HR: 0800h
AN: T41A-1168    [Abstracts]
TI: Initiation of Ridges and Transform Faults
AU: * Nyst, M
EM: mnyst@pangea.stanford.edu
AF: Geophysics, Stanford University, Mitchell Building \# 324, 397 Panama Mall, Stanford, CA 94305-2204 United States
AU: Thompson, G A
EM: thompson@pangea.stanford.edu
AF: Geophysics, Stanford University, Mitchell Building \# 324, 397 Panama Mall, Stanford, CA 94305-2204 United States
AU: Parsons, T
EM: tparsons@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AB: No clear consensus has emerged to explain initiation of the strikingly regular pattern of ocean ridges and transform faults. The question is important on the continents also, because a less regular pattern of step-overs on faults such as the San Andreas influences the sources of earthquakes. We explore the question by finite element modeling and a study of observational data on ridges and transforms. We focus on the simplest case, where ridges and transforms seem to self-organize at new plate boundaries as soon as new oceanic (magmatic) crust forms. The South Atlantic supplies a clear example. Continental South America and Africa separated along an irregular break, whose general shape is still preserved in the Mid-Atlantic Ridge. In detail, however, the sea floor magnetic anomalies and satellite gravity show that traces of the ridges and transforms extend to the base of the continental slope, i.e. they formed quickly in the new oceanic crust. The Gulf of California provides another clear example and is notable because of its northward transition into the continental San Andreas fault system. In continental crust, dike segments connected by transform faults provide the clearest analogues of oceanic ridges and transforms. Remarkably, the ridge-transform pattern has been simulated by pulling the crust on molten wax [Oldenburg and Brune, {\it JGR}, {\bf 80}, 1975] and also observed in the crust of a molten lava lake [Duffield, {\it JGR}, {\bf 77}, 1972]. In neither of these models, however, do the spatial and temporal scales permit investigation of the dikes whose repeated emplacement and inflation builds layer 3 of the ocean crust. It is well established that, under a buoyant head of magma, dikes tend to fracture and intrude the crust in planes perpendicular to the least horizontal stress, and they relieve the stress difference as they inflate [e.g. Parsons and Thompson, {\it Science}, {\bf 253}, 1991]. Dikes are commonly used as stress-direction indicators analogous to artificial hydraulic fractures. In a simple 2-D finite-element modeling setup we simulate the initiation of rifting by the dike intrusion and opening of several mis-aligned cracks in a pre-extended elastic basalt-like crust. The design of the model parameterization and the dimensions of the cracks, their mis-alignments and mutual distances resemble beginning rift systems observed in the Gulf of California and the Red Sea. Two major processes are assumed to control the expansion and interaction of the cracks and the subsequent development of transform faults. Tectonic extension and dike inflation widen the cracks and stress concentration causes the tips to propagate. Intruding magma aids the opening by exerting stresses on the crack boundaries. Then stress changes induced by the interacting cracks cause the region between the cracks to break. In our modeling the Coulomb failure criterion controls the development of faults around the cracks. Under plane stress assumptions we study the evolution of the stress regime with time, while varying the spreading rate of the lithosphere, the melt pressure of the intruding magma, the degree of mis-alignment and the distance between the cracks. We propose a process of dike intrusion to explain the orientation of ridges; mis-alignment of dikes propagating from different magma supply centers leads to formation of transforms. The hypothesis is supported by the discovery of magma-poor, ultra-slow spreading ridges that are spreading obliquely and generally lack transforms [Dick, Lin and Schouten, {\it Nature}, {\bf 426}, 2003].
DE: 8434 Magma migration
DE: 8109 Continental tectonics--extensional (0905)
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting