T159 Assembling Laurentia: Mesoproterozoic to Early Neoproterozoic Tectonic Evolution of Laurentia and Its Role within the Supercontinent Rodinia II

Wednesday, 28 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

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

Paper number: T159-7
Program: Technical Sessions
Time: 6:25 PM – 6:35 PM

Authors

  • Emily Wolin
    Northwestern University, Evanston, IL
  • Suzan van der Lee Presenting Author
    Northwestern UniversityEarth and Planetary Sciences Tech F379, Evanston, IL

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-359794

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