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Quaternary Exhumation of the Verdugo Mountains? Constraints from (U-Th)/He Ages and Geomorphology

Jeanette C. Arkle

Advisor: Dr. Phillip A. Armstrong

Quaternary Exhumation of the Verdugo Mountains? Constraints from (U-Th)/He Ages and Geomorphology

Jeanette C. Arkle

Advisor: Dr. Phillip A. Armstrong

Undergraduate Thesis

Bachelor of Science in Geology

Department of Geological Sciences California State University, Fullerton

May 2008

Table of Contents Abstract . . . . . . . . . 1 Introduction . . . . . . . . . 2 Geologic Background . . . . . . . 3 Methods . . . . . . . . . 4 Low-temperature Thermochronology Methods . . . 4 Tectonic Geomorphology Methods . . . . . 6 Results and Interpretations . . . . . . . 8 AHe age Results . . . . . . . 8 AHe age Interpretations . . . . . . 11 Geomorphology Results . . . . . . 12 Geomorphology Interpretations . . . . . 14 Discussion . . . . . . . . . 16 Conclusions . . . . . . . . . 19 Acknowledgments . . . . . . . . 21 References Cited . . . . . . . . 22 Figures . . . . . . . . . 24 Appendix A . . . . . . . . . 37 Grain Dimension and Morphology Data Appendix B . . . . . . . . . 48

Individual Apatite Grain Descriptions and Photographs

Appendix C . . . . . . . . . 58 Apatite (U-Th)/He Data Appendix D . . . . . . . . . 61 Geomorphic Analysis

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Abstract

The Verdugo Mountains are bound to the south by the Verdugo blind-thrust and to the

north by the Sierra Madre thrust of the San Gabriel Mountains. This is a key location to

assess the rates of movement on a fault located beneath a major metropolitan area.

Evidence from low-temperature thermochronometry, geomorphic properties, and

stratigraphic sequences suggest that the VMB was recently (late Pliocene-Quaternary)

uplifted and exhumed. Apatite (U-Th)/He ages (AHe), from samples collected along

transects across the 950-m-relief VMB antiform, range from 13±4 to 78±2 Ma.

Elevation-projected AHe ages suggest approximately 2 km of erosion since 2.0 to 0.5 Ma

at an exhumation rate of 1 to 4 mm/yr, respectively. Geomorphic analyses support and

are consistent with young and rapid uplift suggested by the AHe ages. This block is a part

of the complex fault system that generally youngs southward away from the San Gabriel

Mountains and suggests that the range-front fault system has stepped southward into the

basin so that the basin-bounding fault may now be the Verdugo Fault.

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Introduction

The Verdugo Mountain block (VMB) (Verdugo Hills) is situated in the Los

Angeles basin and abuts the Transverse Ranges to the south (Figure 1). The VMB is

bound to the north by the Sierra Madre fault and the Verdugo fault to the south. The

structural geology and tectonic evolution of the Los Angeles basin is well documented,

but is very complex. Ingersoll and Rumelhart (1999) model the evolution of the Los

Angeles basin in three stages that include: (1) transrotation (18-12 Ma), (2) transtension

(12-6 Ma), and (3) Transpression (6-0 Ma). They interpret that exhumation of Mesozoic

middle-crustal rocks were a result of extension along detachment faults due to

transrotation. The activation of the San Andreas Fault occurred during transpression,

which formed positive inversion structures such as the Puente Hills, San Jose Hills, and

the Verdugo Hills.

The San Gabriel Mountains are a major block of the Transverse Ranges in

Southern California and have relatively well-constrained exhumation rates based on

apatite (U-Th)/He and fission-track data (Blythe et al., 2002). Meigs et al. (2003)

estimated uplift and exhumation of the VMB by measuring stratigraphic thickness across

structural highs and lows; they coupled these data with the San Gabriel Mountains (U-

Th)/He data from Blythe et al. (2000) to conclude that the VMB experienced uplift of

2.5-4.0 km and exhumation of ~1.5-2 km at rates of 1.1 km/Myr and 0.9 km/Myr,

respectively. These estimates assume that the Saugus Formation is complete and

continuous over the VMB. However, no quantitative data have been published for the

VMB to positively constrain exhumation. In this study, apatite (U-Th)/He ages (AHe)

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and geomorphic data are used to better constrain timing and rates of exhumation in a

region associated with the major fault systems in California.

Geologic Background

The VMB is expressed as an east-west trending antiform that is ~15 km long and

~6 km wide, has a maximum elevation of 953 m, and is composed mainly of late

Mesozoic quartz monzonite-granodiorite (Dibblee 1989; 1991; 2001) (Figure 2). The

Verdugo fault is a blind thrust that dips north under Miocene strata and is associated with

mid-crustal decollement (Fuis, 2001), which is part of a generally southward-propagating

zone of contractional deformation related to the “Big Bend” region of the San Andreas

Fault (Figure 3) (Luyendyk, 1991; Yeats, 1981). The structure between the hanging wall

of the Verdugo fault to the footwall of the Sierra Madre fault is characterized by an

anticline-syncline pair that form the Verdugo Mountains and Merrick syncline (Figure 4)

(Meigs et al., 2003). The nonmarine Saugus Formation was deposited from about 2.3 to

0.5 Ma and is exposed in the Merrick syncline and within the footwall of the Verdugo

fault (Levi and Yeats 1993; Meigs et al., 2003). The Verdugo fault is completely covered

by sediments, thus it is unknown how active it has been in the Quaternary.

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Methods

Low-temperature Thermochronology Methods

Low-temperature thermochronology has proven to be a powerful tool for

determining thermal histories in the upper 1-6 km of the crust (e.g., Armstrong et al.,

2003; Blythe et al., 2000; Ehlers and Farley, 2003; Green et al., 1989). Specifically,

apatite (U-Th)/He ages may be used to constrain the cooling history of rocks in the upper

1.5-3 km of the crust (Farley, 2000), which allows us to measure uplift and exhumation

due to tectonic or denudation processes based on the accumulation of radioactive isotopes

(Wolf et al., 1996). The spontaneous decay of 235U, 238U, 232Th, and 147Sm, produces α

particles (4He) and when at temperatures of less than 70-75 ˚C, the isotopes behave as in

a closed-system and He daughter products are partially retained in the apatite crystals.

Conversely, at higher temperatures, isotopes behave as in an open-system and He

daughter products are diffused out of the crystal. At temperatures above 85 ˚C there is

complete diffusive loss of He and the AHe age is reset to zero (Wolf et al., 1998). The

retention of He is thermally sensitive, thus the switch from diffusive loss to retention is

gradual within an apatite crystal, and occurs over an interval of ~40-70 ˚C and He is only

partially retained.

Depths that correspond to these temperatures are referred to as the partial

retention zone (PRZ) (Farley, 2000). Assuming a normal geothermal gradient (in

southern California it is ~30 ˚C/km (Wright, 1991)) and a surface temperature of 15 ˚C,

the PRZ corresponds to crustal depths of ~1.5-2.5 km. The specific temperature at which

an apatite crystal retains all He daughter products, which reflects its apparent age, is

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dependant on cooling rate, grain size, and chemical composition. This temperature is

referred to as the closure temperature (Tc) (Ehlers and Farley, 2003). For typical crystal

sizes and compositions, and at rapid monotonic cooling rates (greater then 10 oC/m.y.),

the closure temperature for apatite is ~70 ˚C (Farley, 2000). Thus, AHe ages record a

cooling history from which exhumation rates can be calculated (Farley, 2000).

Apatite is an accessory mineral usually found in medium to coarse-grained

granodiorites or tonalities. After using normal apatite separation techniques, the

following criteria are used to pick individual crystals for dating: (1) the grain should be

greater than 80 µm in diameter to ensure there are substantial amounts of He to work

with and to limit the possibility of α ejection; (2) grains are euhedral, to reduce the error

for imbalanced (or parentless) isotopes; (3) grains should be inclusion free to reduce the

possibility of high-U phases, such as zircon, that would supply excess uranium and

thorium and yield an erroneously high He date (Ehlers and Farley, 2003; Wolf et al.,

1996). Original grain sizes and geometries are measured and AHe ages are multiplied by

a factor of 1.2 to 1.5 to correct for α ejection (Figure 5) (Ehlers and Farley, 2003).

To extract He, individual apatite crystals are heated with a laser to ~900 ˚C, which

degasses the He that is collected and analyzed in a mass spectrometer (House et al., 2000).

A second reheating will reveal inclusions within an apatite crystal if He is re-extracted; in

these cases the sample is omitted. Grains are dissolved in nitric acid and sent through an

inductively coupled plasma mass spectrometer (ICP-MS) where U, Th, and Sm content

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are measured. The apparent age (or AHe age) reflects the amount of He present at time t

and is determined by the equation from Wolf et al. (1998):

4He=8 238U (e λ238t - 1) + 7 (238U/137.88) (e λ235t - 1) + 6 232Th (e λ232t -1)

where:

4He, 238U, and 232Th = the measured present-day amount of these isotopes

λ238, λ235, and λ232 = the decay constants for each isotope

t = the Helium age

Tectonic Geomorphology Methods

The field of tectonic geomorphology provides quantitative methods for evaluating

relative ages of topographic features and for assessing the mechanisms and rates of

geomorphic processes (Burbank and Anderson, 2001). Tectonics and climate are the two

primary Earth processes that tend to shape topography. Inherently, tectonic activity

begets specific geomorphic features that can be associated with a particular type of fault

and to some extent the rates and/or timing of activity. Folds, like the Verdugo anticline,

are geomorphic features that often form in response to buried faults, such as the blind

thrust faults (Montgomery and Brandon, 2002). The morphologic expression of the VMB

shows distinct spatial variation of denudation and topographic dissection. These features

are quantified and compared with AHe ages to make general interpretations of rates of

exhumation across the VMB.

Elevation profiles that trend N-S and E-W were extracted from 10m Digital

Elevation models (DEM) that provided a measure to qualify general slope morphologies

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and to compare the varying regions of the VMB. The gradient for of the individual slope

faces was calculated (change in elevation over the change in distance) along four

transects and a mean value was derived. The gradient values were used to calculate the

angle between the horizontal plane and the surface of the mountain side. This calculation

yields individual slope angles, which were averaged.

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Results and Interpretations

AHe age Results

A suite of nine samples collected along transects across the 950-m-relief VMB

antiform are identify as late Mesozoic quartz monzonite-granodiorite and quartz diorite,

with rock ages of 90 and 120 m.y. (Dibblee, 1989; 1991; 2001), respectively. The

measured AHe data for individual grains from each of the samples are summarized and

given in Table 1. The raw AHe ages are multiplied by a factor of 0.70 to 0.86 (the Ft) to

correct for α ejection (Ehlers and Farley, 2003). The FT values are based on

measurements of original grain size and grain geometry (see Appendix A for these

measurements). Cracks and chipped grains provide conduits where He may diffuse out of

the crystal and thus, is not accounted for in the gas collection process. The FT correction

increases AHe ages slightly (by 2-4 M.y.) to a more accurate corrected AHe age that

accounts for these defects.

The V1 –V8 apatite (U-Th)/He ages were determined from 2-4 grains from the

corrected AHe ages for each sample (Figure 6). Uncertainties are reported as the standard

error (at 1σ). Problem grains were excluded based on three criteria: (1) helium was re-

extracted; (2) the AHe age was older than the rock age; and (3) the age was an outlier (i.e.

the crystal contained an inclusion(s)). The grains V1c and V1d both re-emitted He upon a

second heating and the latter was not apatite. These grain ages were therefore excluded

from sample V1 and the mean AHe age is 13±4 Ma. Grains from sample V2 show a large

variation in AHe ages (~27 m.y.) however, none of these grains violate the given criteria

for a “bad” grain and were all used for a mean AHe age of 56±6 Ma. All four grains

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from sample V3 have AHe ages that are close in range (~ 6 m.y.); however, He was re-

extracted from samples V3a and V3c and thus, they were not included in the mean

sample AHe age of 13±3 Ma. Sample V4 contained two grains, V4c and V4d, with AHe

ages that are older than the rock, 135 and 196 Ma, respectively. These grains were not

included in the mean AHe age for V4. Grains V4a and V4b contained very low uranium

concentrations that may have attributed to a younger AHe age respective to the other

grains in this sample. Regardless, the AHe age was calculated with these two grains and

is 78±2 Ma. Sample V5 contained four good grains that average to an AHe age of 32±3

Ma. Sample V6 contained three grains that are within age range of one another; however,

grain V6c had a re-emission of He upon a second heating and grain V6a is an outlier, thus

both were omitted. The mean AHe age for sample V6 is 17±1 Ma. A possible inclusion

was identified in grain V7d during the final inspection of the grains. This grain was not

processed and the mean AHe age, 36±7 Ma, was calculated from the three other grains.

Grain V8d has an AHe age of 40 m.y., which is about 21 m.y. older than the other three

grains in sample V8. This grain is considered an outlier and was omitted. Grains V8a-

V8c have AHe ages that are close in range (~3 m.y.) and have a mean AHe age of 17±1

Ma. Sample V9 contains four grains that have AHe ages that are within 4 m.y. of one

another. The mean AHe age for this sample is 16±1 Ma.

These corrected mean AHe ages are plotted with respect to elevation (Figure 7)

and located on a three dimensional digital elevation model of the VMB (Figure 8). These

data range from 13±4 to 78±3 Ma and show increasing age with increasing elevation with

the exception of sample V2. Sample V2 has an AHe age of 56±6 Ma and is located about

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400 m above sea level. Three other samples from this study (V1, V3, and V9) were

collected at approximately the same elevation and have AHe ages ~ 40 m.y. younger than

sample V2. The relatively older age could be considered an outlier; however, we suggest

that the AHe age reflects the location of sample V2. Sample V2 was collected on the far

west flank of the VMB and lies to the west of a small fault that separates it from the other

eight samples. This side of the block may not have had the same uplift and exhumation

history as the east side of the VMB where the majority of the samples were collected.

Since sample V2 lies in a different structural domain we have omitted it from the general

trends that will be discussed in out interpretations.

AHe ages at the highest elevations from samples near the summit (~910 m) to

lower elevations (~766 m) decrease substantially from ~78±3 to 17± 1 Ma, respectively,

with a slight decrease in elevation (Figure 8). These data suggest that during this period

these rocks were stalled and/or very slowly being exhumed as they traveled through the

PRZ. Samples V6, V8 and V9 show little age variation at ~17 Ma with a sharp decline in

elevation starting at ~770 to ~380m. This implies that at ~17 Ma cooling was initiated (or

the first phase of rapid cooling). At similar elevations, but slightly below ~380m, AHe

ages deviate from this mean age to ~4 m.y. younger. These samples (V1 and V3) have

AHe ages of ~13 Ma and are the lowest elevation samples collected (~350m). These data

allude to a shallowing trend in age versus elevation which suggests the initiation of slow

cooling; however there is the possibility that these data show a natural age range.

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AHe age Interpretations

Distinct breaks in slope of the elevation-age data show two possible, Miocene to

recent, cooling histories: (1) a single phase of slow/steady cooling since the Miocene (13-

17 Ma); and (2) two punctuated periods of rapid cooling with the first occurring during

the Miocene (~17 Ma), and the second occurring subsequent. In order to evaluate when

this later cooling event may have occurred, predicted AHe ages were plotted at lower

elevations following different data trends that reflect two possible cooling histories

(Figure 9).

Interpretation (A) (Figure 9) is constrained by the work from Tsutsumi and Yeats

(1999) and Meigs et al. (2003). Their work shows that Miocene-Quaternary strata project

over the Verdugo Fault to the footwall (Figure 4) and implies that exhumation of the

VMB was concurrent with and/or began after deposition of the Saugus Formation.

Considering this stratigraphic control, samples were projected to the ages of base and the

top of the Saugus Formation, 2.3 to 0.5 Ma, respectively. This interpretation (A) suggests

that granitic rocks slowly cooled prior to ~17 Ma and a punctuated period of rapid

cooling occurred for about 4 My during the Miocene. This initial period of rapid

exhumation occurring ~17 Ma may be associated with the start of transrotation of the Los

Angeles basin, which Ingersoll and Rumelhart (1999) suggest occurring between 18-12

Ma. The break in slope around 13-17 Ma suggests slow cooling to between 2.3 and 0.5

Ma, when the second phase of rapid cooling and exhumation ensued. Initiation of rapid

exhumation at these time periods accounts for ~2 km of erosion and VMB exhumation

rates of between 1 and 4 mm/yr.

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Interpretation B (Figure 9) treats the lowest elevation samples as being essentially

the same age as indicated by the overlapping uncertainties and disregards the break in

slope at 16 Ma at the lowest elevations. Predicted AHe ages are plotted along a best fit

line between the lowest samples (V1 and V3) and the upper break in slope at 17 Ma (V8).

This trend in projected data suggests that granitic rocks were stalled in the shallow crust

slowly cooling prior to the early Miocene (~17 Ma), and since the late Miocene, ca. 13-

17 Ma, they have been rapidly cooling. These data suggest that at these ages and

corresponding elevations denudation of about 1.8 km started 13-17 Ma at an exhumation

rate of ~0.1 mm/yr.

Geomorphology Results

Mean relief and mean slope angles are calculated from topographic profiles of the

VMB 10-meter digital elevation model or DEM (Figure 10). An east-west trending

topographic profile (A-A’) was constructed over the VMB that extends approximately 16

km (Figure 11). This topographic profile was constructed generally along the ridge line

that separates the north and south faces of the range, indicated by the cross-section line.

The cross-section A-A’ was measured from east to west, reaching a minimum elevation

of about 300 m on both sides, and a maximum elevation near the peak of about 940 m.

This topographic profile shows a steeper gradient on the east (107 m/km) than the west

side (74 m/km) (Figure 11).

Three north-south topographic profiles (B-B’, C-C’ and D-D’) reveal that the

south side of the range has a steeper mean gradient (248 m/km) than the north facing side

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(152 m/km) (Figure 11). The asymmetry between the north and south sides of the VMB

is typical of fault-propagation folds where the steeper dipping side is the forelimb (the

south face) and the more shallow dipping side lies over the decollement zone (the north

face) (Keller and Pinter, 2002). These topographic profiles were constructed over the

inner-fluves that generally follow along the indicated cross-section lines. The cross-

section B-B’ was measured from south to north, reaching minimum elevations of about

250 m and 380 m, respectively, and a maximum elevation of about 870 m. This was the

widest of the three cross-sections, extending approximately 5400 m. The cross-section C-

C’ was measured from south to north, reaching minimum elevations of about 470 m and

500 m, respectively, and a maximum elevation near the peak of about 920 m. The cross-

section D-D’ was measured from south to north, reaching minimum elevations of about

340 m and 375 m, respectively, and a maximum elevation of about 590 m. The VMB

anticline displays a flat ramp geometry that is common of thrust-folded orogens, which is

most likely responsible for a higher elevated basin on the north side of the range.

The gradient values were used to calculate the mean slope angle for the east and

west flank as well as the north and south faces of the VMB. Mean slope angles are often

used in lieu of slope gradients and are useful when comparing slopes angles to well-

developed geomorphic models (e.g. Blythe et al., 2000). It is generally accepted that the

magnitude and rates of erosion are slope dependant and can therefore, be determined by

slope angle (Burbank and Anderson, 2001).

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Mean slope angles calculated on the east and west flanks are 6.1˚ and 4.3˚,

respectively. The mean slope angle difference from east to west is ~2˚. These data

suggest the steeper east flank has been uplifted and exhumed more than the west flank,

which perpetuates higher rates of denudation. Increased denudation on the east slope

may be responsible for steeper slope angles on the east flank of the range compared to the

west flank.

The mean slope angles calculated on the north and south face of the VMB are 8.7˚,

14.0˚, respectively. The south face of VMB has a considerably larger mean slope angle (a

difference of 5.3˚) than the north. These data are consistant with well-developed models

that show that the magnitude of erosion is higher on the forelimb of a fold compared to

the backlimb (Burbank and Anderson, 2001). These slope angles may indicate that rates

of denudation and exhumation on the south side are higher than the north side.

Geomorphology Interpretations

In regions of tectonically active mountain blocks indicators of geologically recent

uplift events, are associated with steep relief (Blythe et al., 2000; Burbank and Anderson,

2001). It is generally recognized that overall mountain front gradients increase when they

are uplifted. Consequently, rates of downslope transport increases as the threshold slope

angle for failure is approached (Burbank and Anderson, 2001). Numerous factors affect

slope angle, however as slope angles increase mass wasting processes increase and result

in highly dissected landforms (Blythe et al., 2000; Montgomery and Brandon, 2002).

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The mean north (152 m/km) and south (248 m/km) gradient values are indicators

of steep topography which may be attributed to relatively recent uplift. The calculated

mean slope angles on the south (14.0˚) and west (6.1˚) flanks indicate high rates of

denudation that have resulted in moderate-highly dissected topography in these areas,

which also indicates recent uplift. Furthermore, these data imply that the south and

southeast flank of the VMB, adjacent to the Verdugo fault, has been uplifted more rapidly

than the north and northwest flank. These geomorphic features are consistent with

younger AHe ages on the south and southeast flanks and in combination suggest that

uplift and exhumation has been relatively rapid and recent at these locations.

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Discussion

Apatite (U-Th)/He ages from this study are consistent with rapid exhumation of

the Verdugo Mountains above the Verdugo Fault. Geomorphic analyses and AHe ages

imply that the south side of the VMB, adjacent to the Verdugo Fault, have been uplifted

and exhumed more rapidly than farther north into the hanging wall; thus, this antiform is

propagating to the southeast. Of the two interpretations outlined earlier in the AHe

section, the Quaternary rapid exhumation interpretation (A) is preferred based on regional

stratigraphic constraints and geomorphic indicators. The Saugus Formation is exposed in

the Merrick syncline and within the footwall of the Verdugo fault (Figure 4), which was

deposited from about 2.3 to 0.5 Ma, implying that exhumation of the VMB was

concurrent with and/or began after deposition of the Saugus Formation (Levi and Yeats,

1993; Meigs et al., 2003). Moderate-highly dissected and steep slopes characterize the

south and southeast flanks of the VMB and are consistent with AHe ages, which may

indicate relatively recent uplift as well. This rapid exhumation interpretation relies on

whether or not some or all of the Plio-Pleistocene Saugus Formation (2.5 – 0.5 Ma) was

deposited prior to uplift (similar to Meigs et al., 2003 interpretation). More samples,

however, at lower elevations are needed to confirm this interpretation.

Alternatively, it is quite possible that the VMB has a simpler and slower one

phase exhumation history, as suggested by interpretation (B). An important observation

needs to be addressed with regards to the relationship between the stratigraphic

information and the thermochronology data. This must be considered since the initiation

of exhumation that we infer relies heavily on the local stratigraphy; in-particularly, the

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Saugus Formation. The Saugus Formation has important implications for the timing of

exhumation; however, there is a possible inconsistent relationship between the location of

the cross-section and the complementing AHe age where it overlies. The cross-section,

used in this study, is constructed through the western flank of the range and projects north

through the Merrick syncline. The only sample located in this area, sample V2, has an

AHe age of 56±6 Ma. We suggest this age anomaly is a product of being located in a

distinct structural boundary that is not associated with the other AHe ages. If the west

side of the range has in fact experienced a different exhumational history than the east

side, it is possible that these stratigraphic sequences in the interpreted cross-section do

not project over the eastern flank of the VMB. If so, perhaps the VMB was a topographic

high during the Quaternary so that Saugus Formation could not have been deposited

overtop.

In contrast, it is possible that the rate of lateral propagation of the fold might have

been greater than vertical slip rates along the fault. If this were the case, then the Saugus

Formation could have been deposited over the area of the present day VMB and since

eroded away. A later transition from predominant strike-slip motion to a more dominate

thrust motion would promote vertical growth and increase denudation, concentrated over

the main decollement. This could be the reason why we see a structural boundary with an

older AHe age on the west flank and accelerated exhumation rates on the eastern flank.

These interpretations show that we have not yet fully constrained the exhumation history

of the VMB; however, we suggest that ca. 17 M.a. is a maximum bound for the initiation

of exhumation for this range.

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The youngest AHe ages in Verdugo Mountains (this study) are within the range of

the ages for the western San Gabriel block of the San Gabriel Mountains (Blythe et al.,

2000), but are considerably younger than the AHe ages from the Tujunga block located

just north of the Verdugo Mountains (Figure 12). Data from Blythe et al. (2000) show

apatite FT and apatite He ages that range from 16±2 to 63±6 Ma in the western San

Gabriel block. They suggest that slow cooling lasted from ca. 40 – 15 Ma for this block.

AHe ages in the VMB show similar middle Miocene cooling that indicates a relatively

static thermal history from ca. 78 to 17 Ma. Further to the west and just north of the

VMB, the Tujunga block yielded older AFT and AHe ages ranging from ca. 47 to 60 Ma

and 33 to 42 Ma (Blythe et al., 2000; 2002), respectively. Blythe et al. (2002) suggest that

these older ages represent more recent exhumation (ca. ~7 Ma) when the Sierra Madre

fault transitioned from a strike-slip to a more dominate thrust and increased exhumation

rates along the San Gabriel strand in the Tujunga region. AFT and AHe ages from the

Sierra Madre block range from ca. 12 to 13 Ma and 3 to 7 Ma (Blythe et al., 2000; 2002),

respectively. Blythe et al., (2000; 2002) suggest that these data represent a slower

cooling period from ca. 13 to 3 Ma preceded by the onset of rapid exhumation from ~3

Ma to the present. It may be reasonable to consider that the VMB had a similar slow

cooling phase followed by a recent and rapid cooling phase, as suggested by

interpretation (A) in this study (from ~12 to 2 Ma).

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Conclusions

A suite of quantitative data consisting of apatite (U-Th)/He ages have been obtained for

the Verdugo Hills of Southern California. In this study, apatite helium ages and

geomorphic data were used to better constrain the timing and rates of exhumation of the

VMB within the Los Angeles Basin. Stratigraphic controls and well-constrained

exhumation rates of the surrounding inversions (based on apatite (U-Th)/He and fission-

track data from Blythe et al., (2000; 2002)) have been used to place constraints and make

interpretations of the exhumational history of the VMB. Our major conclusions are:

(1) The earliest phase of rapid cooling seen in the VMB began ~17 Ma. This cooling may

be associated with transrotation within the L.A. basin from 18-12 Ma.

(2) These data from this study show two possible cooling histories from about 12 Ma to

the present. Interpretations (A) and (B) have been identified as rapid exhumation of ~ 4

mm/yr since ~2 Ma, and slow, steady exhumation of ~ 0.1 mm/yr since ~17 Ma.

Interpretation (A) is the preferred interpretation due to stratigraphic (the Saugus

Formation) and geomorphic controls from this study. Furthermore, data and regional

kinematic interpretations from other studies support a rapid exhumation interpretation.

(3) Interpretation (A) - Rapid exhumation: The VMB shows a similar exhumational

history to that of adjacent orogens in the L.A. basin. For example, during transtension

about 12-6 Ma, the Sierra Madre block experienced slow cooling. As the L.A. basin

transitioned to a transpressional region about 6-0 Ma strain was accommodated for along

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range-front thrusts. This may be reflected by the on-set of rapid cooling since ~3 Ma in

the western region of the Sierra Madre block. The projected AHe ages in this study seem

to mirror these data from surrounding regional studies; however, more data from the

VMB are needed to positively confirm this interpretation.

(4) Interpretation (B) – Slow and steady exhumation: It is possible that the VMB has

exhumed slowly since 17 Ma and sat as a topographic high as the Saugus Formation was

being deposited from 2.3 to 0.5 Ma. Younger AHe ages in the VMB compared to the

Tujunga block suggest that strain imparted along regional decollements within the L.A.

basin may have been partially accommodated for more recently by the Verdugo thrust

(Tsutsumi et al., 2001).

(5) The relationship between the Tujunga, Sierra Madre and Verdugo blocks, regardless

of which interpretation is favored, suggests that the range-front fault system has stepped

southward into the basin in the Verdugo area and that the basin-bounding fault may now

be the Verdugo Fault. Farther east, the basin-bounding fault is the Sierra Madre thrust

and the main phase of deformation has not stepped southward in that area.

(6) Based on the conclusion above, the two very different interpretations presented may

be considered non-unique and thus better constrained with more AHe ages and possibly

AFT data from the VMB.

Arkle 21

Acknowledgments

I would like to thank: Dr. Phillip Armstrong for his continued guidance, insight, and

support. His dedication and motivation that he presents to students to achieve is

incredible and contagious; Dr. Kenneth Farley and Lindsey Hedges (California Institute

of Technology) for sample processing, providing (U-Th)/He data and valuable

instruction; and Brian Kohl for his aid with graphics. This project was funded by a

CSUF undergraduate research grant and a scholarship from the Victor Valley Gem and

Mineral Club.

Arkle 22

References

Armstrong, P. A., Ehlers, T. A., Chapman, D. S., Farley, K. A., and Kamp, P. J. J., 2003, Exhumation of the Central Wasatch Mountains, 1: Patterns and timing deduced from Low-temperature Thermochronometry data: Journal of Geophysical Research, v. 108, p. 2172.

Blythe, A. E., Burbank, D. W., Farley, K. A., and Fielding, E. J., 2000, Structural and

topographic evolution of the central Transverse Ranges, California, from apatite fission-track, (U-Th)/He and digital elevation model analyses: Basin Research, v. 12, p. 97-114.

Blythe, A. E., House, M. A., and Spotila, J. A., 2002, Low-temperature

thermochronology of the San Gabriel and San Bernardino Mountains, southern California: Constraining structural evolution: Geology, p. 231-250.

Burbank, D. W., and Anderson, R. S., 2001, Tectonic Geomorphology: Oxford,

Blackwell Science, 274 p. Dibblee, T. W., Jr., 1989, Geologic map of the Pasadena Quadrangle quadrangle: Dibblee

Geological Foundation; Map DF-23, scale 1:24,000. Dibblee, T. W., Jr., 1991, Geologic map of the Sunland and Burbank (North 1/2)

quadrangles: Dibblee Geological Foundation; Map DF-32, scale 1:24,000. Dibblee, T. W., Jr., 2001, Geologic map of the Hollywood and Burbank (South 1/2)

quadrangles: Dibblee Geological Foundation; Map DF-30, scale 1:24,000. Ehlers, T. A., and Farley, K. A., 2003, Apatite (U-Th)/He thermochronometry: Methods

and applications to problems in tectonic and surface processes: Earth and Planetary Science Letters, v. 206, p. 1-14.

Farley, K. A., 2000, Helium diffusion from apatite: General behavior as illustrated by

Durango fluorapatite: Journal of Geophysical Research, v. 105, p. 2909-2914. Fuis, G. S., Ryberg, T., Godfrey, N. J., Okaya, D. A., and Murphy, J. M., 2001, Crustal

structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California: Geology, v. 29, p. 15-18.

Green, P. F., Duddy, I. R., Laslett, G. M., Hegarty, K. A., Gleadow, A. J. W., and

Lovering, J. F., 1989, Thermal annealing of fission tracks in apatite 4: quantitative modeling techniques and extension to geological timescales: Chemical Geology (Isotope Geoscience Section), v. 79, p. 155-182.

Arkle 23

House, M. A., Farley, K. A., and Stockli, D., 2000, Helium chronometry of apatite and titanite using Nd-YAG laser heating: Earth and Planetary Science Letters, v. 183, p. 365-368.

Keller, E. A., and Pinter, N., 2002, Active Tectonics: Earthquakes, Uplift, and

Landscape: Upper Saddle River, New Jersey, Prentice Hall, 362 p. Levi, S., and Yeats, R. S., 1993, Paleomagnetic Constraints on the Initiation of Uplift on

the Santa Susana Fault, Western Transverse Ranges, California: Tectonics, v. 12, p. 688-702.

Luyendyk, B. P., 1991, A model for Neogene crustal rotations, transtension, and

transpression in southern California: Geological Society of America Bulletin, v. 103, p. 1528-1536.

Meigs, A., Yule, D., Blythe, A., and Burbank, D., 2003, Implications of distributed

crustal deformation for exhumation in a portion of a transpressional plate boundary, Western Transverse Ranges, Southern California: Quaternary International, v. 101-102, p. 169-177.

Montgomery, D. R., and Brandon, M. T., 2002, Topographic controls on erosion rates in

tectonically active mountain ranges: Earth and Planetary Science Letters, v. 201, p. 481-489.

Tsutsumi, H., and Yeats, R. S., 1999, Tectonic setting of the 1971 Sylmar and 1994

Northridge earthquakes in the San Fernando Valley, California: GSA Bulletin, p. 1232-1249.

Tsutsumi, H., Yeats, R. S., and Huftile, G. J., 2001, Late Cenozoic tectonics of the

northern Los Angeles fault system, California: GSA Bulletin, v. 113, p. 454-468. Wolf, R. A., Farley, K. A., and Kass, D. M., 1998, Modeling of the temperature

sensitivity of the apatite (U-Th)/He thermochronometer: Chemical Geology, v. 148, p. 105-114.

Wolf, R. A., Farley, K. A., and Silver, L. T., 1996, Helium diffusion and low-temperature

thermochronometry of apatite: Geochimica et Cosmochimica Acta, v. 60, p. 4231-4240.

Wright, T. L., 1991, Structural geology and tectonic evolution of the Los Angeles basin:

in Biddle, K. T., ed., Active Margin Basins, American Association of Petroleum Geologists, p. 35-134.

Yeats, R. S., 1981, Quaternary flake tectonics of the California Transverse Ranges:

Geology, v. 9, p. 16-20. Yeats, R. S., 2004, Tectonics of the San Gabriel Basin and surroundings, southern

California: GSA Bulletin, v. 116, p. 1158-1182.

Arkle 24

Figures: Verdugo Mountain Block

Lowlands

Upper Cretaceous -Cenozoic rocks

Santa Monica Formation

PR-SM basement

San Gabriel basement

119 W34 N

0 10 20 30

KILOMETERS

118 W

118 W

119 W

Ocean

PacificDume Fault

? ?

Malibu Coast Fault

P.V.Hills

Palos Verdes F.

Newport-Inglew

ood Fault

Los

Angeles

Basin CH

Puente Hills Thrust

Whittier Fault

Puente

HillsSAR

Santa Ana

Mountains

Chino F. PerrisBlock

Elsinore F.

Chino

Basin

JurupaMts.

San Jose Hills

SJF

San GabrielValley

WCF

SierraMadre Fault

Hollywood

SMF

SanFernando

Valley

F.

Verdugo F.

Verdugo Mts.

Santa Monica Mountains

Simi HillsSimi F.

SantaSusana

OakRidge Fault

Ventura

Basin

SanCayetano

F.

TopatopaMountains

Canton

Fault

Ridge Basin

San Gabriel F.

Soledad Basin

F. SFF

Sierra PelonaSanta Ynez Fault

SanGabriel

Mountains

San Gabriel Fault

VCF

Punchbowl

San

Andreas

Fault geology notshown

Fault

Cucamonga F.

SAC

F

SJcF

Study Area

S

N

Figure 1: Generlized geologic map of the central Transverse Ranges, San Gabriel Valley, Los Angeles Basin, and surrounding regions. Faults shown in heavy lines, dashed where covered or blind. Abbreviations: CH – Coyote Hills; PV Hills – Palos Verde Hills; RF – Raymond fault; SACF – San Antonio Canyon fault; SAR – Santa Ana River; SFF – San Fernando fault; SJF – Sand Jose fault; SJcF – San Jacinto fault; SMF – Santa Monica fault; VCF – Vasquez Creek fault; WCF – Walnut Creek fault. Regional map modified from Yeats (2004). Bold dotted line, S-N, shows location of cross section in Figure 3.

Arkle 25

N

S

Figure 2: Simplified geologic map of the study area. Faults are shown in heavy black lines and include: the Verdugo (Vf), Sierra Madre (SMt), San Gabriel (SGf), Raymond (Rf), Lakeview (Lt), Santa Susana (SSf). Other key features include: the Western San Gabriel Mountains (WSG), the San Fernando Valley (SFv), the Big Tujunga Wash (BTw), and the Merrick syncline (Ms). Figure from Meigs et al. (2003).

Arkle 26

SaugusFormation

SaugusFormation

SN

STUDY AREA

NS

Figure 3: Simplified crustal section across the Verdugo and San Gabriel Mountains showing approximate mid-crustal detachment depth. Location is from Figure 1 and geology is from Figure 2. Faults are shown in heavy black lines and include: the Verdugo (Vf), San Gabriel (SGf), and the San Andreas (Saf). Figure from Meigs et al. (2003).

Figure 4: Generalized cross section across the Verdugo Mountains. Faults are shown in heavy black lines and include the Verdugo (Vf), Sierra Madre (SMt), San Gabriel (SGf), and Lakeview (Lt). Key folds include the Verdugo anticline (projected strata) and the Merrick syncline (Ms). Base of the Saugus Formation (dark) is 2.3 Ma and the top is 0.5 Ma. Cross section modified from Meigs et al. (2003). A

rkle 27

Figure 5: From Ehlers and Farley, 2003. Diagram showing how α ejection effects the helium age of an apatite. When helium is produced kinetic energy allows it to travel through the crystal lattice, and it will remain within a diameter of ~20 µm to the parent. Isotopes that are located near an edge of a crystal have the potential to eject daughter products. The prism cross section must be measured and then multiplied by an appropriate correction factor in order to yield a more precise age.

Arkle 28

Table 1: Summary of (U-Th)/He Data Arkle 29

Sample Number

Location UTM

Elevation (m)

Raw AHe Age (Ma)

Ft Correction

Corrected AHe Age (Ma) Notes

V1(a):laser 379826 E 350 6.43 0.70 9.18 Very low uraniumV1(b):laser 3785922 N 12.76 0.72 17.79 Very low uraniumV1(c):laser 21.98 0.72 30.58 Re-emmsionV1(d):laser 3.65 0.65 5.60 Not apatiteMean AHe Age 13±4

V2(a):laser 376081 E 404 35.41 0.74 47.68V2(b):laser 3787518 N 56.38 0.79 71.48V2(c):laser 32.94 0.73 44.90V2(d):laser 47.80 0.78 60.86Mean AHe Age 56±6

V3(a):laser 385866 E 371 9.96 0.77 12.92 Re-emmsionV3(b):laser 3782421 N 9.17 0.84 10.89V3(c):laser 7.31 0.70 10.41 Re-emmsionV3(d):laser 12.14 0.74 16.47Mean AHe Age 13±3

V4(a):laser 381817 E 910 63.82 0.79 80.92 Very low uraniumV4(b):laser 3786946 N 59.19 0.79 75.11 Very low uraniumV4(c):laser 108.88 0.80 135.33 Older than rockV4(d):laser 155.31 0.79 196.57 Older than rockMean AHe Age 78±3

V5(a):laser 383188 E 749 27.43 0.76 35.86V5(b):laser 3786882 N 14.29 0.69 20.59V5(c):laser 27.64 0.76 36.51V5(d):laser 28.26 0.78 36.02Mean AHe Age 32±4

V6(a):laser 384342 E 504 28.36 0.73 38.59 Outlier AHe ageV6(b):laser 3786723 N 15.37 0.84 18.24V6(c):laser 11.11 0.78 14.22 Re-emmsionV6(d):laser 12.55 0.78 16.17Mean AHe Age 17±1

V7(a):laser 383315 E 812 16.67 0.78 21.44V7(b):laser 3785524 N 37.32 0.77 48.41V7(c):laser 29.30 0.77 38.15Mean AHe Age 36±8

V8(a):laser 383927 E 766 15.20 0.78 19.36V8(b):laser 3784888 N 13.42 0.84 16.07V8(c):laser 14.46 0.86 16.73V8(d):laser 31.40 0.78 40.40 Outlier AHe ageMean AHe Age 17±1

V9(a):laser 385358 E 383 10.50 0.76 13.73V9(b):laser 3784073 N 13.47 0.79 16.95V9(c):laser 11.55 0.71 16.17V9(d):laser 13.56 0.79 17.24Mean AHe Age 16±1*Location UTM coordinates: Datum is NAD 84, UTM zone 11* Mean AHe age: see text for the mean AHe age calculation. Error is given as the standard error.

Explanation for single apatite grains

Inclusion ?Re-extract

Older than the rock

Apatite grains used

0

20

40

60

80

100

V7-1 V7-2 V7-3

Sample V7 Helium Ages

(U-T

h)H

eAg

e(M

a)

Grain Number

Average: 36.0 ± 7.9

0

20

40

60

80

100

V8-1 V8-2 V8-3 V8-4


( U-T

h)H

eAg

e(M

a)

Grain Number

Average: 17.4 ± 1.0

0

20

40

60

80

100

V9-1 V9-2 V9-3 V9-4


(U-T

h )H

eAg

e( M

a)

Grain Number

Averge: 16.0 ± 0.8

0

20

40

60

80

100

V3-1 V3-2 V3-3 V3-4


(U- T

h)H

eAg

e(M

a)

Grain Number

Average: 13.7 ± 2.8

0

20

40

60

80

100

V2-1 V2-2 V2-3 V2-4


( U-T

h)H

eAg

e(M

a )

Grain Number

Average: 56.2 ± 6.1

40

60

80

100


Grain Number

Average: 13.5 ± 4.3

(U-T

h)H

eA

ge(M

a)

0

20

V1-1 V1-2 V1-3 V1-4

0

20

40

60

80

100

V6-1 V6-2 V6-3 V6-4


(U-T

h)H

eAg

e(M

a)

Grain Number

Average: 17.2 ± 1.0

0

20

40

60

80

100

V5-1 V5-2 V5-3 V5-4


(U-T

h)H

eAg

e( M

a)

Grain Number

Average: 32.2 ± 3.9

0

50

100

150

200

V4-1 V4-2 V4-3 V4-4


(U-T

h)H

eAg

e(M

a)

Grain Number

Average: 78.0 ± 2.9

Arkle 30

Individual Apatite Grain He Ages

Figure 6: Individual apatite grain He ages for samples V1-V9. Mean AHe ages given and uncertainties (shaded region) within the chosen suite of a sample are given as the standard error (at 1σ).

0

200

400

600

800

1000

0 10 20 30 40 50 60 70 80

V4

V1V2

V3

V5

V6

V7V8

V9

Sample from a different structural domain

(U-Th)/He Age (Ma)

Helium Ages vs. ElevationVerdugo Mountains, Southern California

Ele

vatio

n (m

)

Figure 7: Plot of average (U-Th)/He ages verses elevation for all samples. Error bars are reported as one standard error.

Arkle 31

Figure 8: Three dimensional digital elevation model of the Verdugo Mountains showing sample locations and faults as heavy black lines.

Arkle 32

Interpretation B: Slow Exhumation

T~ 45ºCTop of the Present-Day PRZ

Tc~ 60ºCEffective Closure Temperature

Base of the Present-Day PRZT~ 75ºC

-2000

-1500

-1000

-500

0

500

1000

0 5 10 15 20 25 30 35 40

Ele

vatio

n (m

)

AHe Age (Ma)

V5

V6

V7V8

V9V1

V3

EF

G

G1G2

~1.8 km of erosion in 13 – 17 My

Interpretation A: Rapid Exhumation

Tc~ 72ºCEffective Closure Temperature

Base of the Present-Day PRZT~ 75ºC

T~ 45ºCTop of the Present-Day PRZ

-2000

-1500

-1000

-500

0

500

1000

0 5 10 15 20 25 30 35 40

Ele

vatio

n (m

)

AHe Age (Ma)

V5

V6

V7V8

V9V1

V3

AB

C

D

~2 km of erosion in 0.5 – 2 My

D1

D2

Figure 9: Verdugo AHe ages (solid diamonds) and predicted AHe ages (open diamonds). Predicted age trends reflect two possible cooling histories. Interpretation A: (A) At highest elevations, ages decrease substantially with a slight decrease in elevation (granitic rocks slowly cooled). (B) Break in slope at 17 Ma – little age variation with decrease in elevation (first phase of rapid cooling). (C) Break in slope at 16 Ma – ages decrease with slight decrease in elevation (slow cooling). (D) Predicted AHe ages, 2.3 (D1) and 0.5 (D2) Ma (the base and top of the Saugus Formation) and break in slope (second phase of recent and rapid cooling). Interpretation B: (E) At highest elevations, ages decrease substantially with a slight decrease in elevation (granitic rocks slowly cooled). (F) Break in slope at 17 Ma – little age variation with decrease in elevation (cooling). (G) Cooling since ~13 Ma. Projected cooling from 13 (G2) – 17 (G1) Ma to present. Note that Interpretation A shows much more rapid cooling in the Quaternary than interpretation B.

Arkle 33

118º 15’

34º 15’

Figure 10: A 10-meter digital elevation model of the Verdugo Mountian block showing topographic profile locations and sample locations.

Arkle 34

B’B

800

600

400

0 1000 2000 3000 4000 5000

Ele

vatio

n (m

)

Distance (m)

South North

300

500

700

900

0 5000 10000 15000Distance (m)

Elev

atio

n (m

)

A A’EastWest

900

500

600

700

800

0 1000 2000 3000 4000

Ele

vatio

n (m

)

Distance (m)

SouthC C’

North

350

400

450

500

550

0 500 1000 1500 2000 2500

Ele

vatio

n (m

)

Distance (m)

SouthD’D

North

Arkle 35

Figure 11: Topographic profiles measured from the VMB 10-meter digital elevation model. Note vertical exaggeration varies for each profile.

Western San Gabiriel Block

SAf

VERDUGO BLOCKAvg. Helium Age: 30 Ma

Range: 13 - 78 MaN = 9

TUJUNGA BLOCKAvg. Helium Age: 36 Ma

Range: 33 - 42 MaN = 3

SIERRA MADRE BLOCKAvg. Helium Age: 6 Ma

Range: 3 - 7.6 MaN = 5

WESTERN SAN GABRIEL BLOCKAvg. Helium Age: 35 Ma

Range: 23 - 42 MaN = 3

MT. BALDY BLOCKAvg. Helium Age: 7 Ma

Range: 5.1 - 9 MaN = 2

SMtVf

Figure 12: Three-dimensional elevation model with apatite helium ages of individual blocks within the San Gabriel Mountains. Interpreted block boundaries are dashed lines and faults shown as heavy solid lines. San Gabriel block data is from Blythe et al. (2000). Boundary lines are redrawn and AHe ages are averaged. Red dashed line shows the VMB and AHe results from this study.

Arkle 36

Arkle 37

Appendix A

Grain Dimension and Morphology Data This section contains original laboratory data sheets that have been scanned and a summary of these data.

Arkle 38Grain Dimensions and Morphology

Apatite Lab Data Sheets

Sample # MorphologyLength

(microns)Width

(microns)JAVM06-1A N,P 165.84 88.67JAVM06-1B N,P 176.15 95.58JAVM06-1C N,I 205.38 94.4JAVM06-1D SP,I 162.65 79.55

JAVM06-2A N,N 178.67 104.17JAVM06-2B N,I 233.52 127.89JAVM06-2C N,N 160.73 100.4JAVM06-2D N,P 174.78 136.78

JAVM06-3A N,P 160.32 127.77JAVM06-3B N,P 281.07 176.69JAVM06-3C N,P 170.79 90.55JAVM06-3D N,SP 215.58 100.12

JAVM06-4A N,I 211.44 131.75JAVM06-4B N,P 291.01 132.21JAVM06-4C SP,I 152.75 139.48

JAVM06-5A N,N 237.85 112.35JAVM06-5B N,P 181.9 86.4JAVM06-5C N,P 160.86 120.68JAVM06-5D N,N 258.93 122.71

JAVM06-6A N,P 132.81 105.98JAVM06-6B N,P 205.36 191.05JAVM06-6C SP,I 162.87 126.94JAVM06-6D N,P 203.22 125.18

JAVM06-7A N,N 219.76 122.73JAVM06-7B N,P 238.19 115.08JAVM06-7D P,P 208.75 116.6

JAVM06-8A N,SP 230.56 125.94JAVM06-8B N,SP 380.2 160.98JAVM06-8C N,SP 308.75 208.16JAVM06-8D N,P 164.95 130.42

JAVM06-9A N,P 203.38 119.16JAVM06-9B N,N 303.12 129JAVM06-9C N,P 292 88.09JAVM06-9D SP,I 249.05 123.11

Average: 214.21 122.25Range: 132-380 79-208

*MorphologyN = NormalP = ParallelSP = SubparallelI = Irregular

Arkle 48

Appendix B

Individual Apatite Grain Descriptions and Photographs

Photographs have been resized maintaining the original grain ratios. Note that the scale for individual apatite grains can be determined from the grain dimensions and morphology laboratory data sheets.

JAVM06-1A

GRAIN DESCRIPTIONJAVM06-1A: Picked, measured and packed. No photo.JAVM06-1B: Euhedral, 1 terminated end, clean.JAVM06-1C: Euhedral, 1 terminated end, clean. Cracks and possilbe inclusions.JAVM06-1D: Euhedral, 2 broken ends.

Individual Apatite Grain DescriptionsReflected Light Refracted Light

JAVM06-1A

JAVM06-1B JAVM06-1B

JAVM06-1C JAVM06-1C

JAVM06-1D JAVM06-1D

No Photo No Photo

Arkle 49

JAVM06-2A

GRAIN DESCRIPTIONJAVM06-2A: Euhedral, 2 terminated ends, very clean.JAVM06-2B: Euhedral, 2 terminated ends, very clean.JAVM06-2C: Euhedral, 1 terminated end, 1 clear broken end, very clean.JAVM06-2D: Euhedral, 1 terminated end, 1 clear broken end, clean.

Individual Apatite Grain DescriptionsRefracted Light Reflected Light

JAVM06-2A

JAVM06-2B JAVM06-2B

JAVM06-2C JAVM06-2C

JAVM06-2D JAVM06-2D

Arkle 50

JAVM06-3A

GRAIN DESCRIPTIONJAVM06-3A: Euhedral, 1 terminated end, semi-clean.JAVM06-3B: Euhedral, 2 terminated ends, semi-clean.JAVM06-3C: Euhedral, 1 terminated end, clean.JAVM06-3D: Euhedral, 1 terminated end, semi-clean.


JAVM06-3A

JAVM06-3B JAVM06-3B

JAVM06-3C JAVM06-3C

JAVM06-3D JAVM06-3D

No Photo

Arkle 51

JAVM06-4A

GRAIN DESCRIPTIONJAVM06-4A: Euhedral, 2 terminated ends, semi-clean. Surficial coloration on ends; reflectinos?JAVM06-4B: Euhedral, 2 terminated ends, semi-clean.JAVM06-4C: Euhedral, 1 terminated end, clean.JAVM06-4D: Do not use.


JAVM06-4A

JAVM06-4B JAVM06-4B

JAVM06-4C JAVM06-4C

JAVM06-4D JAVM06-4D

Arkle 52

JAVM06-5A

GRAIN DESCRIPTIONJAVM06-5A: Euhedral, 1 terminated end, very clean.JAVM06-5B: Euhedral, 1 terminated end, clean.JAVM06-5C: Subhedral, 1 terminated end, clean.JAVM06-5D: Euhedral, 2 terminated ends, very clean.


JAVM06-5A

JAVM06-5B JAVM06-5B

JAVM06-5C JAVM06-5C

JAVM06-5D JAVM06-5D

Arkle 53

JAVM06-6A

GRAIN DESCRIPTIONJAVM06-6A: Subhedral, 1 terminated end, semi-clean.JAVM06-6B: Euhedral, 1 terminated end, dirty. Potential inclusion; cloudy surface.JAVM06-6C: Euhedral, 2 broken ends, clean.JAVM06-6D: Euhedral, 1 terminated end, clean.


JAVM06-6A

JAVM06-6B JAVM06-6B

JAVM06-6C JAVM06-6C

JAVM06-6D JAVM06-6D

100 µm 100 µm

100 µm 100 µm

100 µm 100 µm

100 µm 100 µm

Arkle 54

JAVM06-7A

GRAIN DESCRIPTIONJAVM06-7A: Subhedral, 1 terminated end, semi-clean.JAVM06-7B: Euhedral, 1 terminated end, clean. Potential inclusions along edge - reflections?JAVM06-7C: Subhedral, 1 terminated end, clean.JAVM06-7D: Do not use. Potential inclusinos; probably crack in grain.


JAVM06-7A

JAVM06-7B JAVM06-7B

JAVM06-7C JAVM06-7C

JAVM06-7D JAVM06-7D

Arkle 55

JAVM06-8A

GRAIN DESCRIPTIONJAVM06-8A: Euhedral, 1 terminated end, clean. Big crack.JAVM06-8B: *Zircon? Euhedral, 1 terminated end, clean. JAVM06-8C: Euhedral, 1 terminated end, semi-clean. Inclusions or surfical feature?JAVM06-8D: Euhedral, 1 terminated end, clean. Excellent.


JAVM06-8A

JAVM06-8B JAVM06-8B

JAVM06-8C JAVM06-8C

JAVM06-8D JAVM06-8D

Arkle 56

JAVM06-9A

GRAIN DESCRIPTIONJAVM06-9A: Euhedral, 1 terminated end, clean.JAVM06-9B: Euhedral, 2 terminated ends, clean. JAVM06-9C: Euhedral, 2 terminated ends, very clean. Long prismatic crystal.JAVM06-9D: Subhedral, 1 terminated end, very cracked. Irregular edges.


JAVM06-9A

JAVM06-9B JAVM06-9B

JAVM06-9C JAVM06-9C

JAVM06-9D JAVM06-9D

Arkle 57

Arkle 58

Appendix C

Apatite (U-Th)/He Data

These data are from the California Institute of Technology. The “Ref” numbers correspond to the assigned Verdugo individual grains and samples sited in this paper.

Apatite (U-Th)/He Data from the California Institute of Technology

Ref Raw Ma Corr Ma U ppm Th/U Th ppm He nmol/g mass ug Ft Correction

Crystal width (um)

Crystal length (um) RE? #grains

J1(a):laser 6.43 9.18 2.70 2.64 7.12 0.15 2.63 0.70 44.34 248.76 0.00 1.00 V. low U.J1(b):laser 12.76 17.79 2.35 3.47 8.14 0.30 3.24 0.72 47.79 264.23 0.00 1.00 V. low U.J1(c) :laser 21.98 30.58 2.75 3.83 10.53 0.63 3.69 0.72 47.20 308.07 1.00 1.00 V. low U.J1(d):laser 3.65 5.60 0.11 36.85 3.93 0.02 2.07 0.65 39.78 243.98 1.00 1.00 NOT APATITE?

J2(a):laser 35.41 47.68 17.25 1.49 25.72 4.49 3.90 0.74 52.09 268.01 0.00 1.00J2(b):laser 56.38 71.48 6.01 1.68 10.10 2.58 7.63 0.79 63.70 350.28 0.00 1.00J2(c):laser 32.94 44.90 10.73 1.03 11.00 2.39 3.26 0.73 50.20 241.10 0.00 1.00J2(d):laser 47.80 60.86 10.01 4.01 40.17 5.07 6.58 0.78 68.39 262.17 0.00 1.00


J4(a):laser 63.82 80.92 2.54 2.55 6.49 1.42 7.39 0.79 65.88 317.16 0.00 1.00 V. low U.J4(b):laser 59.19 75.11 2.65 2.80 7.40 1.42 7.39 0.79 65.88 317.16 0.00 1.00 V. low U.J4(c):laser 108.88 135.33 20.68 0.86 17.78 14.85 10.24 0.80 66.11 436.52 0.00 1.00J4(d):laser 155.31 196.57 5.65 1.93 10.93 7.03 5.98 0.79 69.74 229.13 0.00 1.00



J7(a):laser 16.67 21.44 32.64 2.42 78.93 4.64 6.67 0.78 61.37 329.64 0.00 1.00

J7(b):laser 37.32 48.41 13.88 1.72 23.90 3.97 6.35 0.77 57.54 357.29 0.00 1.00J7(c):laser 29.30 38.15 25.73 2.02 51.91 6.05 5.71 0.77 58.30 313.13 0.00 1.00



Raw Ma: This column shows the apatite He age (in millions of years) before the Ft correction is accounted for. See text for discussion.Corr Ma: This column shows the corrected apatite He age (in millions of years) that is used for analysis. See text for discussion.RE: The number 1.00 in this column indicates a re-extract. See text for discussion.

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Appendix D

Geomorphic Analysis

Geomorphic Analysis

Slope Face Cross Section X final X initial

Dist. Change X

(m)Y Final Y Initial Elev Change Y (m) Graident

(m/km)Slope

Angle ( )

East-West A-A'West 8500 0 8500 940 300 640 75.3 4.3East 16000 10000 6000 940 300 640 106.7 6.1

East to West Graident Difference (m/km): 31.4 Difference ( ): 1.8

South Face B-B' 2500 0 2500 860 260 600 240.0 13.5C-C' 1600 0 1600 900 500 400 250.0 14.0D-D' 900 0 900 570 340 230 255.6 14.3

Average South Face Graident (m/km): 248.5 Mean Slope ( ): 13.9

North Face B-B' 5400 2500 2900 860 375 485 167.2 9.5C-C' 4200 1800 2400 910 500 410 170.8 9.7D-D' 2700 950 1750 560 350 210 120.0 6.8

Average North Face Graident (m/km): 152.7 Mean Slope ( ): 8.7

North and South Face Difference (m/km): 95.8 Difference ( ): 5.3

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