part 1: kinematic analysis: the next step james pindell ... · part 1: kinematic analysis: ... very...

22
1 Originally published in Geophysical Corner, AAPG Explorer, June 2000. Text and Figures, © Tectonic Analysis Ltd. Part 1: Kinematic Analysis: The Next Step James Pindell, Lorcan Kennan and Stephen Barrett. For more information about Tectonic Analysis Ltd and our reports and services, please visit our website: www.tectonicanalysis.com or email the authors: [email protected], [email protected] An unfortunate fact of geology is that most datasets, including seismic, rarely allow for a unique interpretation of a geological problem. Having to wrestle with multiple working hypotheses is perhaps especially common in the structural arena, where one or another of many theoretical models of "ideal" crustal deformation can be made to fit a given structural pattern. This can be frustrating and potentially costly if the optimum exploration strategy is dependent upon the interpretation finally chosen. Integration of multiple and diverse data sets is one popular approach to reducing the range of possible interpretations, the goal being to minimize exploration risk. But on too many occasions, if your "best" data set can't give you a clear solution, then mixing in diverse secondary data sets can muddle the picture even further. Worse, this multifaceted picture may not be fully understood by anyone on the work team, and the full implications of the "integrated solution," which will provide the basis of the exploration model, might never be recognized. It is widely recognized that broadening the scale of geological assessment to beyond the limits of the block or field can help to constrain a unique solution to a given problem. Indeed, many plate tectonic and structural processes evolve over scales far larger than most blocks, and to ignore the larger scale can lead to serious misinterpretations. But broadening the scale of examination to beyond the block remains, in many cases throughout industry, little more than a matter of describing what is out there. In other words, mapping. As geologists, we all know that mapping is a key part of geology, but it is very important to take the next step and understand how and why a given set of mapped structures developed. Can this help to resolve our interpretation of geological problems? Can it tell us anything more about an exploration play? Can it trigger the identification of new plays altogether? We believe it can. When we shift from trying to address the "what "questions of structural analysis into the "how" questions, we move from static description into time-progressive kinematic analysis. Kinematic analysis can be performed at all scales in geology - from mineral grains to tectonic plates - and it embraces the motions of material undergoing geological change. Defining the motions of the plates and crustal blocks, where possible, can tremendously facilitate understanding how certain types of structures developed. Plate kinematics addresses the history of motion of the plates and blocks that comprise or have comprised the earth's surface. Although plate kinematics is traditionally associated with the oceans, it also can be applied successfully to areas of continental crust and margins of real exploration interest.

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Page 1: Part 1: Kinematic Analysis: The Next Step James Pindell ... · Part 1: Kinematic Analysis: ... Very powerful plate kinematic rules, however, ... the principles of kinematic analysis

1

Originally published in Geophysical Corner, AAPG Explorer, June 2000.

Text and Figures, © Tectonic Analysis Ltd.

Part 1: Kinematic Analysis: The Next Step

James Pindell, Lorcan Kennan and Stephen Barrett.

For more information about Tectonic Analysis Ltd and our reports and services, please visit our website:www.tectonicanalysis.com or email the authors: [email protected], [email protected]

An unfortunate fact of geology is that most datasets, including seismic, rarely allow for a uniqueinterpretation of a geological problem. Having to wrestle with multiple working hypotheses is perhapsespecially common in the structural arena, where one or another of many theoretical models of "ideal"crustal deformation can be made to fit a given structural pattern. This can be frustrating and potentiallycostly if the optimum exploration strategy is dependent upon the interpretation finally chosen.Integration of multiple and diverse data sets is one popular approach to reducing the range of possibleinterpretations, the goal being to minimize exploration risk. But on too many occasions, if your "best"data set can't give you a clear solution, then mixing in diverse secondary data sets can muddle thepicture even further. Worse, this multifaceted picture may not be fully understood by anyone on thework team, and the full implications of the "integrated solution," which will provide the basis of theexploration model, might never be recognized.

It is widely recognized that broadening the scale of geological assessment to beyond the limits of theblock or field can help to constrain a unique solution to a given problem. Indeed, many plate tectonicand structural processes evolve over scales far larger than most blocks, and to ignore the larger scalecan lead to serious misinterpretations. But broadening the scale of examination to beyond the blockremains, in many cases throughout industry, little more than a matter of describing what is out there. Inother words, mapping. As geologists, we all know that mapping is a key part of geology, but it is veryimportant to take the next step and understand how and why a given set of mapped structuresdeveloped.

• Can this help to resolve our interpretation of geological problems?• Can it tell us anything more about an exploration play?• Can it trigger the identification of new plays altogether?

We believe it can.

When we shift from trying to address the "what "questions of structural analysis into the "how"questions, we move from static description into time-progressive kinematic analysis. Kinematicanalysis can be performed at all scales in geology - from mineral grains to tectonic plates - and itembraces the motions of material undergoing geological change. Defining the motions of the plates andcrustal blocks, where possible, can tremendously facilitate understanding how certain types ofstructures developed. Plate kinematics addresses the history of motion of the plates and blocks thatcomprise or have comprised the earth's surface. Although plate kinematics is traditionally associatedwith the oceans, it also can be applied successfully to areas of continental crust and margins of realexploration interest.

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In the late 1960s, one of the most exciting early realizations of the plate tectonic revolution was that theways in which plates move relative to each other, both past and present, are governed by a firm set ofpredictive, or retrodictive, geometric rules. Plate kinematics gave us the power to quantitatively openand close oceans, collide continents and evolve plate circuits in area-balanced models. Earth'sgeological history became an intellectual playground for "plate pushers" who began to decipher Earth'sglobal tectonic evolution. However, all too often, these kinematic rules were either not applied,misapplied or applied to inappropriate places, such that by 1980 many journal articles, no matter whatthe discipline, ended with "bandwagon" Plate Tectonic Interpretation sections, which correctly came tobe viewed as mere arm waving. Similarly, industry decision-makers grew to be suspicious of suchtectonic scenarios - with good reason - and often ignored or discounted them. Thus, the potential ofkinematic analysis often was never reached. Sadly, these very powerful rules are no longer even taughtin many universities, and quantitative plate kinematic analysis is becoming something of a lost art.Very powerful plate kinematic rules, however, do still exist. Here, in this first in a series of threearticles, we review some of these principles to provide the basis for exploring the power of kinematicanalysis.

In Figure 1a, we show a simple two-plate system in which block A moves NNE relative to B with time.Displacement during the particular time interval of concern can be drawn as shown by the red vectorbetween the dots representing the plates. To palinspastically restore the offset back in time, we woulduse the blue vector to retract the accrued measured offset. Progressing to a three-plate system, we mustconsider the motions between the three pairs of plates. A simple analogy of this situation is to consider,in Figure 1b, two runners, A and B, running from home plate to first and third base on a baseballdiamond. The displacement between home plate and runners A and B, respectively, is NE and NW, butthe relative motion between the two runners is east-west. A plate boundary separating platesrepresented by the two runners would be extensional, with net E-W fault displacements.

In the three-plate example of Figure 1c, we can restore, moving back in time, two known offsets (A-C)and (A-B) to determine the unknown offset between the third plate pair (B-C). The measured directionsand displacements of plates B and C are drawn relative to Plate A. Tieline B-C will then approximatethe net direction (NE) and displacement (76km) of the common B-C fault zone. If this happens to be athrust belt with the orientation as shown, then the strike-slip (blue, 30km) and convergent (red, 70km)components of net motion can be inferred by construction of the right-triangle, thereby providing vitalinformation about overall structural style, with the expectation of dextral transpressive (combination ofstrike-slip plus compression) strain partitioning at that thrust belt.

Finally, in the larger two-plate example of Figure 2, plates A and B diverge by seafloor spreading at theridge (red) and transcurrent motions at the transform faults (green). The continuations of the transformsinto adjacent oceanic crust are fracture zones where differential thermal subsidence occurs, but withoutactive strike-slip faulting. Ridge segments lie on great circles to the pole defining the plate separation,whereas the transforms lie on small circles. The rate of plate separation and also of transcurrentdisplacement at the transforms increases with distance from the pole. Transforms also becomestraighter as distance increases from the pole of rotation.

Subsequent articles in this series will apply these principles to two well-known oil provinces,Colombia/western Venezuela and the Gulf of Mexico, showing how formal kinematic analysis canoffer some of the most sound constraints available to guide and to favor certain geologicalinterpretations over others. Further, it can provide the basis for defining or rejecting play concepts,therefore strongly influencing exploration strategy.

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2-PLATE SYSTEM

AB

B

A

AccruedOffset:

A

B

RestoredOffset

A

B

C40km

70km

70km

30km76km

FirstBase

ThirdBase

SecondBase

Home

RUNNERB

RUNNERA

A (?)

B (70)C (40)

3-PLATE SYSTEM

100 km

A

B

C

Figure 1. Examples of vector displacement diagramsfor two and three-plate systems.

Published in AAPG Explorer, v. 21, June 2000

www.tectonicanalysis.com

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Concentric SmallCircles about Pole

RidgeSegments

Transforms

Pole of Rotation

FractureZones

PLATE A PLATE B

Great Circlesthrough Pole

Figure 2. Relationships between pole of rotation,great circles, ridge segments, small circles,transforms, and fracture zones in a two-plate system.

Published in AAPG Explorer, v. 21, June 2000

www.tectonicanalysis.com

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3

Originally published in Geophysical Corner, AAPG Explorer, July 2000.

Text and Figures, © Tectonic Analysis Ltd.

Part 2: Kinematics a Key To Unlocking Plays

James Pindell, Lorcan Kennan and Stephen Barrett.

For more information about Tectonic Analysis Ltd and our reports and services, please visit our website:www.tectonicanalysis.com or email the authors: [email protected], [email protected]

Last month's Geophysical Corner outlined some of the principles and methods of kinematic analysis asa means of better deciphering the structural history of basins. In this second article, we apply some ofthe principles of kinematic analysis to the first of our example areas: the northern Andes of Colombiaand western Venezuela. We also will illustrate some uses and benefits of this analysis to petroleumgeology and exploration in continental settings. When applied to continental areas, kinematic analysisprovides map-view palinspastic reconstructions of deformed regions prior to the deformation(s),analogous to balancing cross sections in the vertical plane. Two very useful applications of continentalblock kinematics for exploration are:

• To allow more accurate plotting of paleofacies for times prior to deformations.• To allow more rigorous reassembly of continental blocks that have become separated during rifting,

enhancing the understanding of the development of hydrocarbon-bearing continental margins.

Here we show a set of simple steps for restoring the northern Andean ranges and basins for EarlyOligocene and earlier time, prior to the majority of "Andean" deformation. Note that variations in thereconstruction will derive from applying different numbers of steps (accuracy can be increased byaccounting for more fault motions between more blocks), and also from adjusting various inputparameters, such as magnitudes of strike-slip offset on certain fault zones. A reference frame is neededto begin: In this example, Andean motions are assessed relative to the Guyana Shield.

First, we address the relative motion of the Maracaibo Block by assessing displacement in the MéridaAndes. Figure 1 shows the ca. 150 km dextral offset across the Mérida Andes of the "Eocenethrustbelt,". In addition, shortening in the Mérida Andes has been estimated as about 40 km. Thus, inthe Early Oligocene, the Maracaibo Block lay significantly farther southwest relative to the Shield thanit does today. In figure 2, we construct a tie line between the Shield and Maracaibo Block byperforming vector addition of the strike-slip (150 km) and thrust (40 km) components. Because wewish to restore the accrued offset (155 km), we draw the tie lines opposite to the real-life sense of faultdisplacements, i.e., moving back in time.

Having defined the Oligocene paleoposition of Maracaibo, our next concern is the Perijá Range,deformation of which accounts for movements between the Maracaibo Block and the Santa MartaMassif. Estimates of post-Early Oligocene shortening are ca. 25 km, as shown by the Perijá vector infigure 2. Thus, displacing Santa Marta Massif to the west-northwest of Maracaibo by 25 km gives theEarly Oligocene position of Santa Marta relative to both Maracaibo and the Shield. Next, the SantaMarta strike-slip fault displaces the Santa Marta Massif Block from the northern part of Colombia'sCentral Cordillera. Left-lateral offset of about 110 km (figures 1,2) is believed to have occurred on thisfault zone since Late Oligocene. This strain is transferred into the Eastern Cordillera along the south-

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southeast continuation of the fault - the Bucaramanga Fault. Interestingly, this fault is flanked by thehigh, compressive topography of Santander Massif - because the Bucaramanga Fault defines theboundary between the Central Cordillera and the Maracaibo Block, not the Santa Marta Block. Forsimplicity in figure 2, the trend shown for the fault (in orange) defines only the total strain betweenthose blocks, i.e. the sum of the strike-slip and orthogonal components of relative motion. Finally, werestore Colombia's Guajira Block, also relative to Santa Marta, by removing about 125 km of dextralshear on the Oca Fault to realign the western edges basement in the two blocks prior to faulting.

With just these simple considerations, and assuming that only minor vertical axis rotation of theseblocks has occurred during their relative motions, we can now fill out other tie lines in the vector "nest"of figure 2 to define offsets between other pairs of blocks in the system. For example, the total strain inthe Eastern Cordillera since the Oligocene is seen to be ca. 200 km toward the east-southeast (red tieline). This can be broken down into components of orthogonal and strike-parallel strain of 180 km(blue line) and 100 km (green line), which translates geologically into shortening (180 km) and dextralshear (100 km). This value of shortening (180 km) falls in the middle of the range of published valuesof estimated shortening in Eastern Cordillera. Thus, vector nests such as figure 2 can be used to helpchoose between alternative balanced cross section models assessing shortening. In addition, it alsoallows detection and estimation of the strike-slip component, which usually cannot be seen in crosssections. Our inferred dextral shear in the Eastern Cordillera is supported by seismicity, GPS data andfield observations.

A pre-Andean (i.e., pre-Late Oligocene) palinspastic reconstruction of the northern Andes continentalregion (figure 3) now can be made by restoring the motions of the blocks defined in figure 2. Theknown limit of pre-Mesozoic continental crust has been identified in figure 3 to show the pre-Andeangeometry of the northern Andes "autochthon," to which a number of oceanic terranes have beenaccreted in the Cenozoic. Additional information can now be added to better focus the picture. We can,for example, draw the occurrence of Eocene formations, sedimentary facies and paleoenvironments onour reconstruction in order to build palinspastically accurate models of regional Eocene depositionalsystems. This practice also allows better sequence stratigraphic interpretation and correlation at theregional scale, which is helpful to determining migration pathways through the strata. Also, thedepositional models can be compared more meaningfully to modern analogues and analyzed forimplications concerning reservoir potential, such as sand body orientation, sinuosity, flow direction,sand grain provenance and sediment maturity. Finally, the reconstruction also allows a betterinterpretation of Cretaceous source rock character, quality and original areal extent.

Using the same block/plate restoration technique, we can depict Eocene-aged structures and the Eoceneposition of the Caribbean Plate relative to South America, to better understand the driving forces ofEocene sedimentation patterns and deformation. Figure 4 thus shows the Caribbean Plate driving anEocene foredeep basin in the northern Maracaibo area - much like today's Persian Gulf - which causedan important early hydrocarbon maturation event in western Venezuela and Colombia's Cesar Basin.Figure 4 also shows depositional systems with important reservoir facies belts at the Middle to LateEocene boundary, as well as the existence, continuity and origin of an Eocene "Maracaibo Tar Belt" inwestern Venezuela (also recognized in Middle to Late Eocene field sections). The concept of this"textbook" foredeep basin for the Eocene of Maracaibo Basin had remained darkly veiled for decadesby today's grossly different geography.

Next month, we will use plate kinematics to reconstruct Africa and South America, and toprogressively close the Atlantic Ocean during Mesozoic times, in order to set the stage for tracing theevolution of the Gulf of Mexico and the Florida/Bahamas region in our fourth article of the series.

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GuyanaShield

-77 -76 -75 -74 -73 -72 -71 -70 -69

-77 -76 -75 -74 -73 -72 -71 -70 -69

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SantaMarta

CentralCordillera

Oca Fault

Rom

eral

Fau

lt

Ibague Fault

FalcónBasin

Venezuela

ColombiaTerranes to

the west ofthe RomeralFault areallochthonous

PerijáAndes

EasternCordillera

MeridaAndes

Maracaibo

Guajira

Santa M

arta-Bucaram

anga Fault Boc

onó,

Cap

aro

Faul

ts

Ca. 150 kmoffset ofCaribbeanNappesand Eocenefacies belts

www.tectonicanalysis.com

Figure 1. Map of northern South America showing main crustalblocks, separated by lithospheric fault zones, under relativemotion during Late Oligocene to Recent Andean Orogeny.

Published in AAPG Explorer, v. 21, July 2000

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155 km (Merida)

40 km

110 km (SantaMarta Ft.)

200 km(Eastern Cordillera)

150 km

180 km

100 km

120 km(Bucara-

manga Ft.)

Maracaibo

GuyanaShield

SantaMarta

NorthernCentralCordillera

Partitioned components,Eastern "Andean" strain

25 km (Perij�)

0 50 100 150

Km

www.tectonicanalysis.com

Figure 2. Vector "nest" restoring displacements of northernAndean blocks along faults during Andean orogenesis. Heavydots denote blocks, tie lines restore net azimuth and magni-tude of fault displacements, moving back in time. MéridaAndes and Eastern Cordillera deformation is shown parti-tioned into strike-orthogonal and strike-parallel components.

Published in AAPG Explorer, v. 21, July 2000

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Ca�oLim�nfield

12°

11°

10°

1°72°

73°

74°75°76°

77°

Orito field

Bogot�

Cusiana field

Bucaramanga

MaracaiboValledupar

SantaMarta

Paleo-positions ofpresent courses ofRios Cauca andMagdalena

Paleo-positions ofLake Maracaiboand border

C a l i

Northern edgeof autochthon

Rom

eral

Medell�n

Honda

Palinspastic GridNorthern Andes, 25 Ma

Faul

t

Western edgeof autochthon

www.tectonicanalysis.com

Figure 3. Oligocene reconstruction of pre-Mesozoic continental basement, northern Andes, based on vector displacementsfrom Figure 2. "Retro-deformed" grid of longitude and latitude lines is created by smoothing the lines across block bound-aries after block restoration. Red outline is present day South America for comparison. Cities, fields, rivers and geographicfeatures (blue) are shown in palinspastic coordinates to help show Oligocene paleogeography.

Published in AAPG Explorer, v. 21, July 2000

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70

6.0°

10.0°

14.0°

-74.0° -70.0° -66.0° -62.0°

?

?

CaribbeanPlate

CaribbeanNappes

AntillesArc

Proto-CaribbeanSeaway

Misoa-PaujiForeland Basin Emergent

Sandy shallow sea

Deep water

Coastal fringeAlluvial plain

Carbonates

Muddy outer shelf

ENVIRONMENT

Caracas

Maracaibo BeltTar

Serrania andTrinidadreentrants

GuyanaShield

After Pindell et al., 1998. SEPM Special Paper 58

FlexuralBulge

JuvenileBarbados

Prism

www.tectonicanalysis.com

Figure 4. Paleogeographic map of western Venezuela and northernColombia, showing the position of the Caribbean Plate and maindepositional units during Eocene time. Note the similarity withtoday’s "Persian Gulf".

Published in AAPG Explorer, v. 21, July 2000

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5

Originally published in Geophysical Corner, AAPG Explorer, July 2000.

Text and Figures, © Tectonic Analysis Ltd.

Part 3: A Removal-Restoration Project

James Pindell, Lorcan Kennan and Stephen Barrett.

For more information about Tectonic Analysis Ltd and our reports and services, please visit our website:www.tectonicanalysis.com or email the authors: [email protected], [email protected]

In our first two articles in this series we showed how vector triangles and rotation poles can be used toconstrain the motions of continental blocks and plates, and we reconstructed the pre-Andean(Oligocene) shape of northern South America. This month we show the importance of removing post-rift sedimentary sections and restoring crustal extension when approximating the pre-rift shapes ofcontinental blocks and margins.

First we'll show how this can be done in a simple way, and then we'll apply the method to a riftedmargin pair - the equatorial margins of Africa and South America - to derive a pre-Aptianreconstruction of the northern parts of those two continents. Prior to the equatorial Atlantic break-upduring the Aptian, the northern parts of these two continents were essentially a single block. We canuse the Euler rotation poles defined by marine magnetic anomalies and fracture zones in the centralNorth Atlantic to rotate the reconstructed shape of Africa/South America back toward North America.This process, when combined with the pre-Andean palinspastic reconstruction of the northern Andesfrom last month's article, provides a quantitative kinematic framework in which to base models for theMesozoic evolution of the Gulf of Mexico, Mexico and nuclear Central America, the Florida/Bahamasregion, the Proto-Caribbean Seaway and northern South America. Continental rifting reflectsdivergence of relatively stable portions of crust. This is accommodated by crustal extension at shallowlevels (typically less than 15 km), by normal faulting and at depth by ductile stretching of the lowercrust and upper mantle. The end result is lithospheric thinning at the rift; we usually see overall tectonicsubsidence of the surface, elevation of the asthenosphere, increased heat flow and, sometimes,volcanism. At the surface, fault-bounded grabens initially fill with red beds, if subaerial, as riftingproceeds. These are then overlapped by "thermal sag" sedimentary sections driven largely by coolingof the asthenosphere, plus the loading effect of the sediments themselves. Where extension issufficiently large, oceanic crust is created and the two portions of continental crust drift apart. Whererifting does not reach this stage, we are left with intra-continental basins. Sediment thickness at therifted margins that flank ocean basins can exceed 16 km.

If sediment supply is sufficient - for instance, near deltas or adjacent to high-relief topography in wetclimates - the position of passive margin features such as the shelf-slope break can change significantlywith time, growing out from the coast and well beyond the original limits of the continental crust(figure 1a). Although used for Bullard's famous reconstruction of the Atlantic margins (1965), this iswhy it is not satisfactory in quantitative kinematic analysis to merely realign a given bathymetriccontour along opposed pairs of passive margins. To gain a much closer approximation of the shapes ofrifted margins to fit together for a more precise pre-rift geometry, we must construct cross sections ofrifted margins that depict the thicknesses of the water column, the sedimentary section, and the crust.Water depth and total sedimentary section are often known from geophysical studies at passivemargins. The position of the Moho (base of the crust) can be crudely estimated by the balancing of

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water, sediment, crust and mantle using Airy isostatic calculations (figures 1b,c) and, where gravitydata or detailed sedimentological data are available, refined by taking into account crustal flexure andsediment compaction. Once the cross-sectional shape of the rifted margin's crust is inferred, the syn-riftextension in basement can be removed by restoring the cross-sectional area of the rifted marginshoulder back to an unstretched beam of continental crust. Again, a crude calculation can assume thisstarted at or near sea-level, and more refined calculations could take account of surface elevation, waterdepth prior to rifting and variations in initial crustal thickness or density. This identifies the positionwithin that cross section that defines the pre-rift edge of the continental block. When plotted at severalpoints along a particular margin, we can estimate the pre-rift shape of the continental margins. This canthen be rotated towards the opposing margin using plate kinematic methods to show pre-rift geologicalrelationships - and to provide a starting point for modeling the ensuing basin evolution.

Figure 2 shows the net result of this method when applied to the rifted margins of the EquatorialAtlantic. The method is particularly important along the shelves at the mouths of the Niger andAmazon rivers, where the sedimentary thickness exceeds 10 km over large areas. Note that the Para-Maranhao Platform is a piece of the West African Craton stranded on South America as the EquatorialAtlantic opened. A satisfactory fit can be achieved to an accuracy of perhaps 50 km. For comparison,the inset of figure 2 shows the classic Bullard reconstruction of the two continents, with the pre-riftshapes of basement shown rather than the 2,000-meter isobath employed by Bullard. The inferredunderfit in the Bullard reconstruction approaches 500 km. Because continental reassembly in the Gulfof Mexico region is achieved by rotating the Africa-South America reconstruction back toward NorthAmerica using Central Atlantic kinematic data, the difference between the two approaches will affectthe final reassembly as profoundly as any other kinematic parameter. Marine magnetic anomalies andfracture zone traces are used in the oceans to track the past velocity and flowpath, respectively, of pairsof plates separated by seafloor spreading.

Figure 3 shows a series of reconstructions of our united Africa-South America supercontinent andNorth America for Aptian and older times, prior to Equatorial Atlantic break up. Some of the positionsare interpolated or extrapolated from the marine data to provide key time slices such as TriassicPangean continental closure, and late Callovian/Early Oxfordian salt deposition in the Gulf. Theanalysis tells us how fast and in what direction the continents separated, which in turn constrains thegeometry of ridge systems between the Americas, and also the size and shape of the inter-Americangap through time. Finally, also shown on figure 3 is the pre-rift palinspastic shape of the northernAndes region superimposed on South America for the Late Triassic time slice. This was drawn bytaking last month's reconstruction (i.e. prior to Cenozoic shortening and strike-slip) and modifying itfor pre-rift time by applying the methodology of figure 1 (assuming an ENE-WSW extensiondirection). The relationship of North and South America at this time is important, because it defines aline separating two parts of Mexico. The part of Mexico overlapped during Late Triassic time by SouthAmerica must have migrated into today's position as a function of Gulf of Mexico evolution,Cordilleran terrane migration, and/or Sierra Madre/Chiapas shortening history. Parts of Mexico notoverlapped by South America during the Triassic may have been in place relative to today's geography,but were not necessarily so. From figure 3, the fact that the formation of the Gulf of Mexico wascompleted by early Cretaceous time implies that Jurassic plate boundary systems active in the Gulfuntil then probably also controlled many primary elements of the evolution of Mexico.

Thus, the stage is set for us next month to use the kinematic constraints developed here to reconstructwestern Pangea and to trace the Mesozoic plate-kinematic evolution of the Gulf of Mexico, easternMexico, the Florida/Bahamas region and the Proto-Caribbean Seaway in our final article of the series.

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Shelf-Slope Break

Continent-Ocean BoundaryPre-stretch Edgeof Continent

Misfit

Note misfit if we matched Shelf-Slope Break insteadof Pre-rift reconstructed continental Edge

Water/sediment

Mantle

OceanCrustUnstretched

Complete removal of all stretching Reconstructedcontinental edge

Continent-Ocean

Boundary

RequiredRestoration

Water/sediment

StretchedContinental

Crust

Post-rift Crustal Profile

A

B

C

StretchedContinental

Crust

www.tectonicanalysis.com

Figure 1. A, Cartoon section showing how passive margin sediments (deltas,turbidites, carbonate banks) can prograde far beyond the original position of the conti-nental edge. B and C, Simple method of estimating and restoring crustal extensionduring rifting and passive margin formation. The cross-sectional area of the stretchedcrust (hatch-pattern) must equal that of the unstretched crust after sediment, water andmantle have been removed from the cross-section.

Published in AAPG Explorer, v. 21, August 2000

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10 5 0 5 10

10

Mismatch (underfit) inBullard fit when crustalstretching and sedimentare removed

AFRICA

500 km

Inset:

SOUTHAMERICA

Bullard (1965)fit coastlines

CentralAtlanticOceanic

Crust

After Pindell 1985,Tectonics v4, p1

PresentCoastlines

Rotational restoration ofPara-Maranhaõ Platform(originally part of W. AfricanCraton) to close MarajoBasin extension and avoidoverlap with Ivory Coast

NORTHEASTSOUTH AMERICA

500 km

Restored crustallimits, reconstructed

prior to rifting

RiverAmazon

RiverNiger

55 1050 45 4015

5

AFRICAWest African Craton

www.tectonicanalysis.com

Figure 2. Pre-Aptian Equatorial Atlantic reconstruction in which the restored pre-rift limitsof continental crust (ie, methodology of Figure 1) are juxtaposed. Note resulting simplegeometry for Aptian rifting. Inset: Bullard (1965) reconstruction of the two continents,which realigned the 2,000 m isobaths of today’s passive margins (not shown), showing thepre-rift limits of continental crust for each, as well as the large region of continental underfitin the absence of the sedimentary sections.

Published in AAPG Explorer, v. 21, August 2000

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www.tectonicanalysis.com

Figure 3. Successive pre-Aptian reconstructions of Gondwana and North America,using the Equatorial Atlantic fit of Figure 2. This analysis provides a quantitativeframework in which to build more locally detailed models of the evolution of the Gulfof Mexico and surrounding areas. Note pre-Andean/pre-rift restoration of the northernAndes on the Triassic position of South America: this defines how much of Mexico isdefinitely allochthonous versus how much is potentially , but not necessarily,autochthonous.

Published in AAPG Explorer, v. 21, August 2000

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Originally published in Geophysical Corner, AAPG Explorer, July 2000.

Text and Figures, © Tectonic Analysis Ltd.

Part 4: Putting It All Together Again

James Pindell, Lorcan Kennan and Stephen Barrett.

For more information about Tectonic Analysis Ltd and our reports and services, please visit our website:www.tectonicanalysis.com or email the authors: [email protected], [email protected]

In the second and third parts of this series we defined a kinematic framework for the evolution of theGulf of Mexico region by restoring Andean deformations and progressively closing the Atlantic Ocean.This month, we further evolve this framework to build a palinspastically quantitative reassembly ofcontinents and continental blocks that were separated during the Mesozoic rifting and subsequent driftin the Gulf of Mexico region - key features of which are shown in figure 1.

Figures 2-4 show primary developmental stages in the Gulf's evolution:• Post-Gulf formation (figure 2).• Post-salt/pre-seafloor spreading (figure 3).• Early syn-rift (figure 4).

The kinematic elements applicable to the reconstructions are as follows.

First, our Oligocene reconstruction of northern South America (article two) is modified for LateJurassic and Cretaceous time by removing island arc and other terranes that accreted to in the LateCretaceous and Early Tertiary (shape shown in figures 2 and 3). We then estimate and restore Jurassicextension in rift basins of the Andes (using principles outlined in the August EXPLORER, which givesus an Early Jurassic shape for the northern Andes that can be closed against North America (figure 4).

Second, figures 2-4 show that Florida, the Blake Plateau and the Bahamas (and the "Cuban autochthon"beneath the Cuban arc) were strongly controlled by fracture zone trends of the early Atlantic. Here,plate separation was achieved by NW-SE stretching of crustal blocks separated by transcurrent faults.Middle Jurassic basalt extrusion was commonplace in zones of high stretching. Each crustal "corridor"between transcurrent faults underwent different amounts of stretching and displacements relative to theothers. The conjugate margin to the Southern Bahamas flank is the transcurrent margin of Guyana.

Third, unlike the Florida region, the Yucatan Block moved independently of the larger continents - intwo distinct stages - as the Gulf opened. At the time of figure 4, there is only a small range of paleo-positions in which Yucatan can fit without overlap of palinspastically restored (i.e., rift-relatedstretching removed) areas of continental crusts. This position can be achieved by rotating present-dayYucatan clockwise about "pole A" (figure 4), which closes most of the Gulf by placing Yucatan snuglyagainst the northeast Mexico-Texas-northwest Florida paleo-margin. It definitely does not, however,close the southeastern Gulf. There, the crust of South Florida - including that of the "Tampa Arch" -must be retracted northwestward against Yucatan and out of an overlap position with Demerara Rise,off the Guyana margin. Thus, the southernmost crustal corridor of the Bahamas must have migratedSE, probably along our "Everglades Fracture Zone" (figure 1) between the times of figures 3 and 4.

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Fourth, the geology of eastern Mexico and the occurrence of Louann and Campeche salt suggest thatthe Gulf opened in two stages. The first, or syn-rift, stage - between the times of figures 3 and 4-involved intra-continental stretching between Yucatan and North America about "pole B1," andbetween Yucatan and South America about "pole B2," (figure 4). This migration defined an arcuatetranscurrent trend of basement contours along the northern Tamaulipas Arch in south Texas. Alsosinistral shear in the Louisiana-Mississippi area, allowed for minor counterclockwise rotation of theWiggins and Middle Grounds arches (figures 1 and 4) and the associated formation of the wedgeshaped East Mississippi and Apalachicola salt basins to the north of each, respectively. This syn-riftstage about "pole B1" can be modeled satisfactorily to Early Oxfordian time to achieve a goodreconstruction of the Louann and Campeche salt provinces flanking the central Gulf (figures 1 and 3).

There is no need to invoke significant salt deposition on ocean crust in the Gulf. Also, during this stage,the southern Bahamas crustal corridor migrated southeast while undergoing internal stretching – withthe Everglades fracture zone and Guyana marginal fault zone both active at this time. The migration ofYucatan to its present position requires that eastern Mexico was a transform rather than a rifted margin.Also, Yucatan did not have the Chiapas Massif attached to it during the syn-rift phase. Why?

First, we cannot satisfactorily fit a combinedYucatan/Chiapas Massif into the northern Gulf, especiallywhen we reverse the effect of Cenozoic shortening in Sierra de Chiapas. Second, we believe that theChiapas syn-rift salt basin is best explained by early transtension along a crustal scale fault beneath it.

The second stage of Yucatan motion began about "pole C" of figure 3, in the Early Oxfordian, at theend of salt deposition. This second stage of motion and its pole of rotation are constrained by:

• Geophysical data along the eastern Mexican margin, which show an abrupt NNW-SSE trendingocean-continent boundary.

• Magnetic anomaly data in the eastern Gulf.• Displacement of the once-adjacent margins of the Louann and Campeche salt basins.

The Chiapas Massif was picked up by Yucatan in this stage as a consequence of the onset of seafloorspreading in the Central Gulf, and because the pole of rotation changed in Stage 2, the orientation andposition of transforms also changed. This new phase of motion had a more southerly direction than theprevious one. The spreading ridge almost reached the Mexican coast and, hence, the new transformalong eastern Mexico picked up an additional wedge of crust - Chiapas Massif - which had beenemplaced there during the syn-rift phase by sinistral transcurrent motions within greater Mexico.

As with the Gulf of Mexico, the creation of the "Proto-Caribbean Basin" also must have involved arotational opening between Yucatan and Venezuela-Trinidad. In figures 2-4, we show the approximateflowlines along which this basin opened, and hypothetical Jurassic rifted margin geometry now whollyoverthrust by Caribbean terranes. Many elements of northern South America's and eastern Yucatan'shydrocarbon potential pertain directly to the geometries of these rifted margins, such as the positions ofmarginal re-entrants that define differing stratigraphic sequences due to differing subsidence histories.

Our working Gulf kinematic model has some interesting implications for exploration.

First, the Eastern Mexican margin (unlike that of Texas) was a Jurassic fracture zone in the north(Burgos-Tampico basins) and a transform - with active structuring until its Early Cretaceous death - inthe south (Veracruz Basin). Heat flow, subsidence history, occurrence of salt, distribution/thickness ofLate Jurassic source rocks and basement controls on future structural development will all vary along

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strike along this margin due to differing crustal properties and histories. In the U.S. Gulf margins, earlysyn-rift stretching was NNW-SSE until Early Oxfordian times, but most of the stretching toward theend of this phase occurred well offshore.

Second, although salt deposition is generally assumed to be of Callovian age, there is little evidence ofopen marine conditions in the Gulf margins until upper Oxfordian (Norphlet-Smackover transition),and thus salt deposition may have continued until Early Oxfordian. Our Early Oxfordian reconstructionaccommodates known salt occurrence in the Gulf ("salt fit"); hence, onset of seafloor spreading, thechange in the Yucatan-North America pole position, separation of Louann and Campeche saltprovinces, and initiation of open marine conditions were nearly coeval and possibly causally related.

Third, although the syn-rift stretching of the Florida Shelf region was NW-SE, the extension directionin the deep eastern Gulf during stage 2 (seafloor spreading) was NE-SW about a nearby pole, such thatsmall circles (transform traces) should be arcuate and convex to the northwest. In the Jurassic, thesouthern Bahamian margin (beneath Cuban overthrust terranes) experienced sinistral strike-sliptectonics along the Guyana margin of South America, followed by the eastward migration of a LateJurassic seafloor spreading ridge (Yucatan/South America boundary) along the western half of theoverthrust zone. The transform nature of this Jurassic margin should be considered in interpretations ofthe Paleogene development of the Cuban thrust belt, Mesozoic source rock paleogeography and oilmigration pathways during Eocene maturation.

In the Proto-Caribbean, the kinematics require westward-propagating Early and Middle Jurassic rifting,followed by Late Jurassic seafloor spreading. The trends of marginal re-entrants such as that defined bythe Urica basement transfer zone are defined by the first stage of Yucatan's motion. Further,Venezuela-Trinidad's passive margin section is predicted to have existed from the end of MiddleJurassic, not Cretaceous as is commonly thought. A several kilometer-thick, probable Late Jurassicshelf section in Eastern Venezuela has not received much attention from exploration, and the"Berriasian or older" salt in Gulf of Paria could be Middle Jurassic (as is the salt in the Bahamas,Guinea Plateau and Demerara Rise and Tacatú Basin). Note the proximity of these areas on figure 4. InSierra Guaniguanico of western Cuba, the conjugate margin of Eastern Venezuela, the lower MiddleJurassic San Cayetano strata indicate the existence of a juvenile passive margin of that age, becomingfully marine for Late Jurassic, as predicted here for Venezuela and Trinidad.

In summary, regional plate kinematic analysis is extremely cost-effective and deserves an importantrole in the exploration of complex areas, both early on and long-term. The kinds of implications wehave drawn here also can be made from kinematic analysis in other parts of the world. When appliedproperly to appropriate areas, it is not arm waving.

Much can be gleaned about:• Fault styles and displacements.• Basement types and associated parameters such as early heat flow.• Systematics of regional reservoir-bearing depositional patterns.• The relative ages of classes of structures, etc.

And all that is gleaned can lead to the creation or dismissal of numerous play concepts. In addition, anexplorationist with a comprehensive kinematic framework available to him or her will work moreconfidently - and therefore, more efficiently - on nearly all other aspects of the exploration process.Finally, in frontier evaluation programs, regional kinematic analysis may not tell you exactly where todrill, but it can often help to tell you where not to drill.

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Pz

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www.tectonicanalysis.com

Figure 1. Present day map of Gulf of Mexico region, showing key geologicalelements addressed in text. Note abrupt terminations of known basementunits in southern Florida. Also note change in trend of East Mexican Margin-al Fault Zone supporting the concept of two stages of Gulf evolution; base-ment structure contour data preclude any E-W faults in Mexico from enteringthe Gulf during the sea-floor spreading stage. Digital bathymetry/relief afterSandwell and Smith (1997), other features from multiple sources.

Published in AAPG Explorer, v. 21, September 2000

Important Note: We have modified Figure 1 since original submission for publication inthe Explorer. The reason is that the magnetic and gravity patterns of the SarasotaArch, off southwest Florida, do not appear to be offset or interrupted along the pro-jected trace of our "Everglades Fracture Zone". Thus, it may be preferable to consid-er that SE-ward motion of the Florida Straits block occurred by sinistral shear jumpingnorthward at the Northeast Basin, which would make that basin a pull-apart basin forMiddle Jurassic time, prior to seafloor spreading in the central Gulf of Mexico.

Relay (pull-apart)

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www.tectonicanalysis.com

105 707580859095100

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Chiapas Massifin final position

Chortis

Jamaica, Cuba

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Simplified from Pindell and Kennan, 2000, in prep.

Reefedge

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Neogene halokinesis (Sigsbee)

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Unstretched Stretched continent

Ocean crust

Early Cretaceous 130 Ma

Figure 2. Early Cretaceous (Valanginian) reconstruction of the Gulfof Mexico and Proto-Caribbean region. Post-Gulf formation stage,when seafloor spreading in the Gulf had ceased but was continuingin the Proto-Caribbean seaway.

Published in AAPG Explorer, v. 21, October 2000

Important Note: We have modified the Figures 2 and 3 since original submission for publication in theExplorer. The reason is that the magnetic and gravity patterns of the Sarasota Arch, off southwestFlorida, do not appear to be offset or interrupted along the projected trace of our "Everglades Frac-ture Zone". Thus, it may be preferable to consider that SE-ward motion of the Florida Straits blockoccurred by sinistral shear jumping northward at the Northeast Basin, which would make that basin apull-apart basin for Middle Jurassic time, prior to seafloor spreading in the central Gulf of Mexico.

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www.tectonicanalysis.com

Figure 3. Late Jurassic (Early Oxfordian) reconstruction of the Gulfof Mexico and Proto-Caribbean region ("salt fit"). Onset of seafloor-spreading stage. Note that Chiapas Massif has been transferred toYucatán Block at this time. Also bulk strain direction in Mexicoshifts from ESE-ward to S-ward at this time, with the opening of theMexican back-arc basin.

Published in AAPG Explorer, v. 21, October 2000

Important Note: We have modified the Figures 2 to 4 since original submission for publication in theExplorer. The reason is that the magnetic and gravity patterns of the Sarasota Arch, off southwestFlorida, do not appear to be offset or interrupted along the projected trace of our "Everglades Frac-ture Zone". Thus, it may be preferable to consider that SE-ward motion of the Florida Straits blockoccurred by sinistral shear jumping northward at the Northeast Basin, which would make that basin apull-apart basin for Middle Jurassic time, prior to seafloor spreading in the central Gulf of Mexico.

Ocean crust Marine incursion

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Seawater spilledthrough here intothe Gulf of Mexico

Max. extentof Salt Basin

Nazas Arc(extinct)

Simplified from Pindell and Kennan, 2000, in prep.

Chiapas

New ridge andtransform

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www.tectonicanalysis.com

Figure 4. Early Jurassic reconstruction of the Gulf of Mexico andProto-Caribbean region. Onset of "syn-rift" stage.

Published in AAPG Explorer, v. 21, October 2000

Important Note: We have modified the Figures 2 to 4 since original submission for publication in theExplorer. The reason is that the magnetic and gravity patterns of the Sarasota Arch, off southwestFlorida, do not appear to be offset or interrupted along the projected trace of our "Everglades Frac-ture Zone". Thus, it may be preferable to consider that SE-ward motion of the Florida Straits blockoccurred by sinistral shear jumping northward at the Northeast Basin, which would make that basin apull-apart basin for Middle Jurassic time, prior to seafloor spreading in the central Gulf of Mexico.

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