Role of fluid overpressures in crustal strength and the form of the brittle–ductile transition

Abstract:

The classic crustal strength-depth model of Brace and Kolhstedt (1980) (see figure) based on experimental rock mechanics depends in the brittle regime on the critical assumption of linearly increasing hydrostatic pore-fluid pressures. This leads to a predicted linearly increasing brittle strength that is well established based on deep borehole stress measurements in crystalline crust.

In contrast, fluid overpressures are widely documented in orogenic belts based on borehole data, seismic velocity analysis and analysis of veins, in some cases showing complex fault-valve pressure fluctuations between lithostatic and hydrostatic. Typical observed overpressure-depth relationships predict a brittle crustal strength that is approximately constant with depth in contrast with the classic model. This constant-strength behavior below the fluid-retention depth (Z_FRD in figure) has been confirmed using deep borehole stress and fluid-pressure measurements (Suppe, 2014). Recent ductile-plastic modeling of disequilibrium compaction suggests that pressure solution promotes further increases in overpressure and weakening, promoting a very prolonged low-strength brittle-ductile transition.

Overpressured conditions can be inferred to exist over a substantial fraction of crustal thickness, spanning the brittle-ductile transition, in several tectonic environments, most straightforwardly in shale-rich clastic sedimentary basins built to sea level on oceanic or highly thinned continental crust such as the US Gulf Coast and Niger Delta. These thick accumulations commonly deform into shale-rich plate boundary mountain belts (e.g. Bangladesh/Miyanmar, Makran, Trinidad/Barbados, Gulf of Alaska, southern Taiwan and New Zealand). There is deep geophysical evidence for near lithostatic pore-fluid pressures existing to depths of 20-30km based on Vp, Vs, Vp/Vs and Q observations. We present active examples from Taiwan and New Zealand, combining borehole data and seismic tomography.