HR: 10:20h
AN: T11E-01 INVITED     [PDF]
TI: Nucleation of an Oceanic Spreading Center in a Continental Rift: The Northern Red Sea
AU: * Cochran, J R
EM: jrc@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Poute 9W, Palisades, NY 10964 United States
AB: The northern Red Sea is an active amagmatic continental rift in which a mid-ocean ridge spreading center is beginning to develop. The morphology of the Red Sea consists of very narrow continental shelves and a main trough with depths of 400-1200 m. The main trough consists of a series of fault bounded terraces 20-30 km wide, stepping down to a 15-30 km-wide axial depression. Free-air gravity anomalies form a pattern of elongate high-amplitude (50 mGal) highs and lows oriented subparallel to the trend of the rift and located on the seaward edges of the bathymetric terraces. Gravity anomalies are systematically terminated or offset across NE-SW trending zones at which the bathymetric contours are also offset. The gravity anomalies display the basement relief under the main trough, interpreted as a series of tilted fault blocks 15-30 km across and roughly 60 km long, separated by accommodation zones. A new data compilation and synthesis shows that the accommodation zones extend continuously across the entire width of the Red Sea. Where accommodation zones have been mapped in the rift mountains on-shore, the off-shore accommodation zones appear to continue into them. However, the trends on land and at sea tend to be different. Accommodation zones in the low-to-moderately extended rift mountains are controlled by pre-existing Precambrian structures, while those in the highly-extended offshore area are oriented parallel to the opening direction. The axial depression is marked by a free-air gravity low with a relative amplitude of 30-60 mGal and by very high heat flow (300-600 mW/m$^{2}$). The axial depression often appears to be fault bounded. Sediment deformation is more intense and concentrated there than in the rest of the main trough. The axial depression is not only the locus of recent deformation, but is the location of a series of small axial deeps, 1400-1600 m deep and associated with large-amplitude dipolar magnetic anomalies. A deep is located near the midpoint of each segment, approximately halfway between the accommodation zones. Water depth and the amplitude of the gravity low decrease away from the deeps toward the accommodation zones. The gravity lows over the axial depression within each segment are offset or overlap without intersecting at accommodation zones. The large-amplitude normally-magnetized dipolar magnetic anomalies at the deeps appear to result from recent volcanic activity. Small volcanoes (a few km across and several hundred m high) can be identified at several deeps. In most cases, the magmatic activity does not occur in the deep itself, but rather immediately adjacent to it along the faults bounding the axial depression. Extension in the northern Red Sea, which has been amagmatic, has reached the point where melt is beginning to be produced. The melt is focused at the center of rift segments where it ascends along faults bounding the axial depression. By analogy with the central and southern Red Sea, it is hypothesized that, with continued intrusion, the deeps will develop into small seafloor spreading cells that propagate and grow together to form an oceanic spreading center. The initial segmentation of the newly formed mid-ocean ridge is thus directly inherited from the rift geometry established early in continental rifting.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 8105 Continental margins and sedimentary basins
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting