HR: 10:20h
AN: T21E-01 INVITED     [PDF]
TI: Constraining Fault Evolution at an Active Extensional Relay: Star Valley, Wyoming
AU: * Gupta, S
EM: s.gupta@imperial.ac.uk
AF: Department of Earth Sciences and Engineering, Imperial College, Exhibition Road, London, SW72AZ United Kingdom
AU: Davis, A M
AF: Department of Earth Sciences and Engineering, Imperial College, Exhibition Road, London, SW72AZ United Kingdom
AU: Densmore, A L
AF: ETH Zurich, Department of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Dawers, N H
AF: Tulane University, Department of Geology, New Orleans, LA 70118 United States
AB: Relay zones, the sites where faults overlap and become linked, provide important insights into the processes by which fault segments coalesce. Whilst numerous studies have been made of the detailed structural geometry of relay zones, our understanding of the temporal evolution of faulting during relay formation remains limited. We focus on the Grover relay, a 4-km-wide en echelon step in the Star Valley fault located at the eastern margin of the Basin and Range province, Wyoming. Several coeval latest Quaternary ruptures have been documented on both segments by paleoseismological studies suggesting that the north and south segments are well linked. We use sedimentological and geomorphic observations to investigate faulting-landscape interactions and to propose a model of relay evolution. Typically relay zones comprise an en echelon fault overlap of inboard and outboard faults. At the Grover relay, the outboard fault loses displacement northward into a N-plunging fault-related anticline that has deformed early synrift deposits. Early syn-rift and pre-rift rocks are also preserved between the segments of the Star Valley faults and are being exhumed in the footwall of the outboard fault suggesting that the relay is not a newly developing one. Stratigraphic relations at the initial point of fault overlap indicate that approximately 2-my-old syn-rift alluvial fan conglomerates derived from erosion of the inboard footwall onlap and are ponded against the hangingwall dipslope of the outboard segment. This indicates that the Star Valley segments were in overlap position and the outboard fault formed topography during conglomerate deposition. This pinning point provides a minimum age for onset of fault overlap and permits estimation of a propagation rate for the outboard fault tip of approximately 9 mm/yr. This is an order of magnitude greater than Holocene fault displacement rates derived from trenching studies. Analysis of drainage patterns indicates that streams on the outboard footwall have incised headward in response to displacement on the outboard fault and captured drainage that originally flowed down the relay. Captured streams show well-developed concave profiles. Streams on the outboard fault tip by contrast show convex-up profiles indicating that tectonics dominates the streams ability to incise and develop an equilibrium profile. Stream capture has resulted in major denudation of the outboard footwall suggesting that this may be an important process in local footwall erosion. Comparison with numerical landscape evolution models (Densmore et al. 2003) suggests that capture of early formed relay zone drainage occurs in situations where fault segments propagate into overlap position and link rapidly, after which displacement addition on the overlapped segments drives the rapid baselevel fall that leads to capture. We hypothesise that the relative timeframe of fault linkage plays a critical role in landscape evolution and sediment dispersal patterns at evolving relays.
DE: 1824 Geomorphology (1625)
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting