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Mapping of rock types, structural geology, and hydrothermal alteration, supported by geochronology and thermochronology, sheds light on the original spatial relationships of hydrothermal systems to intrusions in the northern Shoshone Range in north-central Nevada. Rocks in the Hilltop district are tilted ~35–40°E, as indicated by orientations of flattened pumice fiamme and bedding in sedimentary rocks along a single set of presently low-angle normal faults that initiated at 60–70°W dips. New U-Pb zircon geochronology from two sets of dikes in the Lewis district could suggest late Eocene–early Oligocene extension, but definitive crosscutting relations are lacking to demonstrably support this potential earlier period of normal faulting. Reinterpretation of previously reported apatite fission-track cooling ages with a new palinspastic restoration in the Lewis mining district concurs with middle Miocene extension as documented to the south at the Caetano caldera; however, the depth of burial of the Lewis district—and thus the significance of the apatite fissiontrack cooling ages—is uncertain. The comparable orientations and tilting history, supported by fault scaling relations, suggest that the temporally coincident extension in the Caetano caldera to the south represents the along-strike continuation of the same system of normal faults as in the Hilltop and Lewis districts, with changes in observed offset, percent extension, and fault spacing attributed to the gradual tipping out of the fault system northward. [ABSTRACT FROM AUTHOR]

The Sesia zone in the Italian Western Alps is a piece of continental crust that has been subducted to eclogite-facies conditions and records a complex metamorphic history. The exact timing of events and the significance of geochronological information are debated due to the interplay of tectonic, metamorphic, and metasomatic processes. Here we present new geochronological data using Rb-Sr internal mineral isochrons and in situ 40Ar/39Ar laser ablation data to provide constraints on the relative importance of fluid-mediated mineral replacement reactions and diffusion for the interpretation of radiogenic isotope signatures, and on the use of these isotopic systems for dating metamorphic and variably deformed rocks. Our study focuses on the shear zone at the contact between two major lithological units of the Sesia zone, the eclogitic micaschists and the gneiss minuti. Metasedimentary rocks of the eclogitic micaschists unit contain phengite with step-like zoning in major element chemistry as evidence for petrologic disequilibrium. Distinct 40Ar/39Ar spot ages of relict phengite cores and overprinted rims demonstrate the preservation of individual age domains in the crystals. The eclogitic micaschists also show systematic Sr isotope disequilibria among different phengite populations, so that minimum ages of relict assemblage crystallization can be differentiated from the timing of late increments of deformation. The preservation of these disequilibrium features shows the lack of diffusive re-equilibration and underpins that fluid-assisted dissolution and recrystallization reactions are the main factors controlling the isotope record in these subduction-related metamorphic rocks. Blueschist-facies mylonites record deformation along the major shear zone that separates the eclogitic micaschists from the gneiss minuti. Two Rb-Sr isochrones that comprise several white mica fractions and glaucophane constrain the timing of this deformation and accompanying near-complete blueschist-facies re-equilibration of the Rb-Sr system to 60.1 ± 0.9 Ma and 60.9 ± 2.1 Ma, respectively. Overlapping ages in eclogitic micaschists of 60.1 ± 1.1 (Rb-Sr isochron of sheared matrix assemblage), 58.6 ± 0.8, and 60.9 ± 0.4 Ma (white mica 40Ar/39Ar inverse isochron ages) support the significance of this age and show that fluid-rock interaction and partial re-equilibration occurred as much as several kilometers away from the shear zone. An earlier equilibration during high-pressure conditions in the eclogitic micaschists is recorded in minimum Rb-Sr ages for relict assemblages (77.2 ± 0.8 and 72.4 ± 1.1 Ma) and an 40Ar/39Ar inverse isochron age of 75.4 ± 0.8 Ma for white mica cores, again demonstrating that the two isotope systems provide mutually supporting geochronological information. Local reactivation and recrystallization along the shear zone lasted >15 m.y., as late increments of deformation are recorded in a greenschist-facies mylonite by a Rb-Sr isochron age of 46.5 ± 0.7 Ma. [ABSTRACT FROM AUTHOR]

Mafic granulite and amphibolite in the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt, northwest China, southernmost Central Asian orogenic belt, occur as lenses within the matrix of metapelite and marble, exhibiting typical block-in-matrix fabrics of tectonic mélange. Three stages of metamorphic mineral assemblages were identified in these lenses. Clockwise metamorphic pressure-temperature (P-T) paths were obtained through geothermobarometry and thermodynamic pseudosection modeling, passing from 656 °C and 10.9 kbar through 830 °C and 16.5 kbar to 657 °C and 4.9 kbar for the mafic granulite, and from 564-645 °C and 3.2-9.6 kbar through 634-727 °C and 6.1-14.2 kbar to 615-664 °C and 3.2-4.2 kbar for the amphibolites, respectively. Metamorphic peak P-T conditions in the metapelitic country rocks were estimated to be 635-675 °C and 6.0-6.9 kbar. The metamorphic peak of the mafic granulite approaches the high P-T facies series, indicative of a subduction zone. Secondary-ion mass spectrometry and laser ablation- inductively coupled plasma-mass spectrometry U-Pb dating of metamorphic zircons suggests that the metamorphic event occurred between ca. 420 and 372 Ma. These data further certify that the subduction of the continental margin and subsequent uplift of the Dunhuang orogenic belt represent a longlived tectono-metamorphic event in the Paleozoic. [ABSTRACT FROM AUTHOR]

The high values of La/Yb (>15-20) obtained from late Eocene to Pliocene volcanic rocks of central Tibet indicate that source regions were in a thickened (>35 km) crust. The higher Neogene La/Yb values compared to lower Paleogene La/Yb values required continuous crustal thickening, as La/Yb values are proportional to crustal thickness. Although the Paleogene high La/Yb values can be explained by coeval thrusting, this mechanism is unable explain the Neogene higher La/Yb values because central Tibet is covered by undeformed flat-lying early Miocene strata. Here we correlate the temporal variation of Cenozoic volcanic geochemistry with the regional geology of central Tibet, which was part of a larger Eocene-Oligocene thrust belt and the related foreland basin. The Paleogene foreland basin was partitioned into the southern (Hoh Xil) and northern (Qaidam) subbasins by the Kunlun fault ca. 25-20 Ma. The two subbasins were at similar elevations at the time of their formation, but the southern basin, at ~5 km, is >2 km higher than the northern basin at ~2.8 km. Because the southern basin and the Paleogene thrust belt have been associated with Neogene volcanism and high La/Yb values, we suggest that the elevation difference between the two basins was induced by >15 km of magmatic inflation sourced from mantle melting. The magnitude of magmatic inflation is comparable to that of tectonic thickening in central Tibet, highlighting the important and often neglected role of syncollisional mantle melting and its induced magmatic inflation in constructing the thickened Tibetan crust. [ABSTRACT FROM AUTHOR]

Serpentinization is a widespread process that affects large-scale geodynamic processes along plate boundaries, including continental breakup, seafloor spreading, and subduction. Documenting the timing of serpentinization is critical for our understanding of these processes, but direct dating of serpentinites has been challenging or impossible. We present the first application of magnetite (U-Th)/He chronometry to date stages of alteration and cooling in ultramafic rocks. In order to demonstrate the viability of magnetite He dating in these lithologies, magnetite ages were obtained from two ultramafic lithologies of the Kampos mélange belt, a high-pressure-low-temperature subduction complex on the island of Syros, Greece. Magnetite (U-Th)/He measurements of internal fragments from large grains within a chlorite schist and a serpentinite record Miocene exhumation-related cooling ages, whereas smaller grains from the serpentinite record mineral growth associated with hydrothermal fluid flow along Pliocene normal faults. These age results with magnetite trace element geochemistry reveal evidence for multiple episodes of fluid-rock alteration, which has implications for the cooling history and local geochemical exchanges of this high-pressure-low-temperature terrane. This method provides a new tool that may be expanded to investigate the processes and time scales of serpentinization from a variety of tectonic settings. [ABSTRACT FROM AUTHOR]

Detrital zircon U-Pb geochronology has been used extensively to develop provenance histories for surface outcrops of key stratigraphic localities within sedimentary basins. However, many basins lack sufficiently continuous and widespread exposures of complete successions to evaluate proposed longterm tectonic histories, stratigraphic correlations, and paleodrainage patterns within individual basins. Here, we demonstrate the utility of subsurface detrital zircon U-Pb analysis by integrating ages from three key wells (21 subsurface samples) with previously reported data from six exposed intervals (90 surface samples) within a single basin. Samples from the 5-10-km-thick clastic successions span several structural blocks over an ~300 × 50 km swath of the Middle Magdalena Valley Basin, a north-trending intermontane basin in the northern Andes of Colombia. Available U-Pb age distributions for modern rivers highlight the distinctive signatures of several competing sediment sources, including two major contiguous ranges (Central Cordillera and Eastern Cordillera) and two localized block uplifts (Santander Massif and San Lucas range). U-Pb results from Jurassic through Neogene stratigraphic units spanning the nine surface and subsurface sites, including several type localities, enable comparisons of provenance shifts at specific sites and spatial variations among key stratigraphic intervals across multiple sites. Distinctive age populations for the Andean magmatic arc, retroarc foldthrust belt, and South American craton facilitate correlation of stratigraphic units and reconstruction of the long-term provenance and tectonic evolution of the Middle Magdalena Valley Basin. Nearly all surface and subsurface localities show up-section changes in age spectra consistent with (1) Jurassic growth of extensional subbasins fed by local igneous sources, (2) Cretaceous deposition in an extensive postrift setting, and (3) protracted Cenozoic growth of basin-bounding ranges during Andean crustal shortening. Subsurface samples augment surface samples, highlighting their utility in developing regional source-to-sink relationships, the timing of paleodrainage integration, and tectonic reconstructions. Provenance shifts of mid-Paleocene and latest Eocene-earliest Oligocene age are consistent with incipient uplift of the flanking Central Cordillera and Eastern Cordillera, respectively. However, a well-documented phase of latest Paleocene-middle Eocene beveling of basement uplifts in the Middle Magdalena Valley Basin appears to be largely aliased in the detrital record. Moreover, despite the proximity of the magmatic arc, there is insufficient syndepositional evidence for a proposed Paleogene pulse of magmatism and, in this case, limited utility of U-Pb ages in pinpointing precise depositional (stratigraphic) ages. U-Pb age spectra for Oligocene through Pliocene basin fill underscore complex along-strike (north-south) and cross-strike (east-west) variations reflective of compartmentalized transverse deposystems demarcated by pointsource contributions from the Central Cordillera and Eastern Cordillera. The late Miocene appearance of 100-0 Ma grains and a regional switch to broad, multimodal age distributions suggest the initial integration of the longitudinal proto-Magdalena River, linking the Middle Magdalena Valley Basin with southern headwaters in the Upper Magdalena Valley and likely driving increased sedimentation rates farther north in the offshore Magdalena submarine fan of the southern Caribbean margin. [ABSTRACT FROM AUTHOR]

We present evidence for an extrusion wedge in the Scandian fold-thrust belt of the central Scandinavian Caledonides (Seve nappe complex). Rb-Sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal-sense roof shear zone and at the reverse-sense floor shear zone between 434 Ma and 429 Ma. A Sm-Nd age of 462 Ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction. Pressure-temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at ~670 ± 50 °C from 17.5 to 14.5 kbar in garnet-kyanite mica schists during reverse-sense shearing, and from 15 to 11 kbar in garnet mica schists during normal-sense shearing. These data and the presence of decompression-related pegmatites dated at 434 Ma and 429 Ma indicate that the Seve nappes form a large-scale extrusion wedge. This wedge extends along strike for at least 150 km and marks an early stage of ultrahigh-pressure metamorphism, exhumation, and orogenic wedge formation in this part of the Scandinavian Caledonides predating the major, post-415 Ma ultrahigh-pressure exhumation processes in southwestern Norway. [ABSTRACT FROM AUTHOR]

U-Pb geochronologic and Hf isotopic analy ses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents signifi cant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early-Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2-1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7-0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with signifi cant production of juvenile crust during late Archean (3.0- 2.5 Ga) and Paleoproterozoic (1.78-1.6 Ga) time and phases of mainly crustal reworking at 2.0-1.78, 1.5-1.3, 1.2-1.0, and 0.6-0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3-0.2 Ga. [ABSTRACT FROM AUTHOR]

U-Pb geochronology of detrital zircons has proven useful in identifying potential source terranes of siliciclastic sediments through the identification of distinct age peaks that can often be linked to geographically restricted regions. In cases where a sedimentary source region is geographically distinct enough to be treated as a point source, simple empirical and theoretical relationships between fluvial catchment area and downstream river length (Hack's law) can be convolved with detrital zircon age analysis of fluvial sediment to quantitatively assess the fluvial transport distance of that sediment from its source. Such an approach is particularly germane to studying the fluvial transport distances of syntectonic sediments for which the presentday distance between sediment and source may be a function of both sedimentary and tectonic transport. This technique is illustrated with an example from the Death Valley extended terrane in the central Basin and Range (United States). Middle Miocene clastic sedimentary rocks located east of Death Valley contain a clast assemblage that includes a distinct Early Jurassic leucomonzo gabbro, the nearest outcrops of which are 80 km to the west-northwest. Previous interpretation of these strata as an alluvial fan sequence required restoration of the clastic rocks to a depositional position within 20 km of the source batholith, and corroborated restorations of extension across central Death Valley based on isopachs in Paleozoic strata and alignment of preextensional contractional structures. Recent reinterpretation of these strata as fluvial in origin permits significantly greater sedimentary transport distances than the alluvial fan interpretation, and thus undercuts the convergence of multiple lines of evidence for large-magnitude extension across central Death Valley. I demonstrate that the Middle Miocene strata contain >75% Early Jurassic detrital zircons. Given the modern areal extent of the Early Jurassic batholith from which these zircons were derived, fluvial transport of these sediments could not have exceeded 25 km, and was most likely <12 km. This estimate is independent of any assumptions regarding depositional environment, and confirms previous interpretations that >200% extension has occurred across central Death Valley since Middle Miocene time. [ABSTRACT FROM AUTHOR]

The eastern California shear zone (ECSZ) and Walker Lane represent an evolving segment of the Pacific-North America plate boundary in the western United States. Understanding temporal variations in strain accumulation and release along plate boundary structures is critical to assessing how deformation is accommodated throughout the lithosphere. Late Pleistocene displacement along the Lone Mountain fault suggests that the Silver Peak-Lone Mountain (SPLM) extensional complex is an important structure in accommodating and transferring strain within the ECSZ and Walker Lane. Using geologic and geomorphic mapping, differential global positioning system surveys, and terrestrial cosmogenic nuclide (TCN) geochronology, we determined rates of extension across the Lone Mountain fault in western Nevada. The Lone Mountain fault displaces the northwestern Lone Mountain and Weepah Hills piedmonts and is the northeastern component of the SPLM extensional complex, a series of down-to-the-northwest normal faults. We mapped seven distinct alluvial fan deposits and dated three of the surfaces using 10Be TCN geochronology, yielding ages of 16.5 ± 1.2 ka, 92 ± 9 ka, and 137 ± 25 ka for the Q3b, Q2c, and Q2b deposits, respectively. The ages were combined with scarp profile measurements across the displaced fans to obtain minimum rates of extension; the Q2b and Q2c surfaces yield an extension rate between 0.1 ± 0.1 and 0.2 ± 01 mm/yr and the Q3b surface yields a rate of 0.2 ± 0.1-0.4 ± 0.1 mm/yr, depending on the dip of the fault. Active extension on the Lone Mountain fault suggests that it helps partition strain off of the major strike-slip faults in the northern ECSZ and transfers deformation to the east around the Mina deflection and northward into the Walker Lane. Combining our results with estimates from other faults accommodating dextral shear in the northern ECSZ reveals an apparent discrepancy between short- and long-term rates of strain accumulation and release. If strain rates have remained constant since the late Pleistocene, this could refl ect transient strain accumulation, similar to the Mojave segment of the ECSZ. However, our data also suggest a potential increase in strain rates between ca. 92 ka and ca. 17 ka, and possibly to present day, which may also help explain the mismatch between long- and short-term rates of deformation in the region. [ABSTRACT FROM AUTHOR]

The Middle Stone Age (MSA) of Africa, like the Middle Paleolithic of Europe, is thought to represent a time period wherein toolmakers acquired significant increases in cognitive abilities and physical dexterity. Existing data fail to resolve whether the MSA emerged gradually, abruptly, or discontinuously, and whether this industry reflects the activity of Homo sapiens. Here we present new 40Ar/39Ar geochronological data revealing that advanced MSA archaeology at two sites in the main Ethiopian Rift is older than 276 ka, much older than technologically comparable MSA archaeology from elsewhere. An age of 183 ka for a unit farther upsection, along with the technological stasis observed throughout the section, indicates that similar technology was used here for ∼93 ka. These results suggest that MSA technology evolved asynchronously in different places, and challenge the notion of a distinct time line for either the appearance of the MSA or the disappearance of the earlier Acheulean. These and other recent results indicate that the oldest known MSA consistently predates fossil evidence for the earliest Homo sapiens. [ABSTRACT FROM AUTHOR]

The article presents the discussion on New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid. Topics include marine red beds of the Oligocene Kulapis Formation showing mafic conglomerates and chloritic shales deriving from nearby ophiolitic sources; and ultramafic rocks normally devoiding of primary zircons but zircon crystallizing in ophiolitic rocks during hydrothermal alteration.

The Miocene Paine Granite in the Torres del Paine Intrusive Complex, southern Chile, is an extraordinary example of an upper crustal mafic and granitic intrusion. The granite intruded as a series of three sheets, each one underplating the previous sheet along the top of the basal Paine Mafic Complex. High-precision U/Pb geochronology on single zircons using isotope dilution-thermal ionization mass spectrometry yields distinct ages of 12.59 ± 0.02 Ma and 12.50 ± 0.02 Ma, respectively, for the first and last sheet of the laccolith. This age relationship is consistent with field observations. The zircon ages define a time frame of 90 ± 40 k.y. for the emplacement of a >2000-m-thick granite laccolith. These precise U-Pb zircon ages permit identification of the pulses in a 20 k.y. range. The data obtained for the Paine Granite fill the gap between 100 k.y. and 100-1000 yr pulses described in the literature for crustal magma chambers. [ABSTRACT FROM AUTHOR]

The ∼2800-km-long Snowbird tectonic zone is one of the most controversial tectonic features of the Canadian shield. Metamorphic and in situ geochronologic data reported here reveal that a 1.9 Ga medium- to high-pressure belt extends along most of this tectonic zone. In contrast to recent interpretations, a collisional origin is indicated by the length of this metamorphic belt, tectonic thickening documented in parts of it, geological contrasts across it, and subduction-type microdiamonds within it. This collisional event marks a pre—1.865 Ga phase of the Hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of Laurentia. [ABSTRACT FROM AUTHOR]

Geomorphologic mapping and cosmogenic radionuclide (36Cl) dating of an offset fluvial terrace yield a preferred late Holocene slip rate of 20.5 ± 5.5 mm/yr for the central part of the North Anatolian fault, Turkey; an independent slip rate constrained by 14C ages is 20.5 ± 8.5 mm/yr. These rates are generally similar to, but possibly slightly slower than, the shortterm rate of elastic strain storage of 25 ± 1 measured geodetically across this major strike-slip fault (Reilinger et al., 2006), suggesting that loading and strain release on this part of the North Anatolian fault have been relatively constant when averaged over the past ∼2-2.5 k.y. We attribute this consistency to the relatively simple structure of the Anatolia-Eurasia plate boundary in north-central Turkey, where almost all plate boundary strain is accommodated along the North Anatolian fault. The absence of other moderate to high slip rate faults (and the earthquakes they produce) leads to a relatively simple stress evolution for the fault dominated by steady tectonic loading. [ABSTRACT FROM AUTHOR]

Both Taconian (ca. 450 Ma) and Acadian (ca. 380 Ma) ages of high-grade deformation and metamorphism have been advocated for the Western Blue Ridge, southern Appalachians, United States. To resolve this debate, metamorphic monazite from the Great Smoky Mountains was dated via in situ microsampling plus isotope dilution-thermal ionization mass spectrometry (ID-TIMS). U-Pb ages are 450 ± 5 Ma (±2σ error), and mean 207Pb/206Pb ages are 453 ± 10 Ma. These data indicate that peak metamorphism and high-grade deformation occurred during the Taconian orogeny, although they do allow for a low-grade (subamphibolite-facies) Acadian overprint. Unresolved issues remain regarding the comparability of U-Pb TIMS ages with those obtained via other methods. [ABSTRACT FROM AUTHOR]

The article discusses how plate tectonics can be used as a framework through which we understand the large-scale Phanerozoic history of Earth. Topics discussed include studies of Spherule beds are preserved within Archean age rocks in the Barberton greenstone belt, South Africa and the Pilbara craton, Australia; and use of Geodynamic calculations as an important hypothesis-testing tool, when tool when combined with the Precambrian geological record.

The relative importance of tectonism in the stratigraphic record should be more clearly expressed during greenhouse times of Earth history due to the lack of overmasking effects of high-frequency and high-amplitude sea-level changes typical for icehouse periods. Establishment of the importance of tectonism, especially at temporal scales comparable to durations of typical transgressive-regressive cycles, has been plagued by poor temporal resolution. Our outcrop and subsurface-based study of a Cenomanian shallow-marine siliciclastic interval, constrained by bentonite-based geochronology and detailed biostratigraphy, examines this problem. In a 2.2 m.y. interval, we identified four tectonically driven erosional surfaces that dominate preserved stratigraphy. Biostratigraphic correlation to a sea-level curve for the Cenomanian—where coeval high-frequency lowamplitude eustatic cyclicity has been demonstrated—allows the first direct comparison of the effects of eustasy and tectonics at temporal scales of hundreds of thousands of years during a global greenhouse time. We suggest that minor tectonic pulses locally overshadow the effects of eustasy and exert the dominant control over preserved stratigraphy. While subtle tectonic control on sedimentation has been documented throughout the Cretaceous Western Interior, the results of this study suggest that much of that deformation occurred at sub-million-year frequencies and at time scales comparable to eustatic transgressiveregressive cycles. [ABSTRACT FROM AUTHOR]

New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ± 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ± 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. 685-750 Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the "Sturtian" ice age was markedly diachronous, and/or there was more than one "Sturtian"-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than "snowball" or "slushball" Earth ice ages. [ABSTRACT FROM AUTHOR]

Fissure eruptions may provide important information on the shallow propagation of dikes at volcanoes. Somma-Vesuvius (Italy) consists of the active Vesuvius cone, bordered to the north by the remnants of the older Somma edifice. Historical chronicles are considered to define the development of the 37 fissure eruptions between A.D. 1631 and 1944. The 1631 fissure, which reopened the magmatic conduit, migrated upward and was the only one triggered by the subvertical propagation of a dike. The other 25 fissure eruptions migrated downward, when the conduit was open, through the lateral propagation of radial dikes. We suggest two scenarios for the development of the fissures. When the summit conduit is closed, the fissures are fed by vertically propagating dikes. When the summit conduit is open, the fissures are fed by laterally propagating dikes along the volcano slopes. Consistent behaviors are found at other composite volcanoes, suggesting a general application to our model, independent of the tectonic setting and composition of magma. At Vesuvius, the historical data set and our scenarios are used to predict the consequences of the emplacement of fissures after the opening of the conduit. The results suggest that, even though the probability of opening of vents within the inhabited south and west slopes is negligible, the possibility that these are reached by a lava flow remains significant. [ABSTRACT FROM AUTHOR]

The southern margin of the Pilbara Craton in northwestern Australia underwent regional heating, folding, and thrusting as well as extensive fluid flow during collision in the Paleoproterozoic. However, the precise tinting of this event is uncertain. Previous geochronologic studies of Archean basement rocks have yielded a wide range of younger radiometric dates (from ca. 2.4 Ga to ca. 2.0 Ga), interpreted to record multiple hydro- thermal alteration events. In situ U-Pb geochronology of authigenic monazite and xenotime in very low grade Archean metasedimentary rocks from across the craton suggests that the Pilbara was affected by at least two cryptic thermotectonic events, the first between ca. 2430 Ma and ca. 2400 Ma, and the second between ca. 2215 Ma and ca. 2145 Ma. The older event is restricted to three localities in the west, and its cause is unknown. The younger event affected most of the craton (>100,000 km²), spanning a 70 m.y. period from ca. 2215 Ma nearest the collisional margin in the south, to ca. 2145 Ma toward the craton interior in the north. The widespread geographic and stratigraphic distribution of ca 2.2 Ga phosphates suggests that fluid flow was pervasive. This event was probably driven by the northward-advancing Ophthalmian fold-and-thrust belt that developed during protracted collision. The associated low-grade metamorphic front migrated over ∼350 km at an average rate of ∼5.0 mm/yr, placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings. [ABSTRACT FROM AUTHOR]

The Salinian block in the Salinas Valley region of central California consists of arc granitic and metasedimentary rocks (schist of Sierra de Salinas) sandwiched between coeval high-pressure, low-temperature mé1ange belts. U-Pb zircon ages of three granitic plutons from this region range from 88 to 82 Ma, and coexisting biotite yielded 4°Ar/[sup 39]Ar cooling ages of 76-75 Ma. The U-Pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117-81 Ma igneous provenance. Muscovite and biotite [sup 40]Ar/ [sup 39]Ar cooling ages of 72-68 Ma from the nearby schist are distinctly younger than those from the granitic plutons. These data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks, contradicting the prevailing view that the schist comprises the local framework for the Salinian arc. We propose that the schist of Sierra de Salinas was thrust beneath the Salinian magmatic arc along a Campanian thrust fault that has not been recognized. This hypothesis implies that the Salinian arc originated as a klippe of basement rocks derived from the vicinity of the western Mojave Desert. Thrusting initiated southeastward-migrating Laramide tectonism of a style similar to that which formed the Vincent thrust and the latest Cretaceous and Paleocene Pelona and Orocopia Schists of southern California and southwestern Arizona. [ABSTRACT FROM AUTHOR]

We present the results of volcanological, geochemical, and geochronological studies of volcanic rocks from Malpelo Island on the Nazca plate (15.8-17.3 Ma) belonging to the Galápagos hotspot tracks, and igneous complexes (20.8-71.3 Ma) along the Pacific margin of Costa Rica and Panama. The igneous complexes consist of accreted portions of ocean island and seamount volcanoes and aseismic ridges, representing the missing (primarily subducted) history of the Galápagos hotspot. The age and geochemical data directly link the Galápagos hotspot tracks on the Pacific Ocean floor to the Caribbean large igneous province (ca. 72-95 Ma), confirming a Pacific origin for the Caribbean oceanic plateau from the Galápagos hotspot. We propose that emplacement of this oceanic plateau between the Americas and interaction of the Galápagos hotspot tracks with the Central American Are played a fundamental role in the formation of land bridges between the Americas in Late Cretaceous-Palcocene and Pliocene-Holocene time. The land bridges allowed the exchange of terrestrial faunas (e.g., dinosaurs, mastodons, saber-tooth cats, and ground sloths) between the Americas and served us barriers for the exchange of marine organisms between the central Pacific Ocean and the Caribbean Sea and the central Atlantic Ocean. [ABSTRACT FROM AUTHOR]

Provides information on the extensional tectonism, ocean-chemistry fluctuations and biological diversification during the late Neoproterozoic Era, preserved by the Chuar Group, exposed only in the Grand Canyon in Arizona. Documentation of shallow-marine deposition; Geochronology of the Chuar Group.