Collaborative Research: Comparing Deformation Rates in Wrench Borderlands from Geodetic and Geologic Data to Evaluate the Permanent and Recoverable Components

Oberlin College, Oberlin OH

Investigators

Abstract

The current velocity field associated with plate tectonic motion in western California is well characterized by geodetic measurements using Global Positioning Systems (GPS). The exact nature of what geodetic measurements record in actively defon-ning zones remains a significant problem, as this technique records both recoverable (elastic) and pen-nanent (quasi-plastic) strains in wrench borderlands (blocks adjacent to major strike-slip faults). The goal of this proposal is to evaluate the relative magnitudes of the recoverable and permanent components in the deformation of wrench borderlands, by simultaneously studying two different sections of the San Andreas fault system. Creeping segment, central California: Geodetic measurements alone are generally unable to distinguish between recoverable and permanent deformation, because of the long (>50 yr) interval between slip episodes on faults. In contrast, intervals between slip episodes are short (weeks to months) in the creeping central section of the San Andreas fault. By collecting geodetic data throughout the short seismic cycle in the creeping segment, the PI's will be able to evaluate the relative sizes of the recoverable and permanent components of the displacement field in the vicinity of the fault. They propose to check the estimates of currently accumulating permanent measurements against long-ten-n averages detentiined from geologic and paleogmagnetic data. Prior work suggests clockwise rotation of the palcomagnetic signal of Miocene and younger sediments in the wrench borderlands. Documenting the areal extent, regional distribution, and amount of rotation throughout this area will allow them to calculate a long-ten-n average for pen-nanent deformation. This part of the proposal involves permanent GPS stations and monitoring, campaign-style GPS, paleomagnetism, and geologic mapping. Durmid Hill, Salton Trough, southern California: In this area, previous work has determined the amount of permanent strain. The PI's propose to complete a step-wise, three-dimensional retrodeformation of defon-ned sedimentary rocks. In order to convert these incremental strain data into estimates of defon-nation rates, one must have precise knowledge of time of defon-nation. In this region of excellent exposure, one aspect of deformation timing is provided by the presence of an ash layer correlated with the 0.76 Ma Bishop tuff. Additional information will result from paleomagnetic techniques that record the magnetic field during deposition. Utilizing the variety of ages of the different sedimentary layers to provide differential vertical axis rotations, The PI's can determine the timing of minor structures (e.g., joints, fractures, and small folds) by determining which units are affected. By comparing geodetic rates (which include both recoverable and pen-nanent strain components) with geologic rates (which record only a permanent strain component) they can assess the amount of recoverable strain accumulation in this area. This part of the proposal involves geologic mapping, paleomagnetism, and campaignstyle GPS. By combining the results of geological, geodetic, and paleomagnetic investigations from our two field areas, the PI's will assess the relative contribution of recoverable (elastic) and pen-nanent (quasi-plastic) strains in wrench borderlands. Resolution of this issue has fundamental implications for earthquake mechanics, geological implications of borderland deformation, and potential slip magnitudes on major faults.