Showing papers on "Terrane published in 1971"

Metamorphic zonations on presumably subducted lithospheric plates from Japan, California and the Alps

Abstract: High-pressure blueschist-type mineral parageneses from the Sanbagawa belt of southwestern Japan, the Franciscan terrane of western California and the Sesia zone, Pennine and Helvetic realms of the central Alps may reflect metamorphic conditions attending lithospheric plate descent. The observed progressive metamorphic sequences seemingly have developed chiefly, but not exclusively within the confines of oceanic crust, and evidently mark the suture zones between pairs of convergent lithospheric plates. The downgoing slabs have developed relatively near-surface (1) zeolitized rocks and apparently at successively greater depths (2) pumpellyite-bearing rocks, (3) greenschists and/or blueschists, and (4) albite-amphibolites; eclogitic assemblages are characteristic of the highergrade environments. The sense of metamorphic progression (1)→(2)→(3)→(4) marks the direction of presumed lithospheric underflow. Profound pressure discontinuities revealed by mineral assemblage contrasts across the plate junctions indicate that the high-pressure terranes must have risen great distances subsequent to the blueschist-type recrystallization. This conclusion is supported in California and the Alps by the exposure of rocks interpreted as basal portions of the oceanic or continental crust+upper mantle in the overlying lithospheric slabs; such sections appear to have been dragged upwards adjacent to the plate junctions during the buoyant rise of the underlying and subducted blueschistic slabs subsequent to active plate convergence. The exposed widths of the high-pressure metamorphic belts roughly correlate with the depths of inferred crustal subduction now exhumed of 25–35 km or more.

Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California

Abstract: Isolated blocks of high-grade blueschist and amphibolite facies metamorphic rocks occur within the Jurassic and Cretaceous eugeosynclinal deposits of the Coast Ranges of southwestern Oregon and California. The blocks range in size from individual rock masses commonly 5 to 1,000 ft in diameter to a few larger masses as much as 7 mi long and 2 mi wide. The high-grade blocks are predominantly basaltic in composition and include glaucophane schists, eclogites, and gneissic rocks of the amphibolite facies. Field relationships indicate that the blocks are closely associated with serpentine, that high-grade blueschist and amphibolite blocks, lower grade blueschists, volcanic rocks, and cherts occupy disturbed zones that may be related to thrusting, and that there is no exposed in situ provenance for the high-grade blueschists, eclogites, and amphibolites. Potassium-argon mineral ages of white mica and actinolite from the blueschists and of hornblende from the amphibolites indicate that these minerals crystallized approximately 150 m.y. ago, but the ages measured on glaucophane from the blueschist blocks are commonly younger. These data suggest that the high-grade blue-schist and amphibolite blocks represent fragments of a cryptic metamorphic terrane of pre-Tithonian age that have been tectonically mixed with younger rocks of the Franciscan Formation in California and Otter Point Formation in Oregon. The younger ages for glaucophane probably reflect metamorphic episodes in which lower grade in situ blueschist facies mineral assemblages were developed in the blocks after their emplacement within the Franciscan Formation. This pre-Tithonian cryptic metamorphic terrane probably developed as a result of interaction between oceanic and continental plates. The occurrence of tectonic blocks of this terrane within melange zones in Oregon and California may be related to later plate interaction.

Nature of Ancestral Orogenic Zone in Nuclear Central America

Abstract: The pre-late Paleozoic basement complex of nuclear Central America is exposed in an arcuate zone that extends from the Caribbean Sea to Chiapas, Mexico. The trend of this arc parallels fold trends in younger sedimentary rocks on both sides of the arc, which has led to the widespread, previously unsubstantiated assumption that structural trends in the ancestral orogenic zone also form an arcuate regional pattern. Analysis of structural and lithologic trends in five areas of this arc indicates that the pre-late Paleozoic ancestral orogenic zone has undergone two phases of penetrative deformation. Folds of the first phase, which was most intense, are oriented parallel with the arc, as suspected, whereas folds of the second phase have north-trending vertical axial planes. The distribution of lithologies in the orogenic zone suggests that the premetamorphic provenance was the widespread Precambrian terrane of southern Mexico. The angular relationship of basement complex structural trends on opposite sides of the Cuilco-Chixoy-Polochic fault zone, which appears to be the most likely boundary between the Americas and Caribbean plates during Cenozoic time, limits left-lateral offset along this zone to less than 150 km.

On Place and Time in Orogenic Granite Plutonism

Abstract: In a recently mapped region of northern Nigeria, typically syntectonic (“low-level”) granites, surrounded by gneisses, are exposed at the same crustal level as typically late-tectonic (“high-level”) granites, cutting migmatites and metasediments. Comparison with relationships in high-grade metamorphic terranes elsewhere suggests: (1) that low-level granites are root zones of granite masses which rose to comparatively high crustal levels during an orogenic climax; (2) that high-level granites are roof zones of bodies which were emplaced during waning stages of orogenesis and, therefore, did not rise so high. It is our contention that these terms have been inappropriately used to refer to level of granite emplacement, rather than to level of exposure within a granite mass.

Petrology and geochemistry of the Duzel (Ordovician) and Gazelle (Silurian) formations, northern California

Abstract: Co, REE and to a lesser degree Mn, Ti, and Zr, reflect the same lithologic provenances for sandstones in the Duzel and Gazelle Formations as do rock and mineral fragment distributions in these sandstones. Most other elements show significant depletions or less commonly enrichments relative to the inferred source area composition. Mineralogical and geochemical data indicate quite different source area compositions and tectonic settings for the Duzel and Gazelle Formations. The clastic sediments of the Duzel appear to have been derived primarily from plutonic-metamorphic terranes and to have been deposited on a broad, semi-stable shelf or slope. Gazelle clastic sediments reflect eastward-lying contemporary calc-alkaline volcanic sources mixed in varying amounts with plutonic-metamorphic and sedimentary sources. The Gazelle Formation was probably deposited in one or more small eugeosynclinal basins lying west of an active volcanic chain built on or near the edge of the continent. It is not possible with existing data to determine if the Duzel Formation was deposited near its present geographic location (the in-situ model), or if it represents a fragment of an extinct or more highly evolved Ordovician arc system that was later swept in from the Pacific basin by sea-floor spreading and "welded" onto the continent (the Swept-in model).

Tectonic Implications of Blueschist Facies Metamorphic Terranes in Alaska

Abstract: Recent field investigations by researchers from the University of Alaska and the US Geological Survey have defined several “blueschist facies” metamorphic terranes in Alaska. Although the regional distribution and petrology of these terranes are not yet fully understood, this early report of their findings may be of considerable interest to those concerned with the tectonic framework of the North Pacific and Bering Oceanic basins and the contiguous continental blocks.