Fault Zone Diagenesis: A Key to Understanding Fluid Pathways
James R. Boles
Department of Geological Sciences, University of California, Santa Barbara, CA 93106
Fault zone cements are a geologic record of fluids in faults. They provide evidence for vertical distance of flow, minimum flow volume and velocity, the episodic nature of flow, the chemistry of fluids within fault zones, and absolute age of fluid movement. This presentation will give examples from our studies of cements in late Tertiary through Quaternary age faults of southern California. These studies illustrate how cements can provide unique information on fluids in faults, as well as demonstrate our lack of understanding of some important fault-related processes.
Calcite is the dominant cement in southern California fault zones. Dolomite and silica are much less common. Oxygen isotopic and fluid inclusion studies of calcite show that in many cases crystallization temperatures in the fault zone are higher than ambient temperatures in adjacent country rock. Elevated temperature in the fault zone is evidence of a minimum vertical transport distance and several studies indicate distances of about 500 meters to a kilometer. Strontium isotopes can also be used to infer vertical transport distance in settings where chemical stratigraphy is well known. Results are similar to those from isotopic and fluid inclusion studies. Minimum aqueous flow volume can be estimated from the volume of cements. In one example, a minimum of 108 liters was focused through about 340 cubic meters of a fault zone. The velocity of fluids has been calculated from the size of clasts transported within the fault zone, presumably during short term rapid flow events. Evidence for episodic movement of fluid can be inferred from bands of cements separated by layers of detrital minerals. Dating of cements using U-series has been successful in cases where the cementation events span the appropriate time interval. In some cases dating of deformed fault zone cements provides unique ages of fault activity.
Fluid inclusion salinities show that mixing of basin fluids with shallow ground waters has occurred in cases where faults were active near the surface and mountains were being uplifted nearby the fault. Hydrocarbon-aqueous fluid inclusion assemblages in carbonate cements document the movement of hydrocarbons with aqueous fluids in the fault. In some cases, extremely light carbon isotopes (13δCPDB approx -40) demonstrate that carbon in the carbonates was derived from oxidation of hydrocarbon gases.
The distribution of permeability within fault zones can be spatially complicated and evolves with progressive deformation. For example, our studies have shown that fluid transport, as inferred from the location of fault zone cements, can be highly localized in a fault zone. In one example of a fault 24 km long, the cements occur within less than 1.5 km of the fault tips. Most of these cements are within a few hundred meters of the tips and formed within fault dilation openings, at least 10cm in width parallel to the fault. In another fault zone, cements occur in the deeper part of the fault (>1km depth), whereas the shallow part of the fault is uncemented. The distribution of hydrocarbon families adjacent to the fault confirms the vertical heterogeneity of the fault zone permeability. In one case, we are monitoring an offshore fault that currently has a normal slip configuration, but was probably once a reverse-slip fault. Presently, the fault is an avenue for reservoir gases to escape to the sea bed and at the same time is allowing sea water to recharge the sub-hydrostatically pressured reservoir. The permeability of this fault has been calculated from the rate of recharge.
Deformation in Southern California from transpressional plate motion (convergence and strike-slip), has caused crustal rotation, changes in slip direction on fault planes, and ultimately the breaking of fault seals and release of overpressure in the offshore basin. In general, we do not have a good understanding of this time-space variation in faulting nor the ability to predict the evolving fault permeability architecture in this structurally complicated region. Cementation of the fault zone is a feedback from faulting processes and adds further complication to the permeability history.
As for cement mineralogy, it is not really clear as to why calcite is the dominant cement in these fault zones. Calcite has retrograde solubility, implying that upwelling and cooling of fluids should inhibit calcite crystallization. The hypothesis of rapid CO2 degassing from wall rock adjacent to the fault, which could be a driving mechanism for precipitation, is not supported by stable isotopic studies or very large crystal morphologies. In most cases calcite crystallization is slow enough to be in isotopic equilibration with the pore water and does not show kinetic isotopic fractionation effects. Large crystals with relatively uniform isotopic compositions require a large supply of water tapped from well mixed sources, rather than a limited supply of locally-derived fluid. Calcite might form by slow CO2 degassing during the ascent of over pressured water as it moves from narrow confined fault pathways to larger open spaces within the fault zone.
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