An analytical solution for the exhumation of an orogenic wedge and a comparison with thermochronology data

Thermochronology data is key for quantifying the exhumation history and dynamics of mountain belts. Here we present a new analytical solution for the steady-state exhumation of an orogenic wedge that undergoes transport along a basal detachment, uniform internal deformation, basal and frontal accretion. The solution predicts an increase in exhumation towards the interior of the wedge, with the rate of increase dependent on the degree of internal deformation. Application of the solution to a cross section in the Himalayas shows that in spite of its simplicity the solution provides a good fit to thermochronology data, with a coefficient of determination (R2) of 0.75. This implies that, although the solution does not capture the effects of individual faults and folds, at a large scale deformation can be described by uniform compression and transport. The results also imply that this part of the Himalayas may be in steady-state. The equations presented here can be used to quantify exhumation, deformation and shortening rates in mature orogens that are in steady-state.

Transect across the External Pamir thrust belt and Main Pamir Thrust along the Altyn Darya valley, Kyrgyzstan

<p>The ca. 35 km long, N-S-trending Altyn Darya valley in Kyrgyzstan exposes a nearly complete cross-section of the External Pamir thrust belt (EP), extending from the active Pamir Frontal Thrust in the north to the Main Pamir Thrust (MPT) and some distance into its hanging-wall. The EP comprises a northward imbricated stack of Carboniferous to Late Neogene rocks. From north to south, young clastics of the Alai Valley foreland basin are overthrust by an intensely folded and thrust-repeated frontal stack of Upper Cretaceous to Paleogene limestone, shale and evaporite. Lower Cretaceous red sandstones first emerge above north- and south-verging thrusts forming a triangle zone whose core comprises spectacular isoclinal folds in Upper Cretaceous strata. Towards the south, another thrust imbricate of Lower Cretaceous is overthrust by Late Triassic-Jurassic sandstones and mafic volcanics which are themselves overthrust by an internally deformed, Carboniferous to Triassic succession of, from bottom to top, greywacke and shale, limestone, volcanoclastic conglomerates, variegated sandstone-shale and pink conglomerates. The Carboniferous units in the south are truncated by the MPT which emplaces a succession of greenschist, marble and chert overlain by a km-thick sequence of metamorphosed and deformed, pillow-bearing lavas of Carboniferous age. Structural geometries and fault preference indicate that the basal detachment of the EP deepens southward very gently, stepping down from a detachment in Upper Cretaceous shale to another one near the base of the Lower Cretaceous and eventually a third one in Triassic shale. Cross-section balancing suggests minimum shortening of 75 km for units in the MPT´s footwall. The displacement on the MPT is poorly constrained due to eroded hanging-wall cutoffs, but must exceed 15 km. The basal detachment cuts into basement no earlier than 100 km from the present thrust front, too far south to link up with the top of the Pamir slab.</p><p>The stratigraphic succession exposed in Altyn Darya can be readily correlated with less deformed and less metamorphosed transects in westernmost China (Qimgan and Kawuke), some 250 km to the east. A marble-greenschist sequence similar to that carried on the MPT in Altyn Darya has been identified there as a tectonic nappe of the Karakul-Mazar unit, emplaced from the south already in an Upper Triassic to Lower Jurassic (Late Cimmerian) event. If the correlation is correct, then the MPT had a Mesozoic precursor structure extending over much of the E-W striking segment of the Northern Pamir.</p>

Lithological and erosional controls on orogen width in the Bolivian Andes

<p>Orogen fold-and-thrust belts (FTBs) often have a tapering wedge geometry in cross section, which develops as a result of the balance between stresses acting along the detachment fault beneath the wedge, its internal strength, and the average slope of the surface topography from the back of the wedge to its toe. The geometry of these critical wedges is thus sensitive to changes in factors that influence stress along the wedge base or the surface slope, including changes in the mechanical strength of the detachment fault or variations in surface erosional efficiency. The Andes of eastern Bolivia have differences in the basal detachment strength, resulting from a thinning of the weak Paleozoic sediments that host the basal detachment, and average annual rainfall north and south of the bend in the orogen at ~18°S. In addition, the orogen and active Subandean FTB are ~50% narrower in the north, where both the detachment layer strength may be higher and the average annual rainfall is around eight times that in the south. This raises the question: What controls orogen width in the Bolivian Andes?</p><p>We explore the effects of variations in the mechanical strength of the basal detachment and surface erosional efficiency on FTB width using 3D numerical geodynamic models with lateral variations in these parameters along strike. Our numerical experiments calculate the orogen geometry using the DOUAR geodynamic modelling software (Braun et al., 2008) coupled to the FastScape surface process model (Braun and Willett, 2013). The model design includes an elevated plateau region that is thrust over a weak frictional plastic detachment layer, resulting in growth of an orogenic wedge at the distal plateau margin. The plateau geometry is also bent, including a 40° change in margin orientation along strike; changes in the erosional efficiency and detachment strength are varied on either side of this bend. We find that changes in detachment strength result in significant differences in FTB width, while changes in erosional efficiency have little effect. Increasing the detachment strength by two results in limited forward propagation of the thrust front and a reduction in the FTB width by roughly 50% compared to the weaker side of the model. In contrast, increasing precipitation by a factor of three (as a proxy for enhanced erosional efficiency) does not significantly effect the FTB width. These results compare well with the observed variations in orogen width in the Bolivian Andes, suggesting the FTB width may be controlled by the detachment strength, while variations in erosional efficiency have a limited effect. Ongoing work is exploring how changes in detachment strength and erosional efficiency may affect thermochronometer ages predicted from the numerical experiments, and how the predicted ages compare to ages observed in the Bolivian Andes.</p>

The influence of upper-plate advance and erosion on overriding plate deformation in orogen syntaxes

Focused, rapid exhumation of rocks is observed at some orogen syntaxes, but the driving mechanisms remain poorly understood and contested. In this study, we use a fully coupled thermomechanical numerical model to investigate the effect of upper-plate advance and different erosion scenarios on overriding plate deformation. The subducting slab in the model is curved in 3-D, analogous to the indenter geometry observed in seismic studies. We find that the amount of upper-plate advance toward the trench dramatically changes the orientation of major shear zones in the upper plate and the location of rock uplift. Shear along the subduction interface facilitates the formation of a basal detachment situated above the indenter, causing localized rock uplift there. We conclude that the change in orientation and dip angle set by the indenter geometry creates a region of localized uplift as long as subduction of the down-going plate is active. Switching from flat (total) erosion to more realistic fluvial erosion using a landscape evolution model leads to variations in rock uplift at the scale of large catchments. In this case, deepest exhumation again occurs above the indenter apex, but tectonic uplift is modulated on even smaller scales by lithostatic pressure from the overburden of the growing orogen. Highest rock uplift can occur when a strong tectonic uplift field spatially coincides with large erosion potential. This implies that both the geometry of the subducting plate and the geomorphic and climatic conditions are important for the creation of focused, rapid exhumation.

Structural Evolution of the Kohat Fold and Thrust Belt in the Shakardarra Area (South Eastern Kohat, Pakistan)

The Kohat fold and thrust belt, located in North-Western Pakistan, is a part of Lesser Himalaya developed due to the collision between the Indian and Eurasian plates. The structural evolution records of this area indicate that it consists of tight anticlines and broad syncline structures. Previous studies show that the structural pattern of this area has been produced due to multiple episodes of deformation. In the present research, 2D seismic data has been integrated with our field surveys to clarify the role of active strike-slip faulting in reshaping the surface structures of Shakardarra, Kohat. At the surface, doubly plunging anticlines and synclines are evolved on evaporites as detachment folds, truncated by thrust faults along their limbs. Seismic data show that the thrust faults originate from basal detachment located at the sedimentary-crystalline interface and either cut up section to the surface or lose their displacement to splay or back thrusts. At the surface, the Shakardarra Fault, the Tola Bangi Khel Fault, the Chorlaki Fault, and the axial trend of fold change their strike from EW to NS showing that the thrust and axial trend of folds are rotated along the vertical axis by the influence of the Kalabagh strike-slip fault. Strike-slip motion dominates the style of deformation at the northern segment. The current deformation is concentrated on the splay faults in the northern segment of the Kalabagh Fault. We propose that Shakardarra is sequentially evolved in three episodes of deformation. In the first phase, the detachment folds developed on Eocene evaporites, which are truncated by thrust faults originated from the basal detachment in the second phase. In the third phase, early formed folds and faults are rotated along the vertical axis by the influence of Kalabagh strike-slip fault.

To what degree the geometry and kinematics of accretionary wedges in analogue experiments is dependent on material properties

Cohesion and friction coefficients are fundamental parameters of granular materials used in analogue experiments. Thus, to test the physical characteristics and mechanical behaviour of the materials used in the experiments will help to better understand into what degree the results of experiments of geological processes depend on the material properties. Our test suggests significant differences between quartz sand and glass bead, in particular the shape factors (~ 1.55 of quartz sand to ~ 1.35 glass bead, angular to rounded) and grain sorting (moderately to well sorted). The glass beads show much better grain sorting and smaller shape factors than the quartz sand. Also they have smaller friction coefficient (~ 0.5 to ~ 0.6) and cohesion (20–30 Ma to 70–100 Ma), no matter of the grain size in our tested samples. The quartz sand shows much smaller friction coefficient (~ 0.6 to ~ 0.65), and smaller cohesion (~ 70 Pa to ~ 100 Pa) than that of smaller grain size sand. We have conducted four sets of analogue experiments with three repeats at the minimum. Our models show that material properties have important influence on the geometry and kinematics of the accretionary wedge. Although the difference in geometries are small, models with larger grain size develop wedges with higher wedge height, larger taper, shorter wedge length and less number of faults under the same amount of bulk shortening. In particular, models with basal detachment (even with 1 mm thickness), show significant difference in geometry and kinematics with that of quartz sand. We thus argue that the geometry and kinematics of the wedge appear to be significantly influenced by relative brittle and ductile strengths, and, to a lesser degree by the layering anisotropy. The basal detachment (even of tiny thickness) determines the first-order control on the location and development of accretionary wedge, in a contrast to the physical properties of brittle materials.

THE CALLOVIAN UNCONFORMITY AND THE OPHIOLITE OBDUCTION ONTO THE PELAGONIAN CARBONATE PLATFORM OF THE INTERNAL HELLENIDES

The carbonate-platform-complex and the oceanic formations of the central Pelagonian zone of the Hellenides evolved in response to a sequence of plate tectonic episodes of ocean spreading, plate convergence and ophiolite obduction. The biostratigraphies of the carbonate platform and the oceanic successions, show that the Triassic-Early Jurassic platform was coeval with an ocean where pillow basalts and radiolarian cherts were being deposited. After convergence began during late Early- Jurassic - Middle Jurassic time, the oceanic leading edge of the Pelagonian plate was subducted beneath the leading edge of the oceanic, overriding plate. The platform subsided while a supra-subduction, volcanic-island-arc evolved. Biostratigraphic and geochemical evidence shows that the platform and the oceanic floor, temporarily became subaerially exposed during Callovian time. This “Callovian event” is suggested to have taken place as oceanic lithosphere first made compressional, tectonic contact with the carbonate platform, initiating a basal detachment fault, along which the platform was thrust upwards. The central Pelagonian zone became an extensive land area that was supplied with laterite from an ophiolite highland. A similar emergence of Vardar ophiolite most likely took place in the Guevgueli area. The Callovian emergence shows that the initial ophiolite obduction onto the platform took place about 25 million years before the final emplacement of the ophiolite during Valanginian time.