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 further 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, 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.
This paper greatly benefited from edits from and discussion with J. Hegg, E. Mittelstaedt, A. Johnson, L. Moresi, and A. Lenardic.
