One outstanding problem in tectonics is the mechanism of crustal thickening. Where tectonic plates converge, relatively low density crust may become delaminated from its denser mantle keel, or slices of felsic crust may indent or interfinger as crustal wedges. In this case indenting or even mutually indenting structures may lead to thickened crust. The structure of tectonic wedging has received increasing support from the interpretations of geophysical data, but interpenetrating crustal slivers have rarely been observed in the field. Consequently the mechanisms and even the confirmation of interpenetrating wedges remain speculative.
In this collaboration between Indiana University and Brigham Young University geologists are making one of the first attempts to directly characterize and identify the causes of syn-collisional wedging of upper crust from lower crust using surface outcrop. They have discovered an example of tectonic wedging in a tilted lower crustal profile. The outcrops in coastal Connecticut exposure rocks formerly mapped as fold nappes within the Avalon terrane. In addition to Avalonian rocks derived from juvenile sources, our pilot study of neodymium and lead isotopes is showing that the structurally lowest orthogneisses is derived from ancient crustal rocks that correlate with the Gander terrane. Their work in this region shows that Avalonian rocks both overlie Gander basement rocks and underlie rocks known to the north to rest conformably over Gander basement. Thus this map pattern defines the geometry of a tectonic wedge.
The goals of the project are (1) to confirm the identity and correlation of Avalon and Gander zone rocks in southern coastal New England with correlative rocks in maritime Canada where they are well characterized, (2) to test the extent and distribution of Avalon and Gander zone rocks in other inliers of New England, (3) to test single vs. double indenting models, (4) to monitor the timing and rate of approach of Avalon in the cover terranes by establishing the time of quenching of the cover Gander terranes, and (5) to assess the rheology of the rocks in the terrane boundary. Methods to be used include characterization of outcrop in the field, analysis of major and trace element compositions to identify intrusive igneous rocks, and neodymium and lead isotopic analysis of theses igneous rocks to ascertain the origins of the initial magmas.
