SPREE: FIELD EXPERIMENT TO STUDY DEEP STRUCTURE OF THE MID-CONTINENT RIFT

By about 1 Ga North America’s midcontinent region completed a formation process broadly similar to the current west coast, with convergence along the Grenville Orogeny and extension along the contemporaneous Mid-continent Rift (MR). Now buried under platform sediments, enormous volumes of dense igneous rock (> 0.5 Ÿ 10⁶ km³) were deposited in the MR. These rocks generate a 60+ mgal Bouguer gravity anomaly and a correlated magnetic anomaly, which cuts curiously through major geologic units such as the Superior and Yavapai Provinces. Normal faulting accompanied the volcanic activity, but both appear confined to the immediate vicinity of the rift rather than having formed an ocean between Minnesota and Wisconsin. Although the MR is the strongest anomaly in the midcontinent, no evidence has been found for current geologic activity or correlated anomalies in the mantle lithosphere. The installation this year of the first swath of Earthscope-USArray stations east of the Mississippi River allows us to shed light on this enigmatic anomaly. Specifically, our seismic field experiment SPREE (Superior Province Rifting Earthscope Experiment) aims to uncover important details such as the depth and lateral extent of rift-related structures. SPREE is a collaboration between Canadian and US universities (see author institutions). Eighty-three Flexible-Array (FA) seismic stations were installed between April and June this year. The array configuration includes an extension of the US-based Transportable Array (TA) into Ontario north of Lake Superior as well as three lines of 10-km spaced stations along and across the MR west and east of the northern Mississippi. SPREE recorded the M2.5 western Minnesota earthquake (April 29) as well as the M7.2 Fox Islands earthquake (June 24). We aim to use the recorded earthquakes and ground motion noise to detect microseismicity as well as to construct a multi-scale, three-dimensional image of the seismic velocity and discontinuity structure of the study region’s lithosphere and underlying mantle.