Rise and Fall of an Egyptian Oasis: Artesian Flow,Irrigation Soils,and Historical Agricultural Development in El‐Deir,Kharga Depression,Western Desert of Egypt

The present study examines the geoarchaeological history of an oasis in Kharga Depression in central Egypt. El‐Deir is renowned for its Ptolemaic temple and Roman fortress on the road from former Hibis (Kharga) to the Nile Valley. During the survey, spring mounds and irrigation soils belonging to an ancient agricultural zone were discovered, and further documented by ceramics found on the site. Our methodology combines the geomorphological interpretation of landforms (especially yardangs) with ceramics and ¹⁴C‐dated charcoal to distinguish and date former agricultural areas in El‐Deir. The results show that the oasis experienced several phases of soil accretion and destruction through time. Playa sediments were deposited in the humid early Holocene and severely eroded by deflation before the onset of irrigated agriculture between Pharaonic and Persian times. Very fast vertical soil accretion occurred in the Ptolemaic period, but irrigation soils were later destroyed during the Roman period by a combination of wind deflation and flash floods (second to fourth century A.D.), suggesting a period of climate instability. The case of El‐Deir invites reevaluation of constructive agencies for the development of irrigated land and destructive agencies as limiting factors for the sustainability of agricultural practices in late antiquity.     

The North Cycladic Detachment System

Low-angle normal faults accommodate a large part of continental post-orogenic extension. Besides the intrinsic rheological characteristics of the continental crust that may lead to the formation of shallow-dipping shear zones at the brittle–ductile transition, the role of pre-existing low-angle structures such as large thrusts has been proposed by several authors. We explore this question with the example of the North Cycladic Detachment System (NCDS) that is composed of a series of distinct detachments cropping out on the islands of Andros, Tinos and Mykonos, separating the Cycladic Blueschists in the footwall from the Upper Cycladic Nappe in the hanging wall. We show that these extensional structures are part of a single large-scale structure (more than 200 km along strike) that reactivates the Vardar suture zone. It extends eastward on Ikaria and westward offshore Evia and Thessalia where it probably connects to recent shallow-dipping normal faults evidenced on published seismic reflection profiles. The NCDS started its activity in the Oligocene concommitantly with the Aegean extension, and was still active in the Late Miocene. It has exhumed a series of metamorphic domes from southern Evia to Mykonos below low-angle detachment systems, made of low-angle normal faults and low-angle ductile shear zones. The ductile shear zones and the faults were created with a low dip and they kept the same attitude throughout their exhumation. We identify three main detachments that are part of a continuum of extension on the NCDS : Tinos detachment, Livada detachment and Mykonos detachment. A fourth detachment (Vari detachment) is the reactivation of an Eocene exhumation-related structure. Deformation in the footwall is characterized by intense stretching and flattening. Using the spatial evolution observed along strike from Andros to Mykonos we construct a history of formation of the NCDS starting with the reactivation of former thrusts leading to the exhumation of high-temperature metamorphic domes. The Aegean example shows that reactivation of earlier shallow-dipping discontinuities can play a fundamental role in continental post-orogenic extension.     

On the derivation of the Buckley—Leverett model from the two fluid Navier—Stokes equations in a thin domain

We consider a two-phase incompressible immiscible flow, with neglected surface tension, in a thin domain. Using the method of asymptotic expansions with respect to the thickness of the domain, effective 1D equations are derived. Supposing that the fluids are strictly separated at t=0, we show that the derived equations equal the Buckley–Leverett equation for the saturation, in conjunction with the Reynolds law for each of the flowing phases. The fractional flow curve and corresponding relative permeabilities are determined explicitly and their heuristically observed properties verified.     

Constraints on the late Quaternary glaciations in Tibet from cosmogenic exposure ages of moraine surfaces

This contribution provides new constraints on the timing of Tibetan glacial recessions recorded by the abandonment of moraines. We present cosmogenic radionuclide ¹⁰Be inventories at 17 sites in southern and western Tibet (32 crests, 249 samples) and infer the range of permissible emplacement ages based on these analyses. Individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for ¹⁰Be abundance. We consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages: 1) Erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age; and 2) Erosion progressively exposes large boulders with little prior exposure, and so the oldest sample age records the minimum moraine emplacement age. We found that depending on the scenario chosen, the moraine emplacement age can vary by > 50% for ～100 ka-old samples. We consider two scaling models for estimating the production rates of ¹⁰Be in Tibet, which has an important, although lesser, effect on inferred moraine ages. While the data presented herein effectively increase the database of sample exposure ages from Tibet by ～20%, we find that uncertainties related to the interpretation of the ¹⁰Be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies. A reanalysis of published data from moraines throughout Tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the Tibetan moraine age database. Thus, when viewed in a similar way with other samples collected from this area, uncertainties within moraine exposure ages obscure attribution of Tibetan glacial advances to temperature minima or precipitation maxima. Our work suggests that more reliable chronologies of Tibetan glaciations will come from improvements in production rate models for this portion of the world, as well as a better understanding of the processes that form and modify these geomorphic surfaces.     

Experimental and discrete element modeling studies of the trapdoor problem: influence of the macro-mechanical frictional parameters

Granular soils have the inherent ability to develop load transfers in their mass. Mechanisms of load transfers are used as a basic principle of many civil and geotechnical engineering applications. However, their complexity makes it difficult to formulate relevant design methods for such works. The trapdoor problem is one of the ways to reproduce load transfers by the arching effect in a granular layer in non-complex conditions. In addition, many analytical solutions for the prediction of load transfer mechanisms are based on the trapdoor problem. However, some of the parameters required are still being widely discussed, in particular the ratio of horizontal stress to vertical stress. For this paper, an experimental device for trapdoor tests in plane strain conditions was created and several geomaterials were tested. Three phases in the response of the materials were consistently observed. Each of these phases corresponded to a specific displacement of the trapdoor. A first phase of high load transfer was observed followed by a transition phase which was followed by a critical phase for which the load transfer amplitude increased and stabilized. Analytical solutions and experimental values of load transfers were compared. Considerable differences between the stress ratio needed to fit the experimental data and the stress ratio proposed in the analytical models were noted. Based on the conclusions of the experimental study, the discrete element method was used to model the same trapdoor problem. A wide range of granular materials was modeled and tested in the trapdoor problem. The three phases in the response of the layer were also observed in the numerical modeling. In addition, it was shown that the shear strength of the material is the key parameter of load transfers: peak shear resistance for the small displacements of the trapdoor and critical shear strength for the larger displacements. A micro-mechanical analysis showed that the effective stress ratio in the sheared zone does not vary as much with shear strength. Stress ratios here were again greater than those proposed in the analytical solutions. Nevertheless, the relevance of the solution of Terzaghi was confirmed as soon as the stress ratio was correctly chosen.     

The prehistoric Mt Wilberg rock avalanche, Westland, New Zealand

The Mt Wilberg rock avalanche in Westland, New Zealand occurred before 1300 AD and may have occurred as a consequence of an Alpine fault earthquake in ca. 1220 AD or earlier. Its ∼40 × 10⁶ m³ deposit may have briefly obstructed the Wanganui River, but only about 25% of its surface morphology still survives, on terraces isolated from river erosion. The landslide appears to have moved initially as a block, in a direction controlled by a strong rock mass at the base of the source area, before disintegrating and spreading across terraces, fans, and floodplains. Rock avalanche deposits in Westland have relatively short expected lifetimes in the rugged terrain and high rainfall of the area; hence, the hazard from such events is under-represented by their current remnants.     

Impact of range‐parallel sediment transport on 2D thermo‐mechanical models of mountain belts: Application to the Kyrgyz Tien Shan

Evolving mountain belts dynamics is very sensitive to surface processes. The surface processes affect tectonics by enhancing crust exhumation and thermal weakening, and depositing soft yet cold sediments in surrounding basins. While 2D plane strain models approximate cylindrical tectonic structures well, simple 1D mass transfer cannot capture erosion–sedimentation complexity. The Eastern Kyrgyz Tien Shan, where structures, basins and exhumation rates are well constrained, is used here to illustrate this issue. Thermo‐mechanical models demonstrate that 1D transport cannot adjust both basin geometry and Apatite Fission Track exhumation ages. When out‐of‐plane sediment transfer is considered, the amount of evacuated sediment delays or accelerates the formation of new faults, affecting the relative timing of exhumation. For our case study, lateral drainage must evacuate 80% of the sediments to match the geological constraints, which is consistent with other source to sink analyses. This indicates that lateral drainage should not be neglected in regional 2D models.