Northwest-trending Belt Research Articles

Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest−southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast−southwest. The northwest-trending belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

Petrogenesis of Eocene shoshonitic rocks from South-Central British Columbia: Melting of Proterozoic lithospheric mantle

The Terrace Mountain complex is a part of a discontinuous north-west trending belt of Eocene volcanic centres, which stretches for about 1000 km in southern and central interior British Columbia and probably extends into the northwestern United States. The 55–45 Ma chain is related to east-vergent subduction of oceanic plates under the North American continent. The volcanic rocks of the belt are mainly of calc-alkaline intermediate to felsic composition, typical of distal continental arcs. However, close to the Canada-United States border, in the Terrace Mountain volcanic complex, there are alkaline (shoshonitic) mafic and intermediate rocks with an unusual isotopic composition compared to most other rocks of the belt. These rocks, which are hosted in an extensional basin/half-graben associated with regional strike-slip faulting in a continental arc setting, have highly negative ɛNd(t) values, Proterozoic neodymium depleted mantle model ages and elevated initial 87Sr/86Sr ratios. The two rock units, which are >700 m thick were emplaced within a short (<1–2 Ma) magmatic episode at ~52 Ma. The older volcanic pulse was composed of basaltic trachyandesites while the younger pulse included trachyandesites. The rocks are enriched in strongly incompatible trace elements and underwent fractional crystallization. Both suites are related to a parent magma, which was sourced during extension of the Cordilleran orogen by partial melting of heterogeneous subduction-modified sub-continental lithospheric mantle composed of amphibole-phlogopite-bearing peridotite. Nd isotopic characteristics require an old LREE-enriched slab in the mantle suggesting that the source was separated from convecting mantle since the Proterozoic.

Mantle cooling and cratonization of Archean lithosphere by continuous plate subduction: Constraints from TTGs, sanukitoids, and high-K granites, eastern North China Craton

The evolution of plate subduction styles on the Earth is coupled with its cooling history. Continuous subduction was initiated during the Neoarchean, yet its influence on mantle cooling and the cratonization of Archean lithosphere remains unclear. Successive subduction-related magmatism occurred during the late Neoarchean evolution of the Western Shandong Province granite-greenstone belt (WSP) of the North China Craton (NCC) and preserved crucial clues relating to processes and roles of regional continuous subduction. In this study, petrology, whole rock geochemistry, and zircon U–Pb–Hf isotopes are reported for late Neoarchean TTG gneisses, quartz monzodiorites, and monzogranites from the WSP. Zircon U–Pb dating reveals that the TTG gneisses, quartz monzodiorites, and monzogranites were formed at ~2561–2517 Ma, 2540–2533 Ma, and 2541–2513 Ma, respectively. The TTG gneisses with high (La/Yb)N ratios, low K2O/Na2O ratios, and low MgO contents were mainly derived from partial melting of oceanic slab material. In contrast, the quartz monzodiorites show high K2O and MgO content and high (La/Yb)N ratios, consistent with the characteristics of Archean sanukitoids, suggesting that they were the products of partial melting of mantle source metasomatized by slab-derived melts. The monzogranites with systematically high K2O, SiO2, and low MgO contents were formed by crustal reworking of regional TTGs and sedimentary rocks. Rock assemblages, geochemical features, regional field observation, and structure geology indicate prolonged (~2.56–2.49 Ga) continuous magmatism in the WSP across three northwest-trending belts (Belts A, B, and C) related to continuous subduction and slab rollback. These processes led to a tectonic transition from early compression to late extension. The final stabilization of the continental crust in the late Neoarchean in the eastern NCC was a consequence of successive mantle cooling due to regional continuous subduction recorded by subduction-related rock assemblages (i.e., adakites, sanukitoids, high-Mg basalts-andesites, and Nb-enriched basalts), the paired type of metamorphism, and the rise of alkali basalts. Secular geochemical changes of alkaline basalts outline the role of continuous subduction in dramatic mantle cooling and the final stabilization and cratonization of the eastern NCC.

Geochemistry of skarn and porphyry deposits in relation to epithermal mineralization in the Arasbaran metallogenic zone, NE Tabriz, Iran

The Arasbaran metallogenic zone in northern Iran is part of the Alborz–Azerbaijan magmatic zone, which developed along the southern margin of Eurasia during the Early Mesozoic–Late Cenozoic. This region hosts precious and base metal mineralization, including porphyry, skarn, and epithermal copper, molybdenum, and gold deposits. Rare earth element variations across all the deposits are similar, indicating a similar source for these elements. The north-west trending belt comprising the Nabijan to the Sonajil deposits consistently shows chiefly alkaline conditions of formation. Fluid inclusion studies indicate that both high and low temperature hydrothermal fluids participated in the formation of all of the deposits. The mineralization age decreases from north to south and east to west and, although metal zonation is complex, the Cu–Au association post-dated the Cu–Mo mineralization reflecting that the ore fluid evolved in terms of both cooling and chemical changes due to fluid–fluid and fluid–rock interactions. In this region most deposits record a concentric zonation, with the centres preserving porphyry and skarn deposits and deposits becoming progressively epithermal toward the outer parts of the mineralizing system. According to this, the mineralization age decreases from the porphyry and skarn deposits to the epithermal deposits. The homogenization temperature and salinity both decrease from the centre to the outer zone. The pattern of homogenization temperature zonation, which is concordant with salinity zonation, suggests that fluids migrated up-dip and towards the margins of the zonation system.

Geophysical characterization of a Proterozoic REE terrane at Mountain Pass, eastern Mojave Desert, California, USA

AbstractMountain Pass, California (USA), located in the eastern Mojave Desert, hosts one of the world’s richest rare earth element (REE) deposits. The REE-rich terrane occurs in a 2.5-km-wide, northwest-trending belt of Mesoproterozoic (1.4 Ga) stocks and dikes, which intrude a larger Paleoproterozoic (1.7 Ga) metamorphic block that extends ∼10 km southward from Clark Mountain to the eastern Mescal Range. To characterize the REE terrane, gravity, magnetic, magnetotelluric, and whole-rock physical property data were analyzed. Geophysical data reveal that the Mountain Pass carbonatite body is associated with an ∼5 mGal local gravity high that is superimposed on a gravity terrace (∼4 km wide) caused by granitic Paleoproterozoic host rocks. Physical rock property data indicate that the Mountain Pass REE suite is essentially nonmagnetic at the surface with a magnetic susceptibility of 2.0 × 10−3 SI (n = 57), and lower-than-expected magnetizations may be the result of alteration. However, aeromagnetic data indicate that the intrusive suite occurs along the eastern edge of a distinct northwest-trending aeromagnetic high along the eastern Mescal Range. The source of this magnetic anomaly is ∼1.5–2 km below the surface and coincides with an electrical conductivity zone that is several orders of magnitude more conductive than the surrounding rock. The source of the magnetic anomaly is likely a moderately magnetic pluton. Combined geophysical data and models suggest that the carbonatite and its associated REE-enriched ultrapotassic suite were preferentially emplaced along a northwest-trending zone of weakness, which has potential implications for regional mineral exploration.

Geophysical interpretation of the geology of the Stanthorpe region using aeromagnetic, gravity and radiometric data

This study provides an in-depth geophysical interpretation of the geology of the Stanthorpe region with a particular focus on hydrothermal alteration. The radiometric ternary map outlines the main lithological divisions in the region, which together with Th and Th/K ratio maps distinguish various granitic suites. Magnetic signatures delineate a non-magnetic zone within the low to moderately magnetic Stanthorpe Granite. The non-magnetic section, a north–northwest trending belt with a high density of faults and dykes is interpreted as a hydrothermal alteration zone that is narrower in the north, widens southwards, and is much more extensive at depth than is suggested by the surface alteration pattern. Modelling of gravity data reveals a massive low-density granitic intrusion extending to a depth of 10–15 km. The low-gravity section with modelled rock densities between 2450 and 2670 kg/m3 coincides with the low magnetic susceptibility zone in the magnetic model. High K, low Th and low Th/K also characterise the sections of the Stanthorpe Granite that have both low gravity and magnetic responses. Also conspicuous in the geophysical signatures within the non-magnetic zone is an ovoid feature of low magnetic response, high uranium content with moderate density, interpreted as a granitic pluton. Thus, decrease in magnetisation, changes in K concentration and corresponding low gravity with an associated high density of faults, fractures and dykes in the Stanthorpe region are the main features of the quartz–sericite–chlorite–muscovite hydrothermal alteration.

Study on Iron‐Copper‐Gold Mineralization in the Truong Son Metallogenic Belt, Laos‐Vietnam

The Truong son fold belt, located in the Truong son Range and in central Vietnam and eastern Laos, an elongate northwest trending belt,is dominated by a system of strikeslip faults, which forms part of the Indochina block. Rocks in the belt are metamorphosed up to the amphibolite facies grade, predominantly Early to Mid Paleozoic.The tectonic boundaries of the Truong son fold belt, Song Ma suture zone, Dien BienPhu Fault, Khorat Basin and Tamky-Phuoc Son fault lies to north, west and south of the belt respectively(Fig,1). The Truong son tectonic magmatic belt,the same position with fold belt, composed of volcanic rock and granite, which was formed in Caledonian, Variscan and Indosinian periods. Due to the particular geological background, the Truongson metallogenic belt has experienced complex diagenesis and mineralization, and rich mineral resources were formed. There are more than 70 gold, copper, iron, tin, lead and zinc deposits, including some large to super-large –scale deposits, such as Sepon copper-gold deposit, Phu Kham gold-copper deposit in Laos, and Thach Khe iron deposit in Vietnam.

The Gifford Creek Ferrocarbonatite Complex, Gascoyne Province, Western Australia: Associated fenitic alteration and a putative link with the ~1075Ma Warakurna LIP

The Gifford Creek Ferrocarbonatite Complex (GFC), located in the Neoarchean–Palaeoproterozoic Gascoyne Province, Western Australia, comprises sills, dykes, and veins of ferrocarbonatite intruding the Pimbyana Granite and Yangibana Granite of the Durlacher Supersuite and metasedimentary rocks of the Pooranoo Metamorphics. The ferrocarbonatites are associated with complex and irregularly distributed zones of fenitic alteration. These ferrocarbonatites and fenites are also associated with a swarm of ironstone veins, containing magnetite, hematite and goethite. The GFC and associated fenite outcrops are distributed within a ~ 700 km2 area, north of the Lyons River Fault. Ferrocarbonatite sills and dykes are predominant in a northwest-trending belt, along the southern margin of the complex; whereas ferrocarbonatite veins tend to be distributed in a series of sub-parallel west–northwest-trending linear belts, generally associated with the Fe oxide veins with sinuous trends. These veins have margins of Fe-rich carbonates associated with zones of alteration that have a fenitic character. The fenitic haloes are characterised by the presence of Na–K-feldspars and/or Na-amphiboles and magnetite. In some cases monomineralic feldspar zones (orthoclasite) are present. Fenitic alteration is spatially associated with the carbonatites, but it can also form discrete veins and veinlets in basement granitic rocks (Pimbyana and Yangibana Granites). Petrographic, XRD and SEM analyses show that the ferrocarbonatites are dominantly composed of ankerite–dolomite, magnetite, arfvedsonite–riebeckite, and lesser calcite. Alkali amphibole has compositions ranging from potassian magnesio-arfvedsonite to magnesio-riebeckite. Sills and dykes north of the Lyons River, are characterised by a carbonate-rich matrix, containing > 50 vol.% ankerite–dolomite, with accessory quantities of apatite, barite, monazite, and phlogopite. In-situ U–Pb age determination of apatite grains by LA-ICP-MS on a sample of ferrocarbonatite was performed and an average age of 1075 ± 35 Ma was obtained. This age is within the range of ages (~ 1078–1070 Ma) of the Warakurna Large Igneous Province (WLIP) and we suggest that the GFC is related to the mantle plume event that generated the WLIP. This is a significant outcome, because it may lead to the recognition or discovery of other carbonatites within the area covered by the WLIP. In addition, monazite from fenitic rocks associated with the ironstones yielded an age of 1050 ± 25 Ma, suggesting that a second phase of carbonatite magmatism occurred, resulting in the emplacement of the carbonatite-ironstone veins swarm. A model is proposed to explain the formation of the GFC system.

Lithofacies analysis of the Dezadeash Formation (Jura–Cretaceous), Yukon, Canada: The depositional architecture of a mud/sand-rich turbidite system

The Dezadeash Formation is a northwest trending belt of predominantly Jura–Cretaceous deep-marine siliciclastic rocks in southwestern Yukon, Canada. It is truncated to the southwest by the Denali Fault System, a major transcurrent fault along which ∼ 370 km of dextral slip occurred in post-Early Cretaceous time, and it is flanked to the northeast by the Kluane Schist, a mainly mica–quartz schist assemblage of unknown age and origin. This study provides a robust quantitative description of lithofacies comprising a point-source mud/sand-rich turbidite system: approximately 16 335 m of measured strata from 75 sections throughout the Dezadeash Formation indicate that up to 29 lithofacies types are present and these are dominated by thin- to thick-bedded sandstone–mudstone couplets and medium- to thick-bedded sandstone. The various lithofacies are grouped into conglomerate, sandstone, sandstone–mudstone, and siltstone–mudstone lithofacies associations, which are interpreted as slide, channel-lobe transition, lobe, and overbank architectural elements, respectively. Stacked channel-lobe transition and lobe elements predominate, but the depositional architecture displays no apparent, overall vertical trend. The Dezadeash Formation represents mainly the lower middle and lower subdivisions of a point-source, mud/sand-rich turbidite system that records a single stage of fan growth as it prograded transversely from a volcano-plutonic arc into the basin.

Geology and juxtaposition history of the Yukon-Tanana, Slide Mountain, and Cassiar terranes in the Glenlyon area of central Yukon

In central Yukon, the pericratonic Yukon–Tanana terrane (YT) is juxtaposed with the Cassiar terrane (CT, parautochthonous North America) along the Tummel fault zone (TFZ), a 3–4 km wide, northwest-trending belt comprising imbricate fault slices of Slide Mountain terrane (SM, greenstone, chert, serpentinite) and synorogenic clastic rocks. Northeast of the TFZ, the CT comprises Paleozoic metapelitic rocks, marble, and amphibolite of continental margin affinity. To the southwest, the YT consists of a pre-Late Devonian metasedimentary complex overlain and intruded by Mississippian clastic, volcanic, and plutonic successions of continental arc affinity. In the TFZ, Middle to Late Permian ocean-floor basalt of the SM shows evidence of crustal contamination, suggesting deposition at the edge of a marginal ocean basin. Deformation features in the TFZ include early ductile fabrics overprinted by younger brittle structures. Triassic synorogenic clastic rocks in the TFZ, and at the base of a klippen above the YT, suggest that terrane imbrication began shortly after the Early Triassic. 40Ar/39Ar mica ages from the region suggest cooling of the YT, SM, and part of CT below 300 °C by Early Jurassic time. Pervasive brittle structures in the Ragged Lake klippe, which roots into the TFZ, indicate brittle thrusting of the SM over the CT in post-Triassic time. Early Cretaceous plutons intrude the CT (Glenlyon Batholith) and the TFZ (leucogabbro) and impose a contact aureole that extends westward into the YT. Steep brittle structures that deformed the TFZ also affect, in part, the Glenlyon Batholith but do not significantly offset its contact aureole. Consequently, little displacement can have occurred along the TFZ after Early Cretaceous time.

Late Triassic and Early Jurassic palaeogeographic evolution and depositional cycles of the Western Tethys Iberian platform system (Eastern Spain)

Deposition of Upper Triassic and Lower Jurassic sediments on the Iberian platform system took place in a post-rift intraplate basin which formed part of the western Tethys shelf. Nevertheless, there is evidence for the tensional reactivation of a network of faults with associated magmatic activity. Early Jurassic palaeogeographic reconstructions, based on over 70 sections and 9 oil wells, reveal the presence of a series of highs and lows (depocentres). Two main highs, referred to as the El Maestrazgo High, in the east, and the La Mancha High, in the west, where condensed sections were deposited, were mainly controlled by syndepositional faults. The main depocentres were situated between these two highs areas, in a northwest trending belt in the central, northern and southern areas. Despite partial fault control, development of some Early Jurassic depocentres appears to be mainly controlled by thermal contraction of the lithosphere following Triassic main rifting pulse. The Upper Triassic–Lower Jurassic succession of the Iberian platform system was deposited during four transgressive and regressive cycles of 12 to 6 My duration. The lowermost cycle LJ-1 started in late Norian and lasted until the Sinemurian. The second cycle LJ-2 started in the Sinemurian and ended in the Pliensbachian (Davoei Zone), and can be subdivided into two minor cycles LJ2-1 and LJ2-2. The cycle LJ-3 started with an extensive upper Pliensbachian (Davoei Zone) transgression, whereas the top of its regressive cycle is dated to the Toarcian (Variablilis Zone). Within cycle LJ-3, three cycles can be distinguished. Maximum deepening was reached during the Bifrons Zone and active volcanism took place along the Teruel and Caudiel faults. The transgressive phase of the cycle LJ-4 started in the Thouarsense Zone and extended up to the Insigne Zone; its regressive phase developed during the upper Toarcian (Pseudoradiosa and Aalensis Zones) and part of the Aalenian (Opalinum and Murchisonae Zones). This cycle can be subdivided in two minor cycles, with their transgressive peaks occurring during the Insigne Zone, and the Aalensis Zone respectively. Palaeogeographically this corresponds with the expansion of the marginal carbonate platforms over hemipelagic deposits. The top of the cycle corresponds to a major unconformity with regional emersion during the Aalenian Murchisonae Zone.

SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central Idaho: Implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits

Research Article| March 01, 2003 SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central Idaho: Implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits Karen Lund; Karen Lund 1U.S. Geological Survey, Denver, Colorado 80225, USA Search for other works by this author on: GSW Google Scholar John N. Aleinikoff; John N. Aleinikoff 1U.S. Geological Survey, Denver, Colorado 80225, USA Search for other works by this author on: GSW Google Scholar Karl V. Evans; Karl V. Evans 1U.S. Geological Survey, Denver, Colorado 80225, USA Search for other works by this author on: GSW Google Scholar C. Mark Fanning C. Mark Fanning 2Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2003) 115 (3): 349–372. https://doi.org/10.1130/0016-7606(2003)115<0349:SUPGON>2.0.CO;2 Article history received: 14 Feb 2002 rev-recd: 21 Jun 2002 accepted: 20 Aug 2002 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 Karen Lund, John N. Aleinikoff, Karl V. Evans, C. Mark Fanning; SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central Idaho: Implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits. GSA Bulletin 2003;; 115 (3): 349–372. doi: https://doi.org/10.1130/0016-7606(2003)115<0349:SUPGON>2.0.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 In central Idaho roof pendants, a northwest-trending belt of metamorphosed strata, correlative with the Windermere Supergroup, links northern and southern segments of the western Laurentia Neoproterozoic rift belt. Nine newly named formations within the Gospel Peaks sequence-A through Gospel Peaks sequence-D record Cryogenian preglacial, rift-glacial, and postglacial events as well as Neoproterozoic III glacial and rift events. The Edwardsburg Formation of Gospel Peaks sequence B includes interfingered bimodal rift-related volcanic and glaciogenic diamictite strata. Zircons from a rhyodacite flow in the lower Edwardsburg Formation and from a rhyolite flow at its top, dated by using the sensitive high-resolution ion microprobe (SHRIMP), yielded a weighted average of 685 ± 7 Ma and 684 ± 4 Ma. Reevaluation of geochronology and correlations indicates that Cryogenian rifting may have been (1) protracted between 780 and 685 Ma, (2) diachronous along the Cordillera, and/or (3) stepwise with a Cordilleran-wide event at ca. 685 Ma that initiated the formation of the Cordilleran miogeocline and set its geometry. Reevaluation of the Cryogenian glacial record indicates that (1) two associated ca. 685 Ma glacial intervals in the Edwardsburg Formation correlate with the Rapitan glaciation, (2) the Sturtian snowball Earth event must be reevaluated on the basis of revision of Rapitan glaciation from 750–700 Ma to ca. 685 Ma, and (3) there were older Cryogenian glaciations or Cryogenian glaciations were not globally synchronous. New dates and correlations significantly impact the number and synchroneity of possible snowball Earth events and the paleolatitudes of Cryogenian glaciations. Western Laurentian events at ca. 685 Ma particularily affect Neoproterozoic paleocontinental reconstructions by indicating diachronous and multi step breakup of supercontinent Rodinia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Depositlonai Architecture of Lacustrine-Delta and Fluvlal Systems of the Permian EpsiIon and Toolachee Formations at Duilingari Field, Southeastern Cooper Basin, South Australia: ABSTRACT

The Epsilon and Toolachee Formations record a transition from lacustrine deltas to fluvial depositional systems in the southeastern part of the Cooper Basin. Gas reservoirs in these formations occur mainly in narrow (typically less than 2 mi [3.2 km] wide) belts of distributary- and fluvial-channel sandstones in structurally high areas in Dullingari field and neighboring areas. In a study by the Bureau of Economic Geology and Santos to document distribution of Epsilon and Toolachee lithotacies, we mapped multiple depositional axes and projected them into undeveloped areas in the field. We performed a high-resolution sequence-stratigraphic study, defining four operational Epsilon subunits bounded by lacustrine mudstones recording periods of delta-platform abandonment and subsidence. In contrast, six genetic Toolachee subunits are bounded by regionally extensive coal seams recording pervasive sediment-supply shutoff and peat-swamp development over broad areas. Epsilon lacustrine deltas prograded northwestward over a muddy substrate on a broad, shallow basin floor. Distributary-channel axes, defined by narrow, northwest-trending belts of upward-fining sandstones less than 20 ft (6 m) thick, are projected across the field area. However, the younger Toolachee Formation contains northeast-trending sandstones in structural troughs, suggesting tectonic control on deposition. Detailed lithofacies maps of these formations provide a framework for assessing potentialmore » development targets at Dullingari field.« less

Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre‐Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large‐magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.

An Introductory Geology Exercise on Contouring Intensities for the 1989 Loma Prieta Earthquake

Following the October 17, 1989, Loma Prieta earthquake, students in several introductory geology classes were asked to complete a questionnaire that included questions concerning where they were at the time of the quake and the ground shaking effects at that site. As part of the Geology of California class, students used the Modified Mercalli Scale to assign an intensity value on the basis of the questionnaire responses and then plotted the values obtained on regional maps. A compilation of data from 133 responses revealed that ground shaking was most intense near the epicenter, but that higher intensities were experienced in northwest-trending belts of presumably weaker bedrock. The assigned values are probably somewhat higher than those that would be assigned by professional geologists and seismologists. The exercise was a very effective method of teaching students about ground shaking, as indicated by the number of correct responses to a subsequent exam question.

Paleocene plutonism and its tectonic implications, North Cascades, Washington

U-Pb zircon geochronology has identified a northwest-trending belt of Paleocene (68-59 Ma) plutons within the crystalline core of the North Cascades. Most of these plutons show amphibolite facies assemblages and solid-state foiliation. They document extensive Paleogene dynamothermal metamorphism in the core, and combined with other recent studies, demonstrate ongoing metamorphism from mid-Cretaceous to mid-Eocene time. The Paleocene plutons were emplaced during an interval that marked a magmatic lull throughout much of the northern Cordillera. The distribution of latest Cretaceous and Paleocene plutons suggests that magmatism in the North Cascades core records the same eastward retreat that occurred farther north in the Cordillera.

Late Oligocene-Early Miocene Volcanism, Faulting, and Sedimentation in West-Central California: ABSTRACT

Abundant evidence from the rock record indicates that volcanism, active faulting, and sedimentary-basin development were widespread and nearly synchronous in west-central California during the late Oligocene and early Miocene. Volcanic activity occurred about 24-22 Ma at more than 20 eruptive centers in a northwest-trending belt, extending from Point Arena to the Orocopia Mountains. Volcanism was accompanied by marine transgression, rapid bathymetric deepening, and accumulation of thick piles of predominantly marine clastic sediment in the Point Arena, La Honda, San Joaquin, Pescadero-Ano Nuevo, Salinas, Husana-San Luis Obispo, and Cuyama basins, as well as in several other basins presently situated offshore. Concurrent movement along basin-margin normal faults resulted in localized deposition of wedges of very coarse nonmarine and deltaic sediment in the La Honda, Cuyama, Lockwood, Soledad, and Diligencia basins. Also, at about the same time, right-lateral strike-slip faulting apparently caused en enchelon folding in both the Cuyama and San Joaquin basins, and formation of a pull-apart feature in the La Honda basin. Late Oligocene-early Miocene volcanism, faulting, and basin development in west-central California were nearly synchronous over the entire area rather than sequential from south to north, and therefore cannot be explained simply as the effects of the northwestward-migrating Mendocino triplemore » junction. An alternative explanation is that volcanism, faulting, and basin development were caused by an episode of regional extension or transtension (strike-slip with a component of extension) related to a change in plate motions in the circum-Pacific area.« less

Implications of the northwestwardly younger age of the volcanic rocks of west-central California

Research Article| May 01, 1985 Implications of the northwestwardly younger age of the volcanic rocks of west-central California KENNETH F. FOX, JR.; KENNETH F. FOX, JR. 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar ROBERT J. FLECK; ROBERT J. FLECK 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GARNISS H. CURTIS; GARNISS H. CURTIS 2Department of Geology and Geophysics, University of California at Berkeley, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar CHARLES E. MEYER CHARLES E. MEYER 3U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1985) 96 (5): 647–654. https://doi.org/10.1130/0016-7606(1985)96<647:IOTNYA>2.0.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 KENNETH F. FOX, ROBERT J. FLECK, GARNISS H. CURTIS, CHARLES E. MEYER; Implications of the northwestwardly younger age of the volcanic rocks of west-central California. GSA Bulletin 1985;; 96 (5): 647–654. doi: https://doi.org/10.1130/0016-7606(1985)96<647:IOTNYA>2.0.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 Erosional remnants of volcanic fields in west-central California form a linear northwest-trending belt growing younger in age to the northwest. Major fields within the belt are represented by the Neenach Volcanics, Pinnacles Volcanic Formation, Quien Sabe Volcanics, volcanic rocks in the Berkeley Hills, Tolay Volcanics, Sonoma Volcanics, and Clear Lake Volcanics. Dispersion in the age-distance relation is reduced by restoration of inferred offsets on transecting right-lateral fault systems. The offsets include 115 km on the San Gregorio–Hosgri fault, 314 km on the San Andreas fault, 43 km on the Hayward-Rodgers Creek fault, and 28 km on the Carneros-Franklin-Sunol-Calaveras fault. On the basis of the age and restored position of the volcanic rocks, we judge that the locus of initial active volcanism migrated northwestward ∼3.75 cm/yr from 25 to 12 Ma, and ∼1.35 cm/yr from 12 Ma to the present.The volcanic rocks apparently formed south of the northwardly retreating edge of the subducted part of the Juan de Fuca plate, corroborating one corollary of a published model of an expanding hole in the subducted Farallon-Juan de Fuca-Cocos plate. The present position of the locus of melting at Clear Lake, California, requires substantial overthrusting of the Juan de Fuca plate by the Pacific plate, as was postulated on the basis of foreshortening of magnetic anomalies in the Gorda basin. The change in rate of northwestward migration ∼12 Ma reflects a change in spreading direction of the Juan de Fuca plate vis-à-vis the Pacific plate, previously recognized from changes in orientation of oceanic magnetic anomalies. From the migration rates, it can be inferred that the relative movement between the Pacific plate and the westernmost fringe of the North American plate averaged ∼3.5 cm/yr from 27 m.y. ago to the present. 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.

Multiple pulses of Neogene compressive deformation in the Ayacucho intermontane basin, Andes of central Peru

Strata of the Neogene Ayacucho intermontane basin, central Peru, can be subdivided into three tectonostratigraphic units: the Larampuquio volcanics, consisting largely of intermediate lavas and coarse conglomerates of similar lithology; the unconformably overlying Huanta Formation, consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas, tuffs, and conglomerates; and the unconformably overlying Ayacucho Formation, consisting of complexly interfingering volcaniclastic sedimentary rocks, silicic tuffs, and lavas of intermediate to silicic composition. Radiometric ages of 17.3 ± 0.2 m.y. and 18.3 ± 0.6 m.y. have been obtained on tuffs from the upper and lower parts of the Larampuquio volcanics. Determinations on 2 rocks from the lower member of the Huanta Formation indicate an age of ∼11.4 ± 0.5 m.y. and a rock from the upper member is dated at 9.3 ± 0.3 m.y. B.P. Units of the Ayacucho Formation yield ages showing that it was deposited between ∼7.7 and 6 m.y. ago. The intervening Molinoyocc and Puchcas volcanics were erupted between ∼9.9 and 7.4 m.y. ago; these units may postdate the Huanta Formation or may be associated in part with the Huanta and in part with the Ayacucho Formation. Coarse conglomerates of the Larampuquio volcanics represent debris eroded from highlands uplifted during the first, and most intense, pulse of the late Cenozoic “Quechuan” phase of Andean compressive tectonism that began in the early Miocene. Folding of the Larampuquio volcanics may have taken place during this, or a subsequent, pulse of compression. A marked unconformity between the Huanta and Ayacucho Formations demonstrates a period of deformation between ∼8.5 and 9.5 m.y. ago. Compression also was active during at least part of the deposition of the Ayacucho Formation, as shown by inter-fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation. Folded rocks of the Ayacucho Formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3.8 ± 0.4 m.y. B.P. and ∼5.5 m.y. B.P., bracketing a well-defined pulse of deformation slightly younger than 6 m.y. These subsequent episodes of compressive deformation represent recurrent pulses of the “Quechuan” phase. Folding related to these late Quechuan pulses is restricted in the Andes to northwest-trending belts corresponding to reactivated fault zones in the pre-Neogene substratum. A system of north-trending dikes in the Huachocolpa district 70 km west of Ayacucho suggests relaxation of compression in the Pliocene, and compression of Quaternary age has been recognized in the Huancayo area 170 km northwest of Ayacucho. Taken together with the first pulse of Quechuan deformation in the early and middle Miocene, these data indicate at least four pulses of crustal compression during the late Cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension.

Laramide and Neogene Structure of Northern Sangre de Cristo Range, South-Central Colorado: ABSTRACT

The Sangre de Cristo Range, from Blanca Peak northward to the Arkansas River in Colorado, is composed mostly of Precambrian crystalline rocks and upper Paleozoic clastic sedimentary rocks. These rocks were folded and faulted by Laramide compressional forces from the Late Cretaceous to Eocene. Laramide structures are large arcuate thrust plates that intersect and overlap one another to form a northwest-trending belt that extends across the range from Huerfano Park to Valley View Hot Springs. All of the thrust plates within the range are bounded by west-dipping faults, some of which extend into the basement of Precambrian crystalline rocks. Along the east side of the range, the Alvarado fault is interpreted tentatively as an east-dipping thrust, bringing Precambrian crystal ine rocks west over Paleozoic rocks. Thrust plates were folded internally before and during thrusting; some plates of Paleozoic rock contain folds that tighten and decrease in amplitude toward the leading edge of the plate. Stacked plates consisting of Precambrian and Paleozoic strata have been folded concordantly after thrusting. Thrust faults are mainly high to medium-angle reverse faults along the leading edge of thrust plates, but they flatten to about 30° at depth. Total shortening within the range is at least 8 km (5 mi) at the latitude of Westcliffe and at least 14 km (9 mi) farther south near the latitude of the Great Sand Dunes. During the Neogene, the Sangre de Cristo Range was uplifted, and the adjoining San Luis and Wet Mountain Valleys were downdropped by extensional rift faulting. Rifting followed late Oligocene intrusion of stocks, sills, and dikes of mafic to felsic igneous rock into the Precambrian and Paleozoic rocks of the range. The horst of the Sangre de Cristo Range probably began to rise in the late Oligocene, rose rapidly in the early Miocene, and rose rapidly again in the late Miocene and Quaternary. Flows of mafic lava were erupted from faults along the southwest side of the Wet Mountain Valley and in the San Luis Valley. Zones of Laramide thrusts along the east and west sides of the range were reactivated to form the Sangre de Cristo and Alvarado normal faults, respectively. The floor of the Neogene sedimentary and volcanic fill of the San Luis Valley has been downdropped 2,000-7,000 m (6,600-23,000 ft) below the top of the range, and the floor of the Wet Mountains Valley has been downdropped about 2,000 m (6,600 ft) below the range. Rifting is still in progress in the San Luis Valley, west of the range, but may have ceased in the Wet Mountain Valley. End_of_Article - Last_Page 941------------