Geologic transect across the Grenville orogen of Ontario and New York

Revised cross sections of the western Grenville Province incorporate new ge ologic results and reprocessed seismic reflection data. The geology is pres ented in terms of three tectonic elements: (1) "pre-Grenvillian Laurentia a nd its margin" with ca. 1740 and 1450 Ma continental arc plutons and associ ated supracrustal rocks; (2) "Composite Arc Belt" of allochthonous similar to 1300-1250 Ma volcanic arcs and sedimentary rocks; and (3) "Frontenac-Adi rondack Belt" characterized by supracrustal and granitoid rocks, and anorth osites, of uncertain affinity, that may represent a distinctive part of the Composite Arc Belt or an offshore (micro)continent. Rocks of the Composite Arc and Frontenac-Adirondack belts were amalgamated with each other by ca. 1160 Ma, were then thrust over Laurentia during ca. 1080-1035 Ma and ca. 1 010-980 Ma phases of convergence, and were dissected and exhumed by < 1040 Ma normal faults. Penetrative deformation was restricted to that part of th e pre-Grenvillian Laurentian margin that lies to the southeast of the Grenv ille front and parts of the accreted Composite Arc and Frontenac-Adirondack belts. The Laurentian rocks in the Grenville Province are bounded to the n orthwest and southeast by southeast-dipping ductile thrust and (or) normal shear zones. The Composite Arc and Frontenac-Adirondack belts to the southe ast are bounded by ductile and brittle-ductile thrust and (or) normal fault s that separate domains with contrasting cooling histories. Despite a long pre-Grenvillian tectonic and plutonic history, the present crustal architec ture and much of the seismic reflectivity were acquired during 1080-980 Ma phases of compression and extension.