Alpine Rockwall Erosion Patterns Follow Elevation-Dependent Climate Trajectories

Mountainous topography reflects an interplay between tectonic uplift, crustal strength, and climate-conditioned erosion cycles. During glaciations, glacial erosion increases bedrock relief, whereas during interglacials relief is lowered by rockwall erosion. In the first landscape-scale, multi-process investigation of postglacial rockwall erosion patterns, we show that paraglacial, frost cracking and permafrost processes jointly drive rockwall erosion. Field observations and modelling experiments demonstrate that all three processes are strongly conditioned by elevation. Our findings provide a multi-process explanation for the increase of rockwall erosion rates with elevation across the European Alps. As alpine basins warm during deglaciation, changing intensities and elevation-dependent interactions between periglacial and paraglacial processes result in elevational shifts in rockwall erosion patterns. Future climate warming will shift the intensity and elevation distribution of these processes, resulting in overall lower erosion rates across the Alps, but with more intensified erosion at the highest topography most sensitive to climate change.

Orogen-scale transpression accounts for GPS velocities and kinematic partitioning in the Southern Andes

The Southern Andes are regarded as a typical subduction orogen formed by oblique plate convergence. However, there is considerable uncertainty as to how deformation is kinematically partitioned in the upper plate. Here we use analogue experiments conducted in the MultiBox (Multifunctional analogue Box) apparatus to investigate dextral transpression in the Southern Andes between 34 °S and 42 °S. We find that transpression in our models is caused mainly by two prominent fault sets; transpression zone-parallel dextral oblique-slip thrust faults and sinistral oblique-slip reverse faults. The latter of these sets may be equivalent to northwest-striking faults which were believed to be pre-Andean in origin. We also model variable crustal strength in our experiments and find that stronger crust north of 37 °S and weaker crust to the south best reproduces the observed GPS velocity field. We propose that transpression in the Southern Andes is accommodated by distributed deformation rather than localized displacements on few margin-parallel faults.

The influence of laterally varying crustal strength on rift physiography – Combining 3D numerical models and geological observations

<p>Continental rifts form across a mosaic of crustal units, each unit displaying properties that reflect their own unique tectonic evolution and lithology. The physiography of rift systems is largely reflective of this underlying crustal substrate, which may change over short distances along-strike of the rift. Pervasive, well-developed structural heterogeneities represent sites where strain may localise and may thus weaken a crustal volume. In contrast, relatively pristine areas of crust, such as igneous batholiths, contain few heterogeneities and may be considered relatively strong. Characteristic rift physiographies associated with these ‘strong’ and ‘weak’ crustal units, and how rift physiography changes across areas where these units are juxtaposed remain elusive.</p><p>In this study we use the 3D thermo-mechanical numerical code ASPECT to investigate how areas of differing upper crustal strength influence rift physiography. We extend a 500x500x100km volume, within which we define four 125km wide upper crustal domains of either ‘strong’, ‘normal’ or ‘weak’ crust. Crustal strength is determined by varying the initial plastic strain in the model across 5km blocks, producing a static-like pattern. Weak domains contain weakened blocks with large initial plastic strain values, creating large contrasts with adjacent blocks. In contrast, 5 km blocks within the strong domain have relatively low values of initial plastic strain, producing little variation between adjacent blocks.</p><p>Our modelling simulations reveal that strain rapidly localises onto high-displacement structures (equivalent to faults) in the weak domain. Fault spacing and the strain accommodated by each fault decreases in the normal domain, with the strong domain characterised by closely-spaced, low displacement faults approximating uniform strain. When heterogeneities are incorporated into the strong domain, we find that these rapidly localise strain, effectively partitioning the domain into a series of smaller, strong areas separated by faults. Faults are initially inhibited at the boundaries with adjacent stronger domains; as extension progresses, these faults break through the barrier and propagate into the stronger domains.</p><p>Our observations have important implications for rift system development, particularly in areas of highly heterogeneous basement. Studies have shown that the Tanganyika rift developed at high angles to cratonic and mobile belt basement terranes, with localisation inhibted in the stronger cratonic areas. Similarly, extension in the Great South Basin (GSB), New Zealand, initially localised in weak, dominantly sedimentary, terranes, compared to stronger, more homogenous granitic areas. Terrane boundaries in the GSB also inhibit the lateral propagation of faults. Comparing our model results with observations from these and other systems globally, we determine characteristic structural styles and examine how rift physiography varies across ‘strong’ and ‘weak’ crustal volumes.</p>

How Alpine seismicity relates to lithospheric strength

Despite the amount of research focused on the Alpine orogen, different hypotheses still exist regarding varying seismicity distribution patterns throughout the region. Previous measurement-constrained regional 3D models of lithospheric density distribution and thermal field facilitate the generation of an observation-based rheological model of the region. Long term lithospheric strength was then calculated and compared to observed seismicity patterns, showing that the highest strengths within the crust (~ 1 GPa) and upper mantle (> 2 GPa), occur at temperatures characteristic for specific phase transitions (crust: 200–400 °C; mantle: ~ 600 °C) with almost all seismicity occurring in in these regions. Correlation in the northern and southern forelands between crustal and lithospheric strengths and seismicity show different patterns of event distribution, reflecting their different tectonic settings. Seismicity in the plate boundary setting of the southern foreland corresponds to the integrated lithospheric strength, occurring mainly in the weaker domains surrounding the strong Adriatic indenter. However, in the intraplate setting of the northern foreland seismicity instead corresponds to the crustal strength, mainly occurring in the weaker and warmer crust beneath the URG. Results generated in this study are available for open access use to further discussions on the region.

The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed in naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust.

The thermal field across the Alpine orogen and its forelands and the relation to seismicity

<p>The Alpine orogen and its forelands comprise a multitude of crustal blocks from different tectonic providences and different physical properties. This implies that the thermal configuration of the lithosphere would also be expected to vary significantly throughout the region. Temperature is a key controlling factor for rock strength via thermally activated creep and it exerts a first order influence on the depth of the brittle-ductile transition zone, the lower bound to the seismogenic zone and the spatial distribution of seismicity. Here we present new results from INTEGRATE, a project in the DFG priority program Mountain Building in 4 Dimensions, as part of the AlpArray initiative, which aims to gain a better understanding of the structure, temperature and rheology of the crust and the uppermost mantle beneath the Alps and their forelands using multiple 3D modelling techniques. The overall goal is to test different hypotheses on the configuration of the lithosphere and its relation to the distribution of deformation and related seismicity in the Alpine region. We build on previous work of a 3D density differentiated structural model of the region that is consistent with deep seismic data and gravity, to calculate the 3D conductive steady state thermal field of the Alps and their forelands. The model is unique in using different thermal parameters for different tectonic domains and is validated with a dataset of wellbore temperatures from across the region. Comparing recorded seismicity to the calculated thermal field we find a systematic clustering of the deep seismic activity that correlates with different isotherms within individual crustal blocks, reflecting the presence of different dominant lithologies. These inferred lithologies in conjunction with the calculated temperatures and the previous 3D density-structural model of the region, can be used to shed light on the lateral changes in crustal strength within the Alps and their forelands, helping to explain the observed patterns of deformation.  </p>