Dilatant normal faulting in jointed cohesive rocks: a physical model study

Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity are not well understood. We present an analogue modeling study using cohesive powder with pre-formed joint sets in the upper layer, varying the angle between joints and a rigid basement fault. We analyze interpreted map-view photographs at maximum displacement for damage zone width, number of connected joints, number of secondary fractures, degree of segmentation and area fraction of massively dilatant fractures. Particle imaging velocimetry provides insight into the deformation history of the experiments and illustrates the localization pattern of fault segments. Results show that with increasing angle between joint-set and basement-fault strike the number of secondary fractures and the number of connected joints increase, while the area fraction of massively dilatant fractures shows only a minor increase. Models without pre-existing joints show far lower area fractions of massively dilatant fractures while forming distinctly more secondary fractures

Dating the youngest deformation in the Alps with ESR thermochronometry

Low-temperature thermochronology is a useful tool to reconstruct tectonic deformation and landscape evolution within the first 2 km of the crust. It is a suitable tool to investigate deformation associated with cooling and exhumation of the lower crust in orogenic settings. Low temperature thermochronology is applied here to understand the Neogenic post-collisional extensional event that occurred in the Alps, because a gap in previous age dating exists between a thousand and a million years. Quartz is the most common mineral in the crust; occurring in magmatic as well as sedimentary and metamorphic rocks. The potential of quartz electron-spin resonance (ESR) as a radiation dosimeter has been well documented, and many studies applied the method to date sediments and heated rocks (e.g. tephra). In this study, we apply quartz ESR dating as an ultralow-temperature thermochronometer, characterized by a closure temperature of 30°-90°, and dating range of 103-107 years. We show the results of ESR thermochronometry on quartz applied to rocks from crustal-scale faults in the Central (Simplon Fault) and Eastern Alps (Brenner and Salzachtal Faults). Here, the lower crust has been tectonically exhumed, associated with exhumation of the Lepontine Dome and Tauern Window, respectively. Thermochronological data are available from this area, such as fission tracks or U-Th/He data on zircon and apatite. Results of the ESR measurements of 15 samples crossing the Brenner and Salzachtal faults (northern and western border of the Tauern Window) show that the ESR ages of quartz get younger (<1Ma) inside the western part of the Tauern Window, in accordance with fission track and (U-Th)/He ages. In general, younger ages (between 200 and 500 ka) are also obtain closer to the fault zone, localized near (e.g. Simplon Fault) or at the bottom of the valley (e.g. Brenner Fault), compared with the protolithic rocks (600-900 ka). We interpret the trend of the ESR ages as an exhumation of the isotherms due to both recent uplift of the footwall of the fault and for erosion of the valley, where the later overprints the former. These results promise to establish ESR as an ultra-low thermochronometer using quartz for the Quaternary landscape reconstruction of the Alpine chain

Critical taper analysis reveals lithological control of variations in detachment strength: An analysis of the Alpine basal detachment (Swiss Alps)

Although evidence for weak detachments underlying foreland thrust belts exists, very little is known about the lateral variations in effective strength, as well as the geological nature of such variations. Using critical taper analysis, we show that a detailed and systematic measurement of surface slope of the Central European Alps reveals variations in strength parameter F along the detachment, based on the argument that the Alps are close to the critical state. We show that the basal detachment is very weak near the deformation front but strengthens toward the hinterland. Very low F (effective coefficient of friction plus normalized cohesion) values of  0.54) but may also require additional mechanisms of dynamic weakening

Time scale bias in erosion rates of glaciated landscapes

Deciphering erosion rates over geologic time is fundamental for understanding the interplay between climate, tectonic, and erosional processes. Existing techniques integrate erosion over different time scales, and direct comparison of such rates is routinely done in earth science. On the basis of a global compilation, we show that erosion rate estimates in glaciated landscapes may be affected by a systematic averaging bias that produces higher estimated erosion rates toward the present, which do not reflect straightforward changes in erosion rates through time. This trend can result from a heavy-tailed distribution of erosional hiatuses (that is, time periods where no or relatively slow erosion occurs). We argue that such a distribution can result from the intermittency of erosional processes in glaciated landscapes that are tightly coupled to climate variability from decadal to millennial time scales. In contrast, we find no evidence for a time scale bias in spatially averaged erosion rates of landscapes dominated by river incision. We discuss the implications of our findings in the context of the proposed coupling between climate and tectonics, and interpreting erosion rate estimates with different averaging time scales through geologic time

Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

We present a study of pressure and temperature evolution in the passive continental margin under the Oman Ophiolite using numerical basin models calibrated with thermal maturity data, fluid-inclusion thermometry, and low-temperature thermochronometry and building on the results of recent work on the tectonic evolution. Because the Oman mountains experienced only weak post-obduction overprint, they offer a unique natural laboratory for this study. Thermal maturity data from the Adam Foothills constrain burial in the basin in front of the advancing nappes to at least 4&thinsp;km. Peak temperature evolution in the carbonate platform under the ophiolite depends on the burial depth and only weakly on the temperature of the overriding nappes, which have cooled during transport from the oceanic subduction zone to emplacement. Fluid-inclusion thermometry yields pressure-corrected homogenization temperatures of 225 to 266&thinsp;∘C for veins formed during progressive burial, 296–364&thinsp;∘C for veins related to peak burial, and 184 to 213&thinsp;∘C for veins associated with late-stage strike-slip faulting. In contrast, the overlying Hawasina nappes have not been heated above 130–170&thinsp;∘C, as witnessed by only partial resetting of the zircon (U-Th)/He thermochronometer. In combination with independently determined temperatures from solid bitumen reflectance, we infer that the fluid inclusions of peak-burial-related veins formed at minimum pressures of 225–285&thinsp;MPa. This implies that the rocks of the future Jebel Akhdar Dome were buried under 8–10&thinsp;km of ophiolite on top of 2&thinsp;km of sedimentary nappes, in agreement with thermal maturity data from solid bitumen reflectance and Raman spectroscopy. Rapid burial of the passive margin under the ophiolite results in sub-lithostatic pore pressures, as indicated by veins formed in dilatant fractures in the carbonates. We infer that overpressure is induced by rapid burial under the ophiolite. Tilting of the carbonate platform in combination with overpressure in the passive margin caused fluid migration towards the south in front of the advancing nappes. Exhumation of the Jebel Akhdar, as indicated by our zircon (U-Th)/He data and in agreement with existing work on the tectonic evolution, started as early as the Late Cretaceous to early Cenozoic, linked with extension above a major listric shear zone with top-to-NNE shear sense. In a second exhumation phase the carbonate platform and obducted nappes of the Jebel Akhdar Dome cooled together below ca. 170&thinsp;∘C between 50 and 40&thinsp;Ma before the final stage of anticline formation.</p

Linking the northern Alps with their foreland: The latest exhumation history resolved by low-temperature thermochronology

The evolution of the Central Alpine deformation front (Subalpine Molasse) and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the Alps. Its latest exhumation history is poorly understood due to the lack of late Miocene to Pliocene sediments. We constrain the late Miocene to Pliocene history of this transitional zone with apatite fission track and (U-Th)/He data. We used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data. Two investigated sections across tectonic slices show that the Subalpine Molasse was tectonically active after the onset of folding of the Jura Mountains. This is much younger than hitherto assumed. Thrusting occurred at 10, 8, 6–5 Ma and potentially thereafter. This is contemporaneous with reported exhumation of the External Crystalline Massifs in the central Alps. The Jura Mountains and the Subalpine Molasse used the same detachments as the External Crystalline Massifs and are therefore kinematically coupled. Estimates on the amount of shortening and thrust displacement corroborate this idea. We argue that the tectonic signal is related to active shortening during the late stage of orogenesis