Stratigraphy of the Eocene–Oligocene Titus Canyon Formation, Death Valley, California, and Eocene extensional tectonism in the Basin and Range

Geologic mapping, measured sections, and geochronologic data elucidate the tectono-stratigraphic development of the Titus Canyon extensional basin in Death Valley, California, and provide new constraints on the age of the Titus Canyon Formation, one of the earliest syn-extensional deposits in the central Basin and Range. Detrital zircon maximum depositional ages (MDAs) and compiled 40Ar/39Ar ages indicate that the Titus Canyon Formation spans 40(?)–30 Ma, consistent with an inferred Duchesnean age for a unique assemblage of mammalian fossils in the lower part of the formation. The Titus Canyon Formation preserves a shift in depositional environment from fluvial to lacustrine at ca. 35 Ma, which along with a change in detrital zircon provenance may reflect both the onset of local extensional tectonism and climatic changes at the Eocene–Oligocene boundary. Our data establish the Titus Canyon basin as the southernmost basin in a system of late Eocene extensional basins that formed along the axis of the Sevier orogenic belt. The distribution of lacustrine deposits in these Eocene basins defines the extent of a low-relief orogenic plateau (Nevadaplano) that occupied eastern Nevada at least through Eocene time. As such, the age and character of Titus Canyon Formation implies that the Nevadaplano extended into the central Basin and Range, ~200 km farther south than previously recognized. Development of the Titus Canyon extensional basin precedes local Farallon slab removal by ca. 20 Ma, implying that other mechanisms, such as plate boundary stress changes due to decreased convergence rates in Eocene time, are a more likely trigger for early extension in the central Basin and Range.

Supplemental Material: Concurrent MORB-type and ultrapotassic volcanism in an extensional basin along the Laurentian Iapetus margin: Tectonomagmatic response to Ordovician arc-continent collision and subduction polarity flip

Supplemental Material containing a detailed description of analytical methods, additional geochemical and geochronological plots, as well as complete geochemical and geochronological data sets.

Supplemental Material: Concurrent MORB-type and ultrapotassic volcanism in an extensional basin along the Laurentian Iapetus margin: Tectonomagmatic response to Ordovician arc-continent collision and subduction polarity flip

Supplemental Material containing a detailed description of analytical methods, additional geochemical and geochronological plots, as well as complete geochemical and geochronological data sets.

Paleomagnetism on salt-detached syndiapiric overburden rocks from the Northern margin of the Basque-Cantabrian extensional Basin (N Spain)

<p>The Basque-Cantabrian Basin is a hyperextended extensional basin that formed as the result of the opening of the Bay of Biscay at latest Jurassic-middle Cretaceous times. It is formed by upper mantle and crustal rocks affected by both high- and low-angle faults that die against an Upper Triassic salt layer that decouples the deformation and generate salt diapirs. From late Santonian (Late Cretaceous), the Basque-Cantabrian Basin was involved in the Pyrenean orogeny which reactivated the previous faults and salt décollement.</p><p>In this scenario, our study focuses at the northern margin of this basin where the salt overburden (Jurassic to Eocene in age) is displaced several km northwards and appears compartmentalized by several salt walls (Bakio, Bermeo, Guernica and Mungia diapirs). These walls are linked by narrow stripes of variable orientations in which the overburden appears strongly deformed by tight detachment folds and minor thin-skinned thrusts. The piercing salt is composed of Upper Triassic evaporites, red clays and volcanic ophites, and is flanked by Aptian-Albian syn-diapiric carbonate to terrigenous halokinetic sequences limited by angular unconformities that become conformable as distance to the diapir edges increases. Using a paleomagnetic study, we seek to better understand the kinematics of suprasalt deformation trying to detect and quantify vertical axis rotations recorded during both the extensional and later contractional reactivation of the basin margin. For that, 52 paleomagnetic sites have been analyzed in the overburden sequence. Of these, 50 sites were sampled in Aptian, Albian and Cenomanian marls, marly limestones and fine grained sandstones, and 2 sites were sampled in Eocene sandstones and marly limestones. Characteristic components are usually defined between 200-450 ºC pointing to titanomagnetite as the main remanence carrier. They show predominant anticlockwise rotations with some anomalous clockwise and larger anticlockwise rotations near the salt diapirs. All these components yield normal polarity, as expected by the age of the rocks, which (except the Eocene sites) coincide with the Cretaceous superchron C34n. However, some of the sites are clearly remagnetized as they yield negative fold tests, whereas some other sites show a prefolding magnetization. These results are also supported by several hysteresis analyses and back field experiments that confirm a clear remagnetization signal in the Day diagram in part of the studied rocks. However, the spatial location of remagnetized rocks does not show a distinct structural pattern. With the current data, the origin and age of this remagnetization is difficult to assess and further analysis will be necessary. It could be either an earlier Albian-Maastrichtian remagnetization or a remagnetization linked to the Pyrenean compression. Although these uncertainties, the obtained results allow establishing a preliminary kinematic model for the suprasalt deformation together with the underlying decoupled autochtonous materials.</p>

The effect of pre-existing faults on the development of extensional basins: insights from wet clay experiments

<p><span>Most of the present-day extensional systems formed in areas that already experienced an older phase of tectonic activity. Therefore, understanding how a pre-existing structural setting may affect the development of an extensional basin is a crucial interplay to decipher. Depending on the kinematics of these phases, the resulting inherited faults can be extensional, contractional, or transcurrent. Consequently, a new extensional basin forms atop or across pre-existing faults that can dip at a low- (e.g., inherited thrust faults) or high-angle (e.g., inherited extensional faults). Furthermore, the inherited structures can have a non-optimal attitude with respect to the new extensional stress field, thereby determining different instances for reactivation. In this study, we analyzed the impact of dip and strike of inherited faults on the development of an extensional basin using wet clay (kaolin) analogue modeling. We reproduced sixteen different setups by varying the dip (30°, 45°, 60°) and the strike (15°, 30°, 45°, 60°, 75°) of the pre-existing faults that we introduced in the experiments before applying extension. The results show that the orientation of pre-existing faults has a direct effect onto the shape of the new extensional basins. When the pre-existing faults are reused to accommodate the new extensional phase, the formed basins are asymmetric and the rate of growth of the new faults is lower.</span></p>