Thermal Constraints on the Rheology of Segmented Oceanic Transform Fault Systems
|Title||Thermal Constraints on the Rheology of Segmented Oceanic Transform Fault Systems|
|Publication Type||Conference Abstract|
|Authors||Wolfson, M, Boettcher, MS, Behn, MD|
|Conference Name||2012 Fall Meeting, American Geophysical Union (AGU)|
|Conference Dates||Dec 3 - 7, 2012|
Midocean ridge transform fault (RTF) systems may be comprised of two or more fault segments that are physically offset by an extensional basin or intra-transform spreading center. These intra-transform offsets affect the thermal structure underlying the transform fault and may act as barriers to rupture propagation. The seismogenic zone of RTFs is thermally controlled and limited by the 600°C isotherm, as evidenced by earthquake hypocentral depths and laboratory friction experiments. Observations from a recent ocean bottom seismic study found that RTF earthquakes rarely occur above ~2 km depth. These findings suggest that the seismogenic zone on RTFs likely extends from ~2 km to the 600°C isotherm. Here we utilize finite element analysis to model the thermal structure of a RTF system comprised of two transform fault segments separated by an extensional offset. The mantle is assumed to have a viscoplastic rheology to simulate brittle failure at temperatures <600°C. We vary offset length, spreading rate, and degree of hydrothermal circulation to examine how these parameters control the underlying thermal structure of segmented RTFs. Longer offsets and faster spreading rates result in warmer thermal structures. Enhanced hydrothermal circulation efficiently cools shallow regions, resulting in an increased area of brittle deformation, and may have a complex effect on the seismogenic zone due to the possible creation of weak, velocity-strengthening alteration phases such as serpentine and talc, and/or changes in fault zone porosity. Incorporating these processes into our model, we are able to assess the potential for an intra-transform offset to act as a barrier to rupture propagation.
As a case study, we focus on the Discovery transform fault, located at 4°S on the East Pacific Rise. Discovery consists of two subparallel fault segments with lengths of 36 km and 27 km, separated by a 6 km