HR: 08:00h
AN: T41A-01    [PDF]
TI: What can old Rocks Tell us About the Upper Aseismic to Seismic Transition in Subduction Zones?
AU: * Moore, J C
EM: cmoore@es.ucsc.edu
AF: Earth Sciences, UC Santa Cruz, Santa Cruz, CA 95060
AU: Rowe, C
EM: crowe@es.ucsc.edu
AF: Earth Sciences, UC Santa Cruz, Santa Cruz, CA 95060
AB: At subduction zones the upper aseismic transition occurs at 5 - 15 km and at modeled temperatures of 100 to 150 deg. C. Combined studies of the structural and thermal history of subduction complexes thus allow placing rocks above or below this transition. Rocks deformed at temperatures less than 125 deg. C show an array of mesoscopically ductile and compactive deformational features that are associated with loss of intergranular fluid. Deformation zones dewater, expand into adjacent less consolidated sediment, producing intervals of stratal disruption. Stratal disruption is commonly accommodated by cataclastic shear zones or "deformation bands". Clays dominate shear zones. Such distributed deformational features and clay-dominated shear zones probably are velocity strengthening and not seismogenic. Rocks deformed at temperatures greater than 125 deg. C show overprinting of the earlier deformational features by pressure solution, with abundant precipitation of solutes as veining and cements, dominantly as calcite and quartz. Later generations of veins become cleaner with less included particulates, reflecting lithification. Cycles of cataclasis, pressure solution and precipitation repeatedly occur. As temperatures rise into the 200 degree level quartz becomes more dominant over calcite. Quartz coats and cuts across shear zones. Quartz is known to be velocity weakening; its widespread mobility and presence on and across shear surfaces at temperatures correlated to seismogenic depths suggests it may be a major factor in seismic instability. In SW Japan, an example of such repeatedly cataclastized, pressure solved rock develops pseudotachylyte, indicating seismogenic behavior. Precipitates heal fault zones that would allow fluid pressures to rise and trigger fault displacement. Studies of fluid inclusions in quartz veins suggest cyclical variations in fluid pressures although the interpretation of these results is controversial.
DE: 7209 Earthquake dynamics and mechanics
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: 2003 Fall Meeting