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Near-vertical and wide-angle seismic reflection studies of the Moho and sub-crustal lithosphere in NW Canada

University of British Columbia

High quality, coincident near-vertical incidence (NVI) and refraction/wide-angle seismic reflection data (R/WAR) acquired along a profile in the Northwest Territories are used to study the nature of the Moho and subcrustal reflectors. First, we re-examine distinct subhorizontal reflections on NVI data in the uppermost mantle that were interpreted previously as a separate feature from a relict subducted slab. Using forward and inverse traveltime modeling of both data sets along the crooked line, we investigate the origin of the reflections. Our results demonstrate that the subhorizontal reflectors are the continuation of the relict subducted slab, which extends laterally for 300 km at depths from 35 to 90 km. Its base is the source of the R/WAR reflections. The apparent flattening is an artifact of projecting a 3-D geometry onto a 2-D cross section. The shallowly subducted slab probably contributed to the thickening and stabilization of the sub-crustal lithosphere in the region. Second, we examine the detailed structure of the Moho and propose a possible scenario for its formation and evolution. Strong Moho reflections are observed on the NVI data (shot gathers and stacked section). The WA data are characterized by a ~0.5 s coda trailing the PmP (Moho) phase. For analysis of these observed data, we follow two approaches, forward and inverse modeling. In forward modeling, we calculate wide-angle and near-vertical synthetic seismograms using 1- and 2-D wave propagation algorithms. Comparison between synthetic and observed data for shot gathers was made possible through development of a novel noise-removal technique using the curvelet transform. For the inverse method, we use a statistical analysis approach based on the von Karman autocorrelation function. Our results indicate that the Moho is a finite-thickness (~3 km), heterogeneous transition zone. The heterogeneities can be described by laterally discontinuous layering, lamellae structure with randomly distributed ellipses or a von Karman distribution with a lateral correlation length of 936 m. The transition zone separates the lower crust with a lateral correlation length of 732 m from the uppermost mantle with a correlation length of 261 m. The Moho is interpreted as a thermal/metamorphic front, a regional décollement, or both.

Text

2010-10-04

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/28903

Geophysics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/