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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Asia
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oxygen
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Primary terms
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Paleogene mid-crustal intrusions in the Ruby Mountains–East Humboldt Range metamorphic core complex, northeastern Nevada, USA
Numerical models of Farallon plate subduction: Creating and removing a flat slab
Geochemistry and geochronology of Grenada and Union islands, Lesser Antilles: The case for mixing between two magma series generated from distinct sources
Location, location, location: The variable lifespan of the Laramide orogeny
Location, location, location: The variable lifespan of the Laramide orogeny
40 Ar/ 39 Ar ages of muscovites from modern Himalayan rivers: Himalayan evolution and the relative contribution of tectonics and climate
Muscovite 40 Ar/ 39 Ar ages help reveal the Neogene tectonic evolution of the southern Annapurna Range, central Nepal
Abstract We present new muscovite 40 Ar/ 39 Ar ages from thirteen Greater Himalayan rocks and one Lesser Himalayan rock collected from four north-trending transects across the southern Annapurna Range. Combining the new data with previously published ages leads to the following new insight into the tectonic development of the southern Annapurna. Muscovite cooling ages from Greater Himalayan rocks are c. 16–10 Ma in the western Annapurna and c. 6–2 Ma in the eastern Annapurna, revealing a decrease of 4–14 Ma from west to east. Similarly, the muscovite cooling age from one Lesser Himalayan rock in the west is c. 7 Ma and ages from several samples in the east are c. 5–2 Ma, indicating a decline of 2–5 Ma towards the east. Earlier cooling in the western Annapurna can be explained by along-strike differences in the geometry of the frontal ramp on the underlying thrust that carries these Greater and Lesser Himalayan rocks and/or by a NE-striking fault that cut these rocks. In Greater Himalayan rocks from the Modi river valley, one sample yielded muscovite 40 Ar/ 39 Ar ages of 18.0±0.7 and 16.2±0.5 Ma for grain sizes of approximately 750 and 200 µm, respectively. In contrast, a sample collected 200 m structurally lower produced ages of 12.6±0.2 and 9.9±0.1 Ma for these two grain sizes. The north-dipping Bhanuwa fault has been proposed between these samples, with different authors arguing for normal or thrust-sense motion. Our newfound pattern of an older muscovite 40 Ar/ 39 Ar age pair in the hanging wall supports arguments for the existence of the Bhanuwa fault and suggests normal sense motion. Supplementary material: Analytical methods, thin section observations, data tables, and plots of argon data for each sample are available at http://www.geolsoc.org.uk/SUP18774
Abstract The Lobo Formation consists of intermontane lacustrine or palustrine, fluvial, and alluvial-fan deposits. The Lobo near Capitol Dome in the Florida Mountains of southwestern New Mexico is about 116 m thick and consists of a basal pebble and cobble conglomerate containing locally derived carbonate clasts, overlain by very fine to fine-grained sandstone, reddish-brown siltstone and pebbly sandstone, and an upper cobble and boulder conglomerate with basement clasts. In the Victorio Mountains, the Lobo is an upward-fining succession (~325 m thick) composed of alluvial-fan and fluvial deposits. Sediment accumulation slows in the latter half of Lobo deposition as indicated by paleosols in the upper half of the Victorios section; depositional style and paleosols indicate deposition in arid conditions. Basal conglomerate clasts have been derived from diverse sources: a Proterozoic basement, Paleozoic carbonate and siliciclastic strata, Jurassic basalt flows, and Lower Cretaceous strata (Bisbee basin). The Lobo “super-sol,” a paleosol carbonate, is present along a karstic paleosurface at the base of the Lobo in the Florida Mountains section. δ18O values of this carbonate range around -12‰, suggesting a paleo-elevation of ~2400 m above sea level. The Victorio Mountains contain paleosols approximately 140 m up section that have carbonate values that range around -16‰, suggesting a paleo-elevation of about 3500 m above sea level. Preliminary paleomagnetic analysis indicates four brief normal intervals among a mostly reversed section; data suggest deposition occurred between 64 and 51 Ma followed by a post-depositional clockwise rotation of 25-30° about a vertical axis. Lobo data correspond well with the proposed Hot Springs Fault System, which suggests an offset by a dextral strike-slip system of approximately 26 km; this system’s location and displacement is sufficient for our suggested rotation given it occurred post-Lobo time.
Oligocene shortening in the Little Burro Mountains of southwest New Mexico
Oligocene Laramide deformation in southern New Mexico and its implications for Farallon plate geodynamics
The structure and rate of late Miocene expansion of C 4 plants: Evidence from lateral variation in stable isotopes in paleosols of the Siwalik Group, northern Pakistan
Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models
Evidence for a Miocene period of transient shallow subduction under the Neuquén Basin in the Andean backarc, and an intermittent Upper Cretaceous to Holocene frontal arc with a relatively stable magma source and arc-to-trench geometry comes from new 40 Ar/ 39 Ar, major- and trace-element, and Sr, Pb, and Nd isotopic data on magmatic rocks from a transect at ∼36°–38°S. Older frontal arc magmas include early Paleogene volcanic rocks erupted after a strong Upper Cretaceous contractional deformation and mid-Eocene lavas erupted from arc centers displaced slightly to the east. Following a gap of some 15 m.y., ca. 26–20 Ma mafic to acidic arc-like magmas erupted in the extensional Cura Mallín intra-arc basin, and alkali olivine basalts with intraplate signatures erupted across the backarc. A major change followed as ca. 20–15 Ma basaltic andesite–dacitic magmas with weak arc signatures and 11.7 Ma Cerro Negro andesites with stronger arc signatures erupted in the near to middle backarc. They were followed by ca. 7.2–4.8 Ma high-K basaltic to dacitic hornblende-bearing magmas with arc-like high field strength element depletion that erupted in the Sierra de Chachahuén, some 500 km east of the trench. The chemistry of these Miocene rocks along with the regional deformational pattern support a transient period of shallow subduction that began at ca. 20 Ma and climaxed near 5 Ma. The subsequent widespread eruption of Pliocene to Pleistocene alkaline magmas with an intraplate chemistry in the Payenia large igneous province signaled a thickening mantle wedge above a steepening subduction zone. A pattern of decreasingly arc-like Pliocene to Holocene backarc lavas in the Tromen region culminated with the eruption of a 0.175 ± 0.025 Ma mafic andesite. The northwest-trending Cortaderas lineament, which generally marks the southern limit of Neogene backarc magmatism, is considered to mark the southern boundary of the transient shallow subduction zone.
The Cura Mallín basin is part of a chain of sedimentary basins that formed within the Andean volcanic arc between 33° and 43°S during the late Oligocene and early Miocene. Most previous studies of these basins have suggested that they are pull-apart–type basins, produced by strike-slip deformation of the Liquiñe-Ofqui fault zone and other structures, all of which are currently active. However, no direct evidence has been cited for a correlation between formation of the Oligocene-Miocene basins and concurrent strike-slip faulting. The Cura Mallín basin lies more than 100 km north of the modern Liquiñe-Ofqui fault zone and is one of the largest and best exposed of the Southern Andean Oligocene-Miocene basins, making it a promising study area for distinguishing between Oligocene-Miocene tectonic activity that produced the basin and subsequent tectonic activity. Stratigraphic and structural data presented here from the Cura Mallín basin and its surroundings include facies variations, stratal thickness patterns, internal and external structural features, 40 Ar/ 39 Ar radiometric ages, and apatite and zircon fission-track ages. Based on the distribution of sedimentary facies and their relation to geologic structures, we conclude that the Cura Mallín basin formed as a result of normal faulting, with little or no significant strike-slip deformation in the area. Due to the lack of supporting evidence for interpretations of the other Oligocene-Miocene basins as pull-apart basins, we suggest that the entire chain of Oligocene-Miocene sedimentary basins formed in response to extensional tectonics on the Southern Andean margin.
New 40 Ar/ 39 Ar, major and trace element, and isotopic data for ca. 24–15 Ma backarc volcanic rocks from the Sierra de Huantraico, Sierra Negra, and Sierra de Chachahuén–La Matancilla regions (36°S–38°S) in the Neuquén Basin shed light on the early Miocene evolution of the south-central Andes. A model calling for incipient shallowing of the sub-ducting slab under the northern Neuquén Basin and an increase in the rate of westward motion of South America relative to the underlying mantle at ca. 20 Ma can explain many regional features. Early Miocene magmatism in the Neuquén Basin began with the eruption of ca. 24–20 Ma alkali olivine basalts from monogenetic and simple polygenetic centers located up to 500 km east of the trench. Their characteristics (Ta/Hf > 0.45, ε Nd = +3.6–+4.2; La/Ta < 14; Ba/La < 16; 87 Sr/ 86 Sr = 0.7037–0.7040) indicate a backarc mantle devoid of arc-like components. These basalts erupted at a time of extension all along the margin during a period of rapid, near-normal Nazca–South America plate convergence when spreading ridges between the Pacific, Nazca, and Antarctic plates were becoming more parallel to the Chile Trench. Ridge rotation along with slab roll-back in response to slow relative motion between South America and the underlying mantle can explain why isotopically enriched magmas erupted far to the east of the trench in a generally extensional regime. Subsequently, 19–15 Ma basaltic to trachyandesitic backarc lavas with weak arc-like La/Ta (15–26), Ba/La (15–32), and Ta/Hf (0.2–4.5) ratios and a more depleted isotopic signature (ε Nd = +3.9–+4.7; 87Sr/86Sr = 0.7033–0.7037) erupted in a contractional regime. Their chemical features fit with incipient shallowing of the Nazca plate under the northern Neuquén Basin. A contractional regime that extended all along the margin can be explained by westward acceleration of South America over the underlying mantle as Nazca–South America plate convergence slowed.
The evolving chemistry of the Chachahuén volcanic complex provides evidence for transient entry of a subduction zone component into the mantle wedge over a late Miocene shallow subduction zone under the Neuquén Basin. The Chachahuén complex, which is in the backarc of the Andean Southern Volcanic Zone near 37°S and some 500 km east of the Chile Trench, occurs at the intersection of NE and SE fault systems that parallel regional trends. Support for a shallow subduction-zone setting at the time of eruption and during the contractional uplift of the Sierra de Chachahuén comes from K/Ar and new 40 Ar/ 39 Ar ages, mineral assemblages, major and trace element chemistry, and Nd-Sr-Pb isotopic compositions. Importantly, the chemistry of the Chachahuén rocks requires an arc-like component in the mantle that is absent in both early Miocene or Pliocene alkaline lavas erupted in the same region. The oldest Chachahuén volcanic rocks are the ca. 7.3–6.8 Ma Vizcachas group orthopyroxene-bearing andesites to rhyodacites that erupted from fissures and small centers along the NE-trending fault system. Intraplate chemical tendencies in the most silicic samples are attributed to mantle-derived basalts interacting with a lower crust that has a chemical imprint that reflects older alkaline magmatic events. Younger Chachahuén group volcanic rocks erupted at ca. 6.8–6.4 Ma from vents generally aligned along the NE-trending fault system and ca. 6.3–4.9 Ma magmas that erupted from a trap-door–type caldera and flanking stratovolcanoes along the NW-trending fault system. These high-K basaltic to dacitic rocks contain amphibole phenocrysts and show arc-like high field strength element depletions that are the strongest in basaltic andesite lavas. Parallels between Chachahuén volcanic rocks and uplift of the Sierra de Chachahuén with late Miocene Pocho volcanic rocks and uplift of the Pampean Ranges over the modern Chilean flat-slab support transient Miocene shallow subduction zone under the Neuquén Basin.
The margin of northern Venezuela is a complex zone representing the orogenic events from basement formation to subsequent subduction and exhumation during transpressional collision. This boundary zone has six east-west–trending belts that each record a different segment of its development. This geologic complexity requires radiometric ages to unravel, and we herein provide 48 new ages including U-Pb (4), Rb-Sr (2), 40 Ar/ 39 Ar (24), zircon and apatite fission-track (17), and 14 C (1) ages to constrain the evolution of three of these belts. These three belts are the Cordillera de la Costa, Caucagua–El Tinaco, and Serranía del Interior belts. In the Cordillera de la Costa belt, U-Pb geochronologic data indicate portions of the basement igneous and metaigneous rocks formed in the Cambro-Ordovician (513–471 Ma). New 40 Ar/ 39 Ar data from Margarita Island indicate that some of the subduction complex was rapidly cooled and exhumed, whereas other portions indicate slower cooling. This contrasts with new 40 Ar/ 39 Ar data from the Puerto Cabello portion of the subduction complex that has Eocene to Oligocene (42–28 Ma) cooling ages. New fission-track data imply the entire Cordillera de la Costa belt from Puerto Cabello to La Guaira (∼150 km) was uplifted at the same time. In the Caucagua–El Tinaco belt, the oldest 40 Ar/ 39 Ar amphibole ages from the Tinaquillo ultramafic complex are Jurassic (190 Ma). Additional amphibole 40 Ar/ 39 Ar cooling ages are older than previously recorded in either the Tinaco or Tinaquillo complex. One amphibole 40 Ar/ 39 Ar cooling age for the Tinaco complex is similar to previous U-Pb results. New apatite fission-track results from the Serranía del Interior foreland fold and thrust belt are synchronous with exhumation in the Cordillera de la Costa belt. In addition, several zircon fission-track ages in the Serranía del Interior belt are older than their fossil ages, indicating a Cretaceous minimum provenance age for Miocene beds. Significant new findings from these geochronologic studies include (1) several igneous and metaigneous bodies that may be correlated with orogenic events in the Appalachians occur within the subduction mélange; (2) the Tinaquillo complex may record Jurassic rifting; (3) Cretaceous source rocks for the Serranía del Interior sedimentary strata; (4) exhumation of the subduction complex is segmented because two regions have significantly different cooling histories, with Margarita Island exhumed in the Cretaceous, whereas to the west, the Puerto Cabello region has widespread Paleogene cooling and exhumation ages; and (5) earthquake activity in 1812 caused uplift as recorded by exposure of Recent corals.