Snapshots of the earthquake cycle: An approach to subduction zone paleo-seismicity.

摘要

The Mesozoic-Cenozoic Kodiak accretionary complex of south-central Alaska hosts a plethora of ancient subduction thrust faults, representing both decollement and splay fault surfaces active at varied depth and temperature conditions. Detailed structural and mineralogical studies across a range of ancient faults reveal that the interaction between fluid transport, stress cycling, strain localization and deformation mechanism varies with depth, diagenetic-metamorphic grade of host rocks, and fault geometry. Deformation at discrete strain rates preserved in cross-cutting structures and fabrics that suggest a record of the earthquake cycle. Where pressure solution is an integral mechanism in forming fault fabrics, we infer that the rate of deformation in the fault is on par with pressure solution (comparable to fault creep). Where pressure solution plays a negligible role under the same p-t-x conditions, it may be inferred that the strain rate significantly exceeds pressure solution rates. Very large earthquakes are preserved in the rock record, in the form of a previously undescribed type of seismic fault rock, occurring in an ancient decollement zone, which has attributes of both pseudotachylyte and fluidized gouge. The mechanism proposed for the formation of the seismic fault rocks is repetitive melting along thin surfaces in seismically ultracomminuted gouge zones. This mechanism may require very large critical slip distance and well developed cataclasite zones in the decollement to operate, possibly leading to rarity in the rock record. Rarity is also addressed by the observation of cataclasis and recycling of seismic fault rock in aseismic decollement fabrics. Fragments of this type of fault material collected by future ocean drilling projects may serve as certain indicators of sampling of a seismic fault surface.; Splay faults observed in this study differ strongly from decollement faults. The veined damage zone in footwall of a splay fault of several kilometers offset may represent a significant paleo-reservoir for venting fluids along a splay fault. The scale and fracture porosity of the veined zone is on par with the minimum size necessary to be detectable as a negative amplitude reflection in a reflection seismology survey. The studied splay fault may be ancient analog for active venting splay faults in modern environments such as the Nankai Megasplay of southwest Japan.