Paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the Hess Deep Rift: Implications for axial processes at the East Pacific Rise

Studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers. We present paleomagnetic remanence data from 45 fully oriented samples from dikes, gabbros and a small number of basaltic lavas from fast spread crust exposed along the Hess Deep Rift. Over ~25 km along this escarpment, dikes and dike-subparallel fault zones dip consistently away from the East Pacific Rise (EPR) while lava flows dip toward the ridge. Underlying gabbro is less deformed but contains widely spaced, low-angle fractures, tentatively interpreted as shear zones. As expected from the crustal age (~1.07-1.48 Ma), most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site. Overly steep and directionally scattered gabbro remanence and observed low-angle shear structures within this unit are tentatively interpreted as the manifestation of three-dimensional strain along anastomosing shear zones. Although some remanence directions are incompatible with any plausible deformation history, and thus likely reflect orientation errors, the overall data set is consistent with a model involving sequential rotations on (1) outward dipping, EPR-parallel (~N-S) normal faults and (2) Hess Deep Rift-parallel (~E-W) normal faults Average rotations for these sequential events are 22° to the east (defined by the mean dike attitude) and 10° to the south (estimated by bathymetry), respectively. This model best explains the remanence data, observed dikes and lava orientations, presence of dike-parallel fault zones, and the observation of steep, little deformed dikes cutting both èast dipping dikes and faults. The data support a structural model for spreading at the EPR in which outcrop-scale faulting and rotation is linked to subaxial subsidence and to consequent development of dominantly outward facing normal faults close to the spreading axis. Because these faults form within the neovolcanic zone, they are subject to burial and are expected to have subdued to little surface expression.