LITHOSPHERIC EXPRESSION OF THE NORUMBEGA FAULT ZONE IN NORTH COASTAL MAINE

The Norumbega Fault Zone (NFZ) of coastal Maine is a 40 km wide and over 400 km long dextral shear zone eroded down to mid-crustal depths with evidence of motion from mid-Paleozoic to Cretaceous time. The NFZ likely started as a suture between elements of the future Gander terrane, but subsequently acted as a transcurrent boundary, akin to modern analogs like the San Andreas, Anatolian or Denali faults.

Numerous temporary seismic stations were deployed in the northern coastal area of Maine in the framework of the Earthscope project, creating a data set with lateral spacing of 15-30 km. These new data present an opportunity to probe the properties of the lithosphere, from the surface to its boundary with the asthenosphere, with resolution sufficient to characterize structures related to the NFZ, provided they exist at depth.

Studies of these data published to date (Levin et al., (2017) and Li et al., (2018)) provide complementary evidence for significant (up to 10 km) localized crustal thinning in the vicinity of the NFZ. Also, a number of sub-horizontal boundaries in seismic properties are detected in the uppermost 100 km of the mantle.

We utilize a new variant of the receiver function methodology designed to emphasize directional variations in the observedf P-to-S converted waves from distant earthquakes. These observations are ideal for detection of changes in rock fabric and for characterizing non-horizontal boundaries at depth. This region in Maine is well illuminated by the high-frequency P waves from teleseismic distances, making it possible to focus the analysis of features with vertical dimensions of a few kilometers.

We will present a detailed characterization of the crustal and upper mantle seismic structure of the NFZ and its immediate vicinity, and discuss the implications for the depth reach and the nature of deformation associated with this major tectonic boundary.