Basaltic breccia constraints on the shallow rheology of the Gofar transform fault

Abstract

Oceanic transform faults provide a unique opportunity to study fault rupture due to their well-defined constraints: simple lithology, known lengths and rates of slip, and predictable thermal structures. The fast-slipping Gofar transform fault on the East Pacific Rise has been the focus of multiple seismic studies. These have identified along-strike changes, where some areas rupture in large seismic events (Mw > 5.3) and other sections act as barriers to rupture propagation yet experience abundant microseismicity. This implies that the physical properties of the fault change along strike, requiring investigation of fault rocks from Gofar. In this study, we describe the 12 dredges that we collected along the Gofar transform fault, with a focus on 42 representative breccia samples. These samples consist of crackle, mosaic and chaotic breccias, based on the relatively high (>30%) abundance of large (>2 mm) clasts. These clasts consist of altered and unaltered fragments of basalt, diabase, and basaltic glass. The matrices of the breccias are cohesive and consist of broken plagioclase and olivine, and of alteration minerals, including chlorite, clays, and zeolites. Cross-cutting fractures are common and are filled with zeolites, clays, and calcite. Geochemically, the breccias are distinct from basalts from Gofar transform fault, with breccias having increased MgO, Na2O, K2O, and fluid mobile trace element abundances, increased loss-on-ignition (LOI) values, and decreased CaO abundances. The breccias indicate extensive fluid flow in the fault, based upon the abundance of hydrous alteration minerals and the high LOI values (2-12%). The compositional changes within the breccias are consistent with changes associated with hydrothermal alteration of basalts, confirming that extensive hydration occurred during breccia formation. The growth of chlorite in some breccias indicates temperatures up to at least 200 ̊C. Clay and chlorite can be frictionally weak minerals and their presence in the breccias may promote creep, leading to the aseismic behavior along some sections of the Gofar transform fault.