Complex Belt Research Articles

Site preservation along an active meandering and avulsing river: The Red River, Arkansas

The low-gradient Red River is a rapidly migrating, sinuous stream with easily erodible banks. Avulsion is common at many scales, from individual meander bends that are cut off to major sections of the river that form multiple, complex meander belts. The present meander belt can be subdivided into mappable landforms—termed phases—that are associated with river courses of different ages and thus associated with archeological sites of different ages. Within the study area two phases are present. The younger Modern meander belt phase has formed within the past 0.2–0.3 ky, precluding preservation of prehistoric archaeological sites. Any protohistoric artifacts that may have been preserved in this meander belt phase would be deeply buried because as much as 2 m of the vertical accretion sediment has accumulated between artificial levees in <0.1 ky and 1–2 m of sediment has accumulated beyond the artificial levees in <0.2 ky. Archeological site preservation in this highly mobile fluvial end member can be used as a predictor for other, similar streams. A large prehistoric site is preserved on an older (0.5–1 kya) Late Prehistoric meander belt phase associated with an abandoned river course. In the study area a Fourche Maline 7 period (A.D. 800–900) through Caddo IV period (ca. A.D. 1500–1700) archeological site (3MI3/30) is preserved on this slightly higher altitude portion of the flood plain. At locations proximal to the river, the site may be buried by overbank sediment 0.4 m thick, but at more distant locations the site is at the surface or only buried by thin overbank sediment because of low sedimentation rates (0.04 cm yr−1) over the span of a millennium. Sites, such as 3MI3/30, that are occupied contemporaneous with overbank sedimentation may be stratified; however, localized erosion and removal of some archeological material may occur where channelized flow crosses the natural levee. © 1998 John Wiley & Sons, Inc.

Coordination Chemistry of Homoscorpionate Ligands with 3-Cyclopropyl Substituents

In a quest for precisely sterically tuned ligand systems, the new scorpionate ligands hydrotris(3-cyclopropylpyrazol-1-yl)borate, TpCpr, and tetrakis(3-cyclopropylpyrazol-1-yl)borate, pz*TpCpr, have been synthesized. They readily form octahedral homoleptic and heteroleptic complexes with first-row transition metal compounds, but their [M(TpCpr)(X)] complexes are unstable with respect to the octahedral ones. The complex [Mo(TpCpr)(CO)2(η3-CH2CMeCH2)] was also prepared. Structures of the paramagnetic [Co(TpCpr)2], [Co(pz*TpCpr)2], and [Co(TpCpr)(TpPh)] complexes have been determined by 1H NMR and those of [Fe(TpCpr)2] and [Fe(pz*TpCpr)2], as well as that of [Co(TpCpr)(TpPh)], by X-ray crystallography, indicating facile accommodation of six cyclopropyl groups in the equatorial belt of octahedral complexes. In its coordination behavior, TpCpr is quite different from TpiPr and resembles TpMe.

Lysophosphatidic Acid Increases Tight Junction Permeability in Cultured Brain Endothelial Cells

Brain capillary endothelial cells are coupled by a continuous belt of complex high-electrical-resistance tight junctions that are largely responsible for the blood-brain barrier. We have investigated mechanisms regulating tight junction permeability in brain endothelial cells cultured to maintain high-resistance junctions. The phospholipid lysophosphatidic acid (LPA) was found to cause a rapid, reversible, and dose-dependent decrease in transcellular electrical resistance in brain endothelial cells. LPA also increased the paracellular flux of sucrose, which, together with the resistance decrease, indicated increased tight junction permeability. Activation of protein kinase C attenuated the effect of LPA, suggesting that it was mediated by activation of a signalling pathway. LPA did not cause any obvious relocalization of adherens junction- or tight junction-associated proteins. However, it did stimulate the formation of stress fibres, the recruitment of focal adhesion components, and the appearance of tyrosine phosphorylated protein at focal contacts. Our study shows that LPA is a modulator of tight junction permeability in brain endothelial cells in culture and raises the possibility that it triggers blood-brain barrier permeability changes under (patho)physiological conditions.

Structural divergence and transpression in the Teslin tectonic zone, southern Yukon Territory

The structural and tectonic evolution of the Teslin tectonic zone, a complex belt of ductilely deformed rocks forming the southern extension of the Yukon‐Tanana terrane in the northern Canadian Cordillera, is elucidated with new field data and by the reinterpretation of existing data. The zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the Nisutlin and Anvil assemblages characterized by S and L‐S tectonite fabrics. Primary contact relationships and ages show that most rocks in the zone were contiguous by Mississippian time. Mapping identified a number of strain domains which preserve S fabrics dipping northeasterly or southwesterly, variable mineral lineation orientations, and a variety of shear directions including easterly and westerly vergent thrust shear, down‐to‐the‐east and down‐to‐the‐west normal shear, and dextral strike‐slip shear. Regional constraints and microstructures suggest that latest ductile deformation was Late Triassic to Early Jurassic in age. Structural characteristics are explained most effectively by convergent‐dominated transpression in combination with tectonic wedging and associated back thrusting above a low‐angle west dipping detachment. Oblique eastward and upward movement of the rocks over the detachment, which may have coincided with the top of thinned North American continental crust, produced easterly vergent shear, and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear. The transpressive deformation postdates formation of the Nisutlin assemblage by Mississippian time and subduction of the Slide Mountain Ocean beneath the Nisutlin assemblage in Permian to Early Triassic time. The western margin of the Teslin tectonic zone was truncated by Cretaceous strike‐slip faults and translated northward. Rocks of the peraluminous orthogneiss assemblage of the Yukon‐Tanana terrane in central Yukon acted as the tectonic wedge and were overthrust by rocks of the Nisutlin and Anvil assemblages in Early Jurassic time before unroofing by local extension in Cretaceous time.

Crustal and upper mantle velocity structure of the Salton Trough, southeast California

This paper presents data and modelling results from a crustal and upper mantle wide‐angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid‐ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide‐angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (∼21–22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (∼27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (∼24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (∼5–6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (∼2–3 km) basin beneath the Colorado River. A regional 6.9‐km/s layer (between ∼15‐km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low‐velocity (7.6–7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part predates Salton Trough rifting. It may also in part result from migration of magmatic spreading centers associated with the southern San Andreas fault system. These spreading centers may have existed east of their current locations in the past and may have influenced the lower crust and upper mantle to the east of the current Salton Trough.

The Evolution of the Piedemonte Llanero Petroleum System, Cordillera Oriental, Colombia: (1) Structural Evolution and Play Definition: ABSTRACT

The Piedemonte Llanero petroleum trend of the Cordillera Oriental in Colombia has proven to be one of the most prolific hydrocarbon provinces discovered in recent years. The Piedemonte Llanero is a fold and thrust belt of complex, multi-phase structuration and hydrocarbon generation. Following the discovery of the Cusiana and Cupiagua fields in the southern part of the trend, BP and its partners began exploration further to the northeast. Early seismic data showed the existence of two structural trends: the frontal (or basal) thrust trend, with structures similar to Cusiana; and the overthrust (or duplex) trend, with multiple imbricated structures. Improved quality seismic data defined the gross structures and allowed them to be successfully drilled, but did not give a constrained model for the kinematic evolution of the fold and thrust belt nor the petroleum play. This resulted in no clear predictive models for reservoir quality and hydrocarbon phase distribution in the undrilled parts of the trend. A wide variety of geological and geochemical analytical techniques including biostratigraphy, reservoir petrology, petroleum geochemistry, thermal maturity data, basin modelling and fluid inclusion studies were undertaken. These were iteratively integrated into the seismo-structural model to develop a constrained interpretation for the evolution of themore » Piedemonte Llanero petroleum system. This paper summarizes the current understanding of the structural evolution of the trend and the development of a major petroleum system. A companion paper details the reservoir petrography and petroleum geochemistry studies.« less

Late Archean regional deformation and structural controls on gold-quartz vein mineralization in the northwestern Slave Province, N.W.T., Canada

Late Archean crustal accretion in the northwestern Slave Province is suggested to have involved approximately west-northwest–east-southeast directed horizontal compression that produced three episodes of deformation recognizable in the northwestern Anialik River igneous complex (ARIC) and Anialik River greenstone belt (ARGB). Observations show that the ARIC was probably emplaced as a series of synvolcanic sills prior to the earliest deformational event. Regional shortening produced a pervasive foliation, downdip lineations and folding in the ARGB, and an early, subsequently folded, foliation in the northwestern ARIC. Postfold ductile and brittle–ductile deformation produced regional- and outcrop-scale shear zones including the Sheeted Zone, which defines the ARIC–ARGB contact. Younger rocks of the northwestern ARGB appear to have been tectonically juxtaposed along the Sheeted Zone against the older rocks of the northwestern ARIC. Greater brittle response, enhanced permeability, and cyclical increases in fluid pressure led to the development and concentration of an anastamosing network of gold-quartz vein bearing shear zones in the ARIC. Steep to subvertical shear-related linear fabrics show that regional-scale and mineralized shear zones have a large component of vertical shear with predominantly east-side-up movement. The age relationships, proximity, and similarity of deformational structures in the Kangguyak gneiss belt, containing a craton-scale ductile deformation zone, and shear zones within the Arcadia Bay area, suggest contemporaneous development and regional late Archean structural relationships similar to those of shear zone hosted gold-quartz vein mineralization seen in the Abitibi Subprovince (Canada) and Yilgarn Craton (Australia).

Tertiary metamorphic core complexes in Sonora, northwestern Mexico

Several ranges encompassing more than 35,000 km2 of Sonora, Mexico, contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes. Penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed. Prominent northwest trending fractures, dikes, and normal faults are orthogonal to the lineation. Most kinematic indicators in lower plate mylonitic rocks record top‐to‐the‐southwest sense of shear. Upper plate stratigraphic sequences include Mesozoic supracrustal rocks, Tertiary volcanic and sedimentary rocks, and allochthonous Precambrian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously published U‐Pb and K‐Ar ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late Oligocene‐early Miocene. Partitioning of extensional strain in Sonora was influenced by pre‐Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continental crust separated by the N60°W striking Mojave‐Sonora megashear. Portions of the southern Papago block (northeast of the megashear) consisting of Jurassic magmatic arc rocks and Upper Jurassic‐Cretaceous siliciclastic and carbonate strata resting upon a concealed, tectonically fragmented Precambrian basement were especially susceptible to crustal attenuation. Some core complexes of the southern Papago block occur within zones trending northwest that may coincide with Late Jurassic lineaments. In the Caborca block (southwest of the megashear), core complex‐related rocks and structures have not been identified where surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic‐Paleozoic platform strata are common. However, extensional mylonitic fabrics are locally developed along the margins of a Tertiary two‐mica granite batholith. Core complexes on both sides of the megashear appear to be preferentially developed where Tertiary granites have intruded regions of crust with basement disrupted by pre‐Tertiary structures. Sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the United States and Canadian Cordillera. The timing of mid crustal extension in Sonora (25–18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension occurred later in these areas than in the Pacific Northwest‐British Columbia region but earlier than in the Mojave Desert‐Death Valley region. Middle Tertiary mylonitic fabrics of similar style and orientation have not been recognized farther south in Mexico. The southern terminus of the mid‐Tertiary Cordilleran core complex belt appears to be in Sonora.

Zircon and rutile U-Pb geochronology of the Niquelândia layered mafic and ultramafic intrusion, Brazil: constraints for the timing of magmatism and high grade metamorphism

Abstract The Niquelândia intrusion is among the largest layered intrusions in the world. It is part of a 350 km long belt of large mafic-ultramafic complexes in central Brazil. Both intrusion and country rocks were subjected to highgrade metamorphism ranging from amphibolite to granulite facies, and associated ductile deformation. The first U-Pb zircon and rutile ages for this region are presented. Zircon fractions from a meta-anorthosite from the upper gabbro-anorthositic unit are variably discordant (0–28% with regard to the lower intercept) along a mixing line from 777±10 Ma lower intercept and 1583±25 Ma upper intercept. Zircon fractions from a later quartz diorite within the Central Mafic Unit are variably discordant (0–14% with regard to the lower intercept) along a mixing line from 794−6+4 lower intercept and 1565±22 upper intercept. The upper intercepts for both rocks, interpreted to be the magmatic age, overlap and suggest a Mesoproterozoic age for the intrusion. The lower intercept is interpreted to be the high-grade metamorphic event. Zircons from two quartz-rich blastomylonites representing different metamorphic domains and cutting through the layered intrusions were analyzed. A single zircon from the amphibolite facies domain blastomylonite gives a concordant (−0.3%) 207 Pb 206 Pb age of 777±3 Ma, and a single zircon from the granulite facies blastomylonite gives a 207 Pb 206 Pb age of 786±5 Ma (−0.8% discordance). Both results are consistent with the lower intercept ages and set the 770–795 Ma interval for the high-grade metamorphic event. Rutile ages (∼650 Ma) are considerably younger than zircon metamorphic ages and suggest a milder regional tectonomagmatic episode or a complex and long-lasting cooling history for the region. The U-Pb data provide evidence for an important Mesoproterozoic extensional tectonic event in central Brazil. It also sets the 770–795 Ma interval for the high-grade amphibolite to granulite facies prograde metamorphism, interpreted to represent the timing of a continental collision event.

Insights into the kinematic Cenozoic evolution of the Basin and Range-Colorado Plateau transition from coincident seismic refraction and reflection data

Research Article| June 01, 1994 Insights into the kinematic Cenozoic evolution of the Basin and Range-Colorado Plateau transition from coincident seismic refraction and reflection data JILL MCCARTHY; JILL MCCARTHY 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar TOM PARSONS TOM PARSONS 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information JILL MCCARTHY 1U.S. Geological Survey, Menlo Park, California 94025 TOM PARSONS 1U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (6): 747–759. https://doi.org/10.1130/0016-7606(1994)106<0747:IITKCE>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JILL MCCARTHY, TOM PARSONS; Insights into the kinematic Cenozoic evolution of the Basin and Range-Colorado Plateau transition from coincident seismic refraction and reflection data. GSA Bulletin 1994;; 106 (6): 747–759. doi: https://doi.org/10.1130/0016-7606(1994)106<0747:IITKCE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Estimates of surface extension in the southern Basin and Range province and transition into the Colorado Plateau range from a few percent to several hundred percent locally, yet the crustal thickness varies perhaps only 10-15 km across these provinces. Within the southern Basin and Range and the metamorphic core complex belt, extremely extended crust is directly juxtaposed against equally thick (or thinner) crust that underwent far milder extension. Unless preextension crustal thickness varied dramatically over a short distance, the crust must have maintained its thickness during extension, through mechanisms that involve crustal flow and magmatism. We employ a 300-km-long profile of seismic refraction and coincident vertical-incidence reflection data to investigate the geophysical signature of these processes from the extended southern Basin and Range province to the unextended Colorado Plateau. By integrating the seismic velocity with the pattern of reflectivity along the profile, we estimate the amounts of Tertiary magmatism and flow that have occurred. We estimate an upper bound of 8 km of mafic material intruded beneath the metamorphic core complex belt and 4 and 5 km of intruded material beneath the Transition Zone and southern Basin and Range province, respectively. We emphasize that this 8-km estimate is strictly an upper bound, and that the actual amount of magmatism was probably less (3 to 4 km). We further speculate that several kilo-meters of silicic rock was added to the metamorphic core complex belt via ductile flow. As suggested by numerous numerical models of crustal extension, we conclude that a mobile, felsic midcrustal layer accommodated most of this crustal flow. This ductile midcrustal layer appears to be thickest beneath the most extended terranes and thinnest beneath the less extended Transition Zone and Colorado Plateau. In contrast, the lowermost crust appears to have thinned passively in an amount that corresponds more directly to the regional surface extension. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Massive sulfide metallogenesis at a late Mesozoic sediment-covered spreading axis: Evidence from the Franciscan complex and contemporary analogues

The Island Mountain deposit, an anomalous massive sulfide in the Central belt of the Franciscan subduction complex, northern California Coast Ranges, formed during hydrothermal activity in a sediment-dominated paleo-sea-floor environment. Although the base of the massive sulfide is juxtaposed against a 500-m-wide melange band, its gradational upper contact within a coherent sequence of sandstone, siltstone, and mudstone indicates that hydrothermal activity was concurrent with turbidite deposition. Accumulations of sulfide breccia and clastic sulfide were produced by mass wasting of the sulfide mound prior to burial by turbidites. The bulk composition of sulfide samples (pyrrhotite rich; high Cu, As, and Au contents; radiogenic Pb isotope ratios) is consistent with a hydrothermal system dominated by fluid-sediment interaction. On the basis of a comparison with possible contemporary tectonic analogues at the southern Gorda Ridge and the Chile margin triple junction, we propose that massive sulfide mineralization in the Central belt of the Franciscan complex resulted from hydrothermal activity at a late Mesozoic sediment-covered ridge axis prior to collision with the North American plate.

Jurassic accretion of Nisling terrane along the western margin of Stikinia, Coast Mountains, northwestern British Columbia

A metamorphosed assemblage of continental margin-type strata, termed the Nisling terrane, exists as a narrow terrane sliver in the northern Coast Mountains of British Columbia. It could be a fragment rifted from North America and later accreted, or, alternatively, it could be allochthonous and far traveled. The Nisling terrane is tectonically bounded by younger, oceanic, arc-dominated terranes on both sides—the Alexander terrane to the west and meta-morphosed Paleozoic rocks of Stikinia to the east. The Nisling terrane was accreted to Stikinia along a sinistral transpressive ductile shear zone between 185 and 170 Ma, during or before the accretion of Stikinia to the ancestral margin of North America. Along this tectonic boundary, intense deformation was accompanied by lower amphibolite-facies metamorphism. Previous inferences by other workers that the Nisling-Stikinia accretion occurred in the Triassic are incorrect, as is the supposition that the Nisling terrane formed the depositional basement to Stikinia. Although the Nisling terrane exists for ∼1000 km along strike on the west side of Stikinia, its connection to either North America or another continent remains uncertain and tectonically puzzling. The middle Cretaceous collision of the Nisling-Stikinia terrane composite with the Alexander terrane overprinted the Jurassic deformational features and produced a complex belt of tectonic slivers involving all of these terrane fragments.

The Pannanich Hill Complex and the origin of Crinan Subgroup migmatites in the North-eastern and Central Highlands

Synopsis Semipelitic rocks of the Crinan Subgroup Dalradian are, under certain conditions, susceptible to in situ reconstitution to form coarse-grained rock, originally mapped as ‘older granite’. The conditions necessary involve extensive shearing allowing fluids to percolate the rock, and increased heat flow from associated basic magmas. A belt of complexes formed in this way can be traced across the N E and Central Highlands of Scotland, and includes the Pannanich Hill Complex. Though the reconstitution of the complexes was probably diachronous along the belt, they all formed during the later part of the Caledonian Orogeny. During the reconstitution, some silicate melt may have formed and migrated along the shear zones, possibly to give the truly intrusive ‘older granites’.

An axial view of a metamorphic core complex: Crustal structure of the Whipple and Chemehuevi Mountains, southeastern California

A 135‐km‐long, NW‐SE trending, seismic refraction/wide‐angle reflection profile provides a unique along‐strike view of the crustal structure of a belt of metamorphic core complexes in southeastern California: the Whipple, Chemehuevi, and Sacramento mountains metamorphic core complexes. Interpretation of the seismic data was done by two‐dimensional forward modeling of travel times and amplitudes. The final model consists of (1) a thin (&lt; 1.5 km) veneer of upper plate and fractured lower plate rocks (velocities of 1.5–5.3 km s−1) overlying a fairly homogeneous basement with velocities of 6.0 km s−1; (2) a localized, high‐velocity (6.4 km s−1) body, situated directly beneath the Whipple Mountains; (3) a 6.3–6.4 km s−1 middle crust that is thickest beneath the core complexes; (4) a 6.65±0.15 km s−1 lower crust; (5) crustal thickness of 27 km with a deeper crustal root (3 km) beneath the Whipple Mountains metamorphic core complex; and (6) a Pn velocity of 8.0±0.10 km s−1. The crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust. Upper crustal velocities are higher beneath the Whipple Mountains (where velocities increase to 6.4 km s−1 at ∼5 km depth) than beneath the Chemehuevi and Sacramento mountains. In addition, midcrustal discontinuities rise 2–5 km beneath the Whipple complex compared to the other complexes. These observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the Whipple Mountains compared to rocks in the Chemehuevi and Sacramento mountains. Despite the enhanced uplift and extension in the Whipple Mountains, the crust is thicker here (30 km) than anywhere else along the Colorado River extensional corridor. This may be in part a relic of compressional and magmatic thickening during the Mesozoic. However, we suggest that inflation of the crust during Tertiary extension was the dominant mechanism. Both mantle‐derived magmatism and lateral ductile inflow in the crust are proposed.

Anatomy of a metamorphic core complex: Seismic refraction/wide‐angle reflection profiling in southeastern California and western Arizona

The metamorphic core complex belt in southeastern California and western Arizona is a NW‐SE trending zone of unusually large Tertiary extension and uplift. Midcrustal rocks exposed in this belt raise questions about the crustal thickness, crustal structure, and the tectonic evolution of the region. Three seismic refraction/wide‐angle reflection profiles, acquired and analyzed as a part of the U.S. Geological Survey's Pacific to Arizona Crustal Experiment, were collected to address these issues. The results presented here, which focus on the Whipple and Buckskin‐Rawhide mountains, yield a consistent three‐dimensional image of this part of the metamorphic core complex belt. The seismic refraction/wide‐angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles. These phases include refractions from the near‐surface and crystalline basement, reflections from boundaries in the middle and lower crust, and reflections and refractions from the upper mantle. The final model consists of a thin veneer (&lt;2 km) of upper plate and fractured lower plate rocks (1.5–5.5 kms−1) overlying a fairly homogeneous basement (∼6.0 km s−1) and a localized high‐velocity (6.4 km s−1) body situated beneath the western Whipple Mountains. A prominent midcrustal reflection is identified beneath the Whipple and Buckskin‐Rawhide mountains between 10 and 20 km depth. This reflector has an arch‐like shape and is centered beneath, or just west of, the metamorphic core complex belt. This event is underlain by a weaker, approximately subhorizontal reflection at 24 km depth. Together, these two discontinuities define a lens‐shaped midcrustal layer with a velocity of 6.35–6.5 km s−1. The apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift (∼10 km) defined by surface geologic mapping and petrologic barometry studies. The layer thins to the northeast and is absent in the Transition Zone. The 6.35–6.5 km s−1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks. This midcrustal layer is underlain by a higher‐velocity lower crustal layer that is modeled as only 3–6 km thick beneath the metamorphic core complex belt and regions to the southwest. To the northeast, however, this layer thickens to 8–10 km as the midcrustal layer pinches out above it. The velocity of the lower crust is constrained by traveltime modeling and is 6.6±0.15 kms−1 beneath the western Transition Zone and the metamorphic core complex belt; higher velocities may be present farther to the southwest where the layer is thin. The velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust. Instead, we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle‐derived intrusions. The crust‐mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26–28 km or, in the case of the Whipple Mountains, actually deepens; a 3‐km crustal root is modeled. This lack of upward doming of the Moho, together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension‐related structures in the middle and lower crust, suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension, as one would expect if extension occurred along crust‐penetrating shear zones (Wernicke, 1981; Wernicke et al., 1985). Instead, domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust.

The Nadanhada Terrane

Abstract The Nadanhada terrane, a Jurassic disrupted terrane in Heilongjiang Province of China, is principally composed of Permo‐ Carboniferous limestone and greenstone, Triassic bedded chert and middle Jurassic siliceous shale, all enclosed within younger (presumably Late Jurassic‐ Early Cretaceous) clastics. Palaeontological and lithological characteristics and structural features of these formations are entirely identical to those of the Mino terrane of the Japanese Islands. Prior to opening of the Sea of Japan, these terranes formed a single superterrane together with the Western Sikhote‐Alin terrane. Tectono‐stratigraphic terranes very similar to the Nadanhada and Mino terranes are also found in the Ryukyu are, the Philippines and probably in Borneo. All these terranes constituted a belt of accretionary complexes during Late Jurassic and / or Early Cretaceous time along the eastern continental margin of Asia after completion of the Triassic collage of the Chinese continent.

Anorogenic magmatism and the Grenvillian Orogeny

The Grenvillian Orogeny was preceded by extensive anorogenic volcanism and plutonism in the period 1500–1300 Ma in the form of rhyolites, epizonal granites, anorthosites, gabbros, alkaline complexes, and basic dykes. An analogue for the mid-Proterozoic anorogenic complexes is provided by the 2000 km by 200 km belt of anorogenic complexes in the Hoggar, Niger, and Nigeria, which contain anorthosites, gabbros, and peralkaline granites and were generated in a Cambrian to Jurassic rift that farther south led to the formation of the South Atlantic. An analogue for the 1 × 106 km2 area of 1500–1350 Ma rhyolites (and associated epizonal granites) that underlie the mid-continental United States is provided by the 1.7 × 106 km2 area of Jurassic Tobifera rhyolites in Argentina, which were extruded on the stretched continental margin of South America immediately preceding the opening of the South Atlantic. The mid-Proterozoic complexes were intruded close to the continental margin of the Grenvillian ocean and were commonly superimposed by the craton-directed thrusts that characterized the final stages of the Grenvillian Orogeny. The bulk of the Keweenawan rift and associated anorogenic magmatism formed about 1100 Ma at the same time as the Ottawan Orogeny in Ontario, which probably resulted from the collision of the island arc of the Central Metasedimentary Belt attached to the continental block in the east with the continental block to the west. The most appropriate modern equivalent would be the Rhine Graben, which formed at the same time as the main Alpine compression.

Extensional fault uplift of regional Franciscan blueschists die to subduction shallowing during the Laramide orogeny

Research Article| December 01, 1989 Extensional fault uplift of regional Franciscan blueschists die to subduction shallowing during the Laramide orogeny Scot W. Krueger; Scot W. Krueger 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar David L. Jones David L. Jones 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar Author and Article Information Scot W. Krueger 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 David L. Jones 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1989) 17 (12): 1157–1159. https://doi.org/10.1130/0091-7613(1989)017<1157:EFUORF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Scot W. Krueger, David L. Jones; Extensional fault uplift of regional Franciscan blueschists die to subduction shallowing during the Laramide orogeny. Geology 1989;; 17 (12): 1157–1159. doi: https://doi.org/10.1130/0091-7613(1989)017<1157:EFUORF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Regional structural and stratigraphic relations in the northern California Coast Ranges require an episode of extensional tectonic unroofmg of coherent blueschist terranes of the Eastern belt of the Franciscan complex along a crustal-scale, low-angle, normal-fault system, here named the Coast Range detachment. This tectonic unroofmg occurred during the latest Cretaceous and early Tertiary, contemporaneous with the compressional Laramide orogeny of the North American interior. Dynamic models of accretionary wedges predict that shallowing of subduction, as has been inferred during the Laramide orogeny, will impart extensional stresses in the wedge above the subducting plate. We therefore suggest a single causal event—gradual shallowing of subduction in the latest Cretaceous and early Tertiary—to link the extensional fault uplift of regional Franciscan blueschists to Laramide compression in the North American interior. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Granitoids of the Coast Range of central Chile: Geochronology and geologic setting

The Southern Coastal Batholith of central Chile is mainly composed of calc-alkaline granitoids of Late Carboniferous to Permian age. There is no clear evidence of the existence of older plutonic rocks in the batholith. The granitoids are intruded into an accretionary prism which underwent contemporaneous metamorphism. Triassic high-level plutons of limited areal extent occur as post-tectonic bodies in the high P/T metamorphic belt of the subduction complex. Jurassic plutons, which increase in volume toward the northern limit of the area and beyond, are associated with a tectonomagmatic event which, in certain localities, remobilized and re-set the ages of the Paleozoic granitoids. Cretaceous plutons occur to the east of the Southern Coastal Batholith. Initial 87Sr/ 86Sr ratios of the Paleozoic granitoids (0.706 to 0.707) are higher than those of the Mesozoic plutons (∼0.704) which were derived from a source similar to that of the present-day volcanic rocks of this segment of the Andes. The northern limit of the Paleozoic rocks is presumed to be an east-west fault zone, apparently inactive since Jurassic times. The continuity of the Mesozoic granitoids is not affected by this boundary.

Tectonic development of the Southern Belt of the Southern Uplands accretionary complex

Detailed structural studies in the Wenlock age Southern Belt of the Southern Uplands have identified major controls on tectonic development. Original thickness and composition of sediment control the style of faulting: thin, poorly consolidated, interbedded sandstone/mudstone sequences form broad sheared zones, while thicker, well consolidated sandstone-dominated sequences fail along thin, bedding-parallel brittle fractures with vein infill. The structural evolution of the sheared zones provides an insight into melange development. Depth of burial within the accretionary pile governs the development of cleavage and intensity of post Fl deformational phases. The intensity of imbrication within accreted material probably relates to subduction rate with early Wenlock sequences being highly imbricated and mid Wenlock sediments preserved in internally coherent fault-bounded packets. Sinistral strike-slip movement controls the development of steeply plunging folds, sub-horizontal stretching directions and sideways facing bedding cleavage relationships, and strongly influences fabric development in sheared zones. Determination of the relative importance of these factors permits a fuller understanding of the deformational history.