Formational processes of oceanic crust at sedimented spreading centers: Perspectives from the West African continental margin and Middle Valley, Juan de Fuca Ridge.

摘要

Deep-imaging multi-channel seismic reflection (MCS), gravity and magnetic data, acquired in the PROBE West Africa-Venture II survey, were processed and interpreted to delineate the extent and character of oceanic and pseudo-oceanic crust in the thickly sedimented southeastern Gulf of Guinea. Oldest oceanic crust is estimated as Late Aptian (95-112 Ma) in age. Both crustal types are highly reflective, underlain by high-amplitude, sub-horizontal, continuous events interpreted as reflection Moho, and traversed by crustal disturbances. The reflection Moho changes from a few-reflection event to a layered series of events near and across these disturbances. Pseudo-oceanic crust is identified by its reflection character and distinct velocity structure. It has a much thinner lower crustal layer (V{dollar}rmsb {lcub}p{rcub}=6.5{dollar} to 7.7 km/s) and thicker upper crustal section (V{dollar}rmsb {lcub}p{rcub}=4.5{dollar} to 6.3 km/s) than in oceanic crust observed in interval velocity contour maps.; Crustal disturbances are divided into three classes, major fracture zones, minor fracture zones, and small-scale crustal disruptions, according to size. Variations in the offset or thickening of the top of crust, offset of reflection Moho, and amount of intracrustal reflections for each class are observed in MCS profiles. Interval velocity contour mapping shows a sharp decrease in thickness of the lower crustal layer (i.e., oceanic layer 3) across traces of major and minor fracture zones and even disruptions. Most boundaries between oceanic, pseudo-oceanic, and continental crust are associated with fracture zones, and are considered transform-faulted segments of the margin. Major and minor fracture zones are traced across the continental shelf, and some may extend onshore. Crustal disruptions are traced as far landward as pseudo-oceanic crust. Interval velocity analyses show that the reflection Moho does not always correspond to the {dollar}>{dollar}8.0 km/s isovelocity contour in oceanic crust and pseudo-oceanic crust. These results have significant implications for the meaning of oceanic reflection Moho, its use as a datum for crustal thickness calculation, and the geologic structure of pseudo-oceanic crust.; The characterization of hydrothermal processes and deposits at regional scale within a young {dollar}({lcub}sim{rcub}400,000{dollar} yr) sedimented spreading center was the objective of Ocean Drilling Program (ODP) Leg 139 in Middle Valley, northern Juan de Fuca Ridge. Physical properties experiments were designed to quantify the physical changes in cored sections of sediment and basalt resulting from alteration owing to high {dollar}({lcub}sim{rcub}300spcirc{dollar}C) temperatures and hydrothermal fluid flow. Compressional- and shear-wave velocity data, densities, and porosities were measured on samples of igneous and sulfide rocks from Sites 856, 857 and 858. Modal analyses of thin sections showed that numerous open and mineralized multi-directional microcracks caused great variability in the physical properties. The velocity and thermal conductivity data for massive sulfide samples are the first reported in the open literature. Laboratory data for diabase and basalt samples were correlated with multi- and single-channel seismic data, downhole logging measurements, and lithologic units identified visually and analytically throughout the core. Results provide important constraints for estimation of the thermal and hydrologic regimes in a sedimented rift.