Extension and early orogenic inversion along the basal detachment of a hyper-extended rifted margin: an example from the Central Pyrenees (France)

The North Pyrenean Zone (NPZ) inverts remnants of an Aptian-Cenomanian rifting during which subcontinental mantle was exhumed. These remnants contain a syn-rift HT-LP metamorphic domain, the Internal Metamorphic Zone (IMZ). New field and RSCM data and structural cross-sections constrain the structural and metamorphic relationships between the IMZ and the underlying low-grade NPZ. The IMZ is a tectonic nappe that overthrusts the European margin along the 3M Fault. Along this contact, the Tuc de Haurades peridotite is surrounded by tectonic breccia composed of ductilely deformed carbonate and sparse lherzolite clasts that passes upward into foliated marbles. Marbles contain top-to-south ductile shear, recording ongoing extensional deformation that marks the onset of HT metamorphism. During Early Cretaceous rifting, European Mesozoic sedimentary cover metamorphosed and its base brecciated as it slid basinward on Triassic salt onto exhumed mantle. As the exhumed mantle domain closed during early convergence, the detached metamorphosed cover was transported northward and thrust into the distal European margin, sampling lherzolite tectonic lenses. This triggered the first tectonic loading on the European plate. This study highlights the role of the IMZ in the early Pyrenean orogenic phase and gives new insights on the E-W diversity of structural setting of the NPZ peridotites.Table with RSCM temperatures and original and high quality photographs of the samples are available on the GSL Figshare portal https://doi.org/10.6084/m9.figshare.c.5539260.

Investigating the Response of Tectonic Loading in the Himalayas on the Peripheral Forebulge through Drainage Network Analysis

<p>Drainage divide migration is a conspicuous natural process through which a landscape evolves. In response to a forced climatic and tectonic disturbance, susceptible river networks transfer the transient signals to the entire river basin, which results in an incision or aggradation. The Himalayan orogeny and subduction of the Indian plate have resulted in an upward flexure in the Indian lithosphere known as a peripheral forebulge. A forebulge can flexurally uplift and migrate following the variation in tectonic load. The emergence of the central Indian plateau is a consequence of the upwarping of the Indian lithosphere (Bilham et al. 2003).  In this work, we are trying to assess the drainage network dynamics between the Narmada and Ganga river systems, which drain the uplifted central Indian plateau. We have calculated the Chi(χ) metrics, steepness index (Ksn), knickpoints for the channels in the study area. We have generated Topographic swath profiles to analyze the topographic variations on the plateau. It has been observed from the results that the rivers in the study area lack dynamic equilibrium, and river capturing is an evident response to the perturbations. Our analysis shows that the Narmada River tributaries are gaining drainage area and aggressing Northwards by capturing adjacent Ganga river tributaries. The field observations show a variation in the surface slope and presence of knickpoints (waterfalls) along the "aggressor" drainages. We propose a model to show a correlation between the tectonic loading of Himalayas, movement of forebulge, and its feedback to the river systems present on the forebulge.</p>

Interplay of near surface rift evolution and deep-seated lower crustal flow: New findings from fully quantified crustal-scale analogue models

<p>Here we present new results and findings from an analogue modelling series using an extension gradient to simulate continental rifting in rotational settings. We study the effect of a pressure-gradient driven, rift-axis parallel lower crustal flow on rift propagation. The dynamically scaled two-layer models represent a brittle upper and a ductile lower crust. To simulate different crustal set-ups, we use variable ductile/brittle ratios R<sub>DB</sub>, where increasing values indicate a hotter crust with the brittle-ductile transition at relatively shallower depth. An additional package of sand on one part of the model simulates tectonic loading to provoke a pressure-gradient driven lower crustal flow.</p><p>Several factors play a role in dynamic rift propagation such as extension rates, fault evolution and the interplay of vertical motions at the surface as well as model-internal rift-axis parallel horizontal flow. We combine surface and internal deformation analysis using stereoscopic Digital Image Correlation and Digital Volume Correlation applied on surface stereo images and XRCT images, respectively to obtain the fully quantified model deformation.</p><p>Our results show that rift propagation occurs in two consecutive stages: (i) bidirectional step-wise growth in fault length by linkage and (ii) unidirectional linear fault growth. Strain partitioning of bulk extension causes episodic alternative fault growth on conjugate rift margin faults. Over time, fault activity abandons rift boundary faults and migrates inward creating intra-rift faults. This process occurs segment-wise along the rift axis, where different fault generations are simultaneously active. We quantify increasing lower crustal flow parallel to the rift axis with increasing R<sub>DB</sub> as the result of tectonic loading. In return, such lower crustal flow causes vertical and horizontal motions at the surface expressed by dynamic topography and deformation features.</p><p>These results give insights into deformation processes of rifting and highlight the important role of extension gradients on fault growth and strain partitioning in segmented rotational rift systems. Rift-axis parallel lower crustal flow in rotational rift settings may be of relevance when dealing with restorations of 2D crustal seismic sections across rifts.</p>

Effect of tectonic loading on continental rifting: Imaging, quantification and linkage of deep-seated flow and surface deformation

<p>Here we use dynamically scaled analogue experiments to investigate the influence of tectonic loading on continental rifting. Analogue models consist of a two-layer brittle-viscous set up overlying a foam base, which expands homogeneously when extension is being applied perpendicular to the rift axis trend. A layer of quartz sand on top of a viscous silicone/corundum sand mixture layer is used as an analogue for an upper brittle crust and a ductile lower part of the crust, respectively. An additional package of sand on one part of the model simulates tectonic loading.</p><p>The aim of this work is to investigate in detail dynamic rift propagation in such a setting by means of a fully quantitative monitoring of surface and internal deformation, focusing on rift propagation velocity, growth rate and orientation. The evolution of the surface topography (DEM) and deformation (3D displacement field) is monitored and quantified using 3D Digital Image Correlation (3D stereo DIC). Furthermore, we apply an automated fault segment tracer on the surface deformation data to characterize rift evolution. Model internal deformation is investigated by digital volume correlation (DVC) techniques applied on X-ray computed tomography data of the time-series experiment volumes. With the use of such techniques we are able to visualize, quantify and link deep-seated internal flow and surface deformation over time.</p><p>Preliminary results from these experiments suggest that rift propagation in our analogue models is directly influenced by load-induced deep-seated deformation resulting in a horizontal lower-crustal flow opposing rift propagation.</p>

Reconstructing the Early Eocene Sediment Routing System of the south-eastern Pyrenees

<p>The Early Eocene was the period of most intense plate collision during the building of the Pyrenean orogen. Tectonic loading of the overriding European plate caused flexure of the subducting Iberian plate and formation of an elongated foredeep connected westward with the Atlantic Ocean. The uneven distribution of the Triassic evaporites caused the formation of a thrust salient in the central Pyrenees related to tectonic inversion of the pre-existing Mesozoic rift basins. This process ultimately resulted in the partitioning of the foreland basin and the isolation of the Ripoll Basin in the East from the Tremp-Graus and Ainsa-Jaca basins in central and western south-Pyrenees. The precise timing and the surface processes related to this reorganization of the sediment routing system remains not fully understood. Early tectono-stratigraphic reconstructions envisaged a scenario of isolation of the eastern Pyrenean Foreland basin in the early Eocene, while other recent studies on detrital zircon geochronometry suggest that the sedimentary transfer system in the Tremp-Graus basin connected upstream to the Ripoll basin until middle Lutetian times. In this contribution we discuss constraints on the early Eocene paleogeography of the south-eastern Pyrenees in the light of a revised chronostratigraphic scheme. We put forward a scenario that tries reconciling all available structural, stratigraphic, petrologic, geochronologic, and sedimentologic datasets.</p>

Complex spatiotemporal patterns of intracontinental earthquakes: a different game

<p>Intracontinental earthquakes show complex spatiotemporal patterns. In North China, no large (M>7) earthquakes ruptured the same fault segments in the past 2000 years; instead they roamed among widespread fault systems. In Australia, morphogenic evidence indicates clusters of earthquakes separated by tens of thousands of years of dormancy. In central and eastern United States, paleoseismic studies suggest that large Holocene earthquakes occurred in places that are seismically inactive today. Such seismicity does not fit existing earthquake models that assume steady tectonic loading and cyclic stress release on fault planes. Intracontinental fault systems are widespread and collectively accommodate slow tectonic loading. A major fault rupture both transfers stress to the neighboring faults and perturbs loading conditions on distant faults. Thus, the loading rate on each individual fault can be variable. Slow tectonic loading means that local stress variations from fault interaction or nontectonic processes, or changes of fault strength, could trigger an earthquake. Furthermore, large intracontinental earthquakes usually rupture multiple fault segments or faults, which vary for each event. For these earthquakes, commonly used concepts such as recurrence intervals and characteristic earthquakes, all based on earthquake models assuming cyclic elastic rebound, are inadequate or inapplicable. On the other hand, the general patterns of intracontinental earthquakes can be described by the theory of complex dynamic systems, in which all faults interact with each other. The rupture of individual fault or fault segment cannot be predetermined, but the system behavior can be studied based on the records of previous events. We found that large intracontinental earthquakes, either on a fault system or in a region, are usually clustered and separated by long but variable periods of quiescence. The lengths of the quiescence periods inversely correlate with tectonic loading rates, and the characteristics of earthquake clusters depend on fault geometry and crustal rheology, through fault interaction and viscoelastic relaxation. Spatially, large intracontinental earthquakes are not limited to faults that are active recently, although weak regions tend to have more earthquakes. Intracontinental earthquakes require a different approach, one that focuses on stress interactions between faults in a complex dynamic system rather than stress accumulation and release on individual faults.</p>

Study on Coulomb Stress Evolution Along Altyn Fault

Taking the altyn fault zone as the study area, the maxwell layered viscoelastic medium model and the PSGRN / PSCMP program were used to study the evolution of the cumulative coulomb stress over the altyn fault from 1900 to 2020, analyze the future seismic hazards of faults in the altyn fault zone. The results show that: the minfeng earthquake and changma earthquake were mainly caused by the long-term tectonic loading of the altyn fault. The wuzunxiaoer S8 section and the shulehe 4 S18 south section were affected by the combined effects of the changma earthquake and long-term earthquake loading. In particular, the maximum cumulative coulomb stress of shulehe 4 S18 south section is 2.58Mpa,which is a great danger to strong earthquakes.

The Locking Depth of the Cholame Section of the San Andreas Fault from ERS2-Envisat InSAR

The Cholame section of the San Andreas Fault (SAF), which has been considered locked since 1857, has been little studied using geodetic methods. In this study, we propose to use Interferometric Synthetic Aperture Radar (InSAR) to contribute to the improvement of the knowledge of this section of the SAF. In particular, the objective of this work is to provide a description of the transition between the Parkfield and Cholame-Carrizo segments further southeast by producing an estimate of the locking depth of the Cholame segment by combining ERS2 (European Remote Sensing) and Envisat Advanced SAR (ASAR) satellites data. Our results indicate that the locking depth between the Parkfield and the Cholame-Carrizo segments deepens to the southeast. We then use these results as a hint to refine the tectonic loading on this section of the SAF.