Can We Constrain the Evolution of Crustal-Scale Normal Fault Arrays Using Geomorphic and Structural Criteria?

Tulane University, New Orleans LA

Investigators

Abstract

The proposed project is aimed at understanding the relationship between the evolution of large continental non-nal fault arrays and the development of drainage catchment-alluvial fan systems. Previous studies of non-nal fault growth, footwall denudation and catchment development have either ignored spatial and temporal changes in fault array geometry and slip rate, or have focused on portions of the fault array that are far from actively-propagating tips and relay zones between adjacent fault strands. In the proposed work, the PI's and a Tulane University graduate student will carry out a quantitative structural and geomorphological study of active faulting at extensional relay zones, i.e. areas of segment overlap and fault linkage, and at fault tips. The study area is located in the circum-Snake River Plain area of cast-central Idaho, southwestern Montana and western Wyoming. This area is well suited to the aims of the project because the Basin and Range non-nal faults show a wide range in scale; i.e., the faults show varying amounts of total displacement and rock uplift, and hence the footwalls exhibit varying degrees of drainage incision and denudation. Two tasks are proposed. The first task is an analysis of digital topographic data. This will measure a number of geomorphic parameters (catchment spacing, relief and area; and alluvial fan area) along fault strike and across a range of fault length scales. Results of this work will constitute the first quantitative assessment of relations between fault structure, slip rate and footwall morphology, as well as the first evaluation of the fan area/catchment area ratio as proxy for fault slip rate, and thus as a potential neotectonic tool. The second task is a focused field investigation of 3 relay zones along 2 of the largest and most active faults. This work will map the distribution of active faulting through these segment linkage sites, and document the relationships between the spatial fault pattern and catchment-fan parameters (derived in Task 1) in relays. The 3 field sites chosen for the detailed structural and geomorphological work show varying amounts of fault segment separation and linkage development, which is hypothesized to control some of the patterns of catchment evolution. The proposed work is expected to establish predictable relationships between the growth, interaction and linkage of continental-scale normal faults and patterns of denudation and catchment-fan evolution. This research will also lead to an improved understanding of earthquake hazards, because the approach will address issues of fault segment connectivity over a time scale longer than that addressed by paleoseismology alone.