CONTRASTING PROTEROZOIC BASEMENT COMPLEXES NEAR THE TRUNCATED MARGIN OF LAURENTIA, NORTHWESTERN SONORA-ARIZONA BORDER REGION: IMPLICATIONS FOR RODINIA RECONSTRUCTION

Two contrasting Proterozoic basement complexes in the Sonora-Arizona border region have ages and radiogenic isotope signatures that generally overlap with the Yavapai and Mazatzal crustal provinces nearby to the northeast. However, peculiar regional map patterns, discordant structural geometries, and differences in deformational chronology distinguish these border complexes, which appear to be juxtaposed along a NW-striking structure. The paleoposition of the complexes will constrain the pre–750 Ma reconstruction of Rodinia. A variety of configurations is possible depending on the degree to which the complexes have been affected by Phanerozoic strike-slip faulting.

Essential features of the Southwest Complex that must be accommodated by any Rodinia restoration include: 1) 1725–1696 Ma granite-granodiorite augen gneiss with depleted initial Nd isotopic signatures intruded as sheets into previously deformed, layered magmatic arc rocks and 2) SW-vergent noncoaxial fabrics and NW-trending map-scale folds that probably developed between 1696 Ma and 1.1 Ga. Noteworthy pre-750 Ma lithologic and structural elements of the Northeast Complex include: 1) a zoned alkali-syenogranite batholith (1650 to 1640 Ma) intruded into arkosic and quartzose sandstone host rocks with a detrital zircon component predominantly 1665 to 1650 Ma, 2) ESE-trending folds and regional metamorphic fabrics with locally preserved NW-vergent S-C fabric developed at ca. 1.59-1.55Ga, (3) crosscutting 1432 Ma megacrystic granite and (4) 1.1 Ga(?) diabase dikes whose orientations are subhorizontal upon removal of Miocene tilting .

Restoration of hundreds of km of left-lateral movement along the Late Jurassic Mojave-Sonora megashear results in separation of the two discordant complexes and also brings Grenvillian-age rocks in southern Mexico closer to southwestern North American cratonal Cambrian clastic rocks rich in ca. 1.0 Ga. detrital zircons. Depending on how postulated Jurassic translations are accomplished, the ages and rock assemblages of the Sonora-Arizona border complexes offer potentially good Rodinia ties to Australia or Siberia. Distinct regional structures in NW Mexico provide piercing lines important for obtaining a unique match.