Showing papers on "Terrane published in 1976"

Geology of Romania

Abstract: Romania consists of four major areas of Mesozoic and older rocks: the southern Carpathians, eastern Carpathians, Apuseni Mountains, and Dobrogea. Late Tertiary sedimentary rocks occupy areas among these four regions and overlie older rocks in the Pannonian basin, Transylvanian basin, and Skythian (Russian) and Moesian platforms. Although the Carpathian erogenic belt appears to form a continuous easterly arc through Romania, this belt formed through a series of events that began in Triassic time and continue to the present. The southern Carpathians consist of three structural and paleogeographic units. From east to west they are the Danubian terrane, Severin nappe, and Getic terrane. The Danubian and Getic terranes consist of shallow-water, non-marine Mesozoic rocks that overlie Paleozoic and Precambrian sedimentary and crystalline rocks. Rocks of the Getic terrane have been thrust relatively eastward over rocks of the Danubian terrane, along the Getic thrust fault. A slice of Late Jurassic to Late Cretaceous flysch-type rocks occurs between the Getic and Danubian rocks. The Severin nappe may represent remnants of oceanic deposits that probably extended eastward into the flysch terrane of the eastern Carpathians. Emplacement of the Getic and Severin nappes is not well dated but most likely occurred during latest Cretaceous to earliest Tertiary time. The eastern Carpathians consist of two main paleogeographic and structural units, which are, from west to east, the inner crystalline zone and the outer flysch zone. Mesozoic rocks of the inner crystalline zone are mainly shallow marine and nonmarine and overlie crystalline rocks of Paleozoic and Precambrian age. They are similar to rocks of the southern Carpathians, but continuation is not observable because Tertiary rocks of the Brasov depression conceal the relation between the two units. Structurally, the inner crystalline zone consists of a sequence of relatively east-directed thrust slices; all but the uppermost slice contains pre-Mesozoic crystalline rocks. From the base to the top, the main tectonic units are the Bertila unit, sub-Bucovinian nappe, Bucovinian nappe, and Transylvanian nappes. The Transylvanian nappes consist of isolated masses of Mesozoic sedimentary rocks, some incorporated, in wildflysch, that were probably emplaced by gravity during Early Cretaceous time. Thrusting of the lower three units occurred in Late Albian-Early Vraconian time; the crystalline nappes were thrust relatively eastward over the westernmost nappes of the flysch zone. The flysch zone contains sedimentary rocks of Late Jurassic to Late Sarmatian age, and the older rocks (Jurassic and Cretaceous) are present mainly in the western part of the zone. The flysch zone consists of seven nappes, from west to east: Black flysch unit, Ceahleau nappe, Curbicortical nappe, Audia nappe, Tarcau nappe, marginal folds nappe, and sub-Carpathian nappe. Stratigraphic units in the flysch nappes change markedly from one nappe to the next; this change can best be explained in terms of the migration of flysch deposition outward in time. Migration of the axis of the flysch basin resulted from westward underthrusting of the flysch zone beneath the inner crystalline zone. Deformation of the flysch zone extended over a long period of time, probably from Albian to Holocene time. Pleistocene rocks are folded at the south end of the flysch zone; these folds occur within a region of deep (to 200 km) earthquakes, suggesting that underthrusting is still active. The Apuseni Mountains are situated in central Romania, north of the southern Carpathians and west of the Transylvanian basin. They consist of two different structural elements, the northern and southern Apuseni Mountains. The northern Apuseni Mountains consist of Mesozoic shallow marine and nonmarine sedimentary rocks that overlie Paleozoic and Precambrian sedimentary and metamorphic rocks. North-directed thrust faults, nearly all involving pre-Mesozoic basement rocks, cut the southern part of the northern Apuseni Mountains. The nine thrust plates can be divided into two groups: the Codru nappes to the north, containing mainly Mesozoic rocks, and the Crystalline or Biharia nappes of pre-Mesozoic rocks to the south. The nappes were emplaced during Late Cretaceous time, but only the northernmost nappes are clearly dated as Late Turanian-Early Coniacian. The southern Apuseni Mountains consist of several major bodies of mafic and rare ultramafic rocks, probably representing a Middle Jurassic ophiolite sequence, and thick sections of flysch, wildflysch, and molasse of Late Jurassic to Late Cretaceous age. The stratigraphy and structure of the sedimentary rocks are poorly known but appear to result from several different sedimentary basins that were brought together during a series of Cretaceous structural events. Structural events are recorded as Aptian, Albian, Turonian, Santonian(?), and Maestrichtian; the earlier deformations were south-vergent, and the later events were north-vergent, giving the southern Apuseni Mountains a crude fan structure. The Dobrogea area comprises a group of low mountains in southeastern Romania, east of the main Carpathian chain. Dobrogea can be divided into four northwest-trending structural zones, each bounded by faults of probable large displacement. The pre-Dobrogea depression is buried beneath Holocene sediments of the Danube delta, north of Dobrogea proper. Its north flank forms the Skythian platform, and its south flank is marked by a south-dipping thrust(?). The depression, or basin, forms an asymmetric syncline with a steep south flank and is filled with a thick sequence of Jurassic rocks. North Dobrogea contains deformed and metamorphosed basement rocks of Hercynian age, overlain by a thick sequence of alpine-type Mesozoic rocks. These rocks comprise northeast-directed folds and thrusts, probably formed either at the end of Jurassic or in late Early Cretaceous time. Central Dobrogea contains Pre-cambrian metamorphic rocks that are overlain by weakly folded Jurassic rocks. South Dobrogea contains rocks that are similar to those of central Dobrogea; they dip gently southward, forming the east margin of the Moesian platform. The structures in Dobrogea can be traced in the subsurface toward the Carpathians, but here they are buried by late Tertiary rocks of the Carpathian foredeep. Only the pre-Dobrogea depression can be traced east of the Carpathians into Poland; its linear trend is unaffected by the arc of the Carpathians. The Pannonian and Transylvanian basins are superposed on the structural elements of the Carpathians. They clearly represent post-tectonic basins, created partly by extension in middle and late Tertiary time. Unfortunately, these Tertiary basins conceal relations among the structural elements of the Carpathian orogen.

Oxygen and hydrogen isotope studies of a Precambrian granite-rhyolite terrane, St. Francois Mountains, southeastern Missouri

Abstract: Isotopic analyses were made on whole-rock samples and minerals from 25 granites, 21 rhyolites, and 10 basaltic dikes and sills from the l,500-m.y.-old St. Francois Mountains terrane. The δD of chlorite is relatively uniform at −44 to −65, but the whole-rock and feldspar δ18O values systematically increase from +7 in the northeast to +14 in the southwest, correlating with an increasing intensity of “brick-red” alteration of the K-feldspars. Coexisting quartz and feldspar are typically not in isotopic equilibrium, except in the northeast. The coarser grained (>1 mm) quartz (δ18O = 8.8 to 10.6) is isotopically similar to “normal” igneous quartz, but the δ18O of the finer grained quartz correlates with the whole-rock δ18O pattern. Although the St. Francois terrane is geologically similar to many low-18O Tertiary volcanic-plutonic complexes that have interacted with meteoric-hydrothermal fluids at high temperatures, only a single locality (Stono Mountain) contains low-18O quartz and feldspar (δ18O quartz = 4.6 to 5.4). Even at this locality, the feldspars were later enriched in 18O by the same alteration event that affected the rest of the region. This second stage of hydrothermal activity apparently involved aqueous fluids having δ18O ≈ 0 to −6 and δD = 0 to −25. The temperature of alteration may have been as low as 50° to 100°C in the southwest (upper part of the volcanic section). This event affected some of the basaltic dikes and sills, but others were not altered and must have been intruded afterward. Rb-Sr and K-Ar age data suggest that the regional alteration occurred as late as 1,100 to 1,200 m.y. ago, long after the age of primary igneous activity. The hydrothermal fluids apparently originated as meteoric surface waters, indicating that during late Precambrian time such waters were isotopically similar to present-day meteoric waters in warm climates; this interpretation implies that the late Precambrian oceans must have been isotopically similar to present-day ocean.

Lithostratigraphy and structure of the Caples terrane of the Humboldt Mountains, New Zealand

Abstract: Schistose lithostratigraphic units, correlated with non-schistose rocks of the Caples terrane, can be mapped successfully along the western margin of the Haast Schist terrane in the Humboldt Mountains of north-west Otago. The rocks include chert, volcanogenic sediment and lava of the Harris Saddle Formation; the mineralogically immature Momus Sandstone; the new Cosmos Formation, a thin but persistent horizon of green volcanogenic sandstone; and the overlying pelitic and psammitic rocks of the Upper Peak unit, the Mystery Pelite (new), and the Bold Peak unit. They have been reconstituted under metamorphic conditions of the pumpellyiteactinolite and greenschist facies. Three pe1iods of deformation are recognised. The first, synmetamorphic phase resulted in a maior recumbent fold, with an axial surface parallel to the regional schistosity; the second, post-metamorphic phase is defined by folded schistosity and deformed isotects. Major structures of both fold episodes have been verified by direct obs...

Boundary between two Precambrian W terranes in Minnesota and its geologic significance

Abstract: Minnesota lies astride two Precambrian W (lower Precambrian) terranes that differ in age, rock assemblages, metamorphic grade, and structural style. In northern Minnesota, greenstone-granite complexes 2,700 to 2,750 m.y. old, which are typical of the majority of the Canadian Shield, are exposed. These rocks trend northeastward, dip steeply, and typically produce narrow curvilinear aeromagnetic and gravity anomalies. In southwestern Minnesota, much older (3,550 m.y.) granulite-facies granitic and mafic gneisses, which are moderately flat-lying and produce relatively broad magnetic and gravity anomalies, are exposed through a window in the Phanerozoic cover in the Minnesota River valley. Similar gneiss, some of which has been dated radiometrically, is exposed sporadically in central Minnesota and is considered part of the same terrane as the gneiss in the Minnesota River valley. Judged from the outcrop pattern and available geophysical data, the boundary between the two terranes trends diagonally across central Minnesota, from approximately latitude 45°30′N at the western boundary to the vicinity of Duluth, on Lake Superior. We postulate that the volcanic and sedimentary rocks in the greenstone terrane accumulated adjacent to the pre-existing gneiss terrane, which 2,700 m.y. ago was part of a sialic protocontinent of moderate size. There is no geologic or geochemical evidence that these rocks were deposited on a sialic crust. The tectonic environment that existed 2,700 m.y. ago, when the greenstone-granite complexes were formed, is not known; there is no compelling evidence that they were formed in a plate tectonic regime.

The Yukon Crystalline Terrane: Enigma in the Canadian Cordillera

Abstract: The Yukon Crystalline Terrane, an area of Proterozoic and Paleozoic metamorphic rocks intruded by Mesozoic and Tertiary plutons and covered by Tertiary volcanic strata, has been one of the least understood tectonic elements in the Canadian Cordillera. It incorporates characteristics of the Omineca Crystalline Belt, Intermontane Belt, and Coast Plutonic Complex. The metamorphic rocks are not part of a single time-stratigraphic assemblage as the term “Yukon Group” has implied. They can be divided into gross lithologic assemblages which are correlated with Proterozoic and Paleozoic strata of the Omineca Crystalline Belt, and the metamorphic rocks of the Yukon Crystalline Terrane are therefore the continuation of the Omineca Crystalline Belt. Four plutonic suites are recognized in the Yukon Crystalline Terrane. The two oldest (Late Triassic and mid-Jurassic) are related to similar plutons in the Intermontane Belt and mark its continuation through the central part of the Yukon Crystalline Terrane. The mid-Cretaceous suite in the northern part of the Yukon Crystalline Terrane is a westward continuation of plutonic rocks of the Omineca Crystalline Belt. Early Eocene alaskite marks the superposition of Coast Plutonic Complex igneous rocks on the southern part of the Yukon Crystalline Terrane. Tertiary tholeiitic basalts, which cover part of the Yukon Crystalline Terrane, lie on an eroded plateau whose relief and physiography approximate the present terrane.

Turbidites of the Aleutian abyssal plain: Mineralogy, provenance, and constraints for Cenozoic motion of the Pacific plate

Abstract: The Aleutian abyssal plain is a fossil abyssal plain of Paleogene age in the western Gulf of Alaska. The plain is a large, southward-thinning turbidite apron now cut off from sediment sources by the Aleutian Trench. Turbidite sedimentation ceased about 30 m.y. ago, and the apron is now buried under a thick blanket of pelagic deposits. Turbidites of the plain were recovered at site 183 of the Deep Sea Drilling Project on the northern edge of the apron. The heavy-mineral fraction of sand-sized samples is mostly amphibole and epidote with minor pyroxene, garnet, and sphene. The light-mineral fraction is mostly quartzose debris and feldspars. Subordinate lithic fragments consist of roughly equal amounts of metamorphic, plutonic, sedimentary, and volcanic grains. The sand compositions are arkoses in many sandstone classifications, although if fine silt is included with clay as matrix, the sand deposits are feldspathic or lithofeldspathic graywacke. The sands are apparently first-cycle products of deep dissection into a plutonic terrane, and they contrast sharply with arc-derived volcanic sandstones of similar age common on the adjacent North American continental margin. The turbidite sands are stratigraphically remarkably constant in composition, which indicates derivation from virtually the same terrane through a time span approaching 20 m.y. Comparison of Aleutian plain data with the compositions of coeval sedimentary rocks from the northeast Pacific margin shows that the Kodiak shelf area includes possible proximal equivalents of the more distal turbidites. Derivation from the volcaniclastic Mesozoic flysch of the Shumagin-Kodiak shelf is unlikely; more probably the sediments were derived from primary plutonic sources. The turbidites also resemble deposits in the Chugach Mountains and the younger turbidites of the Alaskan abyssal plain and could conceivably have been derived from the coast ranges of southeastern Alaska or western British Columbia. The Aleutian plain sediment most likely was not derived from as far south as the Oregon-Washington continental margin, where coeval sedimentary deposits are dominantly volcaniclastic. This work lends some support to earlier suggestions that the fan-shaped turbidite body originated on the continental margin in the Gulf of Alaska, and it supports models of little or modest motion of the Pacific plate relative to North America. The mineralogy alone cannot refute more ambitious motion models, but when combined with previously published evidence on size of the plain, sediment thicknesses, and nan-nofossil species diversity, the data seriously constrain models requiring large-scale northwestward motion of the Pacific plate in post-Eocene time.

Pre-Cretaceous Rocks of Northwestern Honduras: Basement Terrane in Sierra de Omoa

Abstract: A basement ridge of metamorphic and plutonic rocks trends easterly along the northern border and coast of Honduras. It is bounded on the south by the Chamelecon and Aguan faults, which juxtapose Mesozoic strata against the basement terrane. Comprehensive regional studies have revealed the following chronology of events in the evolution of the basement complex of northwestern Honduras. (1) An undifferentiated sequence of Paleozoic, and possibly Precambrian, sedimentary and volcanic strata was metamorphosed to the almandine-amphibolite facies in the pre-Pennsylvanian. (2) This sequence was intruded by plutons of intermediate composition that are deformed penetratively and metamorphosed regionally; one of the plutons bears a Rb-Sr whole-rock isochron age of 305 ± 12 m.y (3) This complex is overlain unconformably by a thick sequence of pelitic strata that was metamorphosed weakly in the pre-Jurassic. (4) The latter sequence was intruded by other plutons of intermediate composition that are not metamorphosed; one such pluton bears a Rb-Sr whole-rock isochron age of 150 ± 13 m.y. (5) The entire basement complex probably was blanketed unconformably in the Early Cretaceous by sublittoral carbonate deposits which were removed subsequently by erosion and deposited to the south as conglomerates in the Late Cretaceous. (6) Late Cretaceous-early Tertiary granitic plutonism was widespread across northern Honduras and probably resulted in pronounced isostatic uplift of the basement ridge along the Chamelecon and Aguan faults. (7) Late Tertiary faulting along northerly trend dislocated the ridge and the Chamelecon-Aguan faults, and resulted in development of the Ulua Valley.

Early Precambrian tectonic-igneous evolution in the Vermilion district, northeastern Minnesota

Abstract: The Vermilion district and adjacent areas are part of the greenstone-granite terrane of northern Minnesota and consist of a thick succession of subaqueous volcanic rocks, derivative sedimentary rocks, and intrusive granitic rocks, which formed during the interval 2,750 to 2,700 m.y. ago. The volcanic-sedimentary succession now constitutes an eastward-trending, upright, tight anticlinorium between flanking granitic batholiths. The folding was broadly contemporaneous with emplacement of the oldest recognized plutonic rocks, which range in composition from granite to tonalite, and is attributed to compression caused by the relative upwelling and convergence of the buoyant granitic bodies. Metamorphism of the supracrustal rocks to greenschist and, locally, middle-amphibolite facies accompanied the folding. Later, anorogenic monzonite–quartz monzonite and other syenitic rocks intruded the supracrustal rocks, apparently as diapirs. Regional strike-slip faulting followed emplacement of the anorogenic plutons. The principal fault, named the Vermilion fault system, transects several greenstone-granite complexes; within the district, stratigraphic offsets indicate a right-lateral strike-slip separation of 17 to 19 km along the fault. The strike-slip faulting marked the transition from an unstable crust dominated by vertical tectonic movements to a more stable crust capable of sustaining regional fractures.

Southeastern margin of the northeastern Appalachians: Late Precambrian orogeny on a continental margin

Abstract: The evidence for a continental margin on the southeastern side of the Appalachians, analogous to that recognized on the northwestern side, and the nature of the margin are described. Remnants of such a margin are probably best preserved in Newfoundland as a thick wedge of complexly deformed dominantly metasedimentary rocks (Gander Group) resting on a gneissic continental basement. These rocks separate the Avalon zone to the southeast from an oceanic terrane to the northwest, now represented by a Lower Ordovician ophiolite suite and overlying volcanic and sedimentary rocks. The Gander Group suffered orogenic deformation, plutonism, and metamorphism, probably in late Precambrian time. Unconformable relationships in the Avalon zone can also be interpreted as the result of this orogenic episode, termed the Ganderian orogeny. Probable equivalents of the Gander Group are locally preserved in northeastern Cape Breton Island and central New Brunswick, but farther southwest they have not been recognized, although continental basement is preserved. Late Precambrian granitic rocks may represent the Ganderian orogen in southeastern Connecticut and eastern Massachusetts, and possible equivalents of the Gander Group are present in Rhode Island and in the British Isles. These occurrences suggest a widespread erogenic episode or episodes of late Precambrian age on the southeastern side of the Caledonian-Appalachian belt. This orogenic activity is interpreted to have developed at and within a continental margin on the northwestern side of a continental mass and is distinct from later orogenic episodes within the system. The ocean, on one of whose margins this orogenic belt was developed, must have opened considerably before Cambrian time, in contrast to the present northwestern margin of the Appalachians.

Rb/Sr Ages and a profile of initial Sr87/Sr86 ratios for plutonic rocks across the Yukon Crystalline Terrane

Abstract: Nine Rb/Sr apparent ages are reported for igneous rocks of the Yukon Crystalline Terrane. The oldest age (144 m.y.) is from the Triassic? Klotassin quartz diorite and is thought to be a hybrid age ...

Miocene Strata on Santa Cruz and Santa Rosa Islands–A Reflection of Tectonic Events in the Southern California Borderland

Abstract: A thick sequence of early to middle Miocene conglomerate and breccia on Santa Cruz Island is assigned to the Vaqueros Formation, the San Onofre Breccia, and the Blanca Formation. The Vaqueros and San Onofre intertongue with finer grained lithofacies on Santa Rosa and San Miguel Islands, and the Blanca Formation is recognized on all three islands. Each of the three units is characterized by a different suite of clast lithology, and the sequence of changes in clast composition probably reflects a sequence of tectonic events in the source terrane. Uplift and erosion of local basement rocks are indicated by clasts of diorite and greenschist in the Vaqueros Formation conglomerate, the lowest unit of the conglomerate-breccia sequence. The overlying San Onofre Breccia includes abundant blueschist detritus derived from a Catalina Schist meta-morphic terrane. Dacitic volcanic clasts occur first at the base of the Blanca Formation, distinguishing it from the underlying San Onofre. They were probably derived from several volcanic events that were contributing andesitic and dacitic ejecta at that time from an area between Santa Cruz and San Miguel Islands. Basin and ridge topography, with erosion of uplifted crystalline basement rocks, apparently began as early as Vaqueros time in the northern Channel Islands region.

Cenozoic Tectonism on Santa Cruz Island

Abstract: Santa Cruz Island is divided into two different geologic terranes by a westerly trending fault zone. Miocene volcanic rocks exposed on the north side dip homoclinally northward and are overlain by siliceous shale of the Monterey Formation. Strata exposed on the south side of the island include Paleogene fine-grained to conglomeratic marine sedimentary rocks, lower and middle Miocene breccia deposits, and middle Miocene volcaniclastic rocks. The composition and texture of these rocks suggest a complex and varying Cenozoic tectonic history. Several distinct tectonic episodes suggest that the south part of the island in pre-Miocene time was involved in northwest-oriented “basin-and-ridge” style tectonism that more recently has been replaced by east-west structures typical of the Transverse Range province.

The southwest portion of the Dunnage Mélange and its relationships to nearby groups

Abstract: The Dunnage Melange is typified by slumped blocks of clastic sediment and mafic volcanics set in a dark shale matrix. It outcrops for 40 km. southwestward from its type area in Dildo Run, with a maximum outcrop width of 13 km. Previous work has been confined to the Dildo Run area, where the interpretation of stratigraphic relationships of the Dunnage Melange and surrounding units has been controversial. The present study focusses on the southwest portion of the melange in an attempt to resolve this controversy as well as to investigate the character and extent of this portion of the melange. -- The southwest portion of the melange overlies and interdigitates with the gabbro-infested, Ordovician New Bay Formation. The melange contains gabbro blocks that are confined to the area proximal to this transition zone. Similarly, in the northwest portion, the largest mafic volcanic blocks define a wide zone that is correlative westwards with the Lawrence Head Volcanics of the Exploits Group. Along its northwest border, the Dunnage is locally conformable with Caradocian or later, shale and greywacke units, though in most places this contact is fault-modified. These relationships imply that the Dunnage is an easterly chaotic equivalent to part of the Exploits Group. -- The chaotic nature of the melange can be attributed primarily to discontinuous, dominantly extensional, soft rock deformation of strata due to massive slumping. This possibly occurred as a single progressive event and involved rocks in various stages of lithification. Forces responsible for soft rock deformation existed from New Bay depositional time until the Lower Silurian, intensifying prior to the Caradocian, following which they decreased and terminated. The chronology of later penetrative events is uncertain, though most hard rock structures are presumed to be Devonian, as flat lying Carboniferous strata occur elsewhere in the Exploits Zone. -- Regional relationships indicate that the Dunnage Melange occupied a basin on the southeast flank of a Lower Ordovician island arc complex. Previous workers have viewed this fact to indicate that melange formation was related to an active, west dipping, subduction zone. The present study reveals that no valid criteria exist for interpreting the melange terrane to have occupied an active trench. Rather, the tectonic position of the melange and its apparent synchroneity with other Newfoundland melanges suggest it was a massive slump during widespread Taconic events, though the direct cause of slumping remains ambiguous.