Seismic constraints of the crustal structure of the Appalachian Front: An investigation of the St. Lawrence River Valley

English Abstract:

Lithosphere beneath the present day Appalachian Front (AF) experienced three major tectonic events over the last 1.5 Ga. First, the crust to the west of the AF was formed and consolidated by the Grenville Orogeny. At ~620 Ma the Iapetus Ocean began to open, leading to rifting and possible thinning of Grenville crust. Subsequently, the closure of Iapetus and the Taconic, Acadian, and Allegenhian orogens sutured the Applachian crust onto pre-existing Grenville crust.

Most reconstructions of the Appalachian orogeny envision a thin-skin tectonic regime, where thrust sheets of relatively small thickness are emplaced onto the pre-existing continental margin of the thick-skinned Grenville tectonic regime. In this view, the crustal structure on the opposite sides of the modern AF should be very similar, specifically in the deep crust. Alternatively, if the Iapetan rifting event broke up and separated blocks of Grenville basement, then the deep crustal structure of the Appalachians, Iapetan rift and Grenville terrain should have features that reflect their individual geologic setting. The underlying basement of the Appalchians and Iapetan rift does not have to be similar to the Grenville’s, and we would expect a major difference in deep crustal structure on the opposing sides of the AF.

We use seismic data from a group of long-operating seismometers on the banks of the St Lawrence River, and also data from recently installed portable observatories in the region. We employ teleseismic receiver function analysis methodology to construct detailed descriptions of seismic properties in the crust on opposite sides of the AF.

Our preliminary results show that while the thickness of the crust is similar (~50 km), on both sides of the AF, the internal structure of the crust is highly variable on lateral scales of ~20 km. Our density of observations in the St. Lawrence region helps us relate this complexity to the tectonic events that shaped the lithosphere.