Thermal Constraints on the Rheology of Segmented Oceanic Transform Fault Systems

TitleThermal Constraints on the Rheology of Segmented Oceanic Transform Fault Systems
Publication TypeConference Abstract
Year2012
AuthorsWolfson-Schwehr, Monica L., Boettcher, Margaret S., and Behn, Mark D.
Conference Name2012 Fall Meeting, American Geophysical Union (AGU)
Conference DatesDec 3 - 7, 2012
Conference LocationSan Francisco, CA, United States

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
intra-transform spreading center. On a number of intermediate and fast-slipping RTFs, including Discovery, the largest earthquakes are known to rerupture the same fault patch in relatively regular seismic cycles. The
rupture patches are bounded by areas of increased microseismicity, which act as barriers to large rupture propagation. Previously, we used welllocated earthquakes recorded on a NOAA hydrophone array together
with a relative relocation technique to determine the absolute positions for the five rupture patches on Discovery, which host 5.4 ≤ Mw ≤ 6.0 earthquakes. In this study, we combine absolute locations of the largest earthquakes, our detailed analysis of the fault trace of Discovery, and our thermal modeling results to assess how intra-transform offsets on Discovery affect the subsurface thermal structure. Along the 6 km intra-transform spreading center we find the 600°C isotherm is shallower than 2 km, suggesting that the thermal structure of this offset creates a rupture barrier between the adjoining fault segments. By contrast, intra-transform offsets <2 km identified along the surface trace of each segment only minimally affect the depth of the 600°C isotherm, resulting in a continuous seismogenic zone between the fault segments. This suggests that the thermal effect of small intra-transform offsets is not sufficient to explain the locations of rupture patches and rupture barriers on the Discovery transform fault.