Episodic Fluid Flow in Monterey Veins, Coastal California.

EICHHUBL, Peter, and BOLES, James R.
Geological Sciences, University ofCalifornia, Santa Barbara, CA 93106.

Structural relationships of carbonate-filled veins and internal sedimentary structures indicate episodes of rapid fluid flow in the Miocene Monterey Formation. Late veins cross-cutting folds, considered to form during basin emergence, are spatially related to faults, for example at Jalama Beach NW Santa Barbara. Centimeter wide, meter spaced, vertical veins originate along a vertical strike slip fault and extend about 200 m away from the fault. Fault valving is inferred based on the 60° angle between veins and fault, the synthetic arrangement of veins with respect to the fault displacement, and the continuity between vein filling cement and cement within the fault zone. In contrast to the classic fault valve model where high-angle veins form during gradual, pre-seismic pore fluid pressure build-up, we interpret these 60°-veins to form by injection of fluid from the fault into the vein system. Vein formation due to fluid injection is inferred from the veins originating in the fault zone and from branching of the veins consistently away from the fault. Further evidence for fluid flowing from the fault into the vein system is provided by a decrease in inferred fluid temperature of about 10°C over 200 m away from the fault based on del18O measurements of vein filling dolomite. Branching of fractures occurs when energy available for fracture propagation is provided to the system at a faster rate than consumption by the propagating crack tip. For hydraulic fractures, branching is likely to occur when the fluid injection rate exceeds the increase in fracture volume through fracture propagation. High injection rates result in frictional loss in hydraulic head along the fracture walls thus favoring formation of crack branches rather than continuing propagation of earlier formed fractures. For an aqueous fluid, injection rates in the order of 1 to 10 m/s are calculated to initiate branching. Independent evidence for rapid fluid flow are cavities within the fault zone with geopetal infilling of possibly fluidized debris including up to one centimeter large fragments which may suggest flow rates in the order of 0.1 m/s. Flow velocities of these orders of magnitude necessarily imply episodic pulses of fluid movement which may be explained by the sudden elastic expansion of over-pressured fluid during seismic fault slip.

In contrast to late-formed veins, early carbonate-filled veins, formed during basin subsidence, are localized to dolomite and adjacent shale beds. Early vein formation during basin subsidence is indicated by methanogenic carbon isotopes and tilting of geopetal vein fill during later folding. Episodic fluid flow in early veins is inferred on the mutual cross-cutting of vein filling laminae of micritic dolomite, calcite and cryptocrystalline quartz, lining the walls of both horizontal and vertical fractures. Because these veins are lithologically controlled we suggest that early veins, in contrast to late veins, represent local diagenetic conditions of dewatering and precipitation.