HR: 0800h
AN: T41C-1232    [Abstracts]
TI: Crustal Behavior in a Strike-Slip-Related, Mid-Crustal Attachment Zone: Evidence From a Down-Plunge Section in the Chugach Metamorphic Complex, Southern Alaska
AU: * O'Driscoll, L J
EM: ljodrisc@uno.edu
AF: University of New Orleans, Department of Geology and Geophysics 2000 Lakeshore Drive, New Orleans, LA 70148 United States
AB: The western termination of the Chugach Metamorphic Complex in southern Alaska provides an excellent example of a down-plunge crustal section that we are utilizing to examine strike-slip/transpressional plate boundary behavior. This exhumed section was developed in monolithologic forearc accretionary sediments (Valdez group), which eliminates rheologic complications associated with heterogeneous crust. Following an "attachment zone" model of Teyssier et al. (2003), we are developing models linking deformational history between rheologic layers of the crust, with particular focus on the development of mid-crustal decoupling. Previous field studies documented mid-crustal decoupling at the lithologic transition from schist to gneiss with homogeneous deformation below and localized deformation in shear zones above. Our recent detailed structural mapping has revealed high strain bands and a synformal foliation "cusp" that has first order similarities to the attachment zone model. The foliation "cusp" is formed by a 3-5km wide high-strain core of steep foliations with abundant dextral shear sense indicators that can be traced along strike for more than 60km, penetrating more than 4km of the crustal section. These steep zones pass outward, on both sides, through a 10-15km wide roll-over to nearly flat-lying foliation that parallels the underlying decoupling surface. This geometry is broadly consistent with foliation trajectories predicted for areas mid-way between two strike-slip shear zones in the attachment zone, but paradoxically, it appears that the steep-foliation is THE major dextral shear zone that penetrates the exposed crustal section; the reverse of the geometry predicted by the attachment model. Our continued work is emphasizing reevaluation of the attachment model (through redefining assumptions) using observed foliations, lineations, finite strain kinematic axes (to constrain the model), considering variable offset amongst the high-strain zones and variable thicknesses of the attachment zone, and variable crustal level geometry. Ultimately more field work may be needed to assess the continuity of the shear zone through the upper part of the crustal section where the discrepancy between observed and theoretical structures is most pronounced.
DE: 8015 Local crustal structure
DE: 8105 Continental margins and sedimentary basins
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting