QMIII project goals

(written in 2012)

 

The cratonic core of North America is beyond the northern border of the EarthScope Transportable Array (TA) footprint and so is arguably one of the poorest covered parts of the continent. We are carrying out a portable deployment that will supplement the TA and provide coverage of the Superior Province in central Quebec. It will consist of a near-linear array from James Bay, across Quebec and Maine to Nova Scotia. Existing US and Canadian stations anchor the array, and also provide supplementary 2D coverage. 


Our project will use the combined broadband seismic data from permanent sites and our temporary deployment to address three important hypotheses related to continental lithosphere evolution and its interaction with the asthenosphere:

LEFT: A schematic map of major tectonic divisions in the study region.

Thick lines denote major tectonic boundaries:

the Grenville Front (red)

the St. Lawrence Rift / Appalachian Front (blue)

the Norumbega Fault Zone (green).


BELOW: Geophysical constraints on the structure and thickness of the lithosphere in the study region. Colors show shear wave speed at 100 km depth in the model of Nettles and Dziewonski (2008). White contours (labeled in km) are estimates of the depth to the base of the lithosphere from the 1°x1° global model based on heat flow and regional geology constraints (Artemieva, 2006). Faster speed generally corresponds to thicker lithosphere, but not always.

  1. 1)that the lithosphere-asthenosphere boundary is a distinct interface that deepens towards the craton in accordance with inferences from surface wave tomography;

  2. 2)that the seismic properties of the continental lithosphere are regionalized, with tectonic boundaries (sutures) having a signature in the deep lithosphere; and

  3. 3)that the broad scale flow of the asthenosphere, and not the fossil fabric in the lithosphere, is the major source of seismic anisotropy.


We will employ higher resolution methods, including body wave tomography to determine bulk properties and converted phase analysis to map internal interfaces. We will assess the degree of regionalization of the lithosphere and, to the extent that we can identify discontinuities of structure at the sutures, their expression in the deep lithosphere. The thick root of the central craton should cause a long wave perturbation of mantle flow patterns. We will use a combination of surface-wave studies and shear wave splitting to map out multi-layered patterns of anisotropy. Of additional interest is the degree to which the deepest part of the cratonic root perturbs asthenospheric flow and how the actual pattern of flow compares to previously-published flow models.