In our first two articles in this series we showed how
[PFItemLinkShortcode|id:20166|type:standard|anchorText:vector triangles and rotation poles can be used to contrain the mostions of contenental blocks and plates|cssClass:asshref|title:read a related article called Kinematic Analysis The Next Step |PFItemLinkShortcode]
and we
[PFItemLinkShortcode|id:20201|type:standard|anchorText:reconstructed the preAndean Oligocene shape of northern South America|cssClass:asshref|title:article titled Kinematics a Key To Unlocking Plays|PFItemLinkShortcode]
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.