Rates of fluid flow in fault systems; evidence for episodic rapid fluid flow in the Miocene Monterey Formation, coastal California

Abstract

Rates of paleo-fluid flow within a fault zone exposed in an exhumed coastal section of the Monterey Formation are estimated based on: (1) the size of rock fragments apparently transported in moving fluid along the fault; and (2) alternating fault cement layers containing high- and low-temperature fluid inclusions, interpreted to reflect pulses of hot fluid flowing up along the fault. The size of entrained rock fragments provides a minimum estimate of flow velocities on the order of 10 ⁶ m/yr (10 (super -2) m/s), interpreted to represent flow conditions during coseismic fluid expulsion events. Flow velocities of 10 ⁴ to 10 ⁸ m/yr (10 (super -4) to 10 ⁰ m/s) are obtained using a conductive model of heat loss for pulses of hot fluid moving up along the fault. The second flow velocity estimate is interpreted to reflect flow conditions over significant post-seismic time intervals. Sustained flow rates on this order of magnitude require effective focusing of fluid from the source rock volume into the fault conduit. Episodic fluid expulsion along the fault system is also reflected in cyclic cathodoluminescence patterns of fault cement, in the variable stable isotopic composition of cement, and in alternating stages of fault cementation and cement brecciation due to fault slip. The number of cyclic cement layers exceeds the number of brecciation events along this fault, suggesting that the cement cycles are largely the result of fluid pulses triggered by earthquakes along other faults in the basin rather than by the exposed fault itself.