ALBERT

Description: The responses of sedimentary systems to rifting at continental margins are three-dimensional and involve the mixing of various sediment sources through tectonic drivers and sediment response. Such sedimentary responses have not been well studied along magma-poor, hyperextended margins where the crust is stretched and thinned to ≤10 km. The asymmetric Mauléon Basin of the western Pyrenees is the product of such magma-poor hyperextension resulting from lateral rift propagation from the Bay of Biscay during Cretaceous time. After rifting, limited shortening during Cenozoic Pyrenean inversion uplifted the basin, resulting in preservation of outcrops of rift basin fill, upper and lower crustal sections, serpentinized lithospheric mantle, and basic rift-fault relationships. In this study ~5800 new zircon U-Pb ages were obtained from prerift, synrift, and postrift strata; the ages constrain the proximal to distal evolution of the Mauléon Basin and define a general model for sediment routing during rifting. Zircon U-Pb analyses from lower crustal granulites indicate that granulite plutons crystallized at 279 ± 2 and 274 ± 2 Ma, and paragneissic granulites yielded zircon rim ages of ca. 295 Ma. Detrital zircon U-Pb ages from western Pyrenean prerift strata show age modes of ca. 615 and ca. 1000 Ma, suggesting continual recycling and/or well-mixed Gondwanan-sourced sediments throughout the Paleozoic and early Mesozoic; additional Paleozoic age components (ca. 300 and ca. 480 Ma) are also observed. The variation of detrital zircon U-Pb ages in synrift and postrift strata illustrates that during rifting, provenance varied spatially and temporally, and sediment routing switched from being regionally, to locally, and then back to regionally derived within individual structurally controlled subbasins.

Description: We use apatite and zircon (U-Th)/He thermochronometry to evaluate space-time patterns and tectonic drivers of Miocene to Pliocene deformation within the Death Valley area, eastern California. Zircon He ages from the footwall of the Amargosa–Black Mountains detachment in the Black Mountains record continuous cooling and exhumation from 9 to 3 Ma. Thermal modeling of data from the central Black Mountains suggests that this cooling took place during two intervals: a period of rapid footwall exhumation from 10 to 6 Ma, followed by slower (〈5 mm/yr) exhumation since 6 Ma. Cumulative exhumation is estimated to be 10–16 km. Paleodepth reconstruction of cooling ages from the footwall of the Panamint-Emigrant detachment, in the central Panamint Range, also show two periods of cooling. Zircons record late Miocene cooling, whereas apatite He ages show punctuated exhumation at ca. 4 Ma. The results suggest the Panamint Range experienced a minimum of 7.2 km of exhumation since ca. 12 Ma. The new data, when evaluated within the context of published fault timing data, suggest that the transition from Basin and Range extension to dextral transtension is spatially and temporally distinct, beginning at ca. 11–8 Ma in ranges to the east and north of the Black Mountains and migrating westward into eastern Death Valley at 6 Ma. Initiation of dextral transtension was coincident with a major change in plate-boundary relative motion vectors. Data from Panamint Range and several ranges to the west of Death Valley indicate transtension initiated over a large area at ca. 3–4 Ma, coeval with proposed lithospheric delamination in the central and southern Sierra Nevada Range. Our results suggest that the transition from extension to dextral transtension may reflect an evolution in tectonic drivers, from plate-boundary kinematics to intraplate lithospheric delamination.

Description: We use apatite and zircon (U-Th)/He thermochronometry to evaluate the timing, magnitude, and spatial pattern of Miocene strain within the Beaver Dam Mountains, Tule Springs Hills, and Mormon Mountains of southwestern Utah and southeastern Nevada (USA). The region is host to three major low-angle structures, the Castle Cliffs, Tule Springs, and Mormon Peak detachments, the origin and role of which in regional extension are vigorously debated. We analyzed 36 samples collected from Precambrian basement gneisses and Paleozoic to Jurassic siltstones and sandstones exposed in the footwalls of these detachments. Zircon He ages from the footwall of the Castle Cliffs detachment record rapid footwall exhumation ca. 18–17 Ma. At structurally higher positions, apparent ages become progressively older, defining a zircon He partial retention zone. Paleodepth reconstructions of the data using published cross sections suggest 180 °C, or greater, of cooling or 6.8–8.2 km of total exhumation, yielding a maximum of ~13 km of extension across this detachment. In contrast, zircon and apatite ages from the footwall of the Mormon Peak record rapid exhumation at 14–13 Ma and 5.8–7.1 km of vertical exhumation. Using a range of restored fault dips (20°–28°) for the Mormon Peak detachment, the thermochronology data record 10.9–19.5 km horizontal extension. Data from the Tule Springs detachment also show a similar timing of exhumation and indicate that there has been 5.0–6.8 km of vertical exhumation and a minimum of ~5 km of extension. The results demonstrate that extension initiated in the east along the Castle Cliffs detachment and migrated westward with time. Although our data indicate that existing extension estimates across this system of detachment faults are too high (40 km versus 54 km), the pattern of cooling ages and protracted cooling history recorded in these ranges are inconsistent with rootless gravity slide interpretations and low-magnitude extension models.

Description: North-trending rifts and associated strike-slip faults in the Tibetan Plateau suggest Cenozoic east-west extension, but the dominant modes of distributed extensional deformation and basin formation are unclear. The Lunggar basin in west-central Tibet is bounded by a 〈40° low-angle detachment fault, contains active high-angle normal faults, and displays elevated topography toward the central segment of the basin with axial fluvial drainage toward the northern and southern basin terminations. Structural and stratigraphic features are consistent with a high-angle extensional system that evolved into a low-angle fault and supradetachment basin during progressive extension. This study seeks to constrain the depositional and exhumational history of the Lunggar basin and bounding fault system by assessing the sedimentologic, structural, and thermochronologic record of basin fill. Upper Cenozoic facies include alluvial-fan conglomerates and fluviolacustrine sandstones and siltstones. Sandstone petrographic data, conglomerate clast compositions, and detrital zircon U-Pb ages indicate systematic unroofing of the western footwall (including Jurassic–Cretaceous and Miocene granites that intruded Permian–Cretaceous strata). Paleocurrents are orthogonal or opposite to the current dispersal pattern, suggesting that growth of a modern intrabasin high in the central segment of the basin has modified the original basin configuration. As a proxy for footwall cooling histories, four basin-fill sandstones and 11 leucogranite boulders from proximal hanging-wall strata were sampled for low-temperature thermochronometry. Apatite and zircon (U-Th)/He results suggest that extension was underway by 10–8 Ma, with late Miocene–Pliocene exhumation rates of roughly 1 km/Myr. This rapid exhumation generated a conglomeratic unroofing sequence and promoted hanging-wall rebound and erosional recycling of range-front basin fill along the central segment of the detachment fault. The collective results support a model of rift evolution that invokes upper-crustal thinning, supradetachment basin subsidence, and subsequent isostatic rebound along the more-evolved central segments of Tibetan extensional systems.

Description: The Adriatic microplate is a key player in the Western Mediterranean tectonic puzzle, but its Oligocene–Miocene dynamics are not yet fully understood. In fact, even though the timing and magnitude of Adriatic slab rollback and backarc extension in the Apennines have long been established, the timing of progressive Adria indentation beneath the Central Alps and major strike-slip motion along the Insubric fault are still poorly constrained. Here we tackle these issues by utilizing detrital zircon U-Pb geochronology on Adriatic foredeep turbidites, i.e., by comparing the geochronologic fingerprints of the exhuming tectonic domes of the Central Alps (Ticino and Toce subdomes) with those of the Oligocene–Miocene turbidites chiefly derived from their erosion. We analyzed 11 sandstone samples ranging in age from 32 to 18 Ma. The ratio between Variscan and Caledonian zircon grains (which are dominant in the Toce and Ticino subdomes, respectively) sharply increases at ca. 24–23 Ma. This major provenance change marks the westward shift of the Adriatic indenter beneath the Central Alps and the associated right-lateral activity of the Insubric fault. The coexistence of strike-slip motion at the northern boundary of the Adriatic microplate at ca. 24–23 Ma and trench retreat during scissor-type backarc opening to the west requires a near-vertical rotation axis located at the northern tip of the Ligurian-Provençal basin. We propose that the rotation axis position was controlled by the interaction between the European and the Adriatic slabs, which may have collided at depth by the end of the Oligocene, triggering the westward shift of the Adriatic indenter beneath the Central Alps.

Description: New mapping combined with fault-slip and thermochronological data show that Middle Miocene to recent extension and exhumation of the Slate Range, eastern California, is produced by the active Searles Valley fault system and the Slate Range detachment, an older Middle Miocene low-angle normal fault. Offset Middle Miocene rocks record a combined ~9 km of west-directed extension over the past ~14 m.y. for the fault zones. (U-Th)/He apatite cooling ages of samples from the central and southern Slate Range indicate that footwall cooling began ca. 14 Ma; we interpret this as the age of initiation of motion on the Slate Range detachment. This timing is consistent with inferences made using stratigraphic and structural criteria. Data from the northern Slate Range show that rapid fault slip began along the Searles Valley fault ca. 4 Ma; data from the central and southern Slate Range can be interpreted as indicating cooling at 5–6 Ma. This timing correlates to the results of nearby studies, suggesting a strain transition in the surrounding area between ca. 6 and 3 Ma. The data collected are most consistent with a westward migration in the locus of transtensional deformation, and show that the initiation of that deformation commonly lags the timing predicted by plate reconstructions by a few million years.

Description: Records of past topography connect Earth’s deep interior to the surface, reflecting the distribution of heat and mass, past crustal structure, and plate interactions. Many tectonic reconstructions of the North American Cordillera suggest the presence of an Altiplano-like plateau in the location of the modern Basin and Range, with conflicting timing and mechanisms for the onset of surface-lowering extension and orogen collapse. Here we show, through a paleotopographic profile, that from the Eocene to the Oligocene a high, broad orogen stretched across Nevada, with a distinct crest that divided a continuous westward-draining slope extending to central California from an internally drained eastern Nevada plateau. This paleo-orogen maintained demonstrably higher-than-modern elevations, reaching 3500 m in the late Oligocene. Despite the long-term high gravitational potential energy of the crust supporting this topography, surface-lowering extension did not occur until the transition to a transform margin changed the external kinematic framework of the system. Maximum surface lowering was spatially decoupled from brittle upper crustal extension, requiring a large component of mid-crustal flow.

Description: The injection of carbon dioxide (CO2) captured at large point sources into deep saline aquifers can significantly reduce anthropogenic CO2 emissions from fossil fuels. Dissolution of the injected CO2 into the formation brine is a trapping mechanism that helps to ensure the long-term security of geological CO2 storage. We use...

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