skooner Gulch Fonnation. ma­rine sedimentary rocks of Miocene age.

These thin beds consist of blue \0 grayclaystone and siltstone, with some lightbrown to orange-brown shale and minorsandstone. The claystone and siltstoneare generally soft to friable (easilycrumbled), but the shale and sandstoneare somewhat harder.

The nearly-flat terrace is marked by aseries of narrow, linear, parallel ridgesthat are a few inches to several feet wideby several hW1dred feet long. TheseTidges form by differential weathering ofthe strata. The more erosion-resistant

beds are several inches to approximately2 feet (.6 m) higher than the softer beds.

This site is a classic exposure of bed­ding strike (across the terracel and dip(exposed within the bluff). The bluff ispartially aligned as a dip slope {the bluffface is parallel to the bedding strike atroughly the same dip inclination).

A fault runs across the terrace in theupper half of the photo. offsetting thebeds and altering the northwest trend ofthe bedding strike by a few degrees.

20 CALIFORNIA GEOlOGY JANUARY/FEBRUARY 1992

The top of the bluff affords aview of the strata extending fromthe lower terrace up the bluff tothe base of a second terrace. Notethe seepage at the contact betweenthe dipping rocks and the nearly­horizontal terrace deposits.

CALIFORNIA GEOLOGY JANUARY/FEBRUARY 1992 "

I n the phot_ph above. the """. along thebedding strike. from the lower lmace northwesttoward the bluff. This portion of the bluff face isnearly perpendicular to the dip of the bedding.Strata exposed in the bluff dip 58 10 61 degreestOY/ard the southwest.

The terrace on the lOp of the bkiff. appraximalcly90 leet (27 m) aboYe sea level. is covered by 6 to8 feet (±2 m) of light-colored. sha1Iow marine. Pleis­tocene terrace deposits. Nol:e the apparent landslide(left) where the top 01 the bluff tilts back.

The photograph to the right is a close-upof beds offset by the fault visible in the firstphoto. Although the fault movement ap­pears to be right-lateral strike slip, therecould have been a significant (if nol lotal)vertical component (southeast side of thefault upthroum ~ see illustration).

Eros+on aeates the "'soon 01''llhl·latllfal stroke-slop 18u~lng

22 CALIFORNIA GEOlOGY JANUARY/fEBRUARY 1992

This is a IIiew toward the face ofthe bluff where the strata dipparallel to the slope, The ~swirl­

Ing" appearance is due to thedifferential erosion of me rockstrata, Several okler (inner) bedsare visible through "holes" erodedin me younger (outer) beds, 1heerosion is caused by slaking(crumbling due to exposure to airand moisture) of me sedimentaryrock. Small rock particles continuoously drift dO\lJll the bluff facefonning a talus deposit that Isperiodkally wa>hed .woy by hightides and Stonns.

The fault that cuts the bedding(center of photo) is the same faultas the one in the preceding ph0­tographs. The terrace depositscapping the bluff do not appearoffset. so the fault movementmust haw occurred prior to for·mation of the nearly horizontalterraces.

Continuous slaking of the frioable rock strata. combined withperiodic waw erosion at the blufftoe. is causing a relatively highbluff retreat rate at this location.Note the overhanging vegetationon the upper bluff edge. Basedon reconnaissances along thebluff in 1977 and 1989, and onaerial photographs taken in1952,1964, 1972, and 1981,bluff retreat is approximately 4 to6 inches 11 0 to IS an} per year.

Erik E. 0Isb0rg is the Prirt­

d"'" Engineering Geologistand Managing VICe Presidentof BACE Geotechnical, Inc.of Windsor. Califomia. ~

CALIFORNIA GEOLOGV JANUARV/FEBRUARV 1992 23

DMGOFR 91-1 GEOLOGIC MAP OFlHE SUSANVILLE QUADRANGLE.L4SSEN AND PLUMAS COUNTIES.CAUFORNIA. Scale: 1: 100.000.By T.L.T. Grose. G.J, Saucedo. andD.L. Wagner. 1991. $7.00

This (}pen-Rie Report makes existinggeologic data for the Susanville 30 x 60minute quadrangle available to the public.P1ior to its release. only unpublished recon­naissance geologic maps were available forthis area. This report was produced by the[)jvisiorl of Mines and Geology's (DMG) Re­gional Geologic Mapping Project as part of itsfunction - to gather. analyze. and disseminateinformation about Californla's regiOnal ge0­

logic setting. Field work was supported inpan by the U.S. Geological Survey Coopera­live Geologic Mapping Program,

The map area includes parts 01 the ModocPlateau. Sierra evada. and Basin and Rangegeomorphic provinces of northeastern Cali­fornia and covers approximately 1.850square miles (4.800 km2) of northeasternP\umas County and southeastern LlssenCounty. DMG orn 91-1 consists of a ge0­

logic map plate and explanation (26 p.) of thegeologic Wlits. map symbols. references. andthe source data used in the compilation. Thegeologic map Is compiled on a topographicbase at a scale of UOO.OOO (1 inch equalsabout 1.6 miles).

This map shows the general geologicframework of the area and provides basicgeo6ogic infonnatiOn on the age. distribution.and descriptkln 01 the various rock typeS. andthe location of faults and other geologic stnx:­tures. The area is underlain primarily by lateCenozoic volcanic rocks that overlie an okler.and for the most part. concealed basementconsisting of Cretaceous and pre-Cretaceousgranitic and metamorphic rocks. Siena Ne·vada vok:anlc sources were located along thepresent SieTT3n crest and to the east. generat­Ing a sequence of rhyolite. andesite. andbasalt flc:r.vs and pyroclastic deposits generallydistributed along the Tertiary drainage sys­tems. The Honey Lake fault zone, a north­west-trending zone of en echelon. right-lateraloblique slip faults marks the boundary be­!\\;een the Sierra Nevada and the Basin andRange. The Basin and Range and ModocregiOns are characterized by mountalflOl.lSterrain and intervening lowlands, Mountainpeaks are moslly older anciesitic to basalticstratovolcanoes whUe the Iourlying areas areunderlain by younger basalt flows or fluvialand lacustrine deposits.x

DMG OFR 91-10 COMP1£TEBOUGUER GRAVITY MAP OF THEOWLSHEAD MOUNTAINS QUADRANGLE.CAUFORNIA. Scale, 1: I00.000. Compiledby Elise Mattison and L.G. Youngs. 1991.$5_00

DMG OFR 91-10 Is a gravity map 01 partsof San Bernardino and lnyo counties_ Thisgravity map is one ina series of 1100.000­scale quadrangles complied to complementthe regional geologiC map, Data for theOMshead Mountains qwdrangle are compiledfrom the existing literature. The map showscontours of the total intensity 01 the earth·sgravitational fiekl from 382 gravity stations.The contours are lines of equal Bouguer grav­ity intensity in miUigals (a tA'lit of accelerationused with gravity measurements). Gravity con­tours are on a planimetric base and show cui·tural features.x

DMG OFR 91-11 COMPLETEBOUGUER GRAVITY MAP OF THEMONTEREY QUADRANGLE, CAUFOR­NIA Scale: 1:100.000. Compiled by L.C.Youngs and Elise Mattison_ 1991, $5.00

This gravity map Is one In a series of1, loo.000-scale quadrangles compiled tocomplement the regional geologic map. Datafor the Monterey qwdrangle are compiledfrom the existing literature. The map showscontours of the total intensity of the earth'sgravitational fiekl from 1,919 gravity sta­tions. The contours are lines of equal Bou·guer gravity Intensity in milligals (a unit ofacceleration used with gr/lVlty measure­ments). Gravity contours are on a plani­metric base showing coastline and culturalleature5_X

DMG orn 91-12 COMPLETEBOUGUER GRAVITY MAP OF THESUSANVILLE QUADRANGLE, CAUFOR­NIA. Scale: 1-100.000. Compiled by L.G.Youngs and Elise Mattison. 1991 $500

DMG OFR 91-12 is a gravity map ofparts of Lassen and Plumas counties. Thisgravity map is one in a series of 1 100.000­scale quadrangles compiled to complementthe regional geologic map_ Data for theSusanville quadrangle are compiled from theexisting literature. The map shows contoursof the tot<ll IntenSIty of the earth's gravita­tional field from 490 gravity stations. Thecontours are lines 01 equal Bouguer graVityintensity in miUigals (a unit of accelerationused with gravity measurementsl_ Gravitycontours are on a planimetric base with cul­tural featuresY

DMG orn 91-13 COMPLETEBOUGUER GRAVITY MAP OF THEEAGLE LAKE QUADRANGLE, CAUFOR­NIA. Scale, 1:100.000. Compiled by L.G.Youngs and Elise Mallison. 1991. $5.00

DMG OFR 91-131s a gravity map forpart of Lassen County. It is one in a series ofI lOO,OOO-scale qwdrangles compiled tocomplement the regional geologic map. Datafor the Eagle Lake quadrangle are compiledfrom the existing literature. The map shoI.l.'scontours of the total intensity of the earth'sgravitational fiekl from 335 gravity stations.The contours are lines of equal Bouguerintensity In miUigals fa unit of accelerationused with gravity measurements). Gravitycontours are on a planimetric base withculturalleatures.'X

DMG OFR 91-1S COMPLETEBOUGUER GRAVITY MAP OF SODAMOUNTAINS QUADRANGLE. CAUFOR­NlA. Scale: UOO,ooo. Compiled by EliseMattison and L.G. Youngs. 1991. $5.00

DMG OFR 91-15 is a gravity map forpart of San Bernardino County. It is one ina series of 1: lOO.OOO-scale quadranglescompiled to complement the regional ge0­

logic map. Data for the Scxia Mountainsquadrangle are compiled from existingliterature. The map shows contours of thetotal intensity 01 the earth's gravitationalfield from 266 gravity stations. TOe con­tours are lines of equal Bouguer gravityintensity in mUligals (a unit of accelerationused wilh gravity measurements). Gravitycontours are on a planimetric base withcultural features.x

The preceding OFRs can be purchasedat DMG offices in Sacramento and SanFrancisco. Reference copies are available atthe Sacramento. San Francisco. and LosAngeles offices. See page 26 lor addressesand telephone numbers.

----DMG OFR 91-14 COMPI..ETI:

BOUGUER GRAVllY MAP OF lHESANTA ANA QUADRANGLE. CAUFOR­NIA. Scale: 1:100,000. Compiled by L.G.Youngs and Elise Mallison. 1991. $5.00

DMG OFR 9 t -14 is a gravity map lorparts 01 San Bernardino. (flange. Riverside.San Diego, and Los Angeles counties_ It isone in a series of 1:1oo.ooo·scale qwd­rangles compiled to complement the regionalgeologic map. Data for the Santa Ana quad­rangle are compiled from the existing litera­ture. The map shows contours of the totalIntensity of the earth's gravitational fiekllrom2.782 gravity statIons. The contours are bnesof equal Bouguer gravity intensity in miDigals

CALIFORNIA GEOLOGY JANUARY,FEBRUARY 1992

AMOUNT ENCLOSEO (Pnce Includes po$1age and sa.es ta~) $ _

ACHECK OR MONEY ORDER MUST ACCOMPANY THIS ORDER. Aft non'US orders must be pal(!Wllh an internatIonal money ordef Of draft payable In U.S. dollars ancl made out 10 DIVISION OF MINESAND GEOLOGY Sand ordef to DIVISION OF MINES AND GEOLOGY, P 0 Bo~ 2980. Sacramento.Caillorllla 95812·2980,NAME _

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OFR 91-19 is the fifth In a series ofreports on the Turkey Flat data. summariz­ing the results from phase IV Weak~Motion

Blind Prediction Test. 1be report presentspredictions subminoo during phase IV ofthe Turkey Flat experiment. plus compari­sons of those predictions to weak-motionobservations. The report is composed oftwo principal sections: I) a summary ofresults of the statisticaJ analyses and com­parisons with observations. and 2) a seriesof appendices containing all the plots of theanalyses and comparisons. The companionReport 6 (Orn 91·20) presents all theweak-motion OOselVatlons for Turkey Flaland the implications of the simple modelingof these obselVations done at the Divisionof Mines and Geology. Reports 5 and 6(OFRs 91-]9 and 91-20) v.iJl fonn a basisfor interpreting strong-motion results for

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DMG OFR 91-19 11JRKEY FlAT. USA.SITE EFFECfS TEST AREA. REPORT 5,WEAK-MOTION TEST STATISTICALANALYSIS OF SUBMITTED PRfDlcnONSAND COMPARISON TO OBSERVATIONS.By Chris H Cramer and Charles R Real.1991. 230 p., 177 ftgures. 9 tables. and 7appendices, $7.00

Reference copies of OFR 91-17 are avail­able in the Sacramento, Los Angeles. andSan Francisco DMG offICeS. Blueline copiesmay be purchased at the Sacramento andLos Angeles offices. Color copies are avail·able at 1M Los Angeles ollice. For prices call(213) 62Q.3560.Y

the Newport-Inglewood. Elsinore-Whinier,San Jacinto. and Banning fault zones.

Lc~----

OFR 91·14 is available for purchase andreference at Sacramento. San Francisco.and Los Angeles DMG offices.....

The Santa Ana 1:100.000 quadranglelies between 33~ 30' and 34~ north latitudeand 117~ and 118" west longitude andcovers an area of approximately 2.000square miles (5.200 km'Z). The map includesthe densely populated Orange CountyCoastal Plain and the rapidly urbanizingregion of western Riverside County. Thereport is a product of the Division of Minesand Geology's (DMG) Regional GeologiCMapping Program. whkh is charged Withgathering. analyzing. and disseminatinginformation regarding California's regionalgeologic setting. It is also supported byCOGEOMAP, a cooperative mappingproject of DMG and the U.S. GeologicalSurvey. The I: 100.00Ckca1e format per­mits a regional synthesis of detailed geologicmapping (based mostly on 1: 12.000- and1:24.000·sca1e source maps) which pro,vides a useful perspective toward under­standing the stratigraphy. structure. andgeologic history of the region. 'The1: IOO.()()().scaJe presentation is appropri­ate for land-use planning applications suchas de:lineating regionaUy significant mineralresource areas. evaluating geologic hazards.and providing an effective basis for planningfuture geologic work in the region.

(a unit 01 acceleration used With gravitymeasurements). Gravity contours are on aplanimetric base With coastline and culturalfealtlres.

OFR 91-17 consists of three plates: ageologic map. a geologic explanation. andan index to source maps. The geologicmap. compiled on a topographic base.shows the regional franw.vork geology andprovides basic information regarding therelative age. description. and distribution ofthe various rock typeS. and locations offaults and other structural features. The areaconsists of complexly folded Late Creta­ceous through Tertiary marine sedimentaryrocks of the southern Los Angeles Basin,These rocks were deposited on Late Creta­ceous granitic rocks and Late Jurassic toEarly Cretaceous metasedimentary andmetavolcanic roof pendants of the northernPeninsular Ranges. 'The area is broken intostructural blocks bounded by active faults of

DMGOFR 91-17 GEOLOGIC MAPOF 11-IE SANTA ANA 1: 100.000 QUAD­RANGlE. CAUFORNJA. Compiled byRichard B. Greenwood and Douglas M.Morton. 1991. $8.00

CALIFORNIA GEOLOGY JANUARY, FEBRUARY 1992 25

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three subjects; 1) two weak·motion datasets recorded at Turkey Rat and the ob­sented site response (empirical transferfunctions) in tenns of Fourier spectralratios; 2) simple modeling of the ob­sented spectral ratios with site amplifica­tion functions from SHAKE (Schnabel.and others. 1972): and 3) an examina­tion of the character of the Weak·MotionTest Event.

The principal objective of the TurkeyRat Project is to systematically test andcompare all methods of estimating theinfluence of local geology on groundmotion during earthquakes to determinethe reliability and cost effectiveness ofeach. Secondary objectives are to gener­ate a data base for the improvement ofthese methods. or to develop new meth­ods, and 10 address the long-standingdebate on the linearity of site response.The approach is to collect high-qualityweak and strong ground-motion data andgeotechnical data. and to carry out aseries of "blind predictions." Expertsfrom around the world are invited to usetheir preferred method and the acquireddata to predict ground motion at loca­tions where the response will be knownbut held in confidence until all predictionshave been submitted.:'::

Southern California Regional Office107 South Broadway. Room t 065

Los Angeles. CA 90012-4402(2t3) 620·3560"

OFRs91-19 and 91-20 can be pur­chased at the Sacramento office. Referencecopies are available at the Sacramento, SanFrancisco. and los Angeles offices.

DMGOFFICES

3 yrs. $23.00(18 issues)o

DMGOFR 91-20l1JRKEY FLAT.USA. SITE EFFECTS TEST AREA.REPORT 6, WEAK-MOllON TEST:OBSERVATIONS AND MODEUNG. ByChris H. Cramer. 1991. 93 p.. 27 figures.5 tables. $7.00

the acqUired data to predict ground motionat locations where the response will beknOVJT1. but held in confidence until allpredictions have been submilled. x

OFR 91-20 is the sixth in a series ofreports on the Turkey Rat Project. It pre­sents the weak-motion observations madeby the Division of Mines and Geology(DMG) and the simple modeling done byDMG as part of phase IV, Weak·MotionBlind Prediction Test. The report covers

O 2 yrs. $15.50(12 issues)

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Turkey Flat once strong-motion data areobtained.

The principal objective of the TurkeyRat Project is to systematically test andcompare all methods of estimating theinnuence of local geology on ground mo­tion during eanhquakes to determine thereliability and cost effectiveness of each.Secondary objectives are to generate a database for the improwment of these meth­ods. or to develop new methods, and toaddress the long-standing debate on thelinearity of site response. The approach isto collect high-quality weak and strongground-motion data. and geotechnical data.and to cany out a series of "blind predic­tions." Experts from around the world areinvited to use their preferred method and

o

ooo

26 CALIFORNIA GEOLOGY JANUARYIFEBRUARYl992

Quaternary Studies

CROSSING THE BORDERS: Quater­nary Studies in Eastern California andSouthwestern Nevada. Edited by JenniferReynolds. 1991. San Bernardino CountyMuseum Association. 2024 Orange TreeLane. Redlands, CA 92374. 133 p.$15.00. paper cover.

The San Bernardino County MuseumAssociation prepared this special publica­tion of 29 papers in conjunction withthe 1991 Mojave Desert QuaternaryResearch Center Symposium held May17 - 20. A two-day field trip guideexplores the Mojave Desert and Basinand Range provinces of the far easternMojave Desert. western Nevada, andsouthern Death Valley. A half-day fieldhip guide presents an oveTView of thesurficial geology of the Cima volcanicfield, The volcanic center near lathropWells. Nevada is the subject of anotherpaper, Other topics include the geologi­cal history of southern California andNevada. the geology of the KingstonRange area. extension and uplih. thrustfaults. Miocene sedimentation. faujasite(a rare zeolite), Tertiary and Quaternarygravels, the Mountain Pass mine andalkalic complex. and late Quaternarypaleoecology. Several paleontologicalpapers report on the area's fossils, in­cluding a late Pleistocene Shasta groundsloth. One paper deals with archaeologi­cal investigations in northern las VegasValley. while others provide the colorfulhistories 01 the Mescal mine and the OldSpanish Trail.

Extensional Tectonics

GEOLOGICAL SOCIETY OFAMERICA MEMOIR 176: Basin andRange Extensional Tectonics near thelatitudes of las Vegas. Nevada. Editedby Brian P. Wernicke. 1990. The Ge0­logical Society of America. P.O. Box9140. Boulder. CO 80310. 511 p..9 plates. $115.00. hard cover.

The Basin and Range province inthe southwestern United States containssome of the most spectacular exposuresof extensional structures in the world.Many new interpretations and mcxlelsbased on work in this region are pre­sented in this collection of 26 articles.

The volume is dedicated to BennieW. Troxel and lauren A. Wright for theirpioneering work in the recognition oflow-angle nonnal faults in the Basin andRange. They have inspired many geolo­gists now working in the region, severalof whom are contributors to this volume.

The first chapter presents a state-ofthe-art review of the plate tectonic settingfor western North America during theCenozoic. New interpretations of mag­netic anomaly data from the North Pacificprovide insight into this pericxl of transi­tion from a subduction to a transfonnplate boundary.

Subsequent chapters address the prob­lems 01 distinguishing between nonnalfaults and thrust faults and describing theinteractions between nonnal faults andstrike-slip faults. Also. the timing andrelationships of regional extension arecompared with magmatic activity. sedi­mentation. and regional stratigraphy.

Two articles deal with the sensitiveissues regarding the geology of theNevada Test Site and of Yucca Mountain,a proposed site for high-level radioactivewaste.

Several papers focus on areas inCalifornia's Death Valley and MojaveDesert. LANDSAT thematic mapperimagery and detailed mapping are usedto analyze the relationships between fault­ing and uplift of the Panamint Rangewest of Death Valley. Seismic reflectionimages are used to compare the reflectivecharacter of the upper crust in theMojave Desert with that of the DeathValley region.

These articles provide an excellentintrcxluction to the study of extensionaltectonics. detachment faulting, and theCenozoic history of western NorthAmerica. They contain numerous maps.sketch drawings, diagrams. photographs,seismic line interpretations. and crosssections. An extensive index completesthe volume. GSA Memoir 176 would bea timely, concise. and infonnative addi­tion to any geologic reference library.Review by C.L. Pridmore.

Sonoran Geology

STUDIES OF SONORAN GEOLOGY,Special Paper 254. Edited by Efren Perez­Segura and Cesar Jacques-Ayala. 1991.The Geological Society of Amelica. P.O.Box 9140. Boulder. CO 80301-9140.130 p. $32.50. soft cover.

Researchers realize the importance ofthe Sonoran region (northwestern Mexico)in understanding the tectonic evolution ofthe southwestern margin of the NorthAmerican continent. This volume bringstogether important contributions toSonoran geology made primarily in thelast 10 years.

Topics in this book are: the relationbetween the Paleozoic strata on oppositesides of the Gulf of California; UpperTriassic nonmarine and shallow-marinerift-basin deposits; the depositional envi­ronment of the Santa Clara Formation(Upper Triassic Barranca Group); paleon­tology and biostratigraphy of Cretaceousrocks of the lampazos area; the geologyand chemical composition of the Jaralitoand Aconchi batholiths; the geology ofthe Yecora area: Quaternary shorelinesalong the northeastem Gulf of Califomia:and Mesozoic coal deposits.

Paleontology

INLAND SOUTHERN CAUFORNIA:THE LAST 70 MIWON YEARS-A Sell-Guiding Tour of Major Paleontolo­gic Localities from Temecula to Red RockCanyon: Fossils. Structure. and GeologicHistory. Edited by Michael O. Woodburne.Robert E. Reynokls. and David P. Whistler.1991. San Bernardino County MuseumAssociation. 2024 Orange Tree Lane.Redlands, CA 92374. 40 p. $15.00.paper cover.

The Mojave Desert is the dominantgeological province of inland southernCalifornia. The early Tertiary highlandwas eroded and then subjected to intensemid-Tertiary regional north-south exten­sion. Alluvial and lacustrine sediment accu·mulated over about 4.000 rniles (6.500km) before being fokled and faulted.Michael O. Woodbume provides a descrip­tion of the province's stratigraphy andgeologic history. A three-day field tripguide focuses on the Mojave Desert andintrcxluces 16 additional papers relatingto the paleontology of inland southernCalifornia. Two of these papers presentstudies of late Cretaceous (?) Plesiosaursand of Barstovian Oate Miocene} tridactylhorses; others describe numerous fossilfauna of various fonnations and locales:><:

CALIFORNIA GEOLOGY JANUARY/FEBRUARY 1992 27

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Moderately-dipping sandstones and conglomerates in the Whipple Mountains ot southeastern California. Faulted and tilted redbeds like thiSare distinctive 01 detachment taulted terranes. Photo by C.L. Pndmore

28 CALIFORNIA GEOlOGY JANUARY FEBRUARY 1992