Fisher, D.M., 1988, Structural evolution of a thickly sedimented convergent margin; Evidence from macroscopic structures, microstructures, and incremental strain studies: Providence, Rhode Island, Brown University, Ph.D. dissertation, 120 p., illust.
Accretionary prisms can record deformation associated with thousands of kilometers of lithospheric convergence, yet very little is known about the processes of deformation (e.g. mechanisms, kinematics, and rheology) within these wedges. This thesis deals with the structural evolution of the Kodiak Formation, an enormous package of turbidites accreted along a thickly sedimented late Cretaceous-early Tertiary convergent margin. The earliest deformation (D1) in the Kodiak Formation reflects the underthrusting of sediments beneath the accretionary wedge, and D1 resulted in development of sand-shale melange terranes and coherent terranes of layered turbidites. The melange terranes have experienced a structural development involving: (1) layer-parallel shear, (2) changes in the mechanical properties lithification and dewatering, and (3) pervasive hydrofracturing. Observations of D1 structures suport a model for the tectonostratigraphy of an underthrusting sediment pile consisting of a decollement structurally underlain by a sand-shale melange zone that grades downwards into coherent strata. The second deformation (D2) in the Kodiak Formation is marked by fold-and-thrust structures and a slaty cleavage. A third deformation (D3) folded the Kodiak Formation into a regional anticline that exposes the deepest D2 structural levels in the core and progressively higher structural levels on either limb. A fourth deformation event (D4) resulted in development of a landward-verging shear zone along the northwest side of the core of the D3 anticline. Theoretical considerations and microstructural observations suggest that, in well-foliated rocks, the orientation history may be important when considering (1) the sequence of deformation mechanisms and (2) the stress-strain history. The Kodiak Formation contains abundant incremental strain markers such as pyrite pressure shadows and porphyroblasts that allow the strain path to be determined. Regional analyses of these strain markers suggest that displacement paths in the late Cretaceous-early Tertiary wedge varied from lowest to highest structural levels and would have led to an accretionary wedge that was weak at its base and strong internally. The D2 displacement field may have been fixed relative to the geometry of the wedge, and D3 and D4 may represent motion of the lowest structural levels through this displacement field. Regional buckling of the Kodiak Formation underscores the importance of an anisotropy in the structural evolution of convergent margins.
Theses and Dissertations
