Analog Models of Fold-and-Thrust Wedges in Progressive Arcs: A Comparison With the Gibraltar Arc External Wedge

The timing and kinematics of the different types of structures and the associated vertical-axis rotations that permit an arcuate external wedge to acquire progressively its curved shape throughout its deformation history—known as progressive arcs—are key questions in natural cases of arcuate fold-and-thrust belts that we want to address through analog modeling. We present laboratory models of fold-and-thrust belts formed with a backstop that deforms in map view to simulate progressive arcs in a thin-skinned tectonic regime. Our setup makes use of a deformable backstop rigid enough to push from behind the initial parallelepiped but deformable in map view. This innovative design permits us to increase the amplitude of the arc indenting in the model as its radius of curvature decreases, that is, it simulates a progressive arc. Taking the Gibraltar Arc external wedge situated in the western Mediterranean to scale our models in terms of rheology, velocities, and sizes, four types of experiments were made. We varied the type of substratum (sand or silicone), the silicone thickness, and the width and length of the initial analog pack in order to test the influence of each of these parameters on the resulting fold-and-thrust belts. All experiments led to the formation of arcuate wedges where strain was partitioned into: (a) arc-perpendicular shortening, accommodated by thrusts which main structural trend is broadly subparallel to the indenter shape and with divergent transport directions, and (b) arc-parallel stretching, accommodated by normal and conjugate strike-slip faults. The normal and strike-slip faults contributed to the fold-and-thrust belt segmentation and the formation of independent blocks that rotated clockwise and counterclockwise depending on their position within the progressive arc. Our experiments allow to simulate and understand the finite deformation mode of the external wedge of the Gibraltar Arc. Accordingly, they shed light on how an arcuate fold-and-thrust belt can develop progressively in terms of structural trend and transport directions, types and distribution of the structures accommodating strain partition, and timing of vertical-axis rotations.This study was supported by projects RNM-0451, EST1/00231, CGL2017-89051-P, PGC2018-100914-B-I00, and UPO 1259543

Geomorphic signature of segmented relief rejuvenation in the Sierra Morena, Betic forebulge, Spain

The foreland relief of alpine orogenic belts is often rejuvenated due to the intraplate propagation of orogenic deformation. Thus, in these long-lived areas, the localisation of relief rejuvenation may be largely controlled by the reactivation of previous mechanical discontinuities. In this regard, we explored the relationship between the relief rejuvenation pattern and the distribution, geometry, and kinematics of faults in a wide segment of the Betic foreland (Sierra Morena, southern Spain). Specifically, we focused on the forebulge, a WSW–ENE flexural relief that formed, paired to the Betic foreland basin, in response to orogenic load. For this purpose, we applied both qualitative and quantitative geomorphological tools, including geomorphic indices and knickpoint pattern modelling in χ space. We found that the pattern of relief rejuvenation responds to large-scale flexural uplift coupled with the tectonic activity of two groups of faults that often show evidence of reactivation, namely overall WSW–ENE faults contributing to both regional NNW–SSE relief segmentation and vertical extrusion of the forebulge, and NW–SE reverse faults associated with an outstanding WSW–ENE topographic segmentation in the west of the study area. In addition, our knickpoint modelling suggests that the faults related to the southernmost Sierra Morena mountain front have been particularly active in recent times, although their activity span and the relative uplift that they accommodate differ along the Sierra Morena/foreland basin limit. The knickpoint pattern also suggests a significant reorganisation of the analysed drainage basins. The strain partitioning accommodated by the structures involved in relief rejuvenation suggests the intraplate propagation of the transpressional deformation reported from the Betic external fold and thrust belt.</p

Tectónica y paleoceanografá en el sector norte de la península Antárctica: Resultados preliminares de la campaña HESANT 1992/93 a bordo del B/O HESPÉRIDES

11 páginas, 7 figuras.[EN] The boundaries between the plates located in the northern sector of the Antarctic Peninsula were investigated during the B10 HESPERIDES HESANT 92/93 cruise using multichannel seismic profiles, magnetometry, and multibeam echo sounding. The investigation focused on the analysis of the tectonic relationships between the main lithospheric elements of the area and the paleoceanographic evolution of the continental margins and basins during the Upper Cenozoic. The triple junction defined by the southern end of the Shackleton Fracture Zone, the South Shetland Trench and the South Scotia Ridge shows a compressive structure where tectonic blocks and deep rooted diapirs develop. The boundary between the Scotia and Antarctic plates is characterized by a succession of structural highs, which represent continental crustal blocks, and intervening transtensive basins bounded by strike-slip faults. The Powell Basin and the continental margins of the Antarctic Peninsula in the Weddell Sea show several evolutionary styles which may represent an asymmetric opening of the basin and the relative importance of extensional and strikeslip faults in its development.[ES] Durante la campaña HESANT 92/03 con el B/O Hespérides, se han investigado con perfiles de sísmica multicanal, magnetometría y sondas multihaz, las relaciones geodinámicas en el límite de las placas situadas en la parte septentrional de la Península Antártica, así como la evolución paleoceanográfica de los márgenes continentales y cuencas de este sector durante el Cenozóico superior. La unión triple definida por la terminación meridional de la Zona de Fractura Shackleton, la fosa de las Shetland del Sur y la dorsal meridional del Mar de Scotia muestra una estructura compresiva, con el desarrollo de bloques y diapiros profundamente enraizados en la corteza. El límite entre las placas de Scotia y Antártica está ocupado por una alternancia de altos estructurales, correspondientes a fragmentos de corteza continental, y cuencas transtensivas delimitadas por fallas de salto en dirección. La Cuenca Powell y los márgenes continentales de la Península Antártica en el Mar de Weddell presentan varios estilos evolutivos que ponen de relieve una apertura disimétrica y la importancia relativa de las fallas distensivas y de salto en dirección en su desarrollo.This research was conducted within the framework of the 1992/1993 Spanish Antarctic Program, with the endorsement of the B/O HESPERIDES Comisión de Gestión. The project was founded by an Acción Especial de la Comisión Interministerial de Ciencia y Tecnología (CICYT).Peer reviewe

Active landscapes of Iberia

The recent geodynamic evolution of Iberia is recorded in its topography. Geomorphic markers and their dating; morphometric indices estimated through cutting-edge DEM analysis techniques; and the link of all this data with results of geophysical studies allow discussing why Iberia displays the highest average elevation in Europe and shows a particular topography with such diversity of landscapes. For example, the region of the Iberian (or Hesperian) Massif, the western sector of Iberia, shows an anomalous average elevation without a satisfactory explanation. On the other hand, different explanations about the recent evolution of the Alpine mountain ranges of the eastern sector of Iberia have come to light after macroscale landscape analyses. This is strengthening the debate on the driving force behind the actual topography of the Pyrenees, Cantabrian Mountains, Iberian Chain and Betics