FAULT-CONTROLLED FLUID-FLOW REGIMES ALONG THE EAST PACIFIC RISE (EPR): GEOLOGICAL VIEWS THROUGH TECTONIC WINDOWS

Faults and fractures are inferred to be important controls on the permeability of the oceanic crust and the spatial-temporal variability of axial hydrothermal systems. An excellent way to document ocean crustal faults and fractures is with near-bottom studies of tectonic windows, rift walls that afford cross-sectional views of ocean crustal structure. Two such tectonic windows are near Hess Deep at the tip of the Cocos-Nazca spreading center, and near Pito Deep at the NE corner of the Easter Microplate. Hess and Pito Deeps expose 1 and 3 Ma EPR crust that spread at 110 and 140 mm/yr, respectively. Hess-Deep exposures reveal fractures that become more densely spaced and interconnected near dike margins and fault zones. In places, fault zones nucleated on the dike margins, accommodated local tilt, but otherwise provided meter-scale displacements; fault-displacements in this setting are virtually unquantifiable due to volcanic construction during tectonic extension. At the grain scale the fractures are veins filled with chlorite and minor quartz, epidote, sulfides, oxides, and, locally, concentrated titanite. The vein networks, dike margins, and faults were likely conduits to large fluid fluxes and, given the chlorite enrichment, this was a recharge-dominated environment. In contrast, some fault rocks from the more deformed zones are less pervasively altered and thus may have been seals to additional fluid flow. Exposures near Pito Deep show far less damage than the Hess-Deep exposures. However, sheeted dikes in Pito-Deep crust are cut by several meter-wide fault zones, with displacements of upwards of 10-100 meters. Several crosscutting generations of quartz-epidote veins and cataclastic domains suggest alternating increments of faulting and fluid discharge. Thus, tectonic windows near Hess and Pito Deep show two very different, structurally controlled fluid-flow regimes. Pito-Deep crust has faults that formed major conduits to discharge, but lack the fracture networks that provide recharge-conduits such as those found near Hess-Deep. In contrast, some faults were not clearly conduits, but were possible seals to fluid flow. Crosscutting relationships establish that deformation and alteration at both localities occurred near the intrusive center, and thus such fault-seal processes would generally inhibit off-axis fluid flow.