UC San Diego

Multichannel Seismic (MCS) Refraction and Reflection analyses are used to determine the structure of the upper 1-2 km of lithosphere at two distinct seafloor spreading regions at 1) the Atlantis Massif Oceanic Core Complex (OCC) at 30°N on the Mid-Atlantic Ridge (MAR), and 2) the southward propagating tip and pseudofault regions of the Central Lau Spreading Center (CLSC) in the Lau Backarc Basin. The Synthetic Ocean Bottom Experiment (SOBE) downward continuation technique is employed to increase the number of usable near-offset first arrival picks within the data allowing the creation of a continuous 2-D upper crustal tomographic model while also improving resolution in the shallow crust. Seismic velocity structure from the resultant tomography models is analyzed in context of the local surroundings and additional available data, and interpreted for formation history and accretionary mechanism of the lithosphere. Summaries of findings at the two study locations are as follows. Atlantis Massif Oceanic Core Complex. At Atlantis Massif, axis-parallel and -perpendicular seismic tomography models indicate that lithospheric structure deviates significantly from the typical oceanic "layer cake" model. Surface velocities change as much as 3 km/s over several hundred meters lateral distance and local vertical velocity gradients are as high as 6 s^-1. Velocities as high as 5 km/s, characteristic of gabbro and confirmed by a 1.4 km drill hole, outcrop directly at the seafloor on the Central Dome and the eastern Southern Ridge and compose the main internal structure of the core complex. These high velocities are surrounded by sheaths of lower velocity structure interpreted as sepentinized peridotite. This gabbroic core with surrounding altered mantle rock implies a history of detachment faulting and the exhumation of originally deep-seated crustal rock. The geometry of Atlantis Massif and the location and orientation of the gabbroic core are consistent with a southeast dipping detachment fault that nucleated at the boundary of a pluton(s) injected into the predominantly ultramafic lithosphere by a southward propagating magmatic source. Central Lau Spreading Center Propagating Tip. New along-axis tomographic models show a dramatic change in the thickness of layer 2 as the tip of the CLSC propagator is approached. Based on shot gather analysis and tomography, crustal layer 2A (< 3 km/s) maintains a relatively constant thickness of ~150-250 m along the ~140 km of the CLSC immediately north of the propagating rift. Layer 2A transitions to a thickness of 500-600 m (including uppermost velocities < 2 km/s) starting about 15 km north of the bathymetrically defined ridge tip. Likewise, as the propagating tip is approached, the layer 2A/2B transition zone changes, in conjunction with the change in layer 2A, from a sharp to low velocity gradient. These structural changes are correlated with results that show both the AMC reflector and the layer 2A reflector disappearing about 18 and 11 km, respectively, from the bathymetric signature of the propagating tip. These observations reflect a change in accretionary mechanism toward the propagating tip, one interpretation being the transition from narrow, focused dike injection to diffuse, random diking with an intermixing of the extrusive and dike layers. Across-axis profiles also support these findings, but do not exhibit the expected aging properties or a structural signature associated with the pseudofault regions