TOWARDS A UNIFYING LITHOSPHERIC MODEL FOR THE MID-CONTINENT RIFT

Despite four pulses of intensified research on the Mid-continent Rift (MCR) over the past seven decades, confusion around the MCR remains. Among other failed rift structures, the MCR corresponds to a higher gravity high. The extensive volcanic flows suggest chemical differentiation, but the mantle lithosphere beneath the rift has similar seismic properties as the surrounding Precambrian mantle lithosphere. While the rift formed through crustal thinning, some report the crust as anomalously thick. Despite active-source seismic studies showing dense, and seismically somewhat faster volcanic layers in the upper crust, some tomography models show low seismic velocities at those depths.

Here, we attempt to reconcile disparate models from the professional literature, utilizing a range of data sets, starting with geophysics. We consider hypothetical models against the following data sets: Bouguer gravity anomalies (Kucks, 1999), teleseismic delay times (Bollmann et al., 2019), surface wave dispersion curves (Shen et al., 2013), and receiver functions (Zhang et al., 2016). We constructed a simple, conceptual model for the crust beneath the axis of the MCR that consists of 1) a more than10-km thick “layer” of volcanic rocks that are approximately 6% denser, and have bulk and shear moduli that are approximately 2% higher than those of the Precambrian upper crust, 2) a 14-km thick underplated “layer” between a thinned crust and the underlying mantle that is also approximately 6% denser, but has bulk and shear moduli that are over 10% higher than those of the Precambrian lower crust, reflecting intermediate properties between mantle and lower crust, and 3) a standard Precambrian mantle lithosphere.

Through forward modeling we find that this simple model produces anomalous values for the listed observables that are conceptually consistent with published measurements in the data sets. More specifically, our model concept shows differences between the MCR’s axis and non-rift reference locations that amount to a gravity high of around 70 mGal, a teleseismic P-wave delay time of 0.04 s, lower surface-wave phase and group velocities that peak in size for periods between 20 and 25 s, and complex waveforms of P to S converted waves in receiver functions. We welcome input on the origin of these lithospheric units.