ALBERT

Description: 〈span〉The present-day topography in Iberia is related to geodynamic processes dealing with lithospheric-scale deformation. However, little attention has been paid to the role of inherited crustal- or lithospheric-scale structures involved in the recent observed large-scale topographic patterns. Whereas the analysis of brittle structures focuses on the evolution of Mesozoic sedimentary basins and their subsequent response to tectonic inversion, their contribution to mountain building has been underestimated. Large numbers of structures, from ductile to brittle, which affected the whole lithosphere, were developed during the evolution of the Cantabrian orocline (ca. 310–300 Ma). The contribution of these Paleozoic post-Variscan structures, together with lithospheric mantle evolution and replacement during orocline development in the Mesozoic and Cenozoic geological evolution of Iberia, remains unexplored. To explore the role of these inherited structures on the final configuration of topography during N-S Pyrenean shortening, we carried out a series of analogue experiments complemented by surface velocity field analyses. Our experiments indicate that strain was concentrated along preexisting crustal- to lithospheric-scale discontinuities, and they show several reactivation events marked by differences in the velocity vector field. Differences in fault displacement were also observed in the models depending upon preexisting fault trends. The obtained results may explain the different amount of displacement observed during the reactivation of some of the post-orocline structures in Iberia during the Cenozoic, indicating the key role of unveiled structures, which probably have accommodated most of the Alpine shortening.〈/span〉

Description: During opening of a new ocean, magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock, creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g., during the Paleocene-Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as sediment volcanoes, and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black smoker–type chimneys rather than sediment volcanoes. We have evidence for two buried and one active hydrothermal vent systems outside the rift axis. The active vent releases fluids of several hundred degrees Celsius containing abundant methane, mid-ocean ridge basalt–type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Description: The southern end of the Baja California peninsula is cut by a north-striking, left-stepping, active, normal-fault system--the marginal fault system of the oblique-divergent plate boundary within the Gulf of California. We conducted gravity surveys across the normal-fault-bounded basins, and, along with optically stimulated luminescence dating of offset piedmont surfaces and geologic data, we estimated fault-slip rates and assessed fault patterns across basins, gaining insight into basin evolution to better understand the role of upper-crustal processes during development of an obliquely rifted plate margin. Gravity surveys across the La Paz, San Juan de los Planes, and San Jose del Cabo basins revealed basin depths ranging from [~]500 to 3000 m. The La Paz basin is a half graben with two smaller basins that reflect the two main east-dipping splays of the Carrizal fault. Within the San Juan de los Planes and San Jose del Cabo basins, there are buried faults, indicating that during the early stages of basin formation, strain was distributed across these smaller intrabasin faults prior to development of the basin-bounding faults. Slip rates coupled with basin depths suggest that the La Paz and San Juan de los Planes basins began forming ca. 2-5 Ma, overlapping in time with the formation of the main plate boundary at this latitude. The San Jose del Cabo basin has the greatest depth to bedrock (1.6-2.7 km), signifying that it accommodates a greater slip rate or a longer duration of slip than the other faults within this system.

Description: A paleoseismological investigation in Spanish Valley, SE Utah, reveals that faults related to interstratal karstification of salt may show episodic displacement and significantly different parameters than tectonic faults. Spanish Valley is a 25-km-long, 3-km-wide, NW-SE–trending graben formed by the collapse of the crest of a salt anticline. This collapse is related to the karstification of Paleozoic salts, which are several kilometers thick and form the core of the anticline. Differential passive bending of the supra-evaporitic Mesozoic strata produced smaller-scale, NW-SE–trending anticlines and synclines parallel to the axis of the graben on both margins of the collapse valley. Mapping reveals that swarms of synthetic and antithetic normal faults associated with these folds accommodate most of the vertical displacement. A 27-m-long, 4.5-m-deep, trench-like artificial excavation was dug into the hanging wall of the master normal fault of the NE flank with 30–40 m of throw. The excavation exposed a complex structure consisting of a half graben and an asymmetric upper graben separated by a horst. Nine displacement events have been inferred and constrained by consistent AMS (Accelerator mass spectrometry) radiocarbon dates, indicating an anomalously high mean vertical slip rate of 3.07 mm/yr and a very low average recurrence of ~316 yr. The most recent recorded faulting event took place after 2330 cal. yr B.P. Data derived from detailed maps indicate that the faults have aspect ratios (maximum displacement to fault length) comparable to those reported for tectonic faults. However, they show greater aperiodicity, with coefficient of variation values greater than 1, long-term slip rates between 2 and 25 times greater, and displacement per event values up to 30 times higher than those expected for tectonic faults of the same length.

Description: Jurassic-aged strata of Asturias, Spain, contain trace fossils including sauropod, theropod, and ornithopod dinosaur footprints, but their paleoenvironmental context has been relatively unstudied. A coastally exposed continuous section at Playa de Vega shows a clear transition from the marine Middle Jurassic Rodiles Formation to the terrestrial Upper Jurassic Vega Formation. Within the 〉100 m of Vega Formation stratigraphy that was logged there, four distinct types of paleosols were identified: (1) Entisols, (2) Inceptisols, (3) Vertisols, and (4) composite or cumulative paleosols. The paleosol types and their features indicate a floodplain depositional setting with short stature, shrubby vegetation. Theropod and ornithopod tracks have been identified at the base of the section, indicating that a dinosaurian fauna was present at Playa de Vega during the Jurassic. Results from well-characterized climofunctions based on modern soils and paleosol B horizon chemical composition of the Inceptisols and Vertisols yield mean annual precipitation estimates of 400–980 mm yr-1 and mean annual temperature estimates of 8–15 °C. The presence of Vertisols, with both evidence for shrink-and-swell behavior and dispersed pedogenic carbonate, is consistent with a strongly seasonal precipitation regime. The d13C analyses of pedogenic carbonates yield values that range from –7.09‰ to –8.88‰ (relative to Vienna Peedee belemnite [VPDB]) and indicate carbon dioxide levels about six times pre-industrial levels, consistent with previous results. The Asturian vertebrate track assemblage is remarkably similar to that of the Morrison Formation (western United States), but it has greater overall richness. The pattern is reversed for body fossils. The reconstructed paleoenvironmental and paleoclimatic conditions indicate similar depositional settings in both places, but with a cooler, wetter, more seasonal environment in Asturias. The greater seasonality indicated by the Vega Formation relative to the Morrison Formation may explain the observed differences in richness.

Description: Delamination of continental lithosphere in the core of active collisional orogens is a well-established process; however, evidence for its occurrence in ancient orogenic belts is less obvious. The contrasting Sm-Nd isotopic signature between pre- and post-Middle Permian mantle-derived mafic rocks from under the Iberian Massif suggests that most, but not all, of the subcontinental lithospheric mantle (SCLM) was replaced in latest Carboniferous to Permian time. Mantle replacement happened during and after the bending of the Variscan orogenic belt into the horseshoe-shaped Iberian-Armorican orocline. Delamination of thickened continental lithosphere in the core of the orocline triggered replacement of the ancient SCLM, thereby providing an explanation for the contrasting Sm-Nd isotopic characteristics of pre- and post-Middle Permian mafic rocks.

Description: Detrital zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb age data from the Lower Ordovician Armorican Quartzite (deformed passive margin strata of Gondwanan affinity) of the Iberian Massif are presented herein. The S -shaped coupled Iberian oroclines defined within these zones palinspastically restore to a 2300 km linear Variscan orogen with a paleomagnetically constrained Late Carboniferous north-south trend. Detrital zircons are used to assess paleogeography and interpreted geometry of the Iberian portion of the Gondwana passive margin. A common signature is identified by (1) Neoproterozoic (ca. 500–850 Ma), (2) Stenian–Tonian (ca. 0.9–1.1 Ga), and lesser (3) Paleoproterozoic and (4) Archean populations (ca. 1.8–2.15 and 2.5–2.7 Ga, respectively). Minor site-to-site variation in relative proportion of widely ranging age groups suggests near-uniform distribution of a highly varied detrital input. Provenance analysis reveals strong correlations with Cambro-Ordovician clastic rocks from northeast African realms. Similarity with underlying sequences suggests a common paleogeography from the Ediacaran through early Paleozoic and persistence of a provenance distinction within the autochthonous Iberian Massif. Consistent northward paleoflow within widespread northeast African lower Paleozoic sedimentary cover suggests long-distance sedimentary transport across a North African peneplain from outlying basement terranes. We propose that the 2300-km-long Cantabrian–Central Iberian portion of the early Paleozoic Gondwana margin stretched east-west along the northern limits of the then low-lying Saharan Metacraton and Arabian-Nubian Shield. Accepting paleomagnetic constraints, a 90° counterclockwise rotation is required to reorient the Iberian portion to a pre-oroclinal (Late Carboniferous) north-south trend. The mechanisms for accommodating such a rotation are unclear.

Description: We present a comprehensive petrologic study of lower-crust mafic and felsic xenoliths hosted by Quaternary alkaline basalts of the Valle de Santiago monogenetic volcanic field. This is the only locality along the entire Trans-Mexican volcanic belt where the abundance and size of xenoliths allow the understanding in great detail of processes associated with interactions of young subduction-related magmatism and the deep continental crust. Mafic xenoliths (two pyroxene ± spinel granulites and metanorthosites), olivine-rich gabbroic xenoliths, and transitional xenoliths compose the bulk of the population, although a few belong to the charnockitic suite (enderbite and faersundite). Thermobarometric calculations (two-pyroxene, ilmenite-magnetite, Ti-in amphibole, amphibole-plagioclase, and phase equilibria in the system NCMAS (Na-Ca-Mg-Si)) result in pressures around 9 kbar and temperatures of 1000–1100 °C for the granulite-facies metamorphism, which would give a very hot lower crust, ~33 km thick, beneath Valle de Santiago and a mean geothermal gradient of ~30 °C/km. Igneous zircons (Th/U = 0.03–0.87) extracted from one of the felsic granulites yielded a major peak of latest Cretaceous age (67.1 Ma), interpreted as the crystallization age of the granitic protolith, without inheritance from Precambrian or Paleozoic crust. Minor peaks at 45.1 and 25.5 Ma are interpreted as partial Pb losses from some of the Cretaceous zircons. Trace-element geochemistry, as well as Sr, Nd, and Pb isotopic studies performed on two granulites, is consistent with the juvenile and coeval nature of both the mafic metagabbroic xenoliths and the alkaline basaltic magmas that lifted the xenoliths from the lower crust. Two intermediate stages in the thermal evolution of the sampled xenoliths include the emplacement at different depths of volatile and K-Fe-Ti–rich oxidized melts represented by igneous assemblages with kaersutite, biotite, titanomagnetite, spinel, plagioclase, Fe-rich epidote, clinopyroxene, fayalitic olivine, and glasses that pervasively invaded most granulite xenoliths before being taken to the surface. A preferred plumbing system model is presented depicting a protracted Miocene to Quaternary basaltic intraplate magmatic system that sampled former basaltic batches stationed in the lower crust, together with the Late Cretaceous deep-seated granitoids beneath Valle de Santiago in the backarc of the central Trans-Mexican volcanic belt. Both components were later subjected to granulite-facies conditions in the lower crust, most probably related to the continued heating of the crust by basaltic magmas underplated in the central Trans-Mexican volcanic belt backarc region.

Description: A paleoseismological investigation of flexural-slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the potential to generate earthquakes with damaging magnitude. The Carbondale collapse center, in the southern Rocky Mountains of Colorado, is a morpho-structural depression of ~1200 km 2 where Miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite-bearing evaporites. On the western margin of the collapse center, the debuttressing effect related to active evaporite dissolution drives unfolding of the steeply dipping late Laramide Grand Hogback monocline, accompanied by displacement on bedding-parallel faults. These flexural-slip faults rupture unconformable Miocene basalts and Quaternary mantled pediments, generating conspicuous half-graben depressions bounded by antislope fault scarps parallel to the underlying strata of the monocline. Two trenches dug across flexural-slip fault scarps developed in each stratigraphic marker (basalt cap, mantled pediment) revealed unexpected evidence of multiple late Quaternary faulting events (e.g., faulted colluvial wedge, sharp unconformities), with displacement-per-event values of ≥1 m. Three faulting events were inferred from the trench dug in the pediment (〈32 ka, 32–28 ka, 5.6–1.5 ka), and four events from the trench sited in the basalts, all probably older than 20 ka. The probable length (~25 km) and downdip width (~7.5 km) of the flexural-slip faults associated with the Carbondale collapse center suggests that they might have the potential to generate damaging "unfolding earthquakes" with moment magnitude (M w ) around 6.