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RESEARCH FOCUS
Puzzling the pieces
C.M. Cooper
School of the Environment, Washington State University, Pullman, Washington 99164, USA
Alfred Wegener famously argued that the seemingly puzzle piece–like
fit of the Atlantic coastlines was not a mere coincidence, but rather one
line of evidence proving that the continents were once arranged as a single, coherent supercontinent (Wegener, 1912,1920). This puzzle piece observation eventually launched a revolution that changed our understanding
of the Earth from its deep interior to evolutionary processes. Often, however, we think of the supercontinent puzzle in a two-dimensional sense,
neglecting to include or consider how variations of the thickness of the
puzzle pieces might also be at play. How do the puzzle pieces fit together
at depth, and is there more to learn by including lithospheric thickness
in our plate reconstructions? Would thinking three-dimensionally in our
plate reconstructions help resolve some of the outstanding questions about
supercontinents, continental deformation, and the lithosphere in general?
This is the motivation of new research by McKenzie et al. (2015, p. 783 in
this issue of Geology).
The concept of the lithosphere was developed by Barrell in 1914 and
1915 in a series of papers describing the response of the Earth’s surface
to loading (e.g., Barrell, 1914, 1915). He contended that there must exist
a layer of strength (the lithosphere) on top of a layer of weakness (the asthenosphere) to accommodate isostatic adjustments (for a more thorough
exploration of the story behind Barrell’s and his predecessors’ lithosphere,
see A.B. Watts’ book, Isostasy and Flexure of the Lithosphere [Watts,
2001]). Since the concept was introduced, the community has struggled
with adopting a definition more concrete beyond a “layer of strength.”
Part of this struggle is tied to what, exactly, is supplying the strength. The
strength of rocks depends on many factors including composition, temperature, pressure, stress, and grain size (e.g., Ranalli, 1995). Thus, to be a
layer of strength, the lithosphere must encompass the region where these
factors (either singularly or in combination) work to promote strength. For
example, the lithosphere is strong because it is at temperatures sufficiently
cool enough to promote rigid behavior, or the lithosphere is strong not
only because of the lower temperatures, but also because the process that
creates the lithosphere produces compositions (and water contents) with
higher viscosities, etc.
As such, there are numerous ways to describe the lithosphere, its base,
and its total thickness. Some use the thermal boundary layer (TBL), or
the region of rapid temperature increase from the low surface temperature
of Earth to the more subtle adiabatic temperature gradient within the hot,
convecting mantle, to describe the lithosphere, using the base of the TBL
as the base of the lithosphere (Turcotte and Schubert, 1982). The base
of the TBL is often demarcated by an isotherm within the range of the
potential temperature of the mantle (Turcotte and Schubert, 1982). Others
prescribe a competent mechanical boundary layer (MBL), or the region
within the thermal boundary layer wherein heat is only transferred by conduction, and thus, not part of the deforming and convecting mantle, and
ideally, more rigid and “strong” (e.g., McKenzie et al., 2015). The base
of the MBL is also distinguished by an isotherm, though one at lower
temperatures than the potential temperature of the mantle (e.g., McKenzie
et al., 2015). The chemical boundary layer (CBL) is a construct similar
to that of the mechanical boundary layer, but the strength is derived from
chemical processes that increase the viscosity of the material (e.g., Lee
et al., 2005). In other words, the material in the CBL is chemically and
rheologically distinct from the rest of the thermal boundary layer, and the
base of the CBL is defined at that transition. The lithosphere can be fur-
ther defined by its elastic thickness (e.g., Burov and Diament, 1995), its
seismic velocity (e.g., Gaherty et al., 1999), or its electrical conductivity
(e.g., Hirth et al., 2000). Each of these descriptions frames a unique lithosphere thickness. In other words, if using the TBL construct to describe
the lithosphere, the corresponding thickness may not be the same as for
the thickness of the lithosphere as described by a MBL. So, which definition should be used? To some extent, it depends on the question at hand.
For example, we’ve gained considerable insight into the behavior of
oceanic lithosphere by placing it in the conceptual framework of a thermal
boundary layer. In general, the thickness of oceanic lithosphere increases
with age in a predictable relationship (Parsons and Sclater, 1977). In this
construct, it is “easy” to interpret variations in the thickness of oceanic
lithosphere—thinner oceanic lithosphere is most likely thin because it is
relatively young, in closer proximity to the spreading ridge, and warmer.
Variations in the thickness of continental lithosphere are not as straightforward to interpret. The oldest continental regions also correspond to the
areas of the thickest lithosphere (Jordan, 1978). While regions of younger
continental lithosphere do tend to be thinner, the relationship with age is
more speculative than for oceanic lithosphere (Artemieva, 2006). Part of
this discrepancy comes from the more storied lifetime of continental lithosphere compared to its oceanic counterpart. Oceanic lithosphere is created
at the mid-ocean ridges, from which it ages, cools, and thickens until it
meets it demise at subduction zones. Continental lithosphere, on the other
hand, forms in a more complex, less self-consistent fashion (Rudnick,
1995), its evolution more chaotic, depending on the tectonic settings it
encounters, and continents do not, for the most part, recycle back into the
mantle. Rather, they are reworked by processes that can either thicken or
thin the lithosphere. In addition, the oldest and thickest regions (called cratons) might act as a spurious end-member. With few exceptions, cratonic
lithosphere tends to remain thick from its inception onward, regardless
of tectonic setting (Pearson, 1999; Griffin et al., 2003). Indeed, ignoring
cratonic lithosphere, the thickness of continental lithosphere is at best correlated to the last tectonothermal (relating to major tectonic or thermal
disturbances) event (e.g., Artemieva, 2006). But, arguably, much of our
knowledge is seated in the context of the present-day geographic location
of continental lithosphere. Perspective could shift if we looked at lithospheric variations in the configuration of the last global tectonothermal
event—right before the split-up of Pangea.
Prompted by curiosity, McKenzie et al. (2015) rotated the continents
with their imaged lithosphere from today’s location back into their Pangean configuration. They mapped the base of the mechanical boundary
layer (MBL) in the present-day plate configuration using Rayleigh wave
tomography, a technique sensitive to temperature. They then used plate
reconstruction models to move these images into their location right before Pangea began to separate (see McKenzie et al. [2015] for the full
technical description). This led to a peculiar result—the regions of thick
continental lithosphere arrange into a long, contiguous feature parallel
to the ancient Pangean coastline and active margin. This is quite different from the current arrangement of thick continental lithosphere: each
continent possesses areas of thicker lithosphere to which it appears that
thinner continental lithosphere has docked around. There seems to be no
noticeable relationship between the thickness of continental lithosphere
and the location of today’s margins, and certainly, no contiguity between
the thick regions (both within and across each continent). At first glance,
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the contiguous, curvilinear shape of McKenzie et al.’s map seems both
surprising and also somewhat expected—after all, we already know the
puzzle fits at the surface.
But it’s the contiguity of the thickened lithosphere that is the exciting
result. It introduces new questions to consider: first of all, why is this so?
Is this unique to Pangea or would other continental assemblies, such as for
other supercontinents, produce a similar result? Why is the feature curvilinear and parallel to the ancient coastline? Is this a curious artifact due to
the shape of Pangea or is it hinting at a new insight into continental deformation? The authors suggest that the continuous feature formed because
cratonic lithosphere is controlling deformation. The stronger, thicker regions focus deformation on its periphery (Lenardic et al., 2000; Audet and
Bürgmann, 2011) and material trapped between two (or more) regions
during convergence would preferentially thicken. As McKenzie et al.
point out, to be able to capture this in a past event, in addition to a similar
process occurring presently in the Himalayan orogeny, is further evidence
of the important role that cratonic lithosphere is playing. This might also
provide information about the breakup of supercontinents. The weaker
lithosphere accommodating the deformation during shortening also could
serve as the weak regions during the splitting of Pangea, leading to the
more disperse distribution of cratonic lithosphere across multiple continents. Perhaps, cratonic lithosphere not only controls deformation, but
could also determine the total number and configuration of continents.
Clearly, more work is required to address these questions and others
that might arise from this result. For example, the sensitivity to the isotherm chosen as the base of the MLB should be tested. In addition, it
would be interesting to compare images produced by other seismological techniques. Furthermore, choices made in order to do the reconstruction, such as the removal of the regions of thick lithosphere associated
with present-day active shortening, should be re-examined to determine
whether they introduce any bias into the result. In addition, careful geodynamic modeling could address the role of thicker, stronger regions in
continental assembly and breakup. But sometimes it requires looking at
observations from a new perspective (or plate configuration) to reinvigorate old puzzles.
ACKNOWLEDGMENTS
This paper greatly benefited from edits from and discussion with J. Hegg, E. Mittelstaedt, A. Johnson, L. Moresi, and A. Lenardic.
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