ALBERT

Description: A possible cause for accelerated thinning and break-up of floating marine ice shelves is warming of the water in the cavity below the ice shelf. Accurate bathymetry beneath large ice shelves is crucial for developing models of the ocean circulation in the sub-ice cavities. A grid of free-air gravity data over the floating Larsen C ice shelf collected during the IceBridge 2009 Antarctic campaign was utilized to develop the first bathymetry model of the underlying continental shelf. Independent control on the continental shelf geologic structures from marine surveys was used to constrain the inversion. Depths on the continental shelf beneath the ice shelf estimated from the inversion generally range from about 350 to 650 m, but vary from 1000 m. Localized overdeepenings, 20-30 km long and 900-1000 m deep, are located in inlets just seaward of the grounding line. Submarine valleys extending seaward from the overdeepenings coalesce into two broad troughs that extend to the seaward limit of the ice shelf and appear to extend to the edge of the continental shelf. The troughs are generally at a depth of 550-700 m although the southernmost mapped trough deepens to over 1000 m near the edge of the ice shelf just south of 68° S. The combination of the newly determined bathymetry with published ice-draft determinations based on laser altimetry and radar data defines the geometry of the water-filled cavity. These newly imaged troughs provide a conduit for water to traverse the continental shelf and interact with the overlying Larsen C ice shelf and the grounding lines of the outlet glaciers.

Description: Although motion between Arabia and Africa is presently occurring along the entire length of the Red Sea, the morphology and tectonics that result from this motion vary greatly along its length. South of 21°N, the main trough is bisected by a deep axial trough which has formed by sea-floor spreading during the past 4 m.y. and is associated with large-amplitude magnetic anomalies and high heat flow. North of 25°N, an axial trough is not present and the floor of the main trough has an irregular faulted appearance. The magnetic field in the north is characterized by smooth low-amplitude anomalies with a few isolated higher amplitude magnetic anomalies commonly associated with gravity anomalies and in many places probably due to intrusions. Between these regions, the axial trough is discontinuous with a series of deeps characterized by large-amplitude magnetic anomalies alternating with shallower intertrough zones which lack magnetic anomalies. It is argued that the different regions represent successive phases in the rifting of a continent and the development of a continental margin. An initial period of diffuse extension by rotational faulting and dike injection over an area perhaps 100 km (60 mi) wide is followed by concentration of extension at a single axis and the initiation of sea-floor spreading. The main trough in the southern Red Sea, away from the deep axial trough, formed during the Miocene by the same processes of diffuse extension that are still active in the northern Red Sea. This model explains the available geologic and geophysical data and reconciles previous models for the formation of the Red Sea which emphasize either the evidence for considerable motion between Arabia and Africa or the evidence for down aulted continental crust beneath much of the Red Sea. The initial pre-sea-floor spreading stage results in considerable extension (160 km or 100 mi) at 25°N in the Red Sea), can last for several tens of millions of years, and is an important factor in the development of the continental margin. Such an extended phase of rifting and diffuse extension must be taken into account in studies of sedimentation, subsidence, and paleotemperatures.