NEOPROTEROZOIC-PALEOZOIC GEOGRAPHY AND TECTONICS - REVIEW, HYPOTHESIS, ENVIRONMENTAL SPECULATION

The ever-changing distribution of continents and ocean basins on Earth is fundamental to the environment of the planet. Recent ideas regardi ng pre-Pangea geography and tectonics offer fresh opportunities to exa mine possible causative relations between tectonics and environmental and biologic changes during the Neoproterozoic and Paleozoic eras, The starting point is an appreciation that Laurentia, the rift-bounded Pr ecambrian core of North America, could have been juxtaposed with the c ratonic cores of some present-day southern continents, This has led to reconstructions of Rodinia and Pannotia, supercontinents that may hav e existed in early and latest Neoproterozoic time, respectively, befor e and after the opening of the Pacific Ocean basin. Recognition that t he Precordillera of northwest Argentina constitutes a terrane derived from Laurentia may provide critical longitudinal control on the relati ons of that craton to Gondwana during the Precambrian-Cambrian boundar y transition, and in the early Paleozoic. The Precordillera was most l ikely derived from the general area of the Ouachita embayment, and may have been part of a hypothetical promontory of Laurentia, the ''Texas plateau,'' which was detached from the Cape of Good Hope embayment wi thin Gondwana between the present-day Falkland-Malvinas Plateau and Tr ansantarctic Mountains margins. Thus the American continents may repre sent geometric ''twins'' detached from the Pannotian and Pangean super continents in Early Cambrian and Early Cretaceous time, respectively-t he new mid-ocean ridge crests of those times initiating the two enviro nmental supercycles of Phanerozoic history 400 m.y. apart. In this sce nario, the extremity of the Texas plateau was detached from Laurentia during the Caradocian Epoch, in a rift event ca, 455 Ma that followed Middle Ordovician collision with the proto-Andean margin of Gondwana a s part of the complex Indonesian-style Taconic-Famatinian orogeny, whi ch involved several island are-continent collisions between the two ma jor continental entities. Laurentia then continued its clockwise relat ive motion around the proto-Andean margin, colliding with other are te rranes, Avalonia, and Baltica en route to the Ouachita-Alleghanian-Her cynian-Uralian collision that completed the amalgamation of Pangea. Th e important change in single-celled organisms at the Mesoproterozoic-N eoproterozoic boundary (1000 Ma) accompanied assembly of Rodinia along Grenvillian sutures. Possible divergence of metazoan phyla, the appea rance and disappearance of the Ediacaran fauna (ca. 650-545 Ma), and t he Cambrian ''explosion'' of skeletalized metazoans (ca, 545-500 Ma) a lso appear to have taken place within the framework of tectonic change of truly global proportions. These are the opening of the Pacific Oce an basin; uplift and erosion of orogens within the newly assembled Gon dwana portion of Pannotia, including: a collisional mountain range ext ending approximate to 7500 km from Arabia to the Pacific margin of Ant arctica; the development of a Pannotia-splitting oceanic spreading rid ge system nearly 10 000 km long as Laurentia broke away from Gondwana, Baltica, and Siberia; and initiation of subduction zones along thousa nds of kilometres of the South American and Antarctic-Australian conti nental margins. The Middle Ordovician sealevel changes and biologic ra diation broadly coincided with initiation of the Appalachian-Andean mo untain system along >7000 km of the Taconic and Famatinian belts. Thes e correlations, based on testable paleogeographic reconstructions, inv ite further speculation about possible causative relations between the internally driven long-term tectonic evolution of the planet, its sur face environment, and life.