The surface of the earth is shaped by both the constructive force of tectonics, which builds mountains, and the destructive agents of geomorphology, including erosion by rivers, glaciers and landslides, which tear mountains down. My research field, Tectonic Geomorphology is the study of this competition, and how it is reflected in the landscape.
Within the scope of my research there are two faces to Tectonic Geomorphology. One, the subfield of neotectonics, is more traditional is scope, using deformation recorded in the landscape to quantify tectonic processes. The methods are anything but traditional, however, drawing on satellite and other remotely sensed data (GPS, InSAR, LiDAR), paleoseismology, and surface exposure dating of offset landforms (cosmogenic nuclide dating, optically stimulated luminescence). At the longer timescales, neotectonics dabbles with thermochronology ((U-Th)/He, fission-track and 40Ar/39Ar dating) and landscape morphometrics (basin hypsometry, asymmetry, range front sinuosity, etc.). This important research avenue allows us to understand active tectonic processes in increasingly greater detail and has led to important breakthroughs.
Another branch of Tectonic Geomorphology, however, goes a step further, exploring not just how geomorphology reflects tectonics, but rather how they influence each other. Tectonic-climate coupling was, when it was introduced in the 90’s, a radical idea. The motion of tectonic plates, although slow, carries tremendous momentum and is controlled by forces operating literally at a planetary scale. The idea that processes scratching at the surface of the earth – glacial erosion, fluvial erosion, landsliding – could influence tectonic deformation in any way is like imagining that pouring water from a teaspoon onto a speeding freighter can change the course of that ship. Two decades of research have now shown that it is in fact the case that rivers, glaciers and landslides are powerful enough as erosive forces to change the mass balance of a mountain range, and therefore change the pattern and rate of tectonic deformation, all the way at least to the mantle lithosphere. The path of the ship, in other words, is influenced, and some would even argue, controlled, by the amount and location of water thrown on it. Research in this field is partly driven by numerical models, which are capable of capturing the complex interplay of feedbacks mechanisms between tectonic and geomorphic processes. However, detailed field studies are critical in establishing the real world relationships which computer models explore.
One particularly compelling area in which questions of neotectonics and tectonic-climate coupling are relevant is the study of continental plateaus. Continental plateaus, such as in Tibet, the central Andes and eastern Turkey, are some of the most striking features on the surface of the Earth, exerting fundamental influence on regional and global climate. However, they are relatively poorly understood tectonic features. Questions remain as to why they form in certain collisional settings and not others, or why they form at all in places like the Andes, where there is no collision, but only the steady subduction of oceanic lithosphere. They share a common morphology, with steep sides and flat, broad, high-elevation tops, yet they differ significantly in the style and degree of deformation and the condition of their lithosphere. Much of my research centers on trying to understand the life cycle of continental plateaus: why they exist in the first place, what processes sustain them or cause them to grow, and eventually how they collapse or otherwise meet their end. I have other interests as well, including the role of glacial erosion in shaping tectonic deformation, and in the interaction of fault systems. My research is focused on four major questions:
1) What is the role of strike-slip faults in accommodating plateau growth?
Strike-slip fault zones respond to a collision by allowing fragments of lithosphere to slide out of the way (extrude), therefore reducing the amount of crustal thickening that the collision would otherwise have produced. There is no denying the existence of major strike-slip faults in Tibet (Ailao Shan-Red River, Kunlun, Altyn Tagh and Karakorum, just to name a few), but their importance in accommodating extrusion of lithosphere is hotly debated (e.g. Meriaux et al., 2004 vs. Cowgill, 2007). Strike-slip faults play a critical role in the Anatolian plateau as well, as the lithosphere extrudes west, bordered by the North and East Anatolian faults (Sengör and Yilmaz, 1981). As in Tibet, the Anatolian lithosphere has experienced crustal thickening, but strike-slip faults are common in the interior of the plateau as well, including the Central Anatolian fault zone and Tuz Gölü fault (Koçyiǧit and Beyhan, 1998). Complicating matters, it is likely that the strike-slip faults themselves evolve as the orogen grows (e.g., Schoenbohm et al., 2006a), and so their behavior and importance in accommodating collision changes with time. Neotectonic studies have made progress in quantifying the rates of strike-slip faulting, and therefore their role in plateau-forming collision, but much work remains to be done
2) What is the role of climate in initiating, enlarging and sustaining plateaus?
Orographic effects along the steep margins of the major continental plateaus concentrate some of the highest precipitation seen on Earth (e.g. the Asian summer monsoon in Bangladesh and India), yet the plateau interiors are arid and largely internally drained. It has long been argued that this internal aridity sustains plateaus by protecting them from the ravages of the rapid erosion that accompanies high rainfall. In fact, orographic gradients along plateau margins may not only protect plateaus, but may also contribute to their outward growth: as marginal basins are constricted by structural deformation, new, upwind orographic barriers are formed. The resultant increased aridity in the basins may cause them to become internally drained and fill with clastic sediment, leading to their morphologic incorporation into the high, dry, internally drained plateau (Sobel et al., 2003). Whether a basin becomes internally drained or not depends on the interplay between tectonic uplift, forming barriers to drainage, and fluvial erosion, keeping basins integrated (Hilley and Strecker, 2005). Tectonic geomorphology is ideally suited to explore and quantify this interplay of constructive and destructive forces, tectonic-climate coupling, at plateau margins.
3) What happens in the roots of plateaus? How are deep lithospheric processes reflected at the surface?
What’s happening in the lower lithosphere exerts fundamental control on plateau development through the redistribution of mass away from the collision zone. However, because such processes occur below the Earth’s surface, their operation can be difficult to test. Tectonic Geomorphology can detect the effects of lithospheric processes through the pattern of deformation at the surface. It is particularly useful in understanding the lithospheric-scale process of lower crustal flow, in which topographically created pressure gradients in gravitational potential energy drive the down-gradient flow of lower crust (Clark and Royden, 2000). Lower crust may reach the surface or tunnel beneath it, depending on the intensity of erosion (Beaumont et al., 2001). This is a process which is demonstrated through modeling studies, but is difficult to prove in the real world because the processes are operating over long time-scales in the hard to image lower crust. Studies of the pattern of uplift and deformation shed light on this phenomenon.
Another important process operating at the lithospheric scale is foundering, in which mantle lithosphere and potentially lower crust are recylced into the mantle through delamination (peeling off and subduction of the lithosphere) or dripping (formation of Rayleigh-Taylor instabilities) (Houseman et al., 1981; Göǧüs and Pysklywec, 2008a). Detachment of lithosphere leads to uplift, contributing to the formation of continental plateaus and influencing regional and even global climate. Foundering and delamination have been proposed in the Andean, Tibetan and Anatolian plateaus (Kay and Kay, 1993; Molnar and Garzione, 2007; Turner et al., 1996; Göǧüs and Pysklywec, 2008b). Modeling studies suggest the ubiquity of lithospheric foundering (Göǧüs and Pysklywec, 2008a; Elkins-Tanton, 2007), but geophysical imaging provides evidence only for very recent foundering events which are still visible in the upper mantle (e.g. Bianchi et al., 2012; Schurr et al., 2006). An additional, seldom explored avenue is to investigate how foundering processes are expressed at the Earth’s surface, in the form of structural deformation, subsidence or uplift, or magmatism, where it is possible to peer further back in time. Such studies are critical in understanding the geometry and pacing of foundering events.
4) How do glaciers and faults interact (in plateau regions)?
Glaciers imprint the landscape in ways which are exquisitely tuned to both tectonic and climatic factors. For example, studies have shown that glaciers effectively limit the average and maximum elevation of a landscape through a process known as the glacial buzzsaw, regardless of the tectonic uplift rate (e.g. Brozović et al., 1997). In this model, glacial erosion is most effective at the elevation of the snowline, or Equilibrium Line Altitude (ELA), where the flux of ice from the accumulative to ablative regions is the highest. Focused erosion drives the average landscape elevation toward a value slightly below the time-averaged ELA (Egholm et al., 2009). Peak elevation, in turn, is limited by rock strength, valley spacing, and a “base level” set by the glaciers, and thus is limited to a few hundred meters above the ELA as well (Anders et al. 2010; Mitchell and Montgomery, 2006). However, in some cases, particularly where exhumation rates are high, erosion occurs under arid conditions, or glaciers are frozen to their beds, some peaks appear to escape the glacial buzzsaw, rising to tower above the adjacent landscape (Brocklehurst and Whipple, 2007; Foster et al., 2008; Thomson et al., 2010). Glacial erosion has also been found to influence the geometry and deformation rate of exhuming mountain ranges, both in modelling studies (Tomkin, 2007) and in field examples (Berger et al., 2008). Additional data, especially from plateau regions, which experience strong climatic and tectonic gradients, are needed to better understand these glacial-tectonic interactions.
I conduct research into these questions in a number of field locations around the world. My projects in the Puna Plateau of Northwest Argentina and in Central Anatolia are active, as is my remote-sensing based research into Glacier Morphology. I'm winding down projects in the Pamir and Argentine Precordillera. I also include information on my PhD research in Yunnan Province, China, on the southeast margin of the Tibetan Plateau.
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