Appalachian Front is a Shallow Feature: Insight From Studies of Seismic Wave Speed in the Mantle

English Abstract:

The Appalachian Front (AF) is a major feature in the surface geology of eastern North America, separating 0.4 Ga old Appalachian terranes from the 1.2 Ga Grenville Province. North of Quebec City, it strikes northeast-southwest, and is approximately collocated with the St. Lawrence River. We use a group of long-operating seismic observatories that straddle the AF in Charlevoix region of Quebec, and a new portable array of observatories extending northwest and southeast from it, to investigate the seismic wave speed in the upper mantle beneath a region extending from the Archean craton near James Bay, across the AF, to the Appalachian terranes of coastal Maine.

Travel time delays of teleseismic P waves indicate that although the mantle tends to be faster in the craton 100-200 km northwest of the AF and slower in the Appalachian terranes 100-200 km southeast of the AF, there is no significant vertical boundary in the uppermost mantle (100-150 km) directly beneath the AF. Furthermore, the cross-strike variability (relative to the AF) is not in a form of a uniform decrease of mantle P velocity from James Bay to coastal Maine, but instead has two en echelon gradients (neither of which aligns with the AF). The Appalachian upper mantle is very heterogeneous (much faster in New Brunswick and northern Maine than in southern New England), and these heterogeneities are not aligned with the strike of the AF.

Stations on both sides of the AF in the Charlevoix region detect strong birefringence in SV-polarized teleseismic shear waves. The nature of birefringence is complex, with considerable scatter in individual observations. There is, however, no systematic difference in birefringence observed by sites on the "Grenville" and "Appalachian" sides of the AF, confirming that this tectonic boundary does not extend into the upper mantle. We detect a distinct pattern of birefringence in shear waves arriving from a narrow range of north-western back-azimuths (330-360NNW). Directional limits of this anomalous birefringence pattern, and the strength of the teleseismic P wave travel-time delay offset suggest a presence of a distinct upper-mantle feature to the NW that is faster, and has distinct rock fabric. A good candidate for such a feature would be an underplated oceanic slab preserved within the body of the craton since the time of its formation.