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 ophiolites have revealed the importance of normal faulting and tilting of upper crustal rock units during accretion 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 EPR while lava flows dip toward the ridge. Underlying gabbro is less deformed but contains widely spaced, low-angle fractures and 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 site secular variation. Overly steep and directionally scattered gabbro remanence and observed low-angle shear zones within this unit are interpreted as the manifestation of three- dimensional strain along anastomosing shear zones. Although some remanence directions are incompatible with any plausible deformation history, 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. This model explains the remanence data, observed dikes and lava orientations, dike-parallel fault zones, and the observation of steep, little-deformed dikes cutting both east-dipping dikes and faults. The data support a structural model for 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.