James Pindell1, Lorcan Kennan1, and Stephen Barrett1
Search and Discovery Article #40064 (2002)
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. However, 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. However, 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. In the late 1960s, one of the most exciting early realizations of the plate tectonic revolution was that the ways in which plates move relative to each other, both past and present, are governed by a firm set of predictive, or retrodictive, geometric rules. Plate kinematics gave us the power to quantitatively open and close oceans, collide continents and evolve plate circuits in area-balanced models.
Earth's geological history became an intellectual playground for "plate pushers" who began to decipher Earth's global 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 what the discipline, ended with "bandwagon" Plate Tectonic Interpretation sections, which correctly came to be viewed as mere arm waving.
Similarly, industry decision-makers grew to be suspicious of such tectonic scenarios -- with good reason -- and often ignored or discounted them. Thus, the potential of kinematic analysis often was never reached. Sadly, these very powerful rules are no longer even taught in many universities, and quantitative plate kinematic analysis is becoming something of a lost art. Very powerful plate kinematic rules, however, do still exist. Kinematic analysis is a means of better deciphering the structural history of basins.
First, we review some of these principles to provide the basis for exploring the power of kinematic analysis. 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 vector between the dots representing the plates. To palinspastically restore the offset back in time, we would use the blue vector to retract the accrued measured offset.
Progressing to a three-plate system, we must consider 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 baseball diamond. The displacement between home plate and runners A and B, respectively, is NE and NW, but the relative motion between the two runners is east-west. A plate boundary separating plates represented 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 directions and displacements of plates B and C are drawn relative to Plate A. Tieline B-C will then approximate the net direction (NE) and displacement (76km) of the common B-C fault zone. If this happens to be a thrust 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 vital information about overall structural style, with the expectation of dextral transpressive (combination of strike-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 the ridge (red) and transcurrent motions at the transform faults (green). The continuations of the transforms into adjacent oceanic crust are fracture zones where differential thermal subsidence occurs, but without active 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 transcurrent displacement at the transforms increases with distance from the pole. Transforms also become straighter as distance increases from the pole of rotation.
In this article these principles will be applied to two well known oil provinces, Colombia/western Venezuela and the Gulf of Mexico, showing how formal kinematic analysis can offer some of the most sound constraints available to guide and to favor certain geological interpretations over others. Further, it can provide the basis for defining or rejecting play concepts, therefore strongly influencing exploration strategy.
Figure 4. Vector "nest" restoring displacements of northern Andean blocks along faults during Andean orogenesis. Heavy dots denote blocks, tie lines restore net azimuth and magnitude of fault displacements, moving back in time. Mérida Andes and Eastern Cordillera deformation is shown partitioned into strike-orthogonal and strike-parallel components.
Figure 5. Oligocene reconstruction of pre-Mesozoic continental basement, northern Andes, based on vector displacements from Figure 4. "Retro-deformed" longitude/latitude grid is created by smoothing the lines across block boundaries after block restoration. Red outline is present day South America; cities, fields, rivers and geographic features (blue) are shown in palinspastic coordinates
Click here for sequence of present-day map of northern South America (Figure 3) and Oligocene reconstruction (Figure 5).
Figure 6. Paleogeographic map of western Venezuela and northern Colombia, showing the position of the Caribbean Plate and main depositional units during Eocene time. Note the similarity with today's Persian Gulf.
Click here for sequence of present-day map of northern South America (Figure 3), Oligocene reconstruction (Figure 5), and Eocene reconstruction of larger region (Figure 6).
Some of the principles of kinematic analysis noted above are applied to the first of our example areas: the northern Andes of Colombia and western Venezuela. We also will illustrate some of the uses and benefits of this analysis to petroleum geology and exploration in continental settings.
When applied to continental areas, kinematic analysis can provide 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 continental block 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, thereby 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 Early Oligocene and earlier time, prior to the majority of "Andean" deformation. Note that variations in the reconstruction will derive from applying different numbers of steps (accuracy can be increased by accounting for more fault motions between more blocks), and also from adjusting various input parameters, such as magnitudes of strike-slip offset on certain fault zones.
A reference frame is needed to 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 Mérida Andes, which separate the Maracaibo Block and the Shield.
Figure 3 shows the dextral offset across the Mérida Andes of the "Eocene thrustbelt," which came to rest in the Early Oligocene, measured by many as about 150 kilometers. In addition, shortening in the Mérida Andes has been estimated as about 40 kilometers. Thus, in the Early Oligocene, the Maracaibo Block lay significantly farther southwest relative to the Shield than it does today.
In Figure 4, we construct a tie line between the Guyana Shield and Maracaibo Block by performing vector addition of the strike-slip (150 kilometers) and thrust (40 kilometers) components. Because we wish to restore the accrued offset (155 kilometers), we draw the tie lines opposite to the real-life sense of fault displacements, i.e., moving back in time.
Having defined the Oligocene paleoposition of Maracaibo relative to the Shield, our next concern is the Perijá Range, deformation of which accounts for movements between the Maracaibo Block and the Santa Marta Massif Block. Estimates of post-Early Oligocene Perijá shortening are roughly 25 kilometers along an azimuth of east-southeast/west-northwest, as shown by the Perijá vector in Figure 2. Thus, displacing Santa Marta Massif to the west-northwest of Maracaibo by 25 kilometers gives the Early Oligocene position of Santa Marta relative to both Maracaibo Block and Guyana Shield.
Next, the Santa Marta strike-slip fault displaces the Santa Marta Massif Block from the northern part of Colombia's Central Cordillera. Left-lateral offset of about 110 kilometers (Figures 3 and 4) is believed to have occurred on this fault zone since the Late Oligocene. This strain is transferred into the Eastern Cordillera along the south-southeast continuation of the fault, where it is called the Bucaramanga Fault. Interestingly, the Bucaramanga Fault is flanked by the high, compressive topography of Santander Massif; this is because the Bucaramanga Fault defines the boundary between the Central Cordillera and the Maracaibo Block, not the Santa Marta Block.
For simplicity in Figure 4, the trend shown for the Bucaramanga Fault (in orange) defines only the total strain between those blocks, i.e. the sum of the strike-slip and orthogonal components of relative motion.
Finally, we restore Colombia's Guajira Block, also relative to the Santa Marta Block, by removing about 125 kilometers of dextral shear on the Oca Fault in order to realign the western flanks of continental basement in the two blocks prior to fault displacement.
With just these simple considerations, and assuming that only minor vertical axis rotation of these blocks has occurred during their relative motions, we can now fill out other tie lines in the vector "nest" of Figure 4 to define offsets between other pairs of blocks in the system. For example, the total strain in the Eastern Cordillera since the Oligocene is seen to be roughly 200 kilometers toward the east-southeast (red tie line). This can then be broken down into components of orthogonal and strike-parallel strain of 180 kilometers (blue line) and 100 kilometers (green line), respectively, which translates geologically into shortening (180 kilometers) and dextral shear (100 kilometers), moving forward in time.
We note that this value of shortening (180 kilometers) falls in the middle of the range of published values of estimated shortening in Eastern Cordillera. Thus, vector nests such as Figure 4 can be used to help choose between alternative balanced cross section models assessing shortening, because different assumptions of depths to detachments or degrees of basement involvement produce very different modeled shortening values. In addition, it also allows detection and estimation of the strike-slip component, which usually cannot be seen in cross sections. Our inferred dextral shear in the Eastern Cordillera is supported by seismicity, GPS data and field observations.
A pre-Andean (i.e., pre-Late Oligocene) palinspastic reconstruction of the northern Andes continental region (Figure 5) now can be made by restoring the motions of the blocks defined in Figure 4. The known limit of pre-Mesozoic continental crust has been identified in Figure 5 to show the pre-Andean geometry of the northern Andes "autochthon," to which a number of oceanic terranes have been accreted 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 on our reconstruction in order to build palinspastically accurate models of regional Eocene depositional systems. This practice also allows better sequence stratigraphic interpretation and correlation at the regional scale, which is helpful to determining migration pathways through the strata.
Also, the depositional models can be compared more meaningfully to modern analogues and analyzed for implications concerning reservoir potential, such as sand body orientation, sinuosity, flow direction, sand grain provenance and sediment maturity.
Finally, the reconstruction also allows a better interpretation 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 Eocene position of the Caribbean Plate relative to South America, to better understand the driving forces of Eocene sedimentation patterns and deformation. Figure 6 thus shows the Caribbean Plate driving an Eocene foredeep basin in the northern Maracaibo area -- much like today's Persian Gulf -- which caused an important early hydrocarbon maturation event in western Venezuela and Colombia's Cesar Basin.
Figure 6 also shows depositional systems with important reservoir facies belts at the Middle to Late Eocene boundary, as well as the existence, continuity and origin of an Eocene "Maracaibo Tar Belt" in western 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 decades by today's grossly different geography.
Figure 7. a. Cartoon section showing how passive margin sediments (deltas, turbidites, carbonate banks) can prograde far beyond the original position of the continental edge. 7b and 7c show a simple method of estimating and restoring crustal extension during rifting and passive margin formation. The cross-sectional area of the stretched crust (hatch-pattern) must equal that of the unstretched crust after sediment, water and mantle have been removed from the cross section.
Figure 8. Pre-Aptian Equatorial Atlantic reconstruction in which the restored pre-rift limits of continental crust (i.e., methodology of Figure 7) are juxtaposed. Note resulting simple geometry 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 the pre-rift limits of continental crust for each, as well as the large region of continental underfit in the absence of the sedimentary sections.
Figure 9. Successive pre-Aptian reconstructions of Gondwana and North America, using the Equatorial Atlantic fit of Figure 8. This analysis provides a quantitative framework in which to build more locally detailed models of the evolution of the Gulf of Mexico and surrounding areas. Note pre-Andean/pre-rift restoration of the northern Andes on the Triassic position of South America: This defines how much of Mexico is definitely allochthonous versus how much is potentially -- but not necessarily -- autochthonous.
Plate kinematics are used to reconstruct Africa and South America, and to progressively close the Atlantic Ocean during Mesozoic times, in order to set the stage for tracing the evolution of the Gulf of Mexico and the Florida/Bahamas region. We show the importance of removing post-rift sedimentary sections and restoring crustal extension when approximating the pre-rift shapes of continental blocks and margins.
First we show how this can be done in a simple way, and then we apply the method to a rifted margin pair -- the equatorial margins of Africa and South America -- to derive a pre-Aptian reconstruction of the northern parts of those two continents.
Prior to the equatorial Atlantic break-up during the Aptian, the northern parts of these two continents were essentially a single block. We can use the Euler rotation poles defined by marine magnetic anomalies and fracture zones in the central North 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 Andes described above, provides a quantitative kinematic framework in which to base models for the Mesozoic evolution of the Gulf of Mexico, Mexico and nuclear Central America, the Florida/Bahamas region, the Proto-Caribbean Seaway and northern South America.
Continental rifting reflects divergence of relatively stable portions of crust. This is accommodated by crustal extension at shallow levels (typically less than 15 kilometers), by normal faulting and at depth by ductile stretching of the lower crust and upper mantle. The end result is lithospheric thinning at the rift; we usually see overall tectonic subsidence 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 rifting proceeds. These are then overlapped by "thermal sag" sedimentary sections driven largely by cooling of the asthenosphere, plus the loading effect of the sediments themselves. Where extension is sufficiently large, oceanic crust is created and the two portions of continental crust drift apart. Where rifting does not reach this stage, we are left with intra-continental basins.
Sediment thickness at the rifted margins that flank ocean basins can exceed 16 kilometers. If sediment supply is sufficient -- for instance, near deltas or adjacent to high-relief topography in wet climates -- the position of passive margin features such as the shelf-slope break can change significantly with time, growing out from the coast and well beyond the original limits of the continental crust (Figure 7a). Although used for Bullard's famous reconstruction of the Atlantic margins (1965), this is why it is not satisfactory in quantitative kinematic analysis to merely realign a given bathymetric contour along opposed pairs of passive margins.
To gain a much closer approximation of the shapes of rifted margins to fit together for a more precise pre-rift geometry, we must construct cross sections of rifted 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 passive margins. The position of the Moho (base of the crust) can be crudely estimated by the balancing of water, sediment, crust and mantle using Airy isostatic calculations (Figures 7b,c) and, where gravity data or detailed sedimentological data are available, refined by taking into account crustal flexure and sediment compaction. Once the cross-sectional shape of the rifted margin's crust is inferred, the syn-rift extension in basement can be removed by restoring the cross-sectional area of the rifted margin shoulder back to an unstretched beam of continental crust.
Again, a crude calculation can assume this started at or near sea-level, and more refined calculations could take account of surface elevation, water depth prior to rifting and variations in initial crustal thickness or density. This identifies the position within that cross section that defines the pre-rift edge of the continental block. When plotted at several points along a particular margin, we can estimate the pre-rift shape of the continental margins. This can then be rotated towards the opposing margin using plate kinematic methods to show pre-rift geological relationships -- and to provide a starting point for modeling the ensuing basin evolution.
Figure 8 shows the net result of this method when applied to the rifted margins of the Equatorial Atlantic. The method is particularly important along the shelves at the mouths of the Niger and Amazon rivers, where the sedimentary thickness exceeds 10 kilometers over large areas. Note that the Para-Maranha- Platform is a piece of the West African Craton stranded on South America as the Equatorial Atlantic opened. A satisfactory fit can be achieved to an accuracy of perhaps 50 kilometers.
For comparison, the inset of Figure 8 shows the classic Bullard reconstruction of the two continents, with the pre-rift shapes of basement shown rather than the 2,000-meter isobath employed by Bullard. The inferred underfit in the Bullard reconstruction approaches 500 kilometers. Because continental reassembly in the Gulf of Mexico region is achieved by rotating the Africa-South America reconstruction back toward North America using Central Atlantic kinematic data, the difference between the two approaches will affect the final reassembly as profoundly as any other kinematic parameter.
Marine magnetic anomalies and fracture zone traces are used in the oceans to track the past velocity and flowpath, respectively, of pairs of plates separated by seafloor spreading.
Figure 9 shows a series of reconstructions of our united Africa-South America supercontinent and North America for Aptian and older times, prior to Equatorial Atlantic break up. Some of the positions are interpolated or extrapolated from the marine data to provide key time slices such as Triassic Pangean continental closure, and late Callovian/Early Oxfordian salt deposition in the Gulf. The analysis tells us how fast and in what direction the continents separated, which in turn constrains the geometry of ridge systems between the Americas, and also the size and shape of the inter-American gap through time.
Finally, also shown on Figure 9 is the pre-rift palinspastic shape of the northern Andes region superimposed on South America for the Late Triassic time slice. This was drawn by taking pre-Andean reconstruction (i.e. prior to Cenozoic shortening and strike-slip) and modifying it for pre-rift time by applying the methodology of Figure 7 (assuming an ENE-WSW extension direction).
The relationship of North and South America at this time is important, because it defines a line separating two parts of Mexico. The part of Mexico overlapped during Late Triassic time by South America 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 not overlapped by South America during the Triassic may have been in place relative to today's geography, but were not necessarily so.
From Figure 9, the fact that the formation of the Gulf of Mexico was completed by early Cretaceous time implies that Jurassic plate boundary systems active in the Gulf until then probably also controlled many primary elements of the evolution of Mexico. The kinematic constraints developed here may now may be used to reconstruct western Pangea and to trace the Mesozoic plate-kinematic evolution of the Gulf of Mexico, eastern Mexico, the Florida/Bahamas region and the Proto-Caribbean Seaway.
Figure 10. Present day map of the Gulf of Mexico region, showing key geological elements addressed in this month’s article. Note the abrupt terminations of known basement units in southern Florida that we consider were truncated by transcurrent motion on our "Everglades Fracture Zone." Also note the change in trend of East Mexican Marginal Fault Zone supporting the concept of two stages of Gulf evolution; basement structure contour data preclude any east-west faults in Mexico from entering the Gulf during the sea-floor spreading stage. Digital bathymetry/relief after Sandwell and Smith (1997), other features from multiple sources.
Figure 11. Early Cretaceous (Valanginian) reconstruction of the Gulf of Mexico and Proto-Caribbean region. Post-Gulf formation stage, when seafloor spreading in the Gulf had ceased but was continuing in the Proto-Caribbean seaway.
Figure 12. Late Jurassic (Early Oxfordian) reconstruction of the Gulf of Mexico and Proto-Caribbean region ("salt fit"). Onset of seafloor-spreading stage. Note that Chiapas Massif has been transferred to Yucatan Block at this time. Also, bulk strain direction in Mexico shifts from ESE-ward to S-ward at this time, with the opening of the Mexican back-arc basin.
Using the kinematic framework for the evolution of the Gulf of Mexico region defined by restoring Andean deformations and progressively closing the Atlantic Ocean, we further evolve it to build a palinspastically quantitative reassembly of continents and continental blocks that were separated during the Mesozoic rifting and subsequent drift in the Gulf of Mexico region. Key features are shown in Figure 10. Figures 11, 12, and 13 show primary developmental stages in the evolution of the Gulf:
The kinematic elements applicable to the reconstructions are as follows.
First, our Oligocene reconstruction of northern South America (article two) is further modified for Late Jurassic and Cretaceous time by removing island arc and other terranes that were accreted to in the Late Cretaceous and Early Tertiary (shape portrayed in Figures 11 and 12). We can then estimate and restore Jurassic extension in the rift basins of the Andes (using principles outlined in the August EXPLORER, which gives us an Early Jurassic shape for the northern Andes that can be closed against North America (Figure 13).
Second, Figures 11, 12, and 13 show that the entire region of 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.
In this region, plate separation was achieved by NW-SE stretching of crustal elements 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 the others. The conjugate margin to the Southern Bahamas flank is the transcurrent margin of Guyana.
Third, unlike the Florida region, the Yucatan Block moved independently -- in two distinct stages -- of the larger continents as the Gulf opened. At the time of Figure 13, there is only a small range of paleo-positions in which Yucatan could have fit geometrically without overlap of palinspastically restored (i.e., rift-related stretching removed) areas of continental crusts. This position can be achieved by rotating present-day Yucatan clockwise about "pole A" (Figure 13), which closes most of the Gulf by placing Yucatan snugly against 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 migrated SE, probably along our "Everglades Fracture Zone" (Figure 10) between the times of Figures 12 and 13.
Fourth, the geology of the eastern Mexican margin and the occurrence of Louann and Campeche salt suggest that the Gulf opened in two stages.
The first, or syn-rift, stage -- between the times of Figures 12 and 13 -- involved intra-continental stretching between Yucatan and North America about "pole B1," and between Yucatan and South America about "pole B2," in Figure 13. This migration defined an arcuate transcurrent trend defined by basement contours along the northern Tamaulipas Arch in south Texas. It also created a sinistral shear couple in the Louisiana-Mississippi area, which allowed for minor counterclockwise rotation of the Wiggins and Middle Grounds arches (Figures 10 and 13) and the associated formation of the wedge shaped East Mississippi and Apalachicola salt basins to the north of each, respectively.
This syn-rift stage about "pole B1" can be modeled satisfactorily to Early Oxfordian time to achieve a good reconstruction of the Louann and Campeche salt provinces flanking the central Gulf (Figures 10 and 12). In our modeling, we see no need to invoke significant salt deposition on oceanic crust in the Gulf. Also, during this stage, the southern Bahamas crustal corridor migrated southeast in addition to undergoing internal stretching -- hence, the Everglades fracture zone and the Guyana marginal fault zone were both active at this time.
The migration of Yucatan from its pre-rift to its present position requires that eastern Mexico was a transform rather than a rifted margin. We consider that Yucatan did not have the Chiapas Massif attached to it during the syn-rift phase. Why?
We believe that the Chiapas Massif was picked up by Yucatan in this stage as a consequence of the onset of seafloor spreading in the Central Gulf -- and because the pole of rotation changed in Stage 2, the orientation and position of transforms also must have changed. This new phase of motion had a more southerly direction than the previous one. The spreading ridge almost reached the Mexican coast and, hence, the new transform along eastern Mexico would have picked up an additional wedge of crust, which we believe is Chiapas Massif and which had been emplaced there during the syn-rift phase by sinistral transcurrent motions within greater Mexico.
As with the Gulf of Mexico, the synchronous creation of the "Proto-Caribbean Basin" also must have involved a rotational opening between Yucatan and Venezuela-Trinidad. In Figures 11, 12, and 13, we show the approximate flowlines along which this basin opened, as well as a hypothetical geometry of its Jurassic rifted margins -- now wholly overthrust by allochthonous Caribbean terranes.
Many elements of northern South America's and possibly eastern Yucatan's hydrocarbon potential pertain directly to the geometries of these rifted margins, such as the positions of marginal 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 -- in the south (Veracruz Basin). Heat flow, subsidence history, occurrence of salt, distribution/thickness of Late Jurassic source rocks and basement controls on future structural development will all vary along strike along this margin due to differing crustal properties and histories. In the U.S. Gulf margins, early syn-rift stretching was NNW-SSE until Early Oxfordian times, but most of the stretching toward the end of this phase occurred well offshore.
Second, although salt deposition is generally assumed to be of Callovian age, there is little evidence of open 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 reconstruction accommodates known salt occurrence in the Gulf ("salt fit"); hence, we consider that onset of seafloor spreading, the change in the Yucatan-North America pole position, separation of Louann and Campeche salt provinces, 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 direction in the deep eastern Gulf during stage 2 (seafloor spreading) was NE-SW about a nearby pole, such that small circles (transform traces) should be arcuate and convex to the northwest. In Cuba, a significant area of Bahamian crust was overthrust by Cuban arc assemblages in the Paleogene. In the Jurassic, the southern Bahamian margin (beneath Cuba) experienced sinistral strike-slip tectonics along the Guyana margin of South America, followed by the eastward migration of a Late Jurassic seafloor spreading ridge (Yucatan/South America boundary) along the western half of the overthrust zone.
The transform nature of this Jurassic margin should be considered in interpretations of the Paleogene development of the Cuban thrust belt, Mesozoic source rock paleogeography and oil migration 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 by the 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 Middle Jurassic, not Cretaceous as is commonly thought. A several kilometer-thick, probable Late Jurassic shelf 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 13. In Sierra Guaniguanico of western Cuba, the conjugate margin of Eastern Venezuela, the lower Middle Jurassic San Cayetano strata indicate the existence of a juvenile passive margin of that age, becoming fully marine for Late Jurassic, as predicted here for Venezuela and Trinidad.
In summary, regional plate kinematic analysis is extremely cost-effective and deserves an important role in the exploration of complex areas, both early on and long-term. The kinds of implications we have drawn here also can be made from kinematic analysis in other parts of the world. When applied properly to appropriate areas, it is not arm waving. Much can be gleaned about:
And all that is gleaned can lead to the creation or dismissal of numerous play concepts. In addition, an explorationist with a comprehensive kinematic framework available to him or her will work more confidently -- 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 to drill, but it can often help to tell you where not to drill.