MOHO REFLECTIVITY PATTERNS - A COMPARISON OF CANADIAN LITHOPROBE TRANSECTS

The complex nature of the continental Moho may be strongly influenced by the tectonic and magmatic history of the lithosphere. Observations of Moho variability from near-vertical incidence seismic reflection su rveys suggest that it may be possible to correlate Moho reflectivity a nd lithospheric evolution. As an initial step in attempting to determi ne such correlations, we qualitatively compare seismic reflection data from Canadian LTTHOPROBE transects that sample regions of exceptional ly diverse age and tectonic history. The seven selected datasets were collected using consistent acquisition parameters, facilitating effect ive and meaningful comparisons among transects. The Moho reflections e xhibit considerable variability. However, in virtually all recent tran sects, a distinct Moho is observed, even in regions where lower crust is considered to have stabilized in the Early Proterozoic or Archean. This suggests not only that relationships between Moho reflectivity an d tectonics can be extended into the Archean, but that tectonic age is poorly correlated with Moho reflectivity, The qualitative comparisons do not clearly link a specific tectonic process with a distinctive re flection pattern. However, well-defined Moho reflections are associate d with lower crustal deformational processes including compression, tr anspression and extension. This indicates that ductile shear and the r heologies of the lower crust, Moho transition and upper mantle play im portant roles in influencing reflective character. Indistinct Moho ref lectivity may be associated with regions that have experienced little ductile deformation or with processes that have disrupted reflective l aminations. However, the majority of incoherent Moho reflections appea r related to signal degradation associated with data acquisition. The study demonstrates that the qualitative characteristics of Moho reflec tivity are difficult to correlate with tectonic history and thus use a s an interpretive tool. Isolating quantitative reflection characterist ics of the Moho from distortions imposed by the overlying crust is a c hallenging problem but appears necessary to further understanding of t he crust-mantle transition.