The Geophysical Corner is a regular column in the EXPLORER, produced cooperatively by the AAPG Geophysical Integration and SEG Interpretation committees, and edited by M. Ray Thomasson.
This month's column, the third in a four-part series dealing with plate kinematics ("Arm Waving, or Underutilized Exploration Tool?"), is titled "Building Quantitative Plate Kinematic Frameworks for Regional Exploration Assessments."
A Removal-Restoration Project
By JAMES PINDELL
In our first two articles in this series we showed how vector triangles and rotation poles can be used to constrain 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 of continental 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 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 from last month's article, 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 1a).
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 1b,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 2 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 2 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 3 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 3 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 last month's reconstruction (i.e. prior to Cenozoic shortening and strike-slip) and modifying it for pre-rift time by applying the methodology of figure 1 (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 3, 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.
Thus, the stage is set for us next month to use the kinematic constraints developed here 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 in our final article of the series.