Julie C. Fosdick

Kinematic evolution of the Patagonian retroarc fold-and-thrust belt and Magallanes foreland basin, Chile and Argentina, 51°30's

The kinematic evolution of the Patagonian fold-and-thrust belt and cogenetic Magallanes retroarc foreland basin is reconstructed using new geologic mapping, two-dimensional (2-D) seismic-reflection data, and zircon U/Pb geochronology. Results span an ~160-km-wide transect of the thrust belt and Magallanes Basin near 51°30′S and highlight the influence of inherited extensional structures on basin paleogeography, syntectonic sedimentation, and Late Cretaceous–Neogene foreland shortening. South of 50°S, the Patagonian fold-and-thrust belt developed on oceanic and attenuated crust of the predecessor Late Jurassic Rocas Verdes rift basin, resulting in a collisional nature to early fold-and-thrust belt development and foreland sedimentation atop rifted South American crust. We identify six principal stages of development between Late Cretaceous and Neogene time. A palinspastic restoration indicates ~32–40 km (~19%–23%) of retroarc shortening following closure of the Rocas Verdes Basin and incipient growth of the thrust belt. More than half of the estimated crustal shortening occurred synchronously with the deep-water phase of Late Cretaceous foreland basin sedimentation. Subsequent deformation migrated into the foreland, accounting for ~12 km of shortening across the Cretaceous–early Miocene basin fill. Thick-skinned thrust faulting along multiple detachment levels in Paleozoic metamorphic basement resulted in –5 km of foreland uplift and exposure of preforeland basin deposits. The final phase of early Miocene deformation ca. 21–18 Ma may reflect enhanced coupling between the continental and oceanic lithospheres, causing foreland basement uplifts as the Chile Ridge spreading ridge approached the trench. We speculate that Neogene foreland shortening was accommodated by reactivation of Mesozoic normal faults zones and accounts for broad uplift of the Patagonian fold-and-thrust belt.

Miocene extension in the East Range, Nevada: A two-stage history of normal faulting in the northern Basin and Range

The East Range in northwestern Nevada is a large, east-tilted crustal block bounded by west-dipping normal faults. Detailed mapping of Tertiary stratigraphic units demonstrates a two-phase history of faulting and extension. The oldest sedimentary and volcanic rocks in the area record cumulative tilting of ~30°–45°E, whereas younger olivine basalt fl ows indicate only a 15°–20°E tilt since ca. 17–13 Ma. Cumulative fault slip during these two episodes caused a minimum of 40% extensional strain across the East Range, and Quaternary fault scarps and seismic activity indicate that fault motion has continued to the present day. Apatite fi ssion track and (U-Th)/ He data presented here show that faulting began in the East Range ca. 17–15 Ma, coeval with middle Miocene extension that occurred across much of the Basin and Range. This phase of extension occurred contemporaneously with middle Miocene volcanism related to the nearby northern Nevada rifts, suggesting a link between magmatism and extensional stresses in the crust that facilitated normal faulting in the East Range. Younger fault slip, although less well constrained, began after 10 Ma and is synchronous with the onset of low-magnitude extension in many parts of northwestern Nevada and eastern California. These fi ndings imply that, rather than migrating west across a discrete boundary, late Miocene extension in western Nevada is a distinct, younger period of faulting that is superimposed on the older, middle Miocene distribution of extended and unextended domains. The partitioning of such middle Miocene deformation may refl ect the infl uence of localized heterogeneities in crustal structure, whereas the more broadly distributed late Miocene extension may refl ect a stronger infl uence from regional plate boundary processes that began in the late Miocene.

Influence of attenuated lithosphere and sediment loading on flexure of the deep‐water Magallanes retroarc foreland basin, Southern Andes

Flexural subsidence in foreland basins is controlled by applied loads—such as topography, water/sediment, and subcrustal forces—and the mechanical properties of the lithosphere. We investigate the controls on subsidence observed within the Upper Cretaceous Magallanes retroarc foreland basin of southern South America to evaluate the impact of lateral variations in flexural rigidity due to Late Jurassic extension. Conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep-water Cenomanian-Turonian basin strata. However, models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns. Results satisfy paleobathymetric observations in the Magallanes Basin and suggest that lithospheric thinning is necessary to produce both long-wavelength and deep subsidence during Late Cretaceous basin evolution. Results indicate that elastic thickness decreases westward from ~45–25 km in the distal foreland to ~37–15 km beneath the foredeep. These findings are consistent with a westward reduction in crustal thickness associated with the Jurassic extensional history of the Patagonian lithosphere. Our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift. We propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time. These findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories.

Retroarc basin reorganization and aridification during Paleogene uplift of the southern central Andes

U.S. National Science Foundation [EAR-1049605]; Robert R. Shrock Foundation at Indiana University; NSF-EAR Award [1338583]; project PDTS (UNSJ) [E985]

Tectonic Evolution of Paleozoic and Mesozoic Andean Metamorphic Complexes and the Rocas Verdes Ophiolites in Southern Patagonia

The petrology and geochronology of Paleozoic and Mesozoic metamorphic and ophiolitic complexes in southern Patagonia constrain the evolution of magmatic belts, marginal basins, and continental fragments that once formed part of the peripheral realm of the southwestern Gondwanan margin. The upper Paleozoic metasedimentary rocks exposed along the eastern slopes of the Andean ranges contain prominent zircon components of Devonian and earliest Carboniferous age. These strata were deposited in marine basins fed by sediment derived from almost coeval felsic magmatic belts and recycled units with Ordovician and Cambrian components; some of which have been identified to the east, in Argentina. Permian to Triassic metasedimentary complexes, mainly exposed along the western slope of the Southern Patagonian Andes and in the Antarctic Peninsula, show conspicuous peaks of Permian detrital zircons with sparse older age components, supporting their deposition near active magmatic belts in a convergent margin setting. It is proposed that rapid northward migration of Gondwana (between 330 and 270 Ma) promoted crustal extension and the establishment of an arc and back-arc marginal basin configuration during the Permian. Subsequent closure of the marginal basins occurred during the Late Triassic–Early Jurassic Chonide orogeny. Middle Jurassic fragmentation of Gondwana involved continental extension, synrift silicic and bimodal magmatism, and seafloor-spreading, leading to the oceanic-type Late Jurassic to Early Cretaceous Rocas Verdes basin. Once the Antarctic Peninsula microplate migrated southward, subduction was re-established in southern Patagonia in Early Cretaceous time, involving processes of subduction erosion and high-P/T metamorphism in the forearc region. The Rocas Verdes basin started to close in the latest Early Cretaceous involving underthrusting and subduction of its oceanic seafloor. These convergent processes culminated with the Late Cretaceous Patagonian and Fuegian orogeny involving the tectonic emplacement of ophiolitic complexes, collision of drifted crustal slivers against the continent, and development of the Magallanes–Austral foreland basin.

Sedimentary signals of recent faulting along an old strand of the San Andreas Fault, USA

Continental transform fault systems are fundamental features in plate tectonics. These complex systems often constitute multiple fault strands with variable spatio-temporal histories. Here, we re-evaluate the complex history of the San Andreas Fault along a restraining bend in southern California (USA). The Mission Creek strand of the San Andreas Fault is a major geologic structure with ~90 km of strike-slip displacement but is currently mapped as inactive. Quaternary deposits record sediment dispersal across the fault from upland catchments and yield key markers of the fault’s displacement history. Our sediment provenance analysis from the Deformed Gravels of Whitewater and the Cabezon Fanglomerate provide detrital geochronologic and lithologic signatures of potential sources within the San Bernardino Mountains and Little San Bernardino Mountains. Statistical analysis shows that the Cabezon Fanglomerate is most compatible with the Mission Creek and Morongo Valley Canyon sources, rather than the Whitewater Canyon as previously suggested. We propose that displacement since deposition ~500–100 ka across the Mission Creek strand has separated these deposits from their original sources. These findings challenge the current paradigm that the Mission Creek strand is inactive and suggest that the fault continues to be a primary structure in accommodating deformation along the Pacific-North American plate boundary.

Grand Challenges (and Great Opportunities) in Sedimentology, Stratigraphy, and Diagenesis Research

NERC Yorkshire Integrated Catchment Solutions Programme [NE/P011160/1]; NERC National Capability project Climate Linked Atlantic Sector Science Programme (CLASS)