SHARP LITHOSPHERIC LAYERING BENEATH THE PALEO AND MESOPROTEROZOIC MAZATZAL AND GRANITE-RHYOLITE PROVINCES

The North-American Granite-Rhyolite province formed during the first ~200 m.y. of the Mesoproterozoic and was intruded by granites and anorthosites. In seismic-tomographic models the lithosphere of this province usually appears as typical Proterozoic lithosphere: A cool, stiff layer that is nearly 200 km thick, indistinguishable from lithosphere of other Proterozoic terranes. However, the confluence of USArray and an increase in North-American intraplate earthquakes over the past decade inspired us to tomographically study the Laurentian lithosphere in more detail. The lithosphere of Paleo- and Meso-proterozoic parts of Laurentia was illuminated by six moderate-sized earthquakes east of the Rocky Mountains between 2011 and 2014 and several hundreds of well-distributed Transportable-Array and strategically distributed SPREE stations recording their seismic waves. On average the S velocity of Laurentia’s Proterozoic lithosphere increases with depth below the Moho, reaching a maximum near 4.7 km/s around a depth of 75 km, distinctly faster than Paleozoic lithosphere to the east. Lithospheric thickness, defined by S velocities decreasing with depth, ranges from 150 to 200 km. We also mapped our smooth, 3-D, lithospheric S velocity model into a depth-dependent geological map with discrete terranes, or clusters. We used a maximum of seven different clusters, defined by similarity of S velocity as a function of depth. These clusters are not required to map into contiguous terranes; yet they largely do. Strikingly, the mapped terranes show great similarity with Paleo and Mesoproterozoic additions to Laurentia. One cluster comprises most of the Mazatzal and Granite-Rhyolite provinces. This cluster shows extreme vertical layering, with high S velocities from the Moho to 50 km, an lower-velocity zone around 65 km, and a pronounced gradient to velocity anomalies of +6% around 100 km. The cluster that comprises the Yavapai province and the south-eastern portion of the Granite-Rhyolite province, just northeast of the part that would later rift away to form the Argentine Precordillera, is less layered and more typical of Proterozoic lithosphere.