Integrating rates of deformation across an orogen
One of the largest contributions to the advancement of earth sciences in the 21st century will be documenting the rates of geologic processes and understanding how they vary between our observational time-scales (10s of years) and longer, geologic time scales (millions of years). In the field of plate tectonics, the rates of interest are ones that constrain fault motion. Historically faults have been difficult to date precisely. Recent technical advances in thermochronometry allow us to date the time over which rocks cool from high (450 C) to low (60 C) temperatures and may provide a means for dating faults. The shortcoming with this approach is how much of that path is a result of deformational processes and how much is purely erosional. A way to overcome this challenge is to extract the displacement vectors from a balanced cross-section, and use the displacement vectors to predict a range of particle path trajectories and possible velocities. We can then test the impact these trajectories and velocities have on mineral cooling ages by using the estimated velocities as input for an advection-diffusion thermal model (Pecube) that predicts the resulting cooling ages. The truly unique aspect of this research is that it allows us to use thermochronometers as a second test to the viability of balanced cross-sections because the cooling history recorded by a suite of thermochronometers must match that predicted by the kinematics of a balanced cross-section for the cross-section to be valid. Linking the kinematics of cross sections to thermochronologic ages gives us an independent displacement amount and age allowing us to quantitatively document rates of deformation and changes in those rates over times scales of millions of years. This work is in collaboration with Todd Ehlers at the University of Tuebingen.
In Bolivia my collaborators and I have determined cooling ages on minerals and collected structural data that have been combined in a kinematic model depicting how the fold-thrust belt has developed through time (Barnes et al., 2006, Barnes et al., 2008; McQuarrie et al., 2008, tectonics). The displacement along folds and faults was forward modeled using the cooling ages of minerals sampled along the same transect as well as ages of overlapping strata. Tying cooling ages to location and magnitude of shortening, show that most of the shortening (~60%) was early (45-25 Ma) and we suggest an ~10–17 Ma pause or a dramatic deceleration in the rate of deformation and propagation of the fold-thrust belt between 25 and ~15 or 8 Ma. The uncertainty in the pause is based on the uncertainty in the timing of cooling of the frontal Subandean zone. The age of deformation in this region has been linked to multiple climatic and/ or dynamic changes in the Central Andes emphasizing that obtaining a robust age of deformation is critical. This is part of our ongoing research (see CAUGHT below)
Working with Tobgay Tobgay, a former graduate student and a Bhutanese native who was the first geoscientist in his country to receive a PhD, we had unprecedented access to the eastern Himalayan Kingdom of Bhutan. This allowed us to 1) map the frontal, unexplored portion of the Bhutan Himalayas and integrate new mapping with existing maps of the hinterland regions (Long et al., 2011, Journal of Maps), 2) create balanced crustal-scale structural cross-sections (Long et al., 2011, GSAB), and 3) obtain new mineral cooling ages (apatite fission track, zircon U-Th/He, 40Ar/39Ar, monazite Th-Pb) that completely describe cooling patterns from high to low temperature. These integrated cooling curves highlight windows of fast exhumation that vary spatially and temporally. We find that pulses of fast exhumation correlate with the vertical motion of material, as predicted by sequentially restored cross sections. This correlation allows us to place age constraints on structures and their associated shortening amounts and document marked changes in the rates of thrusting with time (Long et al., 2012, Tectonics; McQuarrie et al., 2014 EPSL). These data show that rate and the tempo of shortening in the Bhutan Himalayas varies with time and space, but emphasizes that the last 10 million years of shortening was markedly slower than modern rates of shortening (GPS) or rates determined from paleoseismicity. Similar variability is seen in the Nepalese Himalaya (Robinson and McQuarrie, 2012).
Interaction between erosion and deformation in fold-thrust belts
The topographic expression of modern mountain ranges reflects the interplay between a prior deformational history, active deformation, which is currently raising the mountain range and erosional processes, which remove material and act to lower the elevation. One of the main objectives of my research is to understand the control climate (specifically variations in precipitation) and associated erosion have on the shape and size of convergent mountain ranges as well as the magnitude of shortening. The two mountain ranges that are ideal to test this hypothesis are the Andes in South America and the Himalayas of India and Asia. One of the first order changes in the morphology of the Andes Mountains along strike is variation in width of high elevations. We evaluate the effect of climate on topography and deformation in Bolivia where the high elevations span a pronounced switch in hemisphere–scale Hadley precipitation regimes at ~17 -18 S dividing the Andes into wet (15–16 S) and dry (21 S) regions. In these regions, tectonics, basin geometry, and the deformation style are similar, allowing us to use variations in the width of the orogen (or changes in percent shortening) to evaluate whether the changes in width and morphology are climate driven. Using sequentially restored, balanced cross sections we determined that percent shortening is the same north and south during early (45-20 Ma) deformation, indicating changes in precipitation had very little effect on the width of the orogen. However, the later (~15 Ma to present) deformation is narrower in the north than the south suggesting a coupling between climate and tectonics that began between ca. 19 and 8 Ma, and continues to 0 Ma, (McQuarrie et al., 2008, Geology; Barnes et al., 2012, Geology).
Like the Andes, the precipitation gradient in the Himalayas is a natural place to test the effect of precipitation on deformation. Our work in the Bhutan Himalayas can address the debate by comparing shortening magnitude, percent shortening and magnitude of exhumation to similar studies in the central and western Himalayas. Our work indicates that both the amount of shortening (400 km in Bhutan compared to 541-667 in western Nepal) and percent shortening (56-58% compared to 72-76%) is significantly less (Long et al., 2011 GSAB.), indicating higher modern precipitation has not had a first-order effect on deformation.
Elevation verses deformation
Traditionally the topographic history of mountain ranges has been thought to mimic the deformational history. Thus as compressive forces shorten and thicken the continental crust, the buoyancy forces associated with a thicker lighter crust raises the surface elevation of mountain ranges. Recent analytical advances that capitalize on systematic changes in the ratios of stable isotopes with elevation, particularly the ratio of O18/O16, suggest that the deformation history of a mountain range may be decoupled in time from the elevation history. CAUGHT: Central Andean Uplift the Geodynamics of High Topography is a multi-institutional, NSF-continental dynamics project designed to document the deformation, elevation and erosional history of the central Andes mountains in South America, specifically to evaluate whether the rise of the Andean plateau was (1) slow-and-steady, commensurate with crustal shortening, or (2) rapid, associated with removal of dense lower lithosphere following significant crustal shortening. The CAUGHT team combines geophysics, structural geology, sedimentology, stable isotope geochemistry, thermochronology, and climate modeling to study interactions between climate, erosion, deformation, surface uplift, lithospheric removal. Work with Andrew Leier, and Carmala Garzione shows that early (27 Ma) changes in O18/O16 isotope ratios were just as significant as later (8 Ma) changes that have been used to infer an ~2-3 km rapid change in elevation. Taken together, the data may infer 2 periods of uplift possibly via mantle delamination, albeit each of a smaller magnitude than originally proposed. Our work on the age of deformation requires that most of the shortening and thickening of the crust predated either of these potential changes in elevation. (Leier et al., 2013, EPSL). A critical question is which uplift pulse prompted eastward propagation of the fold-thrust belt? or did both? Our ongoing research at Pitt is determining the age, geometry and rate of shortening as well as understanding the impact the 3D geometry of shortening in a curved orogen has on the crustal thickening and elevation history.
We are also actively looking at the links between deformation, elevation and exhumation on the island of Timor. While exhumation and surface uplift are important parameters in constraining the development of a mountain belt, the varied lithologies necessary to determine both of these parameters are rarely preserved in close proximity. In East Timor arc-continent collision since the late Miocene has uplifted a mountain range containing both deeply exhumed metamorphic belts and piggyback deepwater synorogenic basins. These varied lithologies are separated by a few tens of kilometers, and thus provide an opportunity to examine the spatial patterns of differential uplift and exhumation on Timor by comparing micropaleontology, thermochronology and one-dimensional thermal modeling. Our combined dataset demonstrates an extreme degree of variability in surface uplift and exhumation over small spatial scales. Mapping of structures at the surface combined with the variability in exhumation and uplift suggest that these patterns appear to be driven by subsurface duplexing. We propose that the correlation of the youngest, fastest exhumation rates (centered on the central mountain axis) with the highest average annual rainfall argue for active faulting and duplexing in the subsurface of Timor today and imply continued subduction and underplating of Australian continental crust (Tate et al., 2014, Tectonics).