Kinematic analysis of melange fabrics: examples and applications from the McHugh Complex, Kenai Peninsula, Alaska

Permian to Cretaceous mi lange of the McHugh Complex on the Kenai Peninsula , south-central Alaska includes blocks and belts of graywacke, argillite, l imestone, chert, basalt, gabbro, and ultramafic rocks, intruded by a variet y of igneous rocks. An oceanic plate stratigraphy is repeated hundreds of t imes across the map area, but most structures at the outcrop scale extend l ithological layering. Strong rheological units occur as blocks within a mat rix that flowed around the competent blocks during deformation, forming bro ken formation and melange. Deformation was noncoaxial, and disruption of pr imary layering was a consequence of general strain driven by plate converge nce in a relatively narrow zone between the overriding accretionary wedge a nd the downgoing, generally thinly sedimented oceanic plate. Soft-sediment deformation processes do not appear to have played a major role in the form ation of the melange. A model for deformation at the toe of the wedge is pr oposed in which layers oriented at low angles to al are contracted in both the brittle and ductile regimes, layers at 30-45 degrees to sigma(1) are ex tended in the brittle regime and contracted in the ductile regime, and laye rs at angles greater than 45 degrees to sigma(1) are extended in both the b rittle and ductile regimes. Imbrication in thrust duplexes occurs at deeper levels within the wedge. Many structures within mi lange of the McHugh Com plex are asymmetric and record kinematic information consistent with the in ferred structural setting in an accretionary wedge. A displacement field fo r the McHugh Complex on the lower Kenai Peninsula includes three belts: an inboard belt of Late Triassic rocks records west-to-east-directed slip of h anging walls, a central belt of predominantly Early Jurassic rocks records north-south directed displacements, and Early Cretaceous rocks in an outboa rd belt preserve southwest-northeast directed slip vectors. Although precis e ages of accretion are unknown, slip directions are compatible with inferr ed plate motions during the general time frame of accretion of the McHugh C omplex. The slip vectors are interpreted to preserve the convergence direct ions between the overriding and underriding plates, which became more obliq ue with time. They are not considered indicative of strain partitioning int o belts of orogen-parallel and orogen-perpendicular displacements, because the kinematic data are derived from the earliest preserved structures, wher eas fabrics related to strain partitioning would be expected to be superimp osed on earlier accretion-related fabrics. (C) 1999 Elsevier Science Ltd. A ll rights reserved.