ALBERT

Description: 〈span〉Active fold-and-thrust belts can potentially accommodate large-magnitude earthquakes, so understanding the structure in such regions has both societal and scientific importance. Recent studies have provided evidence for large earthquakes in the Western Transverse Ranges of California, USA. However, the diverse set of conflicting structural models for this region highlights the lack of understanding of the subsurface geometry of faults. A more robust structural model is required to assess the seismic hazard of the Western Transverse Ranges. Toward this goal, we developed a forward structural model using Trishear in MOVE® to match the first-order structure of the Western Transverse Ranges, as inferred from surface geology, subsurface well control, and seismic stratigraphy. We incorporated the full range of geologic observations, including vertical motions from uplifted fluvial and marine terraces, as constraints on our kinematic forward modeling. Using fault-related folding methods, we predicted the geometry and sense of slip of the major faults at depth, and we used these structures to model the evolution of the Western Transverse Ranges since the late Pliocene. The model predictions are in good agreement with the observed geology. Our results suggest that the Western Transverse Ranges comprises a southward-verging imbricate thrust system, with the dominant faults dipping as a ramp to the north and steepening as they shoal from ∼16°–30° at depth to ∼45°–60° near the surface. We estimate ∼21 km of total shortening since the Pliocene in the eastern part of the region, and a decrease of total shortening west of Santa Barbara down to 7 km near Point Conception. The potential surface area of the inferred deep thrust ramp is up to 6000 km〈sup〉2〈/sup〉, which is of sufficient size to host the large earthquakes inferred from paleoseismic studies in this region.〈/span〉

Description: New samples collected from a transect across the summit limestone of Mount Everest (Qomolangma Formation) show that multiple distinct deformational events are discretely partitioned across this formation. Samples from the highest exposures of the Qomolangma Formation (Everest summit) preserve a well-developed mylonitic foliation and microstructures consistent with deformation temperatures of ≥250 °C. Thermochronologic and microstructural results indicate these fabrics were ingrained during initial contractile phases of Himalayan orogenesis, when crustal thickening was accommodated by folding and thrusting of the Tethyan Sedimentary Sequence. In contrast, samples from near the base of the Qomolangma Formation (South Summit) preserve extensional shear deformation, indicate metasomatism at temperatures of ~500 °C, and contain a synkinematic secondary mineral assemblage of muscovite + chlorite + biotite + tourmaline + rutile. Shear fabrics preserved in South Summit samples are associated with activity on the Qomolangma detachment, while the crystallization of secondary phases was the result of reactions between the limestone protolith and a volatile, boron-rich fluid that infiltrated the base of the Qomolangma Formation, resulting in metasomatism. The 40 Ar/ 39 Ar dating of synkinematic muscovite indicates the secondary assemblage crystallized at ca. 28 Ma and that shear fabrics were ingrained at ≥18 Ma. This paper presents the first evidence that Everest’s summit limestone records multiple phases of deformation associated with discrete stages in Himalayan orogenesis, and that the structurally highest strand of the South Tibetan detachment on Everest was initially active as a distributed shear zone before it manifested as a discrete brittle detachment at the base of the Qomolangma Formation.

Description: The Variscan orogen provides the European record of the late Paleozoic continental collisions that culminated with formation of the supercontinent Pangea. An S-shaped pair of isoclinal coupled oroclines characterizes the Variscan orogen of the Iberian Massif. Though oroclines are common features of the world’s orogenic belts, the mechanisms that drive oroclinal formation, and the manner in which these continental-scale vertical-axis folds of orogens are accommodated are poorly understood. The northerly Cantabrian and the southerly Central Iberian oroclines are structurally continuous and pericontemporaneous, suggesting that they formed in the same fashion. Exposures of the Ediacaran Narcea Slates within the so-called Narcea antiform trace a 150-km-long arcuate belt around the 180° Cantabrian orocline. In the western flank of the Narcea antiform, the Narcea Slates are characterized by a penetrative steep to vertical, rough to slaty cleavage (S1) and subparallel 2-km-wide reverse shear zones with a penetrative fabric (S2) that are postdated by asymmetric meso- to outcrop-scale vertical-axis folds (plunge 〉65°) with a dominant vergence toward the oroclinal hinge; i.e., fold geometry is dominantly dextral (Z-shaped) in the southern limb of the Cantabrian orocline and dominantly sinistral (S-shaped) in its northern limb. Axial planes are consistently steeply dipping, but they are typically oriented at a high angle to S1/S2 and are therefore variable in strike about the orocline hinge. Vertical-axis folds affecting the Narcea Slates are of the appropriate scale and geometry to be interpreted as parasitic structures developed in response to a component of flexural shear within the limbs of the forming Cantabrian orocline. A model of formation of the Iberian coupled oroclines by buckling accommodating significant orogen-parallel shortening along an initially linear Iberian Variscan belt is therefore supported, providing new insight into the complexities associated with the final stages of Pangean amalgamation.

Description: The coupled Iberian oroclines of the western European Variscan orogen accommodated 〉1100 km of post-Variscan orogen-parallel shortening at translation rates in excess of 5 cm⋅yr –1 . Palinspastic restoration of the Iberian coupled oroclines reveals a north-south–trending 2300-km-long Variscan ribbon bound by ophiolite-bearing allochthons. The requirements for orocline formation, including continuing subduction and the consumption of vast tracks of oceanic lithosphere, cannot be reconciled within traditional models that view the Variscan as a record of the closure of a single (Rheic) ocean resulting in terminal Gondwana-Laurussia collision and a stable Pangea supercontinent. Paleomagnetic data from the Gondwana-derived Variscan autochthon indicate its mid-Paleozoic separation (as the Armorican microplate) and the growth of a second mid-Paleozoic (Paleotethys) ocean. The Gondwanan stratigraphic and faunal character of the Variscan autochthon can be reconciled by a model within which counterclockwise separation results in an Armorican ribbon continent that remains close or connected to Gondwana to the south and extends north toward Laurasia between an older (Rheic) ocean to the west and a newly opened (Paleotethys) ocean to the east. Geologic and paleomagnetic data further indicate that, despite translating steadily northward, Gondwana remained separated from Armorica throughout the various stages of Variscan orogenesis. We explain Pangean amalgamation as being coincident with buckling of a linear Armorican ribbon continent caught between Laurussia to the north and the northward-migrating Gondwana to the south. In this model, Variscan orogenesis is explained in terms of individual accretionary events generated through continuous, west-dipping subduction along the margins of the Armorican ribbon continent.

Description: The origin of the Cantabrian orocline of the Variscan orogen in NW Iberia remains a topic of debate. We present a structural study of the Ponga Unit, a Cambrian to Carboniferous tectonostratigraphic package within the West European Variscan belt foreland fold-and-thrust belt that lies within the core region of the orocline. Our primary goal was to determine if W-plunging folds of the fold-and-thrust belt are attributable to formation of the Cantabrian orocline, or if they reflect lateral ramps in the underlying Variscan thrust faults. The major lithologic units of the Ponga Unit are the rheologically competent Lower Ordovician Barrios quartzite, and the less-competent, Carboniferous Barcaliente limestone and Beleño shale and sandstone formation. Our mapping and structural analysis within the Ponga Unit focused on the Laviana, Rioseco, and Campo de Caso thrust sheets, and associated bounding thrusts. Over 800 structural orientation measurements were collected across the study area. These data, coupled with data compiled from regional geological maps, allow for analysis of the crustal structure. West-plunging folds of the Laviana, Rioseco, and Campo de Caso thrust sheets form kilometer-scale anticline-syncline pairs, producing a complex fold interference pattern that is characteristic of the Ponga Unit. Our analysis shows that: (1) the geometry of the W-plunging folds is inconsistent with a lateral ramp model; (2) the map pattern defines a mushroom-type fold interference pattern, indicating two distinct deformational events characterized by principal compressive stresses oriented at a high angle (perpendicular) to one another; and (3) paleomagnetic data from the study area are consistent with the secondary model of orocline formation and indicate that there was a short window of time between the end of Variscan orogenesis and the onset of oroclinal buckling. Our results indicate that early N-S–trending folds, which resulted from Variscan orogenesis, were refolded during a post-Variscan orogen-parallel compression event attributable to formation of the Cantabrian orocline.

Description: 〈span〉〈div〉Abstract〈/div〉Active fold-and-thrust belts can potentially accommodate large-magnitude earthquakes, so understanding the structure in such regions has both societal and scientific importance. Recent studies have provided evidence for large earthquakes in the Western Transverse Ranges of California, USA. However, the diverse set of conflicting structural models for this region highlights the lack of understanding of the subsurface geometry of faults. A more robust structural model is required to assess the seismic hazard of the Western Transverse Ranges. Toward this goal, we developed a forward structural model using Trishear in MOVE〈sup〉®〈/sup〉 to match the first-order structure of the Western Transverse Ranges, as inferred from surface geology, subsurface well control, and seismic stratigraphy. We incorporated the full range of geologic observations, including vertical motions from uplifted fluvial and marine terraces, as constraints on our kinematic forward modeling. Using fault-related folding methods, we predicted the geometry and sense of slip of the major faults at depth, and we used these structures to model the evolution of the Western Transverse Ranges since the late Pliocene. The model predictions are in good agreement with the observed geology. Our results suggest that the Western Transverse Ranges comprises a southward-verging imbricate thrust system, with the dominant faults dipping as a ramp to the north and steepening as they shoal from ∼16°–30° at depth to ∼45°–60° near the surface. We estimate ∼21 km of total shortening since the Pliocene in the eastern part of the region, and a decrease of total shortening west of Santa Barbara down to 7 km near Point Conception. The potential surface area of the inferred deep thrust ramp is up to 6000 km〈sup〉2〈/sup〉, which is of sufficient size to host the large earthquakes inferred from paleoseismic studies in this region.〈/span〉

Description: Active fold-and-thrust belts can potentially accommodate large-magnitude earthquakes, so understanding the structure in such regions has both societal and scientific importance. Recent studies have provided evidence for large earthquakes in the Western Transverse Ranges of California, USA. However, the diverse set of conflicting structural models for this region highlights the lack of understanding of the subsurface geometry of faults. A more robust structural model is required to assess the seismic hazard of the Western Transverse Ranges. Toward this goal, we developed a forward structural model using Trishear in MOVE® to match the first-order structure of the Western Transverse Ranges, as inferred from surface geology, subsurface well control, and seismic stratigraphy. We incorporated the full range of geologic observations, including vertical motions from uplifted fluvial and marine terraces, as constraints on our kinematic forward modeling. Using fault-related folding methods, we predicted the geometry and sense of slip of the major faults at depth, and we used these structures to model the evolution of the Western Transverse Ranges since the late Pliocene. The model predictions are in good agreement with the observed geology. Our results suggest that the Western Transverse Ranges comprises a southward-verging imbricate thrust system, with the dominant faults dipping as a ramp to the north and steepening as they shoal from ∼16°–30° at depth to ∼45°–60° near the surface. We estimate ∼21 km of total shortening since the Pliocene in the eastern part of the region, and a decrease of total shortening west of Santa Barbara down to 7 km near Point Conception. The potential surface area of the inferred deep thrust ramp is up to 6000 km2, which is of sufficient size to host the large earthquakes inferred from paleoseismic studies in this region.