New GSA Bulletin Articles Published Online Ahead of Print Now

Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics studied this month include the nature and dynamics of China and Tibet; a possible new record of global organic carbon; and a model for natural avalanches. You can find these articles at .

Long-lived (>100 m.y.) postcollisional exhumation and cooling in the Paleoproterozoic Trans−North China orogen: Evidence from phase equilibria modeling and monazite petrochronology of granulite-facies metapelites in the Fuping Complex
Li Tang; M. Santosh; Richard M. Palin; Li-Hui Jia; Hua-Wen Cao ...
Long-lived collisional orogens that formed over tens to hundreds of millions of years are common in the geologic record. The Trans−North China orogen marks the collision between the Eastern and Western blocks of the North China craton, and it preserves metamorphic rocks with ages between 1.98 Ga and 1.80 Ga. These units allow detailed assessment of the time scale and duration of crustal thickening, exhumation, and cooling associated with a major Proterozoic orogeny. In this study, we present integrated petrography, mineral chemistry, phase equilibria modeling, and texturally controlled in situ mass spectrometry of monazite U-Th-Pb and trace-element analyses performed on a suite of orthopyroxene-bearing pelitic granulites and garnet-biotite gneisses from the Fuping Complex within the Trans−North China orogen. These rocks record clockwise pressure-temperature (P-T) paths involving granulite-facies peak conditions of 9.9−11.0 kbar and 850−880 °C for pelitic granulites, and 10.9−11.6 kbar and 860−880 °C for garnet-biotite gneisses, followed by postpeak decompression to ∼8−9 kbar and later cooling, with final solidification of melt at <840 °C. Four monazite populations were identified in these samples. Group I grains are irregular and elongated and occur in contact with or embay garnet. They have high rare earth element (REE) and Y contents and metamorphic ages of 1.90−1.86 Ga, which correspond to the breakdown of garnet during postpeak decompression. Most monazite grains crystallized from melt are represented by groups II + III + IV and are associated with orthopyroxene, biotite, plagioclase, and quartz in the matrix. They have crystallization ages between 1.86 Ga and 1.76 Ga and relatively low REE and Y concentrations. These data imply a long-lived (>100 m.y.) postcollisional exhumation and cooling involving decompression from 10−12 kbar to ∼9 kbar during 1.90−1.86 Ga, followed by retrograde cooling from 1.86 to 1.76 Ga under prolonged residence in the middle to lower crust. Initial collision and peak metamorphism occurred before 1.90 Ga, ultimately leading to the final cratonization of the North China craton and its incorporation into the Columbia supercontinent.

Reconstructing backwater hydrodynamics from fluvial-deltaic deposits using stratigraphic inversion: An example from the Tullig Sandstone, Western Irish Namurian Basin, County Clare, Ireland
Chenliang Wu; Jeffrey A. Nittrouer; Eric A. Barefoot; Kurtis C. Burmeister
Fluvial-deltaic systems are subject to non-uniform backwater flow where rivers approach receiving basins. This hydrodynamic condition results in sediment aggradation on the channel bed and enhanced downstream fining. In turn, this impacts river channel dynamics, including lateral migration rates and the propensity for avulsion. The imprint of non-uniform flow on stratigraphy has been reported from field, numerical modeling, and experimental studies. This work provides key observations for evaluating the impact of non-uniform flow spanning length scales from those of sediment grains to delta lobes. However, reconstructing paleohydraulic conditions of non-uniform flow from fluvial-deltaic settings remains a challenge. Non-uniform flow is a defining characteristic of fluvial-deltaic environments, but most existing relations linking hydrology and the depositional record rely on the assumption of steady and uniform flow. Herein, we present a novel stratigraphic inversion technique, combining it with morphodynamic modeling and statistical analyses, to evaluate how backwater conditions manifest in the stratigraphy of the Tullig Sandstone, an ancient fluvial-deltaic deposit of the Western Irish Namurian Basin. Our analyses refine estimates of channel properties, including flow depth and bed slope, and are validated by field measurements of sandstone-facies properties, including grain size and stratal architecture. For example, bed sediment fines down-dip, commensurate with increasing cross set and bed thicknesses. These patterns indicate bed aggradation and are consistent with reconstructed morphodynamic conditions. This study pinpoints the extent of non-uniform flow and its influence on fluvial-deltaic stratigraphy and provides a framework for improving reconstruction techniques used to interpret the paleohydrology of ancient fluvial-deltaic systems.

Frontal expansion of an accretionary wedge under highly oblique plate convergence: Southern Indo-Burman Ranges, Myanmar
Peng Zhang; Shao-Yong Jiang; Raymond A. Donelick; Renyuan Li; Cleber J. Soares ...
The formation of accretionary wedges with oversteepened slopes and uplifted axial zones has been demonstrated to be potentially associated with highly oblique plate convergence by numerical and analog studies. The direct role of this mechanism, or other factor(s) in producing the described structural and morphological features in nature, however, has yet to be confirmed. We used seismic reflection sections, detrital zircon U-Pb ages, and detrital apatite fission-track thermochronological data to examine the effects of highly oblique convergence and sediment reworking on accretionary wedge growth in the Indo-Burma Subduction Zone. A detailed subsurface structural analysis of a two-dimensional seismic survey from the outer wedge of the southern Indo-Burman Ranges, Myanmar, yielded three primary characteristics. These are (1) a narrow, steep deformation front (average width 15.6 km) and a vast, low-relief shelf terrace (average width 49 km); (2) a comparatively long-lived growth thrust fault (FT1) with a convex-up geometry at the rear of the deformation front that controlled the vertical stack of the progradational sequences in the shelf terrace; and (3) a group of NE-striking transtensional faults that cut through entire outer-wedge successions and displays as a series of negative flower-like structures. These characteristic features are roughly consistent with the results of laboratory analog modeling of highly oblique plate convergence but significantly differ from those of natural accretionary wedges that formed under highly oblique convergence conditions, such as those in the Sumatra, Hikurangi, Chile, and Cascadia. We further analyzed the sediment provenance of the southern Indo-Burman Ranges and discovered that the outer-wedge rocks are a product of sediment reworking of the hinterland wedge that began to be uplifted and exhumed in the early Miocene (22−12 Ma) due to transpressional motion between the Indian plate and West Burma Terrane. Our analyses indicate that active sedimentation behind the major growth thrust fault (FT1) provided additional basal shear stress that strengthened the coupling of the interface between the wedge base and décollement and promoted the vertical expansion of the outer wedge of the southern Indo-Burman Ranges from the Neogene to the present day. In contrast, the outer wedge of the central Indo-Burman Ranges has experienced stronger forward accretion since the late Miocene, which could be explained by a smaller degree of obliquity and weaker sediment reworking. Our findings demonstrate that both highly oblique plate convergence and sediment reworking were the primary driving forces that triggered vertical development of accretionary wedges. The results of this research have significant implications for understanding the structures and kinematic evolution of wedge systems at other convergent plate margins, in which seamount passage or subduction erosion is often interpreted as the cause of the steeply tapered wedges.

Tracing high-pressure metamorphism in the eastern Himalayan syntaxis using detrital zircon and monazite from modern stream sediments
Yunxuan Zhang; Liang Guo; Hongfei Zhang; Nigel Harris; Wangchun Xu ...
The timing of high-pressure (HP) metamorphism in the eastern Himalayan syntaxis is important for understanding the India-Asia collisional processes, but it remains elusive. To reveal the metamorphic history of the eastern Himalayan syntaxis, we performed a study of geochronology, trace elements, and mineral inclusions of detrital zircon and monazite from modern stream sediments in the eastern Himalayan syntaxis. Detrital zircon comprise magmatic and metamorphic domains with different zoning. Inherited magmatic zircon domains have high Th/U, low (Dy/Yb)N, and retain ages of 1798−360 Ma. Metamorphic zircon domains with low Th/U, high (Dy/Yb) N, and inclusions of garnet, kyanite, and/or clinopyroxene probably formed under HP conditions. They yield age groups of 49−35 Ma, 33−17 Ma, and 12−7 Ma. The low Th/U and low (Dy/Yb)N metamorphic zircon domains probably formed during retrogression and yield age groups of 27−16 Ma and 10−6 Ma. Detrital monazite yield age distributions similar to those of the low (Dy/Yb)N metamorphic zircon except for the 821−402 Ma inherited cores. The (Dy/Yb)N of 31.6−5.7 Ma monazite decreases with increasing Y content, which indicates that it likely formed under the retrograde stage during garnet breakdown. Based on the oldest metamorphic ages, the initial India-Asia collision occurred no later than 50−44 Ma in the eastern Himalayan syntaxis. The multimodal age patterns of the metamorphic zircon and monazite indicate that the Indian continent underwent multistage HP and retrograde metamorphism in the eastern Himalayan syntaxis. The nearly contemporaneous HP and retrograde metamorphism indicate that the Indian continent continued subducting while the earlier HP metamorphic slices detached and exhumed.

Immiscible two-phase model for air blasts created during natural avalanches
Jianbo Fei; Zhankui Liu; Yuxin Jie
An immiscible two-phase model based on the incompressible Navier-Stokes (N-S) equations is used to simulate the air blast generated by an avalanche. For simplicity, the avalanche is treated as an assembly of monodisperse spherical grains and described as a continuous media. The constitutive law of local µ(I) rheology is introduced to model the moving granular material. The motion of the avalanche and the induced air blast fits into a unified framework that combines the N-S−type governing equations with a µ(I)-rheology−based kinematic viscosity and a constant viscosity. The avalanche-air interface is treated using the volume-of-fluid method. A numerical program was developed on the open-source platform OpenFOAM specifically for this model to simulate the entire evolutionary process of the avalanche as well as the air blast generated. The model was validated by comparing the results of numerical simulations with those from inclined-plane laboratory experiments. With terrain input from the Shuttle Radar Topography Mission data, the model was further applied to simulate the air blast generated in two natural avalanches, namely, the Baige and Wenjia valley avalanches fo China, which occurred in 2008 and 2018, respectively. The simulation results were found to be consistent with field observations following a statistical analysis of the properties of the air blast including flow speed and area of impact of the above-mentioned natural events.

Morphometric constraints on the formation of new terrestrial analogs for planetary pits
R. Naor; A. Mushkin; I. Halevy
The origin of geological depressions abounding on Mars and other planetary bodies remains poorly understood, partially due to the limited variability in the geological settings of existing terrestrial analogs. Here, we present a new terrestrial analog that is located at the northwestern margin of the Levantine volcanic field of Harrat Ash−Shaam along the Dead Sea Transform. The analog site consists of tens of geological depressions (locally named “juba”) that morphologically resemble Martian bowl-shaped pits and occur within a Pleistocene basaltic plateau that overlies Meso-Cenozoic carbonates. To constrain plausible formation mechanisms for the juba depressions, we carried out detailed field mapping and morphometric analyses using a 0.25 m/pixel digital terrain model (DTM) derived from airborne light detection and ranging (LiDAR) survey covering 34 km2 of the study area, and centimeter-scale, ground-based LiDAR scans of selected juba depressions. We show that variable magnitudes of slope asymmetry between north- and south-facing walls within the juba depressions, along with different degrees of sediment infilling, provide effective proxies for the relative geomorphic maturity of these landforms, and in turn indicate asynchronous formation of the juba depressions after the Pleistocene emplacement of the Harrat Ash−Shaam basalts in the study area. Our findings preclude formation of the juba depressions by phreatomagmatic explosions and instead point toward collapse into missing subsurface volume. In a broader context, we propose that the morphometric analyses developed herein to distinguish between plausible juba formation mechanisms in the Harrat Ash−Shaam volcanic field can be extended to better constrain the formation mechanisms of similar pit features on Mars and other planetary bodies.

Juxtaposition of different-grade metamorphic rocks in an ancient orogen: Evidence from the Chengde Complex of the Trans-North China Orogen, North China Craton
Hui C.G. Zhang; Guochun Zhao; Chao Wang; Nanqing Xu; Jinlong Yao
Juxtaposition of different-grade metamorphic slices is a typical feature of Phanerozoic orogens but is relatively scarce in Precambrian orogens. Here, we focus on the Chengde Complex at the northern segment of the late Paleoproterozoic Trans-North China Orogen, the North China Craton as a case, to explore how different-grade orogeny-related metamorphism was juxtaposed in Precambrian orogens. The Trans-North China Orogen is a typical Precambrian collisional orogen and records abundant information about the Paleoproterozoic orogeny. High-pressure (HP) mafic granulite and two types of amphibolite samples were collected from the Chengde Complex, and we conducted a combined study involving litho-structural assemblage investigation, metamorphic petrology, and geochronology to decipher their metamorphic P-Tt history. HP mafic granulite and amphibolite occur as enclaves or dikes within felsic gneisses. HP mafic granulite records clockwise P-T paths with isothermal decompression (ITD) segments, and the peak metamorphic P-T conditions are 13.0−14.9 kbar/790−830 °C. Peak metamorphic P-T conditions retrieved from garnet-bearing amphibolite are 8.3 kbar/675 °C, whereas peak metamorphic P-T conditions retrieved from garnet-free amphibolite are 4.0−5.5 kbar/500−510 °C. Zircon U-Pb dating of HP granulite and amphibolite yield different prograde (1914−1871 Ma), peak (1869−1816 Ma), and retrograde (ca. 1830 Ma) metamorphic ages, and all of these data indicate that these metamorphic rocks were diachronously transferred to different depths in the subduction channel and subsequently exhumed at shallower crustal levels. Therefore, we conclude that the Chengde Complex is composed of imbricate rocks with different metamorphic grades and ages, and such an unordered juxtaposition of diverse metamorphism could also be a typical feature of Precambrian orogens.

Intense metamorphism-generated radiogenic Sr regulated Cenozoic water Sr isotope evolution on the NE Tibetan Plateau: A perspective on Qilian orogen denudation and Asian eolian transport
Yibo Yang; Albert Galy; Rongsheng Yang; Yudong Liu; Weilin Zhang ...
The India-Asia collision reactivated the early Paleozoic Qilian orogen with an intense metamorphic belt that promoted the release of metamorphism-generated radiogenic Sr into the drainage systems on the NE Tibetan Plateau. This metamorphic impact on the regional dissolved Sr cycle is well observed at the recent−modern scale, but its onset and evolutionary histories are unclear. We present the first basin-scale 52−5 Ma regional dissolved Sr isotopic record in water on the NE Tibetan Plateau by analyzing well-dated basin fluvial-lacustrine sediments in the Xining, Linxia, and Tianshui Basins. The Xining Basin displays an increase in basin water 87Sr/86Sr ratio and a decrease in the sediment εNd values at ca. 25 Ma. This Sr-Nd isotope-deduced provenance change coincides with the reorganization of drainage and erosion regimes that is suggested by an evident rise in the youngest peak and the lag time of detrital apatite fission-track ages from Cenozoic sedimentary basins surrounding the Qilian orogen. The Qilian-sourced eolian dust during ca. 9−8 Ma significantly elevated the river and lake water 87Sr/ 86Sr ratios in the downwind Linxia and Tianshui Basins, which is consistent with the expansion of the Asian dust system. Our results suggest that large-scale denudation of the Qilian orogen in response to the India-Asia collision initiated in the late Oligocene. Given the remarkable hydrological impact of the Cenozoic reactivation of the Qilian orogen, our study highlights the potentially important role of continental collision−formed metamorphic belts in regulating past regional and even seawater Sr isotope evolution.

Validation of earthquake ground-motion models in southern California, USA, using precariously balanced rocks
Anna H. Rood; Dylan H. Rood; Greg Balco; Peter J. Stafford; Lisa Grant Ludwig ...
Accurate estimates of earthquake ground shaking rely on uncertain ground-motion models derived from limited instrumental recordings of historical earthquakes. A critical issue is that there is currently no method to empirically validate the resultant ground-motion estimates of these models at the timescale of rare, large earthquakes; this lack of validation causes great uncertainty in ground-motion estimates. Here, we address this issue and validate ground-motion estimates for southern California utilizing the unexceeded ground motions recorded by 20 precariously balanced rocks. We used cosmogenic 10Be exposure dating to model the age of the precariously balanced rocks, which ranged from ca. 1 ka to ca. 50 ka, and calculated their probability of toppling at different ground-motion levels. With this rock data, we then validated the earthquake ground motions estimated by the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) seismic-source characterization and the Next Generation Attenuation (NGA)-West2 ground-motion models. We found that no ground-motion model estimated levels of earthquake ground shaking consistent with the observed continued existence of all 20 precariously balanced rocks. The ground-motion model I14 estimated ground-motion levels that were inconsistent with the most rocks; therefore, I14 was invalidated and removed. At a 2475 year mean return period, the removal of this invalid ground-motion model resulted in a 2−7% reduction in the mean and a 10−36% reduction in the 5th−95th fractile uncertainty of the ground-motion estimates. Our findings demonstrate the value of empirical data from precariously balanced rocks as a validation tool for removing invalid ground-motion models and, in turn, reducing the uncertainty in earthquake ground-motion estimates.

Tectonically controlled drainage fragmentation in the southwestern Great Basin, USA
B.M. Lutz; J.R. Knott; F.M. Phillips; M.T. Heizler; K.A. Heitkamp, Jr. ...
The area now occupied by the Great Basin, western USA, contained paleo-fluvial systems that predated the modern-day endorheic (closed) basins. The areal extent of these paleo-fluvial systems within the southwestern Great Basin is known mainly from isolated remnants preserved in the modern mountain ranges. We document the age, extent, and tectonic disruption of Mio-Pliocene fluvial systems of the southwestern Great Basin. Synthesis of new field observations, geochemistry, and geochronology with existing studies defines two latest Miocene to Pliocene east-southeast flowing drainages that predated the modern endorheic basins. The drainage network was ultimately fragmented in Pliocene time (ca. 3.5-4 Ma). Fragmentation of the drainage network led to lake formation, drying of lakes, and the formation of isolated springs. The rapid environmental changes initiated by faulting and volcanism isolated previously interbreeding populations of spring-dwelling taxa and have caused divergent evolution since Pliocene time. Modern endemism within the region’s springs is thus a direct consequence of intraplate tectonism.

Location and shape of the Lhasa terrane prior to India-Asia collision
Weiwei Bian; Suo Wang; Yong Yao; Xianwei Jiao; Wenxiao Peng ...
The precollisional location and shape of the Lhasa terrane are crucial for constraining the closure of the Neo-Tethys Ocean and the ensuing India-Asia collision; however, estimation of these features of the Lhasa terrane remains highly controversial. Here, we carried out a new paleomagnetic investigation on the Lower Cretaceous Duoni Formation red beds in the central-eastern Lhasa terrane. The tilt-corrected site-mean direction is declination (Ds) = 339.0°, inclination (I s) = 26.8°, ks = 78.4, and α95 = 2.3° (k—precision parameter; α95—the radius that the mean direction lies within 95% confidence; s —stratigraphic coordinates) (N = 50), corresponding to a paleopole at 64.2°N, 324.2°E, with A95 = 1.9° ( A95—the radius that the mean pole lies within 95% confidence). These new paleomagnetic data pass a positive fold test and indicate that the studied area was located at 14.3 ± 1.9°N during the Early Cretaceous. No significant inclination shallowing is present in the Lower Cretaceous Duoni Formation red beds. Our new results, combined with previously published reliable Cretaceous paleomagnetic results, show that the Lhasa terrane was located at a paleolatitude of ∼22.9°N to 10.1°N from west to east and was oriented at ∼298°−296° prior to India-Asia collision.

Geodynamic controls in the southernmost Northern Andes magmatic arc: Trace elements and Hf-O isotopic systematics in forearc detrital zircon
César Witt; Marc Poujol; Massimo Chiaradia; Diego Villagomez; Monique Seyler ...
U-Pb dating of single detrital zircon grains by laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) paired with Hf and O isotopic and trace-element analyses provide first-order indicators of the Late Cretaceous−Cenozoic evolution of the southern Ecuadorian magmatic arc. Detrital zircon U-Pb ages define significant clusters that are tentatively interpreted as intense arc magmatism at ca. 72 Ma, ca. 60 Ma, and ca. 43 Ma. A major accretionary event in the Late Cretaceous (75−65 Ma) is marked by a broad range of zircon isotopic values (εHf[ t] > 20 and δ18O > 8‰) that suggest melting of both the lower and upper crust (most likely of continental affinity) as well as enriched mantle components. Highly fractionated signatures in trace-element patterns and Eu/Eu* combined with mantle-like δ18O and juvenile εHf values characterize zircons from 60 to 45 Ma, suggesting that the Late Cretaceous−middle Eocene arc originated from an enriched mantle and likely reflects the persistence of overthickened crust previously attributed to the main Late Cretaceous accretionary period. Subsequently, negative shifts in εHf( t) isotopic composition from 45 to 30 Ma are paired with mantle-like δ18O values as well as decreases in U/Yb and Eu/Eu*. These signatures could be attributed to magma emplacement in a thinner crust and the existence of a broad extensional magmatic arc extending from the current forearc toward areas near the craton; however, other scenarios cannot be excluded. This event was characterized by enriched mantle melt sources with residence times pointing to known crustal events (Sunsás) in the Amazonian craton. From 30 to 10 Ma, the isotopic record slightly evolved toward a depleted mantle signature with a substantial increase in fractionation. Our results combined with previously published isotopic records from detrital zircon grains found in modern rivers suggest that, for at least the last 30 m.y., the southernmost Northern Andes magmatic arc has been segmented, with the emplacement of juvenile magmas to the north and more enriched magmas related to the recycling of ancient continental crust and/or subducted sediments to the south—aspects found in other Northern Andes settings in which the continental arc was constructed in both oceanic and continental crust.

Rapid southeastern Laurentide Ice Sheet thinning during the last deglaciation revealed by elevation profiles of in situ cosmogenic 10Be
Christopher T. Halsted; Paul R. Bierman; Jeremy D. Shakun; P. Thompson Davis; Lee B. Corbett ...
Accurate reconstruction of Laurentide Ice Sheet volume changes following the Last Glacial Maximum is critical for understanding ice sheet contribution to sea-level rise, the resulting influence of meltwater on oceanic circulation, and the spatial and temporal patterns of deglaciation. Here, we provide empirical constraints on Laurentide Ice Sheet thinning during the last deglaciation by measuring in situ cosmogenic 10 Be in 81 samples collected along vertical transects of nine mountains in the northeastern United States. In conjunction with 107 exposure age samples over five vertical transects from previous studies, we reconstruct ice sheet thinning history. At peripheral sites (within 200 km of the terminal moraine), we find evidence for ∼600 m of thinning between 19.5 ka and 17.5 ka, which is coincident with the slow initial margin retreat indicated by varve records. At locations >400 km north of the terminal moraine, exposure ages above and below 1200 m a.s.l. exhibit different patterns. Ages above this elevation are variable and older, while lower elevation ages are indistinguishable over 800−1000 m elevation ranges, a pattern that suggests a subglacial thermal boundary at ∼1200 m a.s.l. separating erosive, warm-based ice below and polythermal, minimally erosive ice above. Low-elevation ages from up-ice mountains are between 15 ka and 13 ka, which suggests rapid thinning of ∼1000 m coincident with Bølling-Allerød warming. These rates of rapid paleo-ice thinning are comparable to those of other vertical exposure age transects around the world and may have been faster than modern basin-wide thinning rates in Antarctica and Greenland, which suggests that the southeastern Laurentide Ice Sheet was highly sensitive to a warming climate.

Bhumichula plateau: A remnant high-elevation low-relief surface in the Himalayan thrust belt of western Nepal
Tshering Z.L. Sherpa; Peter G. DeCelles; Barbara Carrapa; Lindsay M. Schoenbohm; Joshua Wolpert
The Himalaya is known for dramatically rugged landscapes including the highest mountains in the world. However, there is a limited understanding of the timing of attainment of high elevation and relief formation, especially in the Nepalese Himalaya. Anomalous high-elevation low-relief (HELR) surfaces, which exhibit geomorphic antiquity and are possibly remnants of formerly widespread high-elevation paleosurfaces, provide a unique opportunity to assess the attainment of regional high elevation in the Himalaya. The Bhumichula plateau is one such HELR surface (4300−4800 m) in the western Nepalese Himalayan fold-thrust belt. The Bhumichula plateau is situated in the Dadeldhura klippe (also called the Karnali klippe), an outlier of Greater Himalayan Sequence high-grade metasedimentary/igneous rocks surrounded by structurally underlying Lesser Himalayan Sequence low-grade metasedimentary rocks. We assess the origin of the Bhumichula plateau by combining regional geological relationships and zircon and apatite (U-Th-Sm)/He and apatite fission track thermochronologic ages. The HELR surface truncates pervasive west-southwestward dipping foliations, indicating that it post-dates tilting of rocks in the hanging wall of the Main Central thrust above the Lesser Himalayan duplex. This suggests that the surface originated at high elevation by erosional beveling of thickened, uplifted crust. Exhumation through the ∼180−60 °C thermal window occurred during middle Miocene for samples on the plateau and between middle and late Miocene for rocks along the Tila River, which bounds the north flank of the Bhumichula plateau. Cooling ages along the Tila River are consistent with erosional exhumation generated by early Miocene emplacement of the Main Central (Dadeldhura) thrust sheet, middle Miocene Ramgarh thrust emplacement, and late Miocene growth of the Lesser Himalayan duplex. The most recent middle-late Miocene exhumation took place as the Tila River and its northward flowing tributaries incised upstream, such that the Bhumichula plateau is a remnant of a more extensive HELR paleolandscape. Alpine glaciation lowered relief on the Bhumichula surface, and surface preservation may owe to its relatively durable lithology, gentle structural relief, and elevation range that is above the rainier Lesser Himalaya.

The Late Triassic Longmenshan lateral foreland thrusting: New insights from geological evidence and 3-D particle discrete-element simulation
Haonan Zhao; Yuanyuan Zhang; Wei Du; Yang Zhang; Renjie Zhou ...
This work proposes a new lateral foreland thrusting model based on geological evidence and 3-D particle discrete-element simulation to explain the Longmenshan southeastward thrusting during the closure of the Songpan-Ganze basin. The Late Triassic NE−SW compression caused by the northward movement of the Qiangtang Block and the resulting differential shortening within the wedge-shaped Songpan-Ganze terrane produced southeastward topographic gradient. The thick sedimentary pile, driven by the horizontal tectonic force and the deviatoric stress generated from gravitational effect, and decoupled from the subducting basement by the low-strength décollement, was laterally extruded and resulted in the southeastward Longmenshan thrusting. Therefore, the Longmenshan thrust belt is a lateral foreland thrust belt of the Songpan-Ganze terrane. For the first time, 3-D particle discrete-element simulation was used for the geological study of the Longmenshan area, and it clearly reproduces the dynamical process of the Longmenshan southeastward thrusting and well predicts the Xiaojin Arcuate Zone. The particle discrete-element simulation results verify the new model and reveal that two key factors facilitate the lateral foreland thrusting: the wedge-shaped geometry that produces differential shortening and lateral topographic gradient, and the low-strength décollement that decouples the extruded sedimentary pile from the basement. The lateral foreland thrust belt, which is unique in its tectonic location and dynamic behavior, is a new kind of foreland thrust belt that is different from the pro-foreland and retro-foreland thrust belts, and it provides new insight into the tectonic evolution of collisional orogens.

Permian−Middle Triassic floral succession in North China and implications for the great transition of continental ecosystems
Wenchao Shu; Jinnan Tong; Jianxin Yu; Jason Hilton; Michael J. Benton ...
The global pattern of plant evolution through the Permian−Triassic mass extinction is uncertain, and the extent to which land plants were affected is debated. Detailed studies undertaken at a regional scale can help evaluate this floral transition, and thus we provide a detailed account of floral evolution from the Permian to Middle Triassic of North China based on new paleobotanical data and a refined biostratigraphy. Five floral transition events are identified from before, during, and after the Permian−Triassic crisis, including the disappearance of the gigantopterid flora (associated with loss of coal deposits), the end-Permian mass extinction of Paleophytic taxa, and gradual recovery in the Triassic with the stepwise appearance of the Mesophytic vegetation. The record begins with a Cisuralian gigantopterid-dominated rainforest community, and then a Lopingian walchian Voltziales conifer-ginkgophyte community that evolved into a voltzialean conifer-pteridosperm forest community. The last is associated with a change amongst terrestrial vertebrates from the Jiyuan fauna to a pareiasaur-dominated fauna, found in red beds that lack coal deposits due to arid conditions. The disappearance of the voltzialean conifer forest community may represents the end-Permian mass extinction of plants although it could also be a consequence of the non-preservation of plants in sedimentary red-beds. The first post-crisis plants are an Induan herbaceous lycopsid community, succeeded by the Pleuromeia- Neocalamites shrub marsh community. A pteridosperm shrub woodland community dominated for a short time in the late Early Triassic along with the reappearance of insect herbivory. Finally, in the Middle Triassic, gymnosperm forest communities gradually rose to dominance in both uplands and lowlands along with other diverse plant communities, indicating the establishment of the Mesophytic Flora.

Buoyancy-driven exhumation deformation: Evidence from the Sulu orogen, eastern China
Hao Yin; Guang Zhu; Xiaodong Wu; Nan Su; Yuanchao Lu ...
Whether deformation of exhumed crust in a collisional orogen is driven by buoyancy or tectonic stress remains uncertain. The Sulu orogen in eastern China contains slowly subducted and exhumed high-pressure (HP)−ultrahigh-pressure (UHP) terranes, which provide a good opportunity to understand whether the exhumation deformation was driven by buoyancy. We used field and microscopic observations as well as quartz c-axis fabrics to determine the deformation kinematics, temperatures, and evolution of the exhumed crustal slices. Deformed and undeformed dikes were dated by using the zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry method to constrain the timing of deformation. Our data demonstrate that each crustal slice was involved in pervasive top-to-the-NW (hinterland) or top-to-the-SE (foreland) ductile deformation during Late Triassic exhumation, and that the base of each slice records deformation that was superimposed during the subsequent exhumation of the underlying slice. The crustal exhumation of the southern Sulu orogen is consistent with the multistage ductile extrusion model. The kinematics of exhumation deformation within each crustal slice conform to an upward asymmetric flow. The flow velocity and corresponding shear sense were affected by temperature variations in each HP slice, whereas they were sensitive to migmatization within the UHP slices. The southern Sulu orogen examples show that crustal flow deformation during exhumation was driven by buoyancy and controlled by viscosity. Unlike the consistent kinematics of tectonics-driven deformation, the kinematics of buoyancy-driven deformation are characterized by variations in the senses of shear within a single crustal slice, and this can therefore be used to distinguish the two types of deformation.

From ka to Ma: A multi-timescale record of accelerating Cenozoic tectonic uplift between the Qilian Shan and Qaidam Basin, northern Tibetan Plateau
Zhimin Li; Runchao Liu; John He; Wenjun Zhu; Wanhe Wang ...
Constraining tectonic uplift history within the Tibetan Plateau is critical to understanding its deformational response to continental collision. However, it is difficult to extrapolate orogen-scale uplift history from any single method alone. Here, we combined high-resolution deep and shallow seismic imaging (on the order of 103 to 102 meters in depth, respectively) with geologic paleoseismic trenching (on the order of several meters in depth) in the Qilian Shan−Qaidam Basin (QSQB) transition zone within the northern Tibetan Plateau, which provide a fault-to-basin, ka-to-Ma-scale record of mountain building. Tectonic uplift began in the early Cenozoic (>40 Ma), with slow uplift rates persisting until ca. 15.3 Ma. Tectonic tilting with limited thrust faulting along the QSQB transition zone was the predominant form of deformation during this period. Accelerated uplift since the middle Miocene is attributed to the activation of more thrust faults, and an increase in fault vertical slip rates by an order of magnitude, reaching ∼0.2−0.25 mm/a.

Early Cretaceous continental-scale sediment routing, the McMurray Formation, Western Canada Sedimentary Basin, Alberta, Canada
Abdullah M. Wahbi; Michael D. Blum; Caroline Nazworth Doerger
Alternative depositional models for the Early Cretaceous McMurray Formation between a dominantly continental and marginal marine settings remain a controversial topic. The source-to-sink model can be inverted to reconstruct ancient sediment-routing systems by utilizing methods to estimate ancient drainage basins, which can contribute to understanding of sediment routing and testing of alternative depositional models. New detrital zircon U-Pb ages from 31 samples were analyzed to identify source terranes of the McMurray Formation and the overlying Wabiskaw Member of the Clearwater Formation to test first-generation alternative sediment-routing models and estimate maximum depositional ages. In total, 9729 new concordant U-Pb ages identified multiple source terranes for the McMurray Formation within an interpreted continental-scale paleodrainage basin that extended from the SW to SE United States and eastern Canada. The paleodrainage basin then expanded to include the Western Cordillera arc system within the overlying Upper Mannville Group. Multidimensional scaling and mixing models independently support a paleo-upstream mixing of primary and recycled sources in the McMurray Formation axial system in the United States and a paleo-downstream confluence between this axial system and east-derived sediment-routing systems in Canada, which display an evolution from bedrock-confined to alluvial morphology from the lower to middle-upper McMurray Formation. The current dataset constrains the maximum depositional age of the McMurray Formation to the latest Barremian to Aptian, with an age range from ca. 122 to 115 Ma, which is significantly younger than previously reported. Age estimates in the overlying Upper Mannville units from ca. 115 to 110 Ma statistically overlap with the McMurray Formation age, suggesting continuous Mannville deposition that lasted 10−12 m.y. or less.

Emplacement dynamics of a crystal-rich, highly viscous trachytic flow of the Sancy stratovolcano, France
Jean-Marie Prival; Andrew J.L. Harris; Elena Zanella; Claudio Robustelli Test; Lucia Gurioli ...
Emplacement dynamics of highly viscous, silicic lava flows remain poorly constrained due to a lack of consideration of crystal-rich cases. Emplacement models mostly apply to glassy or microlitic, vesiculated rhyolitic flows. However, crystalline, vesicle-free silicic lava can flow differently. We studied the Grande Cascade unit, which is a vesicle-free, phenocryst-rich, trachytic flow in the Monts Dore massif, France. Field work was carried out to define internal structures, and oriented samples were collected for chemical, petrological, and anisotropy of magnetic susceptibility analyses, allowing us to estimate emplacement temperature and viscosity. These data allow us to define a new silicic lava flow subtype that is low in temperature (800−900 °C), high in silica content (up to 66.8 wt%), high in viscosity (109−1011 Pa s), rich in phenocrysts (∼35%), and lacks vesicles. Brittle deformation of the lava occurs upon extrusion, generating a cataclasite basal layer and thin (3-m-thick) shear zone that accommodates all of the stress, allowing most of the flow’s volume to slide over its base as a 40-m-thick plug in which there is no deformation. Blocks are rare, of a single size (10 ± 1 cm), and result from localized break-up of the basal shear zone. Emplacement dynamics are different from those of glassy, pumiceous lava flows. They are closer to glacier dynamics, where most of the volume slides over a thin basal shear zone and till is generated there by abrasion and milling of the underlying layer. For the Grande Cascade lava flow, abrasion means that the flow lacks its classical blocky crust and instead the flow base is marked by a layer rich in fine-grained material. The structures and emplacement dynamics of this crystal-rich flow are consistent with ideal, gravity-driven shear flow. We thus argue for a global reassessment of silicic-rich lava emplacement based on crystal content and using a multidisciplinary approach focused on well-exposed examples in the rock record.

Constraining the crustal composition of the continental U.S. using seismic observables
Siyuan Sui; Weisen Shen; Kevin Mahan; Vera Schulte-Pelkum
The composition of the crust is one of the most uncertain and controversial components of continental estimates due to (1) limited direct access and (2) inconsistent indirect assessments. Here we show that by combining high-resolution shear velocity (Vs) models with newly measured with newly measured ratio of compressional wave velocity (Vp) and Vs, or Vp/Vs ratio, for the crystalline crust, a 3-D composition (SiO2 wt%) model of the continental crust can be derived with uncertainty estimates. Comparing the model with local xenolith data shows consistency at mid and lower crustal depths. The spatial patterns in bulk SiO2 content correlate with major geological provinces, including the footprints of Cenozoic and Mesozoic mafic volcanism in the western U.S., and offer new insight into the composition and evolution of the continental U.S.

Finding a VOICE in the Southern Hemisphere: A new record of global organic carbon?
Ralf J. Weger; Gregor P. Eberli; Leticia Rodriguez Blanco; Maximillian Tenaglia; Peter K. Swart
Variations in the carbon isotopic composition of carbonate and organic carbon (δ13Ccarb and δ13Corg) are generally used to record perturbations in the global carbon cycle, which are in turn closely linked to changes in climate. However, because of climate gradients on Earth, assignment of the “global” signal in ancient records is not straightforward. Here, we report the δ13C values of organic material in the Upper Jurassic to Lower Cretaceous sedimentary record of the Vaca Muerta Formation, situated in the Neuquén Basin, Argentina, which show similar patterns to those observed in several northern latitude basins. This record of δ13C values in the organic material differs from those measured in the early Atlantic Ocean, a record previously considered to be representative of the global values of organic carbon. As a result of the global synchronicity observed in the δ 13C values of organic material from both northern and southern latitudes, we suggest that these patterns may represent the global record of δ13C values in organic material rather than those measured in the proto−Atlantic Ocean. The δ13C values of the organic components show a slight initial decrease of ∼2‰ in the early Tithonian (149−145 Ma) and then another decrease of ∼2‰ before reaching a minimum of −30.29‰ in the late Tithonian (145−143 Ma), followed by a gradual increasing trend throughout the Berriasian (143.1−137.7 Ma). The early Valanginian (137.7−135.5 Ma) was marked by a more substantial increase in δ 13C values up to −23.46‰. These changes mirror those seen in Northern Hemisphere locations during the Late Jurassic and Early Cretaceous, where this perturbation has been termed the Volgian isotopic carbon excursion (VOICE). This difference in the Late Jurassic and Early Cretaceous δ13C values between the early Atlantic Ocean and the Neuquén Basin is interpreted to be the result of the climate gradient at the time, which was characterized by more humid conditions in high latitudes compared to dry conditions in the Atlantic Ocean basin.

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For Immediate Release
4 January 2023
GSA Release No. 23-03

Kea Giles