Calcite twinning fabrics along the Middle America trench, Costa Rica and the Motagua sinistral fault, Honduras and Jamaica: Tectonic implications for the Caribbean plate

摘要

The Cretaceous oceanic crust of the Caribbean plate has a long and complex tectonic history as an indentor plate bounded by transcurrent faults (N and S) and subduction margins (W [east slab dip] and E [west slab dip]) with no strain data. Miocene forearc sediments and accreted ophiolites are exposed along the central Pacific coast of Costa Rica and preserve a variety of folding and faulting styles. These clastic sediments, deposited in the forearc as the Middle America trench evolved, contain calcite cements and are cross cut by calcite veins. Analysis of the mechanical twins in these two calcite groups (8 samples, n = 267 twins) at Punta Dominical and the Nicoya Peninsula ophiolite preserve a consistent sub-horizontal shortening strain that is oriented parallel to the current trench axis, as are most of the accretion-related fold axes. Differential stress magnitudes for the calcite twinning strain average -450 bars, and there is little evidence of a twinning strain overprint (low NEVs). Obduction along the Middle America trench involves, in part, a component of trench-parallel horizontal shortening.The Motagua fault originates in the Middle America trench and strikes east as the northern boundary of the Caribbean plate as a sinistral margin. The Roatan Islands are a series of islands composed of south-dipping rocks along the southern side of the Motagua fault. Recent, deformed clastic sediments are drapped over gneisses, schists (Ar-40-Ar-39 biotite age of 14 Ma), conodont-free marbles and ophiolitic fragments of presumed Cretaceous age. Twinning studies (n = 6 samples, n = 425 twins) of these marbles preserve a N-S, horizontal shortening strain, normal to the sinistral plate boundary. Differential stress magnitudes for the calcite twinning strain average -36 MPa. Only in a combined sample (n = 189 grains) is a strain overprint (high NEVs; n = 47) present, which records horizontal shortening strain at 132 degrees, 5 degrees. Jamaica is also along the eastern Motagua fault (Cayman trough) and twinning strain studies in Eocene limestones and younger calcite veins (n = 4 samples; n = 136 twins) preserve a horizontal shortening strain normal to the sinistral plate boundary.Twinning strain results are compared to regional GPS, focal mechanism and SKS anisotropy results around the Caribbean plate, especially along the Middle America trench on the western margin of Costa Rica with some new insights into subduction dynamics.