Showing papers on "Terrane published in 1983"

Provenance of North American Phanerozoic sandstones in relation to tectonic setting

Abstract: Framework modes of terrigenous sandstones reflect derivation from various types of provenance terranes that depend upon plate-tectonic setting. Triangular QFL and QmFLt compositional diagrams for plotting point counts of sandstones can be subdivided into fields that are characteristic of sandstone suites derived from the different kinds of provenance terranes controlled by plate tectonics. Three main classes of provenance are termed “continental blocks,” “magmatic arcs,” and “recycled orogens.” Sandstone suites from each include three variants, of which the subfields lie within the larger subdivisions. Average modes for sandstone suites can be classified provisionally according to tectonic setting using the subdivided QFL and QmFLt plots. To test the validity of the classification, average modes for 233 Phanerozoic sandstone suites from North America were plotted on the triangular compositional diagrams and accompanying paleotectonic maps. Paired maps and ternary diagrams were prepared for eight different time slices, for each of which the tectonic setting of each major region within the continent remained relatively unchanged. Time slices are unequal in length but are controlled by the timing of major orogenic and rifting events that affected North America during the Phanerozoic. Comparison of the sandstone compositions with inferred tectonic setting through the Phanerozoic indicates that the proposed classification scheme is generally valid and yields satisfactory results when applied on a broad scale. Its application, together with other approaches, in regions of the world where over-all trends of geologic history are less well known could lead to important conclusions about the timing and nature of major tectonic events.

Origin of the Torlesse terrane and coeval rocks, South Island, New Zealand

Abstract: The Carboniferous to Lower Cretaceous Torlesse terrane and Haast Schist derivatives constitute the major part of the complexly deformed facies of the Eastern Province of New Zealand. Strata consist mainly of quartzofeldspathic graywacke and mudstone, intercalated with minor but widely distributed conglomerate, and volcanics with associated chert and limestone. Clastic rocks were deposited largely by sediment gravity-flow mechanisms in a deep-marine environment. Also present are a few highly fossiliferous shallow-marine and terrestrial deposits of limited areal extent, which rest unconformably on or in fault contact with Torlesse flysch. Several periods of deformation are recognized and melange is present on both local and regional scales. Metamorphism ranges from zeolite to greenschist facies. The bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages: Permian ( Atomodesma ), Middle Triassic, early Late Triassic (?) ( Torlessia ), Late Triassic ( Monotis ), and Late Jurassic-Early Cretaceous. Contacts between major fossil zones are mainly tectonic. Petrographic analysis permits subdivision of Torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones. The sandstone petrofacies (mainly arkosic), together with the composition of conglomerate clasts (mainly indurated Torlesse rocks), indicate that the source terrane was a continental volcano-plutonic arc, probably part of Gondwanaland, coupled with autocannibalistic reworking of older uplifted Torlesse rocks. In contrast to the quartzofeldspathic nature of the Torlesse, coeval sedimentary rocks of the Eastern Province are volcanogenic. They are thought to represent related forearc-basin (Maitai-Murihiku terranes) and trench-complex (Caples terrane) deposits derived from a volcanic island arc (Brook Street terrane). Three petrofacies are established for Maitai-Murihiku and Caples sandstones. The petrofacies indicate a common, evolving, immature to submature volcanic island arc source for these terranes. A reconstruction of New Zealand's Eastern and Western Provinces is proposed. In Permian and Triassic times, the Torlesse was deposited in trench, slope, or borderland basins along a trench-transform margin fronting a continental volcano-plutonic arc source (Western Province-Gondwanaland). Deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of Torlesse rock younging outward from the Gondwanaland margin. At the same time, the Brook Street terrane volcanic arc and associated terranes were forming to the west of the Torlesse site, separated from Gondwanaland by a marginal sea. In latest Triassic or Early Jurassic times, the Torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the Gondwana margin. The collision event resulted in tectonic thickening of Torlesse and Caples rocks at the plate interface and metamorphism to Haast Schist. The source was then dominated by older, partly metamorphosed Torlesse terrane, newly uplifted along the collision front. Closing of the marginal sea behind the Brook Street terrane in Late Jurassic-Early Cretaceous times resulted in juxtapositioning with the Western Province (Gondwanaland) along the Median Tectonic Line.

High-frequency seismic wave propagation beneath the Indian Shield, Himalayan Arc, Tibetan Plateau and surrounding regions: high uppermost mantle velocities and efficient Sn propagation beneath Tibet

Abstract: Summary. This paper reports the regional propagation characteristics of high-frequhcy P,, P,, S, and L, waves (0.5 to 2Hz) and P,, Pg and S, velocities for several regions in Central Asia, Tibet and India. An important result is that S, waves propagate efficiently in the uppermost mantle beneath most of the Tibetan Plateau. Also, S, transmission in Tibet is not affected by crossing many physiographic and tectonic boundaries, such as the Indus-Tsangpo suture. These observations are similar to observations of S, propagation in stable continental and shield regions. Efficient S, propagation is also observed across the stable Tarim Basin, along the Tien Shan and Himalayan Mountains and across the Indian Shield. S, is strongly attenuated across the northern Tibetan Plateau (Chang Thang terrane). Such strong attenuation suggests the existence of a low-Q zone in the uppermost mantle beneath that region. This interpretation is supported by recent geological observations of widespread Quaternary to Recent basaltic volcanism in the Chang Thang terrane. L, propagates efficiently across the Tarim Basin, the Indian Shield and along the strike of the Himalayan Mountains. L, is also observed, although less pronounced, for paths crossing large portions of the complicated structures of Tien Shan, Karakoram and Pamir. L, is not observed for paths crossing the Tibetan Plateau. The southern boundary of the region of inefficient L, propagation in Tibet lies in the vicinity of the Indus-Tsangpo suture zone. The lack of L, can be attributed to attenuation within the Tibetan crust; however, other possibilities including scattering by structural discontinuities at the margins of Tibet are equally plausible. Pg and S, are the most prominent features on most short-period WWSSN seismograms at epicentral distances less than about 4". Apparent Pg velocity determined for the area of the Himalayan Mountains is 6.0 ? 0.2 km s-l. Apparent velocities of P, and S, waves that have at least 50 per cent of their paths in the uppermost mantle beneath the Tibetan Plateau are 8.42 It: 0.10 and 4.73 f 0.06 km s-', respectively. These velocities are quite similar to those determined for the Himalayan Mountains (P, = 8.45 f 0.08 km s-I, S, = 4.74 _+ 0.05 km s-') and the Indian Shield (P, = 8.40 0.08 km s-', S, = 4.69 k 0.04 km s-'). P,

Accreted Terranes in the Northern Part of the Philippine Archipelago

Abstract: The Philippine Archipelago consists of a complex array of ophiolites, continental fragments and island arc elements that can be identified as accreted terranes, similar to those increasingly recognized as components of orogenic zones. The northern Philippines, including Luzon, Mindoro and nearby smaller islands, can be divided into at least six of these tectonic elements. From west to east across central Luzon there are the Neogene accretionary prism of the West Luzon Arc, the Eocene Zambales ophiolite, the Cretaceous Angat ophiolite, and a late Cretaceous-early Paleogene volcanic arc built on an older metamorphic basement. This arc terrane is repeated by offset along the Philippine Fault. To the south, the western Luzon terranes are juxtaposed against the metamorphic basement block of Mindoro, which itself is colliding with the North Palawan microcontinental fragment. The ophiolites appear to have originated in back arc basins rather than in oceanic plateaus, and the metamorphic belts have protoliths indicating deformation along active margins, perhaps deep within accretionary prisms. The Philippine terranes are interpreted as fragments that originated within the complex Pacific-Eurasian plate boundary rather than as intra-Pacific basin units. These terranes have been assembling at least since the Oligocene by strike-slip and convergent displacements. Strike-slip displacement, either along transcurrent faults or as a component of convergence played a very important, if not dominant, role in this assembly. Most terranes identified in the northern Philippine appear to be plate fragments, with attached crust and upper mantle. Their characteristics contrast with those usually interpreted for terranes in the North American Cordillera, where near-orthogonal convergence and shallow dipping terrane sutures are favored.

Petrofacies and Provenance of Late Mesozoic Forearc Basin, Northern and Central California

Abstract: Data from the Great Valley Group (sequence) represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc. This information is useful in documenting tectonic and magmatic events within the arc and related terranes, and forms the basis for the establishment of petrostratigraphic units for mapping and correlation. Sandstone and conglomerate compositions are controlled by changes in provenance, many of which were basinwide and synchronous. Clay-mineral composition is controlled primarily by burial metamorphism. Careful attention to sample collection, sample preparation, and petrographic techniques is essential for uniform results. Seven petrographic parameters (P/F, Lv/L, M, Qp/Q, Q, F, and L--listed in decreasing importance to petrofacies discrimination) define eight petrofacies (Stony Creek, Platina, Lodoga, Grabast, Boxer, Cortina, Los Gatos and Rumsey--listed in approximate order of decreasing age). The Upper Jurassic-Lower Cretaceous petrofacies (Stony Creek, Platina, and Lodoga) contain higher lithic contents (supracrustal sources), whereas the Upper Cretaceous petrofacies (especially the Rumsey) contain higher proportions of plutoniclastic components (quartz, feldspar, and micas). The proportion of potassium-feldspar increases from near zero in the Upper Jurassic to nearly 50% of all feldspars in the uppermost Cretaceous. The lower part of the Great Valley Group (Upper Jurassic and Lower Cretaceous) contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes ("tectonic highlands") formed by the arc-arc collision (Nevadan orogeny) that occurred prior to initiation of the Franciscan-Great Valley-Sierra Nevada arc-trench system. The Klamath Mountains area provided a major proportion of this detritus. Ophiolite and serpentinite detritus was deposited locally near the base of the Great Valley Group as a result of deformation along the east side of the growing Franciscan subduction complex. Volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the Sierra Nevada area during the Cretaceous. As the volcanic cover was stripped off, plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin. Crustal components were more "continental" in the southern Sierra Nevada and more "oceanic" in the northern Sierra Nevada, as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south. By the middle of the Late Cretaceous, extensive batholithic terranes provided potassium-feldspar-rich arkosic detritus to the entire forearc basin. By the Paleogene, arc magmatism had migrated eastward sufficiently that deeper levels of the California part of the arc were exposed by erosion, tectonic activity decreased in the forearc basin, and the basin was filled to sea leve in most parts.

Recognition, Character and Analysis of Tectonostratigraphic Terranes In Western North America.

Abstract: The pacific margin of North America is composed of many allochthonous terranes that accreted to the cratonal nucleus during Mesozoic and early Cenozoic time. More than 150 terranes have now been recognized, each of which is characterized by distinctive rock assemblages and geologic histories that differ markedly from those of their neighbors. These terranes are composed of scraps of island arcs, disrupted oceanic basins with abundant radiolarian chert, parts of seamounts, continental-rise prisms, and continental fragments, some of which seem to be of North American affinity. Combined paleomagnetic, paleobiogeographic, and lithologic data substantiate that some terranes have moved northward as much as several thousand kilometers; some terranes now at high latitudes may have been situated in the Southern Hemisphere during Triassic time. Tectonic effects during accretion include polyphase isoclinal folding, large-scale thrust and strike-slip faulting, development of penetrative metamorphic fabrics on a regio...

Model for the origin of the Yakutat block, an accreting terrane in the northern Gulf of Alaska

Abstract: A composite oceanic and continental terrane, the Yakutat block, is currently colliding with and accreting to North America in the northern Gulf of Alaska. Marine geophysical data indicate that the block has moved with the Pacific plate during Pliocene and Quaternary time, but are indeterminant about its previous movement history. A model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the Kula-Farallon spreading center beneath North America. In this model, the spreading center that formed the basaltic basement of the Yakutat block was offset from a spreading center to the south by a left-lateral transform fault 400–500 km long. Subduction of the northern spreading center about 45 m.y. ago sliced off part of the North America continental margin, attached it to the Kula plate, and initiated northward movement of the composite terrane as the Yakutat block. During Eocene and Oligocene time, migration of the southern spreading center toward North America truncated the Yakutat block on the south and left a fossil fracture zone that is now seen as the Transition fault. The southern spreading center subducted about 25 m.y. ago, and the Yakutat block has since traveled with the Pacific plate to southern Alaska.

Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern British Columbia. Part I: Jurassic to mid-Cretaceous

Abstract: K–Ar dates and U–Pb zircon dates define three periods of igneous activity in the southern Kootenay Arc: (1) emplacement of late-synkinematic to post-kinematic granodioritic plutons in mid-Jurassic time (170–165 Ma) accompanying amphibolite-facies regional metamorphism; (2) emplacement of post-kinematic granitic plutons in mid-Cretaceous time (~100 Ma); and (3) emplacement of small bodies of syenite in Eocene time (~50 Ma) in the western part of the area. Micas from mid-Jurassic plutons that yield the oldest K–Ar dates (158–166 Ma) also yield plateau-shaped 40Ar/39Ar age spectra. Age spectra for biotites younger than these but older than 125 Ma reflect thermal overprinting.In southeastern British Columbia, the Kootenay Arc marks the transition from the North American rocks of the Cordilleran miogeocline to the tectonic collage of allochthonous terranes that have been accreted to it.Deformation, metamorphism, and plutonism recorded in rocks of the southern Kootenay Arc commenced in mid-Jurassic time as a co...

Confirmation of the Carolina slate belt as an exotic terrane.

Abstract: An assemblage of Middle Cambrian Atlantic faunal province trilobites has been found in the rocks of the Carolina slate belt near Batesburg, South Carolina. Geologic and paleomagnetic data suggest that the Carolina slate belt and the adjacent Charlotte belt constitute an exotic terrane that was accreted to North America in early to middle Paleozoic time.

A model for the crustal evolution of southern Scotland

Abstract: In the late Silurian, southern Scotland was a mature accretionary active margin of Sumatra-type, on the southeastern edge of North America, while England was on a passive margin on the opposite side of the Iapetus Ocean. Southward subduction below England during the Middle Ordovician produced a magmatic arc which now lies close to the Iapetus suture, and thus argues for a missing forearc terrane. Tectonic erosion or strike-slip removal may be responsible for some of this missing terrane to the north of the Lake District, but we now suggest that underthrusting below southern Scotland may also be an important cause. If the Ordovician forearc terrane of northern England is now below the Southern Uplands, we would have an explanation for the evidence for similar lower crustal foundations on either side of the surface trace of the Iapetus suture (velocity structure from the LISPB study, Sr/U-Pb isotopic signatures and geophysical attributes of the c. 400 Ma granites). The under-thrusting model still leaves room for considerable modification of both margins by large strike-slip faults. Seismic refraction profiles of modern accretionary active margins (Sumatra, SW Japan) allow us to propose a likely velocity profile for late Silurian southern Scotland. We choose a Mesozoic forearc truncated by strike-slip faulting, the Queen Charlotte Islands margin of the NE Pacific, as an analogue for the late Silurian southern Iapetus margin. Following previous suggestions of highly oblique and slow convergence during the final stages of closure of Iapetus, we combine the two profiles by under-thrusting the southern margin to produce a model which involves minimal shortening in the collision zone. The crustal profile resulting from such a collision would be very similar to the modern southern Scotland profile, both in terms of the scale of the tectono-stratigraphic terranes involved and of the resulting crustal velocity structure. The cessation of accretion after the mid-Wenlock may be related to impingement of the English continental crust against the Scottish trench. Subsequent uplift of the Southern Uplands may be connected with the appearance of coarse detritus in the Lake District during the Silurian.

Regional geothermobarometry in the granulite facies terrane of South India

Abstract: In the southern part of the Archaean craton of South India, an approximately 3.4–2.9 b.y. old migmatite–gneiss terrane (Peninsular gneiss complex) has been subjected to granulite facies metamorphism about 2.6 b.y. ago. During this event, the extensive charnockite-khondalite zone of southern India developed. A younger metamorphism (Proterozoic?) led to retrogression of the charnockites and khondalites, mainly under the conditions of the amphibolite facies.The physical conditions of metamorphism have been evaluated by applying methods of geothermobarometry to the widespread charnockitic assemblages with garnet, orthopyroxene, clinopyroxene, plagioclase, and quartz. The interpretation of the P–T estimates includes a critical discussion of potential error sources, e.g. errors of the analytical data and the calibrations of the models, and takes into account the complex metamorphic history of the rocks and the kinetics of the mineral equilibria.P-T estimates were obtained for seven subareas from the rim compositions of the coexisting minerals: Shevaroy Hills 680±55°C—7·4±1 kb; Kollaimalai area 680±40°C—8·6± 1 kb; Nilgiri Hills 680±90°C—6·6±0.8kb (upland massif) and 705±60°C—9·3±0.8 kb (northern margin); Bhavani Sagar area 650±50°C—7·2± 1 kb; Sargur-Mysore area 690±60°C—7·6 kb; Bangalore-Kunigal-Satnur area 760±50°C—6 kb. Except for the last subarea, the P-T model data reflect the conditions of a late annealing stage probably related to the retrogressive metamorphism. Conditions near the peak of granulite facies metamorphism (730–800°C—6·5–9·5 kb) are recorded by the core compositions of the minerals. Although a rather uniform cooling history of the main part of the charnockite-khondalite terrane is suggested from the temperature data, differential uplift of smaller blocks is indicated by the regional variation of the pressure data.

Shuswap terrane of British Columbia: A Mesozoic “core complex”

Abstract: The Shuswap terrane of the northwest Cordillera owes its distinctive characteristics to deformation and high-grade regional metamorphism that occurred primarily during the Mesozoic. Within the terrane, the Monashee Complex, the overlying Selkirk allochthon, and the intervening Monashee decollement record strain and metamorphism that occurred in Middle to Late Jurassic time, during and subsequent to the accretion of a western allochthonous terrane. The Selkirk allochthon moved eastward across the Monashee Complex after emplacement of the western allochthonous terrane but before late Mesozoic to early Cenozoic telescoping of the Rocky Mountain Belt. In this latter event, the Monashee Complex and overlying allochthonous slices were transported eastward relative to the North American craton on a sole fault that developed during listric thrusting of the Rocky Mountain foreland. Synmetamorphic to late-metamorphic elongation lineations and associated fabrics of the mylonitic rocks of the Monashee decollement zone were generated by shear strain during the Middle Jurassic emplacement of the Selkirk allochthon and are not related to any upper crustal extension that occurred in the Tertiary. Uplift, normal faulting, brittle reactivation of the mylonitic decollement zone, widespread resetting of K-Ar and Rb-Sr mineral dates, and arching of the terrane are events that culminated in the Eocene.

Sedimentary record of subduction to forearc transition in the rotated Eocene basin of western Oregon

Abstract: Systematic changes in compositions of Eocene sandstones in the southern part of the Oregon Coast Range constrain proposed models of tectonic accretion and rotation of this crustal fragment. Three basic petrofacies of lithic, arkosic, and volcanolithic composition can be defined sequentially from the base to the top of the section. The lithic petrofacies was deposited prior to rotation and was derived from the nearby Klamath Mountains during collision of the subjacent seamount terrane. The transition to arkosic composition coincided with the end of collision and the initiation of forearc basin subsidence. The arkosic sands contain muscovite and potassium feldspar derived from a continental-plutonic source area; their presence in the forearc sequence suggests that the Coast Range block lay farther to the east and closer to interior parts of the Mesozoic batholith belt prior to tectonic rotation. Volcanolithic sandstones were deposited after rotation of the Coast Range had begun, and they mark the beginning of volcanic activity in the adjacent Cascade arc.

Comparison of terrane accretion in modern Southeast Asia and the Mesozoic North American Cordillera

Abstract: The western part of the North American Cordillera has been divided into numerous suspect terranes. Some of these terranes have traveled hundreds to thousands of kilometres between the time of their formation and their final incorporation into the accretionary belt of the Cordillera. Most of the terranes appear to be relics of island arcs, oceanic plateaus and islands, continental margin fragments, and complex accretionary terranes, the latter including melange belts, ophiolite fragments, and thrust-faulted forearc provinces. Because the terranes have complex histories and diverse stratigraphies, it has been difficult to envisage an actualistic setting for the Mesozoic Cordillera. Here we propose an analogy with the tectonic setting of the Indo-Pacific region, from the Tonga trench on the east to eastern Indonesia on the west. This region comprises several distinct island arcs, several large oceanic plateaus, numerous accretionary terranes, melange zones, ophiolite fragments, and a variety of continental fragments, all moving toward the central collision zone. Like the Cordillera, this part of the Indo-Pacific region is undergoing oblique convergence, driven by relatively high rates of movement between the oceanic plates and the collision zone. Also analogous to the Cordillera is the formation of a foreland fold and thrust belt, developing between the collision zone and the Australian continent, and a zone of basement-rooted foreland folds, suggestive of an early Laramide style of deformation.

Paleomagnetism of the Paleocene Ghost Rocks Formation, Prince William Terrane, Alaska

Abstract: The mean paleomagnetic inclination of Paleocene volcanics from two regions on Kodiak Island in the Ghost Rocks Formation suggests that the Prince William terrane originated at mid latitudes, about 25° south of its ‘expected’ Alaskan latitude in Paleocene time. The remanent magnetization of these rocks passes both the fold and reversal tests and is well constrained in age. The mean declinations of the two regions, however, differ by approximately 120°, suggesting they have rotated with respect to each other, perhaps during emplacement. We suggest that, despite the lack of evidence for a major Tertiary suture zone between the Prince William terrane and central Alaska, the Prince William and perhaps adjacent terranes may have lain substantially south of their present position in the Early Tertiary.

Deep crustal carbonates as CO 2 fluid sources: Evidence from metasomatic reaction zones

Abstract: Metasomatic reaction zones which developed at marble-pelitic schist contacts in a granulite facies terrane in West Greenland contain a consistent sequence of five mineralogical zones. Outward from the carbonates the zones are characterized by the assemblages grossular-diopside-meionite (I), meionite-anorthite-diopside (II), anorthite-diopside-edenitic hornblende (III), anorthite-enstatite (IV), plagioclase-almandine-sillimanite (V). Sphene is superceded by ilmenite between zones (II) and (III); quartz is present in all zones except zone I. Scapolite, plagioclase, clinopyroxene and mica exhibit a small degree of compositional variation which correlates with distance from the carbonate. These small compositional variations are superimposed on a strong CaO chemical potential gradient. Compositional features, zone distributions and CaO activity calculations demonstrate that the zones developed in response to CaO diffusion along a chemical potential gradient of 2 kcal/m. The CaO source appears to be carbonate rocks which release calcium as decarbonation reactions proceed. The maximum volume of CO2 released in this process, and that released during discontinuous reactions in the marbles, will contribute a total volume of CO2 approximately equivalent to the volume of carbonate in the rock. Calculations demonstrate that a terrane consisting of as little as 8% carbonate will release sufficient CO2 to result in complete dehydration of an amphibolite terrane, at deep crustal conditions. Dehydration through CO2 release will be accomplished either through rapid burial, which would prevent both equilibration of mineral assemblages and CO2 release at intermediate crustal levels, or through diffusion-driven metasomatic reactions which would lead to CO2 release primarily at the high temperatures of deep crustal environments. The latter process would be the dominant CO2 source at deep crustal levels if carbonate rocks occur predominately as relatively thin layers.

Correlation of metamorphosed Paleozoic strata of the southeastern Mojave Desert region, California and Arizona

Abstract: Isolated outcrops of deformed, regionally metamorphosed Paleozoic strata are scattered within the southeastern Mojave Desert region of California and western Arizona. These strata unconformably overlie a basement of Proterozoic crystalline rocks and are overlain in turn by metamorphosed Mesozoic sedimentary rocks. The strata can be correlated lithostratigraphically with the classic cratonal Paleozoic section of the western Grand Canyon, Arizona, and with nonmetamorphosed Paleozoic sections transitional between cratonal and miogeoclinal in the Ship, Marble, and Providence Mountains, California. The strata evidently were once continuous with Paleozoic epicontinental strata exposed throughout the southern Great Basin and Colorado Plateau. Outcrops of Paleozoic strata and of the underlying Proterozoic basement in the southeastern Mojave Desert region define a terrane that has been disrupted by Mesozoic thrust faults and by Tertiary detachment faults but that nevertheless retain a gross paleogeographic coherence. This coherent terrane extends at least as far west and southwest as the Big Maria, Palen, and Calumet Mountains, and possibly beyond to include Paleozoic exposures in the San Bernardino Mountains and near Victorville. Poorly understood tectonic boundaries separate the area of paleogeographic coherence from known or suspected allochthonous terranes in the western Mojave Desert and the eastern Transverse Ranges.

Evolution of a Forearc Basin, Luzon Central Valley, Philippines

Abstract: The Cenozoic history of the 14 km-thick Luzon Central Valley sequence illustrates the development of a forearc basin. Forearc basins are important both as major sediment traps and as sites of hydrocarbon accumulations. The Luzon basin is floored by oceanic crust on the seaward (western) side and older accreted terranes on the arc (eastern) side. Initial sedimentation on this oceanic crust occurred during early Tertiary northward translation and emplacement of the crust as an ophiolite along a strike-slip or oblique-slip zone. The basal sediments consist of pelagic limestones and thin ash layers overlain by sandy turbidites derived from uplift and progressive dissection of the ophiolite. A sequence of arc-derived sediments at least 26,000 ft (8 km) thick was shed into the astern (arc) side of the basin during late Paleogene to Quaternary convergence along the western margin of Luzon. By the middle Miocene, the Central Valley became a continuous, elongate basin fringed by extensive shelf deposits along both the uplifted seaward and arc sides of the basin. Detritus shed from both flanks filled the subsiding basin and resulted in progressively shallower depths. Nonmarine deposition began in central portions of the basin in the Pliocene and migrated with time both north and south along the basin axis. Late Miocene to Holocene movement along the Philippine fault zone caused uplift and folding of adjacent parts of the basin. Exploration models for the Central Valley predict gas-prone hydrocarbon generation in central portions of the basin at times that coincide with and postdate the formation of both structural and stratigraphic traps. Previous drilling in the basin has either been in areas with thermally immature source rocks or has failed to reach prospective intervals where thermal maturation is inferred. The hydrocarbon potential of the Central Valley has not been determined adequately.

Cretaceous Sinistral Strike Slip Along Nacimiento Fault in Coastal California

Abstract: The San Andreas and Nacimiento faults of coastal California both separate granitic and metamorphic basement rocks of the Salinian block from partly coeval but contrasting Mesozoic terranes underlain by the Franciscan subduction complex. By analogy with Neogene dextral strike slip along the San Andreas fault, Cretaceous sinistral strike slip can be inferred along the Nacimiento fault in preference to hypotheses for tectonic erosion during subduction or for dextral strike slip of unspecified amount. Following restoration of known San Andreas and inferred proto-San Andreas dextral displacements, reversal of about 560 km (350 mi) of postulated sinistral slip on the Nacimiento fault brings four major Mesozoic lithotectonic belts of California and Baja California into simple al gnment as subparallel terranes related to Mesozoic subduction along the continental margin. Neogene deformation within the San Andreas transform system involved (a) elongation of the Salinian block by dextral slip along subsidiary faults that branch from the San Andreas fault, and (b) dextral rotation of crustal panels within the Transverse Ranges. Latest Cretaceous and/or earliest Paleogene dextral slip along a proto-San Andreas fault followed the San Andreas course in central California, but diverged westward in southern California. Nacimiento sinistral displacements occurred in mid-Cretaceous to early or medial Late Cretaceous time, after Cretaceous emplacement of plutons now within the Salinian block but prior to deposition of uppermost Cretaceous sedimentary sequences in central California. Available data on Mesozoic relative and absolute plate motions in the Mesoameric n region support the likelihood of Cretaceous sinistral strike slip subparallel to the California continental margin. Paleotectonic reconstruction of crustal blocks in California and Baja California to their inferred mid-Cretaceous relative positions shows the Salinian block inserted on a bias between the flanking Mojave and Peninsular Ranges blocks. Salinian granitic rocks thus formed an interior part of the Mesozoic batholith belt, and their initial strontium isotopic ratios are compatible with the gradients displayed by values from the adjoining blocks. The similar Mesozoic terranes that now lie east and west of the Salinian block were then adjacent to one another west of the Sierra Nevada block. Available paleomagnetic data neither support nor preclude the reconstruction, but additional work together with future detailed lithotectonic comparisons potentially can confirm or refute the hypothesis i represents. A correct interpretation of the Nacimiento fault is important for understanding the overall tectonic framework of petroliferous basins both onshore and offshore in coastal California.

The ophiolitic North Fork terrane in the Salmon River region, central Klamath Mountains, California

Abstract: The North Fork terrane is an assemblage of ophiolitic and other oceanic volcanic and sedimentary rocks that has been internally imbricated and folded. The ophiolitic rocks form a north-trending belt through the central part of the region and consist of a disrupted sequence of homogeneous gabbro, diabase, massive to pillowed basalt, and interleaved tectonitic harzburgite. U-Pb zircon age data on a plagiogranite pod from the gabbroic unit indicate that at least this part of the igneous sequence is late Paleozoic in age. The ophiolitic belt is flanked on either side by mafic volcanic and volcaniclastic rocks, limestone, bedded chert, and argillite. Most of the chert is Triassic, including much of Late Triassic age, but chert with uncertain stratigraphic relations at one locality is Permian. The strata flanking the east side of the ophiolitic belt face eastward, and depositional contacts between units are for the most part preserved. The strata on the west side of the ophiolitic belt are more highly disrupted than those on the east side, contain chert-argillite melange, and have unproven stratigraphic relation to either the ophiolitic rocks or the eastern strata. Rocks of the North Fork terrane do not show widespread evidence of penetrative deformation at elevated temperatures, except an early tectonitic fabric in the harzburgite. Slip-fiber foliation in serpentinite, phacoidal foliation in chert and mafic rocks, scaly foliation in argillite, and mesoscopic folds in bedded chert are consistent with an interpretation of large-scale anti-formal folding of the terrane about a north-south hinge found along the ophiolitic belt, but other structural interpretations are tenable. The age of folding of North Fork rocks is constrained by the involvement of Triassic and younger cherts and crosscutting Late Jurassic plutons. Deformation in the North Fork terrane must have spanned a short period of time because the terrane is bounded structurally above and below by Middle or Late Jurassic thrust faults. The North Fork terrane appears to contain no arc volcanic rocks or arc-derived detritus, suggesting that it neither constituted the base for an arc nor was in a basinal setting adjacent to an arc sediment source. Details of the progressive accretion and evolutionary relationship of the North Fork to other terranes of the Klamath Mountains are not yet clear.

Pre-Cretaceous crystalline rocks of the western Chugach Mountains, Alaska: Nature of the basement of the Jurassic Peninsular terrane

Abstract: A narrow belt of Mesozoic and(or) Paleozoic crystalline rocks exposed immediately north of the Border Ranges fault of southern Alaska is the southern (trailing) edge of the Peninsular terrane, an exotic micro-continental block that collided with North America during the Cretaceous. The tectonic significance of this crystalline belt has been debated because previous K-Ar geochronology suggested that the belt contains both Early Jurassic high P/T metamorphic rocks—an apparent subduction complex—and Early Jurassic intermediate plutonic rocks—an apparent magmatic arc. Three major observations from detailed mapping in the western Chugach Mountains pertain directly to this apparent problem of mixed rocks association. First, despite major postmetamorphic faulting along the Border Ranges fault, all of the metamorphic rocks have been subjected to a regional, apparently low P/T metamorphism at the greenschist-amphibolite facies transition. The metamorphism is imprinted on rocks ranging from nearly undeformed ultramafic rocks to highly deformed amphibolites, yet regardless of structural histories of fault-bounded slices, the culmination of metamorphism appears to postdate deformation. Second, the metamorphic rocks are locally an argillite matrix melange in which main-phase structural fabrics appear to have formed at relatively low grade but are overprinted by growth of postkinematic, transitional greenschist-amphibolite facies minerals. Third, despite ambiguities in correlation across faults and equivocal absolute age relationships—presumably due to Cretaceous resetting of K-Ar ages—the transitional greenschist-amphibolite facies rocks are intruded by plutons that yield Early Jurassic K-Ar ages. Thus, the metamorphic protolith is presumably pre-Jurassic. In addition, the close association of low P/T metamorphic rocks with a large plutonic complex implies that the regional metamorphism is probably Early Jurassic in age and is associated with extensive plutonism. New age data from the western Chugach Mountains imply that a belt of Early Jurassic plutonism (and associated low P/T metamorphism) probably extends throughout the southern edge of the Peninsular terrane. The entire terrane probably represents a long-lived magmatic arc, because this Early Jurassic magmatic arc is separated from a parallel belt of Early to Middle Jurassic plutons by an Early Jurassic andesitic-volcanic pile. In the western Chugach Mountains, emplacement of alpine ultramafic bodies and development of chaotic rock bodies is premetamorphic, presumably pre-Jurassic in age. These observations of structural style, together with lithologic associations and regional occurrences of blueschist-facies rocks, suggest that a pre-Jurassic subduction complex may have formed parts of the basement for the Peninsular terrane arc. As a corollary of this conclusion, the blueschists of the southern Peninsular terrane are probably pre-Jurassic in age and represent scraps of the pre-Jurassic basement, not parts of an Early Jurassic high-pressure metamorphic belt.

Seven Devils terrane: Is it really a piece of Wrangellia?

Abstract: The Seven Devils terrane of eastern Oregon and western Idaho has been correlated with the Wrangellia terrane, which is best exposed on Vancouver Island, British Columbia, and in the Wrangell Mountains, Alaska. However, geologic histories of these two terranes are different. The Triassic section of the Seven Devils terrane represents a volcanic arc of probable calc-alkalic affinity. It is characterized by abundant volcaniclastic lithologies, a complete suite of flow rocks, and fractionated rare-earth-element distribution patterns. The sequence of similar age in Wrangellia includes thick accumulations of tholeiitic pillow basalts, subaerial basalt flows, tuffs, and breccias. The tectonic environment in which these rocks were extruded is not known, but they are not an island-arc assemblage. Paleomagnetic and faunal evidence suggests that the Seven Devils terrane and Wrangellia could have formed close to one another. This evidence does not override the geologic arguments against correlation of these terranes. Lithologic, geochemical, and stratigraphic data all militate against consanguinity of the Seven Devils arc and Wrangellia.

Intracrustal complexity in the United States midcontinent: Preliminary results from COCORP surveys in northeastern Kansas

Abstract: Unusually clear indications of complex structure in the mid-to-lower crust is revealed by seismic reflection surveys in northeastern Kansas. This complexity contrasts markedly with the layer-cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central United States. Seismic sections collected by COCORP (Consortium for Continental Reflection Profiling) as part of a major east-west traverse across the Neniaha Ridge and Midcontinent Geophysical Anomaly indicate that below a thin, relatively flat layered Paleozoic sedimentary section, the deep crust is characterized by numerous dipping and arcuate reflections and diffractions. In many places layered and crosscutting, these reflections suggest convoluted three-dimensional folded, faulted, and intruded structures. Specific identification of these deep features may be possible if future surveys can trace them to accessible depths. The basement above these reflection complexes contains significantly fewer reflections—consistent with, but not necessarily diagnostic of, the granitic terrane that dominates basement drill-hole samples in the region. Among the events at these shallower basement depths are several east-dipping reflections, some of which may be major faults. Travel times corresponding to expected Moho depths (about 36 km) are characterized less by specific reflections than by an apparent decrease in the density and number of reflections. While evidence of crustal heterogeneity is common among deep reflection studies, the Kansas seismic results outlined in this brief report stand out as being unusually clear representations of such.

Paleomagnetism of the Lower Devonian Traveler Felsite and the Acadian orogeny in the New England Appalachians

Abstract: A suite of felsic extrusive rocks from the Lower Devonian Traveler Felsite, north-central Maine (46.1°N lat., 68.9°W long.) was collected for paleomagnetic study: 28 samples from 5 sites in the (lower) Pogy member, 56 samples from 10 sites in the (upper) Black Cat member, and 13 samples from 10 cobbles in the basal conglomerate of the overlying Trout Valley Formation. Characteristic magnetizations based on AF demagnetization analyses and supported by thermal studies give a formation mean direction, after simple correction for bedding tilt, of D = 25.2°, I = − 20.2°, α 95 = 10.6° for 13 sites in the Traveler Felsite. The occurrence of normal and reversed magnetozones under stratigraphic control and the antiparallel directions of the polarity units following the tilt correction indicate an acquisition of remanence near the time of formation of the rock units. The mean direction corresponds to a (south) paleopole position of 29°S lat., 262°E long. (δ p , δm = 6°, 11°) for the Traveler Felsite. This pole position is in close agreement with poles from two other rock units of similar age from the same lithostratigraphic belt in the northern Appalachians. The mean of these three poles is distinct from Late Silurian and Middle Devonian poles from cratonic North America (Laurentia) and from Early Devonian results from the Acadia displaced terrane, suggesting that this area constituted a separate tectonic unit during the Early Devonian. We propose a tectonic model in which a “Traveler terrane” is rotated 20° to 30° while converging with Laurentia and undergoing deformation during the Early to Middle Devonian Acadian orogeny.

Wallrocks of the central Sierra Nevada batholith, California; a collage of accreted tectono-stratigraphic terranes

Abstract: ...................................