George E. Hilley, PhD, Stanford University
We have explored the role of pre-existing geologic heterogeneities in determining the dynamics of deformation in active mountain belts. First, analytical approximations relating topographic loading to the stress state along active faults were used to determine situations in which tectonic motions may be distributed between multiple, simultaneously active structures. Second, numerical work-energy balances that consider body forces, fault friction, and far-field tectonic loading were used to study the kinematics of crustal deformation in areas where pre-existing geologic structures may largely dictate patterns of deformation. Third, we used the Gale geodynamic visco-plastic model to simulate the dynamics of fault motion along pre-existing geologic structures and within a deformable crust to understand how simple predictions made by the work-energy model differ from those expected under more realistic constitutive rules for the pre-existing faults and surrounding crust. Fourth, we have used the analytical, work-energy, and Gale geodynamic model to contrast the kinematics and dynamics expected for a situation in which the deforming crust is relatively homogeneous. The contrasting dynamics we have studied encompass those expected for tectonic styles spanning basement-cored uplift provinces to fold-and-thrust belts. We are currently incorporating bedrock erosion and alluvial transport into these models to study the impact of erosional mass redistribution on deformation within the simulated mountain belt and deposition within the adjacent foreland basin. Additionally, we continue to work with scientists at Potsdam and Syracuse Universities to compare the qualitative aspects of our modeling results with observations from the basement-cored and fold-and-thrust-belt provinces of the central Andes.
Key Results of PRF-Funded Research
This project seeks to understand the role that inherited geologic structures and erosional mass removal play in the dynamics of active contractional mountain belts, and how these dynamics influence patterns of deposition within foreland basins adjacent to these areas. First, we studied fold-and-thrust belts, which constitute end-member tectonic conditions in which pre-existing geologic heterogeneities and anisotropies are likely subordinate to the effects of topographic loading in dictating orogenic dynamics. We interrogated these dynamics by coupling rules describing erosional mass removal of material to a static mechanical model in which topographic and tectonic loading is exactly balanced to produce frictional failure everywhere within a cohesionless material. Additionally, we used the Gale geodynamic model to understand the detailed kinematics of individual thrust sheets, and how these may be influenced by factors such as erosional mass removal. We found that the number of shear zones that formed over time, the total slip accommodated along individual shear zones, and the spatio-temporal patterns of deformation within fold-and-thrust belts was sensitive to erosion of mass within the orogen. Specifically, more vigorous erosion is generally associated with fewer, often simultaneously-active out-of-sequence thrust sheets. The impoundment of sediment, especially near the front of the fold-and-thrust belt, may promote the migration of deformation into the foreland, while removal of this load can, in some modeled circumstances, cause deformation to step back into the orogen.
In a second step, we modified our approach to study the impact of crustal-scale geologic heterogeneities and anisotropies on the dynamics of active mountain building in cases where these material contrasts are large. This situation represents a second end-member tectonic style that is manifest by basement-cored uplift provinces. We explored these dynamics using an analytical force balance, a work-energy-based model to predict the relative sequence of activity along a set of pre-existing structures, and a modified version of our Gale models in which a series of crustal-scale zones of weakness were embedded. We found that these pre-existing, weak structures provide a seed for localizing deformation and constructing topography. However, the kinematics strongly depend on spatio-temporal patterns of topography constructed in the models. Frictionally weak, pre-existing geologic structures initially fail and build topography; however, activity eventually ceases as the topographic load becomes large and deformation is accommodated along other weak structures in the model. In these cases, frictionally stronger structures without overlying topography may fail as large topographic acting along weaker structures prevent their further motion. These observations explain the simultaneous activity of faults throughout basement-cored uplifts, despite the differences in frictional strength that exist along these structures. Our models reveal that the structure of basins formed between, and partitioned by these uplifts are strongly determined by the sequence of erosion of individual basement ranges.
Third, we have studied the how topographic loading, erosion, and pre-existing litho-tectonic structure of crust adjacent to mountain belts interacts to produce accommodation space in these areas. We found that the crustal structure underlying the adjacent foreland basin plays a first-order role in dictating patterns of accommodation for sediment deposition. Specifically, the presence of pre-existing weaknesses within crust adjacent to orogens may localize, and create persistent gradients in subsidence that are poorly predicted by current tectono-stratigraphic models. In these cases, zones of deposition may remain fixed in space, despite the advance of the topographic load into the flexing foreland basin. These patterns were quantitatively compared to the southern Andean foreland basins, which demonstrated the importance of these pre-existing structures in producing accommodation. In all of the cases we studied, the geologic template of active orogens and surrounding crust plays a central role in determining the spatio-temporal patterns of deformation and accommodation.
We are currently coupling a bedrock incision and sediment deposition model into the Gale geodynamic model. This will allow us to explore the details of feedbacks between mass redistribution within active orogens, the kinematics expected within them, and the patterns of accommodation and deposition in the adjacent foreland basins. The qualitative aspects of these fully coupled models will be compared to two end-member sites in the central Andes, where field observations suggest that excavation and filling of intramontaine basins may be linked to the initiation and cessation of deformation in these areas.