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1999 Projects: Benthic processesConvergent margin processes: Submarine canyonsProject lead: Gary
Greene This project addresses questions related to the transform margin of offshore Central California. Specifically, it deals with relationships between fluid flow, turbidity current processes, and the transport of tectonic blocks of tectonostratigraphic terrain along a transform boundary. The Monterey Bay region is an ideal place to study submarine processes associated with the oblique convergence of tectonic plates, as the North American and Pacific plates exemplify this type of margin. Related to this margin are offshore fault zones (the Palo Colorado-San Gregorio and Monterey Bay fault zones) that define the western boundary of the tectonic Salinian granite basement rock block. This "allochthonous" block (of distant origins) has been "slivered" into its present position off Central California by the northward movement of the Pacific Plate along its boundary with the North American Plate, the San Andreas Fault. While the western edge of the Salinian block is well defined in Monterey Bay, little is known about its boundary to the north. The submarine canyons that dissect the continental margin of the region form two major systems: the Ascension-Monterey system and the Sur Submarine Canyon and sea valley system. The Monterey Canyon group, composed of Monterey, Soquel, and Carmel Canyons, is by far the most dominant physiographic feature of the Central California coastal region. The two most active canyons, Monterey and Carmel, serve as major conduits for sediment transport from the continental shelf to the deep-ocean floor. This geological setting has created distinct areas of transpression, or compression, (expressed by the uplift of the Santa Lucia Range and the Santa Cruz Mountains and adjoining offshore areas) and transtension (manifested by subsidence of the Salinas Valley and central Monterey Bay). The EM-300 bathymetric data recently collected by MBARI suggests that many of the canyons in the Monterey Bay region have heads in various tectonic settings and appear to have formed by different processes. At the heads of Monterey, Carmel, and Soquel canyons (all formed by transtension and general subsidence) fine-grained sediments are currently accumulating. In contrast, within the Sur submarine canyon system (located in an area of transpression) coarse-grained materials are introduced to oversteepened canyon heads. Ascension canyon system, also located in an area of transpression, is void of shelf sediments, but appears to be eroded by active, fluid-induced mass wasting. One cause of such mass wasting, turbidity currents, is a primary erosive force in submarine canyons and is responsible for the extensive offshore deposition of coarse-grained sediments (turbidites) throughout the world. The suspected origins of turbidity currents range from storms to earthquakes, but the frequency and timing of such events are not well understood. In Monterey Canyon, the complex evidence of mass-wasting processes seems to indicate that turbidity currents occur in stages rather than all at once; that is, deposits are often dammed behind slumps and not released again until some external force breaks the dam, triggering another turbidity current. Under this project we will make ROV dives over a total of ten days to:
Next: Geochemistry and mapping of oceanic fracture zones and spreading centers Last updated: 07 October 2004 |
