Stress and temperature in subduction shear zones: Tonga and Mariana

Abstract

Because there is secondary sea-floor spreading in the Tonga and Manana subduction systems, the island arcs are separate plates. Horizontal forces on the two sides of the arc must balance, and the maximum force on the back-arc side can be calculated from a lithostatic ridge model. This, in combination with gravity data, allows calculation of the average shear stress in the top 100 km of the subduction shear zone. Stress in Tonga is 220k 100 bar, and in the Manana it is 165 * 75 bar. These low stresses are probably made possible by a fluid pore pressure almost equal t o the least compressive stress.Knowledge of stress allows approximate calculation of temperature in the shear zone by integration of a single differential equation. These temperatures are too low t o activate most dehydration reactions in the subducted crust. As it approaches the volcanic line, this crust is at 150-350°C in Tonga and 150-300°C in the Manana. Shear melting of the crust is ruled out, and conductive melting of the slab by contact with the asthenosphere meets with geochemical objections. Magmas in these systems are probably produced by partial melting of asthenosphere, triggered by a sudden release of water from the slab.

Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

“…We observe the same effect, especially when the crust is made more silicic in rheology than our assumption for the coast ranges. Bird (1978; similarly found subduction shear limits of 10-20 MPa in the western Pacific and Alaskan subduction zones. Our limits and those of Geist (1998) for strike-slip fault friction in Cascadia compare well with low values found for southern California and Alaskan strike-slip faults by Bird and Kong (1994) and Bird (1996) respectively.…”

Interaction of metamorphism, deformation and exhumation in large convergent orogens

“…Quantitative analysis of channel flows between rigid moving or stationary boundaries (e.g. Batchelor, 1967; Turcotte & Schubert, 1982) has been applied to subduction zone flow regimes in both lithostatic and overpressured conditions (Bird, 1978; Shreve & Cloos, 1986; Mancktelow, 1995). Grujic et al .…”

Earthquake Hydrology

“…That fault zones may be porous and saturated at seismogenic depths has important implications for a host of issues related to the dynamics of faulting (e.g., Andrew, 2003;Brodsky and Kanamori, 2001;Lachenbruch, 1980;Mase and Smith, 1987;Sibson, 1973;Sleep and Blanpied, 1992;Wibberley and Shimamoto, 2005), the strength (or weakness) of faults (e.g., Lachenbruch and Sass, 1977;Mount and Suppe, 1987;Rice, 1992;Zoback et al, 1987), and the forces that move lithospheric plates (e.g., Bird, 1978). To obtain direct information on the pore pressure in fault zones, drilling into active faults has been attempted around the world in the past decades (San Andreas Fault Observatory at Depth, SAFOD, near Parkfield in Central California (Zoback et al, 2010), the Nojima Fault Zone Probe in Japan, the Chelungpu Fault Drilling Project in Taiwan (see Brodsky et al, 2010, for a summary of the Nojima and Chelungpu projects), and an ongoing project to drill into the New Zealand South Island Alpine Fault, Gorman, 2011).…”