How Accretionary Prisms Elucidate Seismogenesis in Subduction Zones

Earthquakes occur along the plate-boundary thrusts underlying accretionary prisms and along out-of-sequence thrusts that cut through prisms. Thermal models suggest that the earthquakes on the plate-boundary thrusts initiate in a temperature range of 125ºC to ~350ºC. Because syndeformational diagenetic and metamorphic alterations recorded in accretionary prisms have specific temperature ranges, the alterations and the associated deformation can be correlated to the temperature range that accretionary prisms are seismogenic. Comparison of accreted rocks deformed above, within, and below the seismogenic zone suggests characteristics of rocks at seismogenic depths that may make them earthquake prone. During passage through temperatures from 50º to 150ºC, accretionary prism sediments become rocks, undergoing diagenetic reactions including the transformation of smectite to illite, albitization of detrital feldspar, dehydration of opal, and the generation of hydrocarbons. Although the smectite to illite transition does not change the frictional properties of the prism so that it becomes seismogenic, water and cations (calcium, magnesium, iron) released during this transition and the albitization process foster cementation. Cementation and veining by carbonates becomes common by 125ºC, perhaps due to the above-mentioned release of cations. Pressure solution fabrics begin to be apparent at ~150ºC, with well-developed cleavages and quartz veining common by 200ºC. Pressure solution may be facilitated by the diagenetic formation of illite. Quartz veining and cementation in the 150º–300ºC range facilitates the change from a velocity-strengthening, clay dominated to a velocity-weakening, quartz-influenced, earthquake-prone rheology. The diagenetic-metamorphic reactions occurring at temperatures from 125º to ~300ºC cement and add rigidity to the thickening upper plate of the accretionary prism. This developing elastic strength of the upper plate is required to store the elastic strain energy required for an earthquake. In accretionary prisms, brittle fabrics are progressively replaced by ductile fabrics through a temperature range of ~150º– 325ºC. Although rocks in the seismogenic zone have lost most of their intergranular fluid through consolidation, vein geometries and fluid inclusions suggest high fluid pressures, approaching lithostatic. Strain localization in the form of discrete shear surfaces occurs across the lower aseismic to seismic transition. Strain localization is observed both at outcrop and map scale. At map scale, the seawardmost occurrence of out-of-sequence thrusts define the leading edge of the rigidified accretionary prism that is capable of storing elastic energy.