Showing papers in "Geological Society of America Bulletin in 2017"

Tectonic evolution of the Qilian Shan: An early Paleozoic orogen reactivated in the Cenozoic

Abstract: The Qilian Shan, located along the northeastern margin of the Tibetan Plateau, has experienced multiple episodes of tectonic deformation, including Neoproterozoic continental breakup, early Paleozoic subduction and continental collision, Mesozoic extension, and Cenozoic intracontinental orogenesis resulting from the India-Asia collision. In the central Qilian Shan, pre-Mesozoic ophiolite complexes, passive-continental margin sequences, and strongly deformed forearc strata were juxtaposed against arc plutonic/ volcanic rocks and ductilely deformed crystalline rocks during the early Paleozoic Qilian orogen. To better constrain this orogen and the resulting closure of the Neoproterozoic–Ordovician Qilian Ocean, we conducted an integrated investigation involving geologic mapping, U-Th-Pb zircon and monazite geochronology, whole-rock geochemistry, thermo barometry, and synthesis of existing data sets across northern Tibet. The central Qilian Shan experienced two phases of arc magmatism at 960–870 Ma and 475–445 Ma that were each followed by periods of protracted continental collision. Integrating our new data with previously published results, we propose the following tectonic model for the Proterozoic–Paleozoic history of northern Tibet. (1) Early Neoproterozoic subduction accommodated the convergence and collision between the South Tarim–Qaidam and North Tarim–North China continents. (2) Late Neoproterozoic rifting partially separated a peninsular Kunlun-Qaidam continent from the southern margin of the linked Tarim–North China craton and opened the Qilian Ocean as an embayed marginal sea; this separation broadly followed the trace of the earlier Neoproterozoic suture zone. (3) South-dipping subduction along the northern margin of the Kunlun-Qaidam continent initiated in the Cambrian, first developing as the Yushigou supra-subduction zone ophiolite and then transitioning into the continental Qilian arc. (4) South-dipping subduction, arc magmatism, and the convergence between Kunlun-Qaidam and North China continued throughout the Ordovician, with a trenchparallel intra-arc strike-slip fault system that is presently represented by high-grade metamorphic rocks that display a pervasive right-lateral shear sense. (5) Counterclockwise rotation of the peninsular KunlunQaidam continent toward North China led to the closure of the Qilian Ocean, which is consistent with the right-lateral kinematics of intra-arc strike-slip faulting observed in the Qilian Shan and the westward tapering mapview geometry of Silurian flysch-basin strata. Continental collision at ca. 445–440 Ma led to widespread plutonism across the Qilian Shan and is recorded by recrystallized monazite (ca. 450–420 Ma) observed in this study. Our tectonic model implies the parallel closure of two oceans of different ages along the trace of the Qilian suture zone since ca. 1.0 Ga. In addition, the Qilian Ocean was neither the Protonor Paleo-Tethys (i.e., the earliest ocean separating Gondwana from Laurasia), as previously suggested, but was rather a relatively small embayed sea along the southern margin of the Laurasian continent. We also document >200 km of Cenozoic north-south shortening across the study area. The observed shortening distribution supports models of Tibetan Plateau development that involve distributed crustal shortening and southward underthrusting of Eurasia beneath the plateau. This India-Asia convergence-related deformation is focused along the sites of repeated ocean closure. Major Cenozoic left-slip faults parallel these sutures, and preexisting subduction-mélange channels may have facilitated Cenozoic shortening and continental underthrusting.

Late Paleozoic closure of the Ob-Zaisan Ocean along the Irtysh shear zone (NW China): Implications for arc amalgamation and oroclinal bending in the Central Asian orogenic belt

Abstract: The largest accretionary orogen in the world, the Central Asian orogenic belt, has evolved through the assembly of various oceanic and continental blocks. Understanding the processes associated with the development of this orogenic belt relies on precise recognition of the boundaries between various terranes. One such major suture zone, which records the collisional history of the Siberian marginal arc system (Chinese Altai) with intra-oceanic arc systems (East/West Junggar), is the Irtysh shear zone in NW China. The spatial continuity and the tectonic nature of this shear zone are still poorly understood, but its development has supposedly made a significant impact on the architecture of the western Central Asian orogenic belt and the formation of the Kazakhstan orocline. Here, we provide new insight into the evolution of this shear zone based on detrital zircon ages, Hf isotope composition, structural data and Ar/Ar age constraints on the timing of deformation. Our results show a major discrepancy in detrital zircon populations and Hf model ages across the southern Chinese Altai and the northern East/West Junggar, thus allowing us to map the exact location of the tectonic boundary. Detrital zircon data constrain the initial closure of the Ob-Zaisan Ocean to the late Carboniferous (<323 Ma), and new structural and Ar/Ar geochronological data shed light on the subsequent collisional processes. We propose that the collisional zone between the Chinese Altai and the East/West Junggar was initially subjected to crustal thickening at ca. 323-295 Ma, followed by orogen-parallel extension (ca. 295 Ma) and transpressional deformation (ca. 286-253 Ma). The closure of the Ob-Zaisan Ocean in NW China postdates the initial phase of oroclinal bending in the western Central Asian orogenic belt, thus indicating that oroclinal bending initiated during subduction. Based on our new constraints and other available geological data, we suggest that the early stage of oroclinal bending was likely driven by slab rollback.

Latest Permian to Middle Triassic redox condition variations in ramp settings, South China: Pyrite framboid evidence

Abstract: A detailed, 10 m.y. redox history of Changhsingian to Anisian (latest Permian to Middle Triassic) oceans in ramp settings is reconstructed based on framboidal pyrite analysis from South China. The result shows that the well-established phenomenon of intense ocean euxinia-anoxia is faithfully recorded in pyrite framboid data. Three major euxinia-anoxia episodes, namely, the end-Changhsingian to end-Smithian, middle to late Spathian, and early to middle Anisian, have been recognized from the ramp successions. The first reducing episode is subdivided into four subepisodes: Permian-Triassic boundary, Griesbachian-Dienerian boundary, earliest Smithian, and end-Smithian. Redox variations broadly track other oceanographic proxies. Euxinia-anoxia episodes coincide with positive excursions of conodont ΩCe anomalies, negative excursions of δ34Scas (carbonate-associated sulfate), increases in sea-surface temperature, and negative excursions of δ13C in most cases. However, euxinia-anoxia near the Dienerian-Smithian boundary coincided with positive excursions of δ13C and a general cooling period. This exception may be the result of locally developed water-column anoxia. The Permian-Triassic boundary subepisode witnessed two ephemeral euxinia-anoxia events separated by a dysoxic to oxic period. The former, together with a rapid increase in sea-surface temperature (up to 8 °C), may have been responsible for the biodiversity crisis, while the latter anoxic event destroyed ecosystem trophic structures. In addition to the Permian-Triassic boundary euxinia-anoxia event, which spread over habitats in all oceans, the Spathian and Anisian euxinia-anoxia episodes also prevailed in global oceans. Variation of the oxygen minimum zone are suggested as the driving mechanism that facilitated the movement of oxygen-poor water columns in various paleogeographic settings over this critical period.

Aquifer recharge, depletion, and connectivity: Inferences from GRACE, land surface models, and geochemical and geophysical data

Abstract: Data from the Gravity Recovery and Climate Experiment (GRACE) and outputs of the CLM4.5 model were used to estimate recharge and depletion rates for large aquifers, investigate the connectivity of an aquifer’s subbasins, and identify barriers and preferred pathways for groundwater flow within an aquifer system. The Nubian Sandstone Aquifer System and its subbasins (Dakhla, Northern Sudan Platform, and Kufra) in northeast Africa were used for demonstration purposes, and findings were tested and verified against geological, geophysical, remote sensing, geochronologic, and geochemical data. There are four major findings. (1) The average annual precipitation data over recharge areas in the southern Kufra section and the Northern Sudan Platform subbasin were estimated at 54.8 km 3 , and 32.8 km 3 , respectively, and knowing the annual extraction rates over these two areas (∼0.40 ± 0.20 km 3 ), recharge rates were estimated at 0.78 ± 0.49 km 3 /yr and 1.44 ± 0.42 km 3 /yr, respectively. (2) GRACE-derived groundwater depletion rates over the Dakhla subbasin and the Northern Kufra section were estimated at 4.44 ± 0.42 km 3 /yr and 0.48 ± 0.32 km 3 /yr, respectively. (3) The observed depletion in the southern parts of the Dakhla subbasin is apparently caused by the presence of the east-west−trending Uweinat-Aswan basement uplift, which impedes the south-to-north groundwater flow and hence reduces replenishment from recharge areas in the south. (4) A major northeast-southwest−trending shear zone (Pelusium shear system) is apparently providing a preferred groundwater flow pathway from the Kufra to the Dakhla subbasin. Our integrated approach provides a replicable and cost-effective model for better understanding of the hydrogeologic setting of large aquifers worldwide and for optimum management of these groundwater resources.

Emergence and evolution of Santa Maria Island (Azores)—The conundrum of uplifted islands revisited

Abstract: The growth and decay of ocean-island volcanoes are intrinsically linked to vertical movements. While the causes for subsidence are better understood, uplift mechanisms remain enigmatic. Santa Maria Island in the Azores Archipelago is an ocean-island volcano resting on top of young lithosphere, barely 480 km away from the Mid-Atlantic Ridge. Like most other Azorean islands, Santa Maria should be experiencing subsidence. Yet, several features indicate an uplift trend instead. In this paper, we reconstruct the evolutionary history of Santa Maria with respect to the timing and magnitude of its vertical movements, using detailed field work and 40Ar/39Ar geochronology. Our investigations revealed a complex evolutionary history spanning ∼6 m.y., with subsidence up to ca. 3.5 Ma followed by uplift extending to the present day. The fact that an island located in young lithosphere experienced a pronounced uplift trend is remarkable and raises important questions concerning possible uplift mechanisms. Localized uplift in response to the tectonic regime affecting the southeastern tip of the Azores Plateau is unlikely, since the area is under transtension. Our analysis shows that the only viable mechanism able to explain the uplift is crustal thickening by basal intrusions, suggesting that intrusive processes play a significant role even on islands standing on young lithosphere, such as in the Azores.

Mantle and geological evidence for a Late Jurassic–Cretaceous suture spanning North America

Abstract: Crustal blocks accreted to North America form two major belts that are separated by a tract of collapsed Jurassic–Cretaceous basins extending from Alaska to Mexico. Evidence of oceanic lithosphere that once underlay these basins is rare at Earth’s surface. Most of the lithosphere was subducted, which accounts for the general difficulty of reconstructing oceanic regions from surface evidence. However, this seafloor was not destroyed; it remains in the mantle beneath North America and is visible to seismic tomography, revealing configurations of arc-trench positions back to the breakup of Pangea. The double uncertainty of where trenches ran and how subducting lithosphere deformed while sinking in the mantle is surmountable, owing to the presence of a special-case slab geometry. Wall-like, linear slab belts exceeding 10,000 km in length appear to trace out intra-oceanic subduction zones that were stationary over tens of millions of years, and beneath which lithosphere sank almost vertically. This hypothesis sets up an absolute lower-mantle reference frame. Combined with a complete Atlantic spreading record that positions paleo–North America in this reference frame, the slab geometries permit detailed predictions of where and when ocean basins at the leading edge of westwarddrifting North America were subducted, how intra-oceanic subduction zones were overridden, and how their associated arcs and basement terranes were sutured to the continent. An unconventional paleogeography is predicted in which midto late Mesozoic arcs grew in a long-lived archipelago located 2000–4000 km west of Pangean North America (while also consistent with the conventional view of a continental arc in early Mesozoic times). The Farallon Ocean subducted beneath the outboard (western) edge of the archipelago, whereas North America converged on the archipelago by westward subduction of an intervening, major ocean, the Mezcalera-Angayucham Ocean. The most conspicuous geologic prediction is that of an oceanic suture that must run along the entire western margin of North America. It formed diachronously between ca. 155 Ma and ca. 50 Ma, analogous to diachro nous suturing of southwest Pacific arcs to the northward-migrating Australian continent today. We proceed to demonstrate that this suture prediction fits the spatio-temporal evidence for the collapse of at least 11 Middle Jurassic to Late Cretaceous basins wedged between the Intermontane and Insular-Guerrero superterranes, about half of which are known to contain mantle rocks. These relatively late suturing ages run counter to the Middle Jurassic or older timing required and asserted by the prevailing, Andean-analogue model for the North American Cordillera. We show that the arguments against late suturing are controvertible, and we present multiple lines of direct evidence for late suturing, consistent with geophysical observations. We refer to our close integration of surface and subsurface evidence from geology and geophysics as “tomotectonic analysis.” This type of analysis provides a stringent test for currently accepted tectonic models and offers a blueprint for similar, continental-scale investigations in other accretionary orogens.

Evolution of fluvial meander-belt deposits and implications for the completeness of the stratigraphic record

Abstract: The fragmentary nature of the stratigraphic record is particularly evident with respect to fluvial deposits, which are characterized by a hierarchy of depositional units deposited over a wide range of time scales and sedimentation rates. We quantified stratigraphic completeness in meander-belt deposits through deducing the total area of bar sedimentation versus what is ultimately preserved in the depositional record, using area as a surrogate metric for sediment volume. Data sets were evaluated for a numerical model, the modern Mississippi River valley, and the Cretaceous McMurray Formation. In each data set, the evolutionary history of a series of meander-belt elements was discerned. Migrated area between successive reconstructed paleochannel positions was measured, representing: total area of net bar migration (MA), the area of bar preserved (PA), and percent of bar preserved (PA/MA), at the accretion package, bar, and meanderbelt scale. Results of our analysis show that the average preservation percent ranges from 27.3% to 67.8% for an accretion package, 35.0% to 85.1% for a bar, and 38.2% to 67.6% for a meander belt. The processes that lead to a decrease in preservation include intra-meanderbend erosion (due to downstream translation or bar rotation), and increasing meanderbend sinuosity and eventual cutoff (neck and chute), as well as inter-meander-bend erosion due to avulsion and subsequent migration of the meandering channel. The results of this study document a decrease in preservation over time that follows a natural logarithmic function of decay; we have termed this the “survivability” curve. The results presented here document a systematic, monotonic decrease in preservation over time, which is consistent regardless of the spatial or temporal scale and agrees with probabilities of preservation at long time scales proposed by previous workers. A comparison between data sets allows for an estimation of the time span represented by meander-belt deposits in the deep time record.

Fluvial stratigraphy of valley fills at Aeolis Dorsa, Mars: Evidence for base-level fluctuations controlled by a downstream water body

Abstract: Aeolis Dorsa, a large sedimentary basin on Mars, contains an array of fluvially dominated sedimentary deposits. These deposits preserve a record of fluvial erosion and deposition during early Martian history. We present evidence that some of these fluvial deposits represent incised valleys carved and filled during falls and rises in base level, which were likely controlled by changes in water-surface elevation of a large lake or sea. The valley stratigraphy consists of three lowalbedo, channelized corridors, each several tens of kilometers long in the streamwise direction. Deposits composing the basal valley fills are characterized by laterally amalgamated point-bar strata confined between valley walls that preserve scoop-shaped segments cut by the erosive outer banks of meandering river bends. Both the point-bar deposits and valley walls were produced by a neterosional river system. Subsequent valleyfilling deposits are defined by both channels and associated overbank strata. The stacked channel-filling deposits are sinuous in form, but unlike the basal strata, they preserve no evidence of river migration. Within each valley, there are multiple sinuous ridges ranging from a few meters to several tens of meters thick, which we interpret as channel-belt deposits that have been topographically inverted via differential erosion. Evidence for channel avulsions and reoccupations, the overall cutting and filling patterns, and consistent up-section decreases in recorded channel migration support the interpretation of the low-albedo corridors as valley stratigraphy cut and filled in the presence of a migrating backwater zone. Crosscutting valleys require at least two episodes of base-level fall and rise at >50 m per episode. These base-

Paleosols and associated channel-belt sand bodies from a continuously subsiding late Quaternary system (Po Basin, Italy): New insights into continental sequence stratigraphy

Abstract: Previous sequence-stratigraphic work has emphasized the key role of paleosols and associated sand-dominated fluvial bodies as key features for interpreting alluvial architecture. The temporal resolution of the ancient record is, however, insufficient to fully explain the complex relationship between soil formation and the evolution of fluvial systems under changing sea-level and climate conditions. In this paper, we present a detailed record of paleosol−channel belt relationships reconstructed from the subsurface of a rapidly subsiding region (Po Plain, Italy) that spans almost all of the last glacial-interglacial cycle (∼120 k.y.). The studied succession preserves a systematic bipartite zonation into a thick paleosol-bearing segment close to the basin margin and a sand-dominated interval, with vertically amalgamated channel belts, in an axial position. Individual paleosols are weakly developed and represent key stratigraphic markers that can be traced basinwide into adjacent, essentially contemporaneous, unconfined channel-belt deposits. Unlike conventional models of late Quaternary alluvial−coastal plain systems, no persistent incised valley was established in the Po system during the last glacial-interglacial cycle. Continuous accommodation was the key depositional control on alluvial stratigraphy during the prolonged (∼90 k.y.) phase of late Pleistocene sea-level fall, which led to the deposition of a thick, dominantly aggradational alluvial succession. The development of shallowly incised, short-lived valley systems took place only at the transition to glacial stages associated with substantial sea-level drop (marine oxygen isotope stage [MIS] 3-2 transition, and possibly MIS 5-4 transition). This study shows that in rapidly subsiding settings with high rates of sedimentation, incised valley systems may be replaced by aggradationally stacked, essentially nonincised fluvial bodies. In these cases, overbank packages bounded by immature paleosols represent the most likely alternative to the highly weathered interfluve paleosol predicted by classic sequence-stratigraphic models. Fourth-order sequence boundaries and lower-rank erosional surfaces may be easily confused at the ∼100 k.y. scale, and transgressive surfaces, defining the onset of retrogradation, may become the most readily identifiable sequence-stratigraphic surfaces.

New insights into the onset and evolution of the central Apennine extensional intermontane basins based on the tectonically active L’Aquila Basin (central Italy)

Abstract: Study of the tectonically active L'Aquila Basin offers new insights into both the creation of the extensional intermontane basins of the central Apennines of Italy and their tectono-sedimentary evolution through time. The combination of large mammal remains, ostracods, molluscs, Mousterian tools, and 14 C dating allows better definition of the onset and stratigraphic evolution of the L'Aquila Basin. Interpretation of a seismic reflection profile and well-log data allow evaluation of the subsurface setting of this sedimentary basin and its tectono-sedimentary evolution. The occurrence of a wedge-shaped seismic unit at the base of the basin sedimentary succession defines the first phase of basin fill during a late Piacenzian–Gelasian synrift stage. Activity along the main fault of the extensional fault system responsible for the onset and subsequent development of the western sector of the L'Aquila Basin (L'Aquila– Scoppito subbasin) migrated from southwest to northeast, reaching the presently active Mount Pettino normal fault only in the late Pleistocene–Holocene. The onset of sedimentation in the L'Aquila Basin was synchronous with the onset in the Tiberino Basin, and so the idea that these extensional GSA Bulletin; Month/Month 2017; v. 129; no. intermontane basins become progressively younger from the Tyrrhenian toward the Adriatic side of the central Apennines is rejected. In the northern and central Apen-nines, only two major syndepositional ex-tensional domains can be recognized: a late Miocene rifting area, which includes all the late Miocene extensional basins in Tuscany, and a late Pliocene to earliest Pleistocene rifting area, which possibly includes all the intermontane basins from the Tiberino Basin to the Sulmona Basin. The different time gaps between compressional and exten-sional deformation at any given locality in the central Apennines could indicate a partial decoupling of processes responsible for the migration of shortening and extension toward the foreland. Diachroneity between the eastward migration of shortening in the foreland and extension in the inner part of the orogen supports the notion that the central Apennines were created as a result of a partially decoupled collision zone. Study of the onset of the central Apennine extensional intermontane basins, together with their seismic activity, indicates that the central Apennine postorogenic extensional domain represents an archive of ~3 m.y. of continued crustal extension. These findings help to refine models of the long-term extensional rate of the central Apennines, and they provide a basis for more reliable seismotectonic models for one of the most seismically active sectors of the central Mediterranean area.

New age constraints for early Paleogene strata of central Patagonia, Argentina: Implications for the timing of South American Land Mammal Ages

Abstract: Fil: Krause, Javier Marcelo. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Museo Paleontologico Egidio Feruglio; Argentina. Universidad Nacional del Sur. Departamento de Geologia; Argentina

Intracontinental subduction beneath the Pamir Mountains: Constraints from thermokinematic modeling of shortening in the Tajik fold-and-thrust belt

Abstract: American Philosophical Society; American Association of Petroleum Geologists; Geological Society of America; Exxon Mobil Corporation

Timing of deep-water slope evolution constrained by large-n detrital and volcanic ash zircon geochronology, Cretaceous Magallanes Basin, Chile

Abstract: Deciphering depositional age from deposits that accumulate in deep-water slope settings can enhance understanding of shelf-margin evolutionary timing, as well as controlling mechanisms in ancient systems worldwide. Basin analysis has long employed biostratigraphy and/or tephrochronology to temporally constrain ancient environments. However, due to poor preservation of index fossils and volcanic ash beds in many deepwater systems, deducing the timing of slope evolution has proven challenging. Here, we present >6600 new U-Pb zircon ages with stratigraphic information from an ~100-kmlong by ~2.5-km-thick outcrop belt to elucidate evolutionary timing for a Campanian– Maastrichtian slope succession in the Magallanes Basin, Chile. Results show that the succession consists of four stratigraphic intervals, which characterize four evolutionary phases of the slope system. Overall, the succession records 9.9 ± 1.4 m.y. (80.5 ± 0.3 Ma to 70.6 ± 1.5 Ma) of graded clinoform development punctuated by out-of-grade periods distinguished by enhanced coarse-grained sediment bypass downslope. Synthesis of our results with geochronologic, structural, and stratigraphic data from the basin suggests that slope evolution was largely controlled by an overall decline in basin subsidence from 82 to 74 Ma. In addition to providing insight into slope evolution, our results show that the reliability of zircon-derived depositional duration estimates for ancient sedimentary systems is controlled by: (1) the proportion of syndepositionally formed zircon in a strati-

Late Quaternary climatic controls on erosion rates and geomorphic processes in western Oregon, USA

Abstract: Climate regulation of erosion in unglaciated landscapes remains difficult to decipher. While climate may disrupt process feedbacks that would otherwise steer landscapes toward steady erosion, sediment transport processes tend to erase past climate landforms and thus bias landscape evolution interpretations. Here, we couple a 50 k.y. paleoenvironmental record with 24 10Be-derived paleo-erosion rates from a 63-m-thick sediment archive in the unglaciated soil-mantled Oregon Coast Range. Our results span the forested marine oxygen isotope stage (MIS) 3 (50–29 ka), the subalpine MIS 2 (29–14 ka), and the forested MIS 1 (14 ka to present). From 46 ka through 28.5 ka, erosion rates increased from 0.06 mm yr–1 to 0.23 mm yr–1, coincident with declining temperatures. Mean MIS 2 erosion rates remained at 0.21 mm yr–1 and declined with increasing MIS 1 temperatures to the modern mean rate of 0.08 mm yr–1. Paleoclimate reconstructions and a frost-weathering model suggest periglacial processes were vigorous between 35 and 17 ka. While steady erosion is often assumed, our results suggest that climate strongly modulates soil production and transport on glacial-interglacial time scales. By applying a cosmogenic paleo-erosion model to evaluate 10Be concentrations in our sedimentary archive, we demonstrate that the depth of soil mixing (which is climate-dependent) controls the lag time required for cosmogenic erosion rates to track actual values. Our results challenge the widely held assumption that climate has minimal impact on erosion rates in unglaciated midlatitude terrain, which invites reconsideration of the extent to which past climate regimes manifest in modern landscapes.

Synthesis of the 780–740 Ma Chuar, Uinta Mountain, and Pahrump (ChUMP) groups, western USA: Implications for Laurentia-wide cratonic marine basins

Abstract: The upper Tonian Chuar, Uinta Mountain, and middle Pahrump (ChUMP) groups of present-day western Laurentia collectively record the early breakup of Rodinia, large-scale perturbations in the carbon cycle, and eukaryotic evolution, all of which preceded the onset of global glaciation by tens of millions of years. The spectacularly preserved and shale-rich Chuar Group of the Grand Canyon Supergroup stands out as one of the best global records of this time period, particularly for paleobiology. A new U-Pb age of 782 Ma on detrital zircons ( n = 14 young grains) from the underlying Nankoweap Formation refines the Chuar Group’s maximum depositional age to younger than 782 Ma. A new 40 Ar/ 39 Ar age of 764 ± 16 Ma (2σ) from K-feldspar within early diagenetic marcasite nodules from the upper Chuar Group (Awatubi Member) helps calibrate the rich Chuar microfossil record and constrain the large-magnitude shift in δ 13 C org (up to 18‰; referred to here as the Awatubi positive carbon-isotope excursion or APCIE) to between ca. 764 and ca. 742 Ma, the date of an ash near the top of the Chuar Group. In addition to the maximum depositional age of ca. 782 Ma, U-Pb detrital zircon analyses ( n = 826 grains) on sandstone beds from the underlying Nankoweap Formation indicate the presence of multiple older Laurentian age peaks. The similarity of detrital zircon populations and sedimentary character to that of the overlying Chuar Group ( n = 764 grains) suggests that the Nankoweap Formation should be included as the lowermost unit in the Chuar Group. This revised geochronological framework indicates a 300 Ma unconformity between the Chuar Group (including the Nankoweap Formation) and the underlying 1.1 Ga Cardenas Basalt of the Unkar Group. Chuar Group detrital zircon populations share similarities with those of the Uinta Mountain Group and especially the middle Pahrump Group, including ca. 780 Ma grains. Biostratigraphic correlation using microfossils enhances the ChUMP connection and shows a trend of higher acritarch diversity in the lower Chuar and Uinta Mountain groups, and the presence of vase-shaped microfossils in the upper intervals of all three ChUMP units. Comparisons of δ 13 C org and δ 13 C carb among ChUMP successions suggest a combination of local and regional controls. Thus, ChUMP successions are coeval within the 780−740 Ma range, show similar fossil and C-isotope trends, and derived sediments from similar Laurentian sources or source types. In light of recent age constraints and compiled paleontology in other Neoproterozoic basins, our high-resolution correlation of ChUMP successions can be extended to the Callison Lake dolostone of NW Canada and the Akademikerbreen-Polarisbreen groups of Svalbard. Biostratigraphic correlation with poorly age-constrained strata such as the Akademikerbreen-Polarisbreen groups and, farther afield, the Visingso Group of Baltica suggests that ChUMP units record continentwide—and perhaps global—evolutionary patterns. The δ 13 C org and δ 13 C carb values in the Chuar Group and its equivalents in Canada and Svalbard show broadly similar trends, including the APCIE, suggesting that δ 13 C org values from organic-rich shale record variations in the C-isotope composition of late Tonian oceans. Intracratonic basins and contiguous rift margins of ChUMP age are inferred to have been important locations for microbial productivity and significant organic carbon burial that induced large positive shifts in δ 13 C and changes in global carbon balance prior to the onset of snowball Earth.

Early Miocene continental-scale sediment supply to the Gulf of Mexico Basin based on detrital zircon analysis

Abstract: The early Miocene was a period of major continental margin progradation in the Gulf of Mexico Basin that accompanied prominent tectonic and climatic changes in North America. However, sediment pathways from continental upland sources to deep basinal sinks remain poorly constrained. This study presents 2192 new detrital zircon U-Pb analyses from 19 Lower Miocene samples spanning the entire northern Gulf of Mexico margin to elucidate early Miocene sediment provenance and paleodrainage systems. The U-Pb age patterns indicate that the Great Plains, southern Rocky Mountains, and mid-Cenozoic volcanic field were the major source terranes for the western-central Gulf of Mexico coast, whereas the Appalachian foreland basin and Appalachian Mountains mainly contributed sediment to the eastern Gulf of Mexico coast. Local source terranes included the Llano uplift and Edwards Plateau in central Texas and the Ouachita Mountains and foreland basin in Oklahoma and Arkansas. A comparison to previous detrital zircon studies around the Gulf of Mexico indicates that sediment recycling was important during the early Miocene. Sediment associated with major paleorivers, including the Rio Bravo, Rio Grande, Houston-Brazos, Red, Mississippi, Tombigbee, and Apalachicola Rivers, can be differentiated using the detrital zircon U-Pb analyses. These data help to better define the early Miocene source-to-sink system in the northern Gulf of Mexico, by relating the basin fill to hinterland tectonic and geological evolution. In comparison to the Paleocene−Eocene Wilcox drainage system, the early Miocene drainage system of the northern Gulf of Mexico was smaller and received less input from western Mexico arc terranes and Archean basement in Wyoming. This drainage area reduction, related to regional thermal uplift and Basin and Range−Rio Grande rifting, likely explains the reduced sediment volume of the Lower Miocene strata in the Gulf of Mexico relative to the Paleocene−Eocene Wilcox Group.

Hadean origins of Paleoarchean continental crust in the central Wyoming Province

Abstract: The scarce remnants of Earth’s earliest history make it challenging to describe the crust-forming processes that operated during that time, and all evidence that survived subsequent tectonism and recycling deserves to be studied closely. We present geologic, petrologic, geochemical, and isotopic descriptions of Paleoarchean gneisses in the central Wyoming Province. We identify two groups of gneisses: a bimodal suite of amphibolite and tonalite-trondhjemite-granodiorite (TTG) layered gneisses (3385−3450 Ma), and a suite of massive trondhjemite and granite gneisses (3300−3330 Ma). Here, 3.82 Ga inherited zircon components are present in several samples. Negative bulk rock initial e Nd values also indicate that older crust was involved. Oxygen isotopic compositions of zircon mainly fall within the range of mantle zircon δ 18 O values, but several analyses extend up to ∼6.5‰. Initial Hf isotopic compositions of 3.82 Ga zircon are negative and require derivation from Hadean crustal sources. Our data reveal the record of a 3.82 Ga differentiation event during which Hadean crust was partially melted, and zircon crystallized from those melts. Hadean crust must have persisted in the central Wyoming Province until 3.45−3.37 Ga, when mafic crust partially melted to form the TTG layered gneisses, which also incorporated the 3.82 Ga zircons. Subsequent intracrustal recycling at 3.33−3.30 Ga produced calc-alkalic granites. The central Wyoming Province provides a significant addition to the sparse record of Hadean crust being magmatically reworked to form the abundant quartzofeldspathic gneisses common in Archean terrains worldwide, which effectively transformed Earth’s evolving continental crust from mafic to felsic in composition.

Lower Cretaceous cold snaps led to widespread glendonite occurrences in the Sverdrup Basin, Canadian High Arctic

Abstract: Dramatic global climate change in the Early Cretaceous suggests that numerous boreal cool events perturbed otherwise warm conditions. Abundant glendonites in Valanginian and Aptian strata are thought to be key markers of cold conditions; however, their use as climate indicators has been questioned. Therefore, a detailed study of glendonites in the context of host-rock geochemistry was conducted on Cretaceous strata exposed on Ellef Ringnes Island, Sverdrup Basin, Canadian High Arctic, to elucidate paleoenvironment controls on periodic glendonite occurrence throughout the Early Cretaceous. Two prominent glendonite zones were identified in Valanginian and Aptian strata. Data for carbon stable isotopes show δ 13 C carb values of −29.5‰ to −10.5‰, consistent with a carbon source from organic matter within the surrounding shale. Trace metal data suggest deposition occurred under an overall oxidizing water column in a setting with higher than average phosphorus and highly degraded organic matter. There is no variation in the geochemical parameters of the mudstones with or without glendonites. Glendonite-bearing zones are coincident, however, with regional evidence for brief periods of colder climate conditions within the otherwise warm Early Cretaceous. We conclude that while the overall conditions for glendonite formation were prevalent throughout the Cretaceous, it was the brief cold periods that provided the final range of stability for their preservation in discrete zones of Valanginian and Aptian strata. These results support the model of an overall warm Early Cretaceous climate being punctuate by several “cold snaps” of as-yet uncertain origin.

Late Devonian to early Carboniferous arc-related magmatism in the Baolidao arc, Inner Mongolia, China: Significance for southward accretion of the eastern Central Asian orogenic belt

Abstract: The Central Asian orogenic belt, formed in response to consumption of the Paleo–Asian Ocean, is one of the largest and most complex accretionary collages in the world and was responsible for considerable Phanerozoic juvenile crustal growth in Central Asia. The timing of subduction-accretion processes and closure of the Paleo–Asian Ocean is controversial. The Xilingol complex, composed of deformed quartzofeldspathic rocks and lenticular or quasi-lamellar amphibolites, is located on the northern section of the eastern Central Asian orogenic belt in Inner Mongolia, China. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of an amphibolite and an epidote amphibolite from the complex. The protolith of the amphibolite is a gabbro or gabbroic diorite with a laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb age of 382 ± 4 Ma and uniform eHf(t) values (–1.06 to +1.25). The protolith of the epidote amphibolite is a quartz diorite with a LA-ICP-MS zircon U-Pb age of 327 ± 5 Ma and uniform positive eHf(t) values (+0.78 to +4.11). The primitive magma of the Devonian gabbroic dike was generated by partial melting of a spinel lherzolite lithospheric mantle that was modified by fluids and melts from subducted slab components. A newly enriched lithospheric mantle is a possible source region for the Devonian mafic rocks. Fractionation of olivine and hornblende played a dominant role in magma differentiation with little or no crustal contamination. Amphibolite-facies metamorphism affected the Devonian gabbroic rocks at 321.6 ± 3.1 Ma, and the quartz diorite underwent epidote amphibolite-facies metamorphism at 279.4 ± 5.3 Ma, based on hornblende 40Ar/39Ar dating. The Devonian to Carboniferous intrusive rocks in the eastern Central Asian orogenic belt likely formed during the collapse of a mature arc at the southern margin of the South Mongolian microcontinent. Combining our results with previous data, we identify an initial phase of postcollisional extension (382–340 Ma) that occurred after earlier compression related to intra-oceanic subduction (484–469 Ma), ridge subduction (440–434 Ma), and arc-continent collision (427–383 Ma). We also constrain increasing extension accompanying extensive collapse of the mature arc (340–309 Ma), northward subduction of the forearc oceanic crust (322–274 Ma) coeval with development of the Hegenshan back-arc basin (354–269 Ma), and final collision (246–228 Ma). The presence of an accretionary belt along the southern margin of the South Mongolian microcontinent reflects the importance of continental growth by accretion of an arc chain during the Late Cambrian–Middle Triassic in the eastern Central Asian orogenic belt.

An Antarctic stratigraphic record of stepwise ice growth through the Eocene-Oligocene transition

Abstract: Earth's current icehouse phase began ?34 m.y. ago with the onset of major Antarctic glaciation at the Eocene-Oligocene transition. Changes in ocean circulation and a decline in atmospheric greenhouse gas levels were associated with stepwise cooling and ice growth at southern high latitudes. The Antarctic cryosphere plays a critical role in the ocean-atmosphere system, but its early evolution is still poorly known. With a near-field record from Prydz Bay, Antarctica, we demonstrate that Antarctic ice growth was stepwise and had an earlier onset than previously suggested. Prydz Bay lies downstream of a major East Antarctic Ice Sheet drainage system, and its sedimentary records uniquely constrain the timing of ice-sheet advance onto the continental shelf. We investigated a detrital record extracted from three Ocean Drilling Program drill holes within a new depositional and chronological framework spanning the late Eocene to early Oligocene (ca. 36?33 Ma). The chemical index of alteration (CIA) and the S index, calculated from the major-element geochemistry of bulk samples, yielded estimates of chemical weathering intensities and mean annual temperature on the East Antarctic continent. We document evidence for late Eocene mountain glaciation along with transient warm events at 35.8?34.8 Ma. From 34.4 Ma, associated with the Eocene-Oligocene transition precursor ?18O excursion, glaciers advanced into Prydz Bay, coincident with a decline in chemical weathering and temperature. We conclude that Antarctic continental ice growth commenced with the Eocene-Oligocene transition “precursor” glaciation, during a time of Subantarctic surface ocean cooling and a decline in atmospheric pCO2. These results call for dynamic high-latitude feedbacks that are currently poorly represented in Earth system models and emphasize the need for additional near-field glacio-sedimentological, high-latitude sea-surface temperature and pCO2 records across the Eocene-Oligocene transition.

Proterozoic–Mesozoic history of the Central Asian orogenic belt in the Tajik and southwestern Kyrgyz Tian Shan: U-Pb, 40Ar/39Ar, and fission-track geochronology and geochemistry of granitoids

Abstract: Multimethod geochronology (U-Pb zircon; 40 Ar/ 39 Ar hornblende, biotite, feldspar; apatite fission track) on granitoids, gneisses, and Cenozoic intramontane basin clastics of the Gissar-Alai ranges, South Tian Shan collisional belt, west of the Talas-Fergana fault, elucidates a history of Neoproterozoic magmatism, late Paleozoic magmatism and metamorphism, and Mesozoic−Cenozoic thermal reactivation. Zircon-core and grain-interior U-Pb ages of ca. 2.7−2.4, 2.2−1.7, 1.1−0.85, and 0.85−0.74 Ga tie the early evolution of the Gissar-Alai ranges to that of the Tarim craton. At least part of the Gissar range crystalline basement—the Garm massif—shows U-Pb zircon crystallization ages of ca. 661‒552 Ma (median ca. 609 Ma), again suggesting a Tarim craton connection. Tarim collided with the Middle Tian Shan block at ca. 310‒305 Ma, completing the protracted formation of the South Tian Shan collisional belt. The massive Gissar range granitoids intruded later (ca. 305‒270 Ma), contemporaneous with peak Barrovian-type metamorphism in the Garm massif rocks. Major- and trace-element compositions suggest that the Gissar granitoid melts have continental arc affinity. Zircon e Hf and whole-rock e Nd values of −2.1 to −6.9 and −2.7 to −7.2, respectively. and Hf-isotope crustal model and Nd-isotope depleted mantle model ages of ca. 1.0‒1.2 and ca. 1.1‒2.2 Ga, respectively, suggest significant input of Precambrian crust in the Gissar granitoid and Garm orthogneiss melts, consistent with the U-Pb ages of inherited and detrital zircons. The distinct ca. 661‒552 Ma Garm gneiss crystallization ages and the ca. 1.0−2.2 Ga model ages (and the lack of 2.4−3.4 Ga model ages) tie the Garm gneisses and the reworked crust of the Gissar range to the northern rim—the Kuqa and Kolar sections—of the Tarim craton, suggesting a united Karakum-Tarim craton. Although about contemporaneous with widespread postcollisional magmatism in the entire Tian Shan, the large volume and short duration of the Gissar range magmatism, including crustal thickening and prograde metamorphism during Tarim craton‒Middle Tian Shan block collision, and formation and closure of an oceanic back-arc basin (the Gissar basin), indicate its origin in a distinct setting. Combined, this likely resulted in midcrustal melting and upper-crustal batholith emplacement. Mafic dikes and pipes intruded at ca. 256−238 Ma (median ca. 241 Ma); the source region of the parental melts was within the asthenospheric mantle. The simplest interpretation for these basanites is that they were part of the Tarim flood basalt province; this would extend this province westward from the Tarim craton into the southwestern Tian Shan and imply that the relatively short-lived flood basalt event (ca. 290‒270 Ma) was followed by much less voluminous but longer-lasting hotspot magmatism. The 40 Ar/ 39 Ar and detrital apatite fission-track dates outline post−Gissar-Alai range granitoid emplacement cooling, Cimmerian collision events at the southern margin of Asia, Late Cretaceous crustal extension and local magmatism, and early Cenozoic shortening and burial in the far field of the India-Asia collision.

Structural relationships along a Neoarchean arc-continent collision zone, North China craton

Abstract: The Archean North China craton is composed of the Western block, Eastern block, and the intervening Central orogenic belt. A 4–10-km-wide and 85-km-long tectonic melange belt informally called the Zanhuang tectonic melange is documented in the Zanhuang Massif of the Central orogenic belt, separating the Eastern block from an Archean arc terrane in the Central orogenic belt. The melange belt contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles, and quartzites mixed with exotic tectonic blocks of volcanic, mafic, and ultramafic rocks, metabasalts that locally include relict pillow structures, and tonalite-trondhjemite-granodiorite (TTG) gneisses. The Zanhuang tectonic melange marks the suture of an arc-continent collisional zone between the Western Zanhuang Massif in the Central orogenic belt and the Eastern block of the North China craton, and it is one of the best-preserved Archean tectonic melanges in the world. Here, using zircon U-Pb dating of various types of blocks from the Zanhuang melange, we show that the formation and associated deformation of the Zanhuang melange occurred in the Neoarchean (ca. 2.5 Ga). High-precision (1:20–1:200) lithostructural mapping of three key outcrops reveals details of the internal fabrics and kinematics of the melange and regional structural relationships along the arc-continent collisional zone. A synthesis of studies on the tectonic evolution of the North China craton, coupled with our new fabric and kinematic analysis of the Zanhuang melange, further constrains the initial amalgamation timing and geometry of the arc-continent collision between the Fuping arc terrane in the Central orogenic belt and the Eastern block with a northwest-dipping subduction polarity. The asymmetric structures and mixture of different blocks and matrices with folding and thrusting events in the Zanhuang melange record kinematic information that is consistent with the tectonic setting of an accretionary wedge that was thrust over the passive margin of the Eastern block by 2.5 Ga. Lithostructural mapping shows that the classic melange and fold-and-thrust structures along the Neoarchean arc-continent collisional zone are broadly similar to Phanerozoic collisional belts.

Stratigraphic and provenance variations in the early evolution of the Magallanes-Austral foreland basin: Implications for the role of longitudinal versus transverse sediment dispersal during arc-continent collision

Abstract: Resolving the role of longitudinal versus transverse sediment dispersal in ancient sedimentary basins is paramount for understanding filling history and the timing of source area exhumation. The southern Patagonian Andes provide a unique opportunity for constraining these relationships because Upper Cretaceous shallow- and deep-marine strata that record the longitudinal filling history of the Magallanes-Austral foreland basin are exposed along a 500+ km outcrop belt. New stratigraphic, sedimentologic, and facies analyses of the Cenomanian-aged Lago Viedma Formation indicate a protracted phase of a dominantly shoreface and foreshore depositional setting at the northern end of the basin (Austral basin sector), whereas 200 km to the south, age-equivalent strata of the Punta Barrosa Formation are characterized by southward-flowing deep-water fan systems. New sandstone compositional data from both formations are rich in intermediate volcanic grains and suggest an undissected to transitional volcanic arc source. However, detrital zircon populations from the shallow-marine Lago Viedma Formation are dominated by arc sources (126−75 Ma), whereas deep-water strata of the Punta Barrosa Formation contain much greater abundances of Jurassic (199−161 Ma) and pre-Jurassic (>200 Ma) ages. This indicates that deep-water fan systems were not linked to a shelfal sediment dispersal system by a simple northward point-source model, despite consistent southward-directed paleocurrents in deep-water strata. Time-transgressive provenance variations continue southward (along strike), where lithostratigraphic equivalents in the Ultima Esperanza and Fuegian sectors of the basin contain a mix of arc and pre-Jurassic metamorphic basement sources and a paucity of Jurassic ages. We interpret these along-strike provenance variations to be the result of significant local sediment contributions from transverse sources. The influence of transverse tributaries during the early history of the Magallanes-Austral foreland basin suggests that the diachronous onset of coarse clastic deposition in the basin was likely due to the progressive delivery (and initiation) of more locally derived coarse clastic sediment. We attribute this to southward-progressing thrust-belt development associated with progressive north to south collision (or suturing) of the parautochthonous Patagonian arc with attenuated continental crust of South America.

High-resolution, millennial-scale patterns of bed compensation on a sand-rich intraslope submarine fan, western Niger Delta slope

Abstract: Near-seafloor core and seismic-reflection data from the western Niger Delta continental slope document the facies, architecture, and evolution of submarine channel and intraslope submarine fan deposits. The submarine channel enters an 8 km long x 8 km wide intraslope basin, where more than 100 m of deposits form an intraslope submarine fan. Lobe deposits in the intraslope submarine fan show no significant downslope trend in sand presence or grain size, indicating that flows were bypassing sediment through the basin. This unique dataset indicates that intraslope lobe deposits may have more sand-rich facies near lobe edges than predicted by traditional lobe facies models, and that thickness patterns in intraslope submarine fans do not necessarily correlate with sand presence and/or quality. Core and radiocarbon age data indicate that sand beds progressively stack southward during the late Pleistocene, resulting in the compensation of at least two lobe elements. The youngest lobe element is well characterized by core data and is sand-rich, ~ 2 km wide x 6 km long, > 1 m thick, and was deposited rapidly over ca. 4,000 yr, from 18-14 ka. Sand beds forming an earlier lobe element were deposited on the northern part of the fan from ca. 25 to 18 ka. Seafloor geomorphology and amplitudes from seismic reflection data confirm the location and age of these two compensating lobe elements. A third compensation event would have shifted sand deposition back to the northern part of the fan, but sediment supply was interrupted by rapid sea level rise during Meltwater Pulse 1-A at ca. 14 ka, resulting in abandonment of the depositional system.

Facies and stratigraphic architecture of the late Pleistocene to early Holocene tide-dominated paleo-Changjiang (Yangtze River) delta

Abstract: This paper reexamines the late Pleistocene to early Holocene transgressive succession (ca. 19.0–8.0 cal. k.y. B.P.) beneath the modern Changjiang delta plain, one of the world’s great rivers, by means of a detailed process-oriented investigation of the sedimentary facies and stratigraphic architecture in two newly drilled cores (ZK01 and ZK02), supplemented by correlation with previous work and comparison with the modern deltaic deposits. Results suggest that the study interval, which has previously been considered to have accumulated in an estuary, instead represents a back-stepping delta. The target succession shows a general finingand muddier-seaward trend for both the channel-bottom and adjacent tidalor point-bar sediments, from coarse sand and gravels of the purely fluvial channel, through tide-influenced fluvial-channel deposits dominated by medium to coarse sand, to fine-grained sandy or muddy deposits of the (terminal) distributary channels, and finally to prodeltaic mud; the sand-dominated deposits that characterize transgressive estuary mouths are not present. The prodeltaic and delta-front deposits of the paleoand modern Changjiang deltas display strong similarities, including the pervasive interbedding of tideand wave-generated fluid-mud deposits and muddy tidal-bar deposits, which are punctuated by terminal-distributary-channel facies that are not expected in an estuary. In more detail, three superimposed successions are recognized, each of which displays an upwardshallowing trend indicating a progradational nature, which is also indicated by the presence of seaward-dipping seismic reflectors in the correlative seaward part of this stratigraphic interval. The three successions show an overall retrogressive, fining-upward trend as a result of stepwise transgression like that seen in the Mississippi and GangesBrahmaputra River deltas. This architecture indicates that the large sediment supply of the Changjiang was able to keep pace with or exceed the rate of relative sea-level rise at certain times and locations during the later part of the postglacial transgression. We also propose that the maximum flooding surface occurred at ca. 9.0 cal. k.y. B.P., which is earlier than previously thought; this is also a reflection of the high rate of sediment input of the Changjiang, which causes a turnaround from net transgression to net progradation to occur when the rate of sea-level rise is higher. This study shows that we should not just simply interpret any transgressive succession in an incised valley as estuarine. This study provides significant new insights into the interpretation and sequence-stratigraphic reconstruction of ancient deltaic deposits, and it advances our understanding of the nature of tide-dominated delta successions.