Structure of uppermost fast‐spread oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise

摘要: The uppermost 2 km of the oceanic crust created at fast spreading (135 mm yr−1, full rate) equatorial East Pacific Rise (EPR) is exposed for tens kilometers along escarpments bounding Hess Deep Rift. Mosaics large-scale digital images from remotely operated vehicle (ROV) Argo II and direct observations submersible Alvin document a degree geological complexity variability that not evident most studies ophiolites or prevailing models seafloor spreading. Dramatic variations in thickness internal structure are documented both basaltic volcanic sheeted dike rock units. These units characterized by extensive faulting, fine-scale fracturing, rotations coherent crustal blocks meters to across. lavas essentially undeformed have overall gently inclined flow surfaces. Through lava unit, however, contacts dip (20°–70°W) toward EPR generally increase downward section. Dikes cutting underlying unit (90°–40°E) away EPR. Deeper level gabbroic rocks show little evidence intense fracturing typical overlying We interpret this upper as result subaxial subsidence within 1–2 accommodated thickening ∼500 m. Variations spatially related structures rift suggest temporal fluctuations magma supply. results indicate substantial brittle deformation accompanied waxing waning volcanism during accretion section If type generated EPR, these processes may be common mid-ocean ridges.