Structure of the lithosphere within the Trans-Hudson Orogen (results of the 1993 LITHOPROBE Trans-Hudson refraction experiment)

Abstract

Data from three refraction profiles of the 1993 LITHOPROBE refraction experiment were used to investigate the structure of the lithosphere of the Trans-Hudson Orogen. Novel digital processing of the wide-angle reflections arrivals with standard type reflection processing techniques revealed significant crustal thickness and Moho reflectivity variations within the orogenic belt. The obtained information was than complemented with the results of near-vertical incidence seismic sections to estimate the crustal thickness variations over the entire study area. The detected crustal thickness and Moho reflectivity changes could not be correlated to the location and extent of geological domains; they appear to reflect the complex deformation and metamorphic history of the orogen. The P-wave velocity image of the crust and upper mantle was established through ray-tracing and inverse modelling of the primary and secondary crustal and mantle arrivals. Successful modelling of the observation required the incorporation of non-standard inversion techniques into the processing sequence. Although the detected crustal velocity variations appear to correlate well with the changes of Moho reflectivity. These variations in the property of the crust are interpreted to be a consequence of differences between the tectonic evolution of orogenic units in the north-western and south-eastern parts of the study area. The transition belt separating these two areas appear to coincide with an anomalous zone located in the upper mantle. This mantle region exhibits strong P-wave velocity anisotropy, determined primarily from modelling of the mantle refraction arrivals. I interpret this anomalous mantle region as a highly deformed zone, a possibly suture, between the two collided Archean plates of the orogen. Additional information on the structure of the mantle was obtained by analyzing the secondary mantle phases, observed at offsets larger than 400 km on the shot records. The broad structure of this deeper mantle region was established by incorporating the results of regional teleseismic studies into the velocity models and refining the models with raytracing. Detailed acoustic properties of the mantle were investigated after introducing random perturbations into the models and comparing the computed finite-difference synthetic-seismic responses to the observations.