Emilio Vera

Seismic imaging of oceanic layer 2A between 9°30′N and 10°N on the East Pacific Rise from two‐ship wide‐aperture profiles

We analyze two-ship multichannel seismic wide aperture profiles (WAP) acquired on the East Pacific Rise (EPR) between 9°30′C and 10°N in May and June 1985. For offsets between 2 and 5 km, most of the WAP common depth point (CDP) gathers exhibit a strong retrograde, reflection-like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2A. Stacking of this postcritical arrival for hundreds of CDP gathers along the WAP lines yields remarkably clear, and virtually continuous, images of the bottom of layer 2A for several tens of kilometers along and across the EPR. An innovative combination of this image with others of the seafloor (SF) and of the axial magma chamber (AMC) provides an unusually clear composite picture of young upper crust. We combine these images with detailed seismic velocity information, also obtained from the CDP gathers, to achieve an accurate determination of the layer 2A thickness in the area. Along the EPR axis between 9°30°N and 10°N, layer 2A shows a relatively constant thickness between 100 and 150 m (80–120 ms two-way travel time). Across the axis, however, a line near 9°30′N indicates that layer 2A thickness rapidly increases by approximately a factor of 2 within 2–4 km of the axis and remains nearly constant afterwards. Thus the final thickness of layer 2A is attained before the development of large-scale tectonic faulting, strongly suggesting that the thickening of layer 2A away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top, rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2A downwards as the crust moves away from the axis.

Thermal uplift and erosion across the continent-ocean transform boundary of the southern Exmouth Plateau

Thermal evolution of the continental lithosphere at a continent-ocean transform margin is examined using a two-dimensional heat conduction model. All heating is assumed to result from the emplacement of new oceanic ridge against the continent. The assumed initial continental temperature gradient is probably best applied to regions having experienced finite-duration periods of rifting lasting tens of millions of years. A deep seismic reflection profile across the southern paleo-transform margin of the Exmouth Plateau (northwest Australia) tests the predictions of the model. Using this reflection profile, it is estimated that up to 3.5 km of sediments have been eroded from the continental rim, diminishing to almost no erosion at 60 km from the continent-ocean transform boundary. This trend and these values for erosion can be matched approximately using the model under the condition of local isostasy. A finite-difference scheme is employed, where the surface elevation is the result of the competing (1) thermal uplift (up), (2) surficial erosion (down), and (3) local isostatic rebound (up) in response to the erosion. The model predicts that most of the erosion ceases by 40 Ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length.