Erosion as a driving mechanism of intracontinental mountain growth

In nature, mountains can grow and remain as localized tectonic features over long periods of time (> 10 m.y.). By contrast, according to current knowledge of lithospheric rheology and neglecting surface processes, any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years. For example, assuming a quartz-dominated crustal rheology, the relief of a range initially 3 km high and 300–400 km wide is reduced by half in about 15 m.y. as a result of lateral spreading of its crustal root. We suggest that surface processes might actually prevent such a “subsurface collapse.” Removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny. We performed a set of numerical experiments in order to validate this hypothesis. A section of a lithosphere, with a brittle-elasto-ductile rheology, initially loaded by a mountain range is submitted to horizontal shortening and to surface processes. If erosion is intense, material is removed more rapidly than it can be supplied by crustal thickening below the range, and the topography is rapidly smoothed. For example, a feature 3 km high and 300–400 km wide is halved in height in about 15 m.y. for an erosion coefficient k = 10^3 m^2/yr (the erosion rate is of the order of a few 0.1 mm/yr). This regime might be called “erosional collapse.” If erosion is not active enough, the crustal root spreads out laterally and “subsurface collapse” occurs. In the third intermediate regime, removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow. In this “mountain growth” regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law. The erosion rate may be high (e.g., 0.5–0.9 mm/yr), close to the rate of tectonic uplift (e.g., 0.7–1.1 mm/yr), and few times higher than the rate of topographic uplift (0.15–0.2 mm/yr). These experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain. Surface processes must therefore be taken into account in the interpretation and modeling of long-term deformation of continental lithosphere. Conversely, the mechanical response of the lithosphere must be accounted for when large-scale topographic features are interpreted and modeled in terms of geomorphologic processes

On the growth of normal faults and the existence of flats and ramps along the El Asnam active fold and thrust system

The combination of detailed topographic leveling on the southwest segment of the El Asnam thrust fault with existing seismic and geologic data implies that the geometry of this fault involves shallow dipping flats and steep ramps. The fault appears to be growing along strike toward the southwest end, where the main shock initiated in 1980. From a depth of about 10 km, the main thrust appears to ramp to the basement-Cenozoic cover interface on a plane striking N40°E and dipping 50°–55° to the northwest. Along the southwest segment where folding has not yet developed, the thrust continues steeply through the Cenozoic cover to the near surface where it flattens, causing normal faulting. Along the central and northeast segments, which display a more evolved fold structure, the deep thrust probably flattens at a depth of 5–6 km, into a decollement along the Cenozoic-Jurassic interface before ramping to the surface. The Sara El Marouf and Kef El Mes anticlines have thus formed as fault propagation folds. Normal faults at Beni Rached probably branch with the thrust to maintain kinematic compatibility between the deep ramp and decollement. The greater separation (∼7 km) between the normal faults at Beni Rached and the thrust where it crosses Oued Cheliff than along the southwest segment (∼1 km) reflects the greater depth of the ramp to flat bend. We infer that the September 9, 1954, earthquake activated only the central deep segment of the main thrust together with the Beni Rached normal faults, while that of October 10, 1980, activated the whole system of flat decollements, ramp thrusts and compatibility normal faults. Further complexities of the faulting in map view are related to changes of strike of the thrust (in particular north of Oued Cheliff)

Constraints from rocks in the Taiwan orogen on crustal stress levels and rheology

Taiwan's Hsüehshan range experienced penetrative coaxial deformation within and near the brittle-plastic transition between ∼6.5 and 3 Ma. This recent and short-lasting deformation in an active, well-studied orogen makes it an ideal natural laboratory for studying crustal rheology. Recrystallized grain size piezometry in quartz and Ti-in-quartz thermobarometry yield peak differential stresses of ∼200 MPa at 250–300°C that taper off to ∼80 MPa at ∼350°C and ∼14 MPa at ∼400–500°C. Stress results do not vary with lithology: recrystallized quartz veins in slates and metasiltstones yield equivalent stresses as recrystallized grains in quartzites. A minimum strain rate of 2.9 × 10^(−15) s^(−1) associated with this deformation is calculated by dividing a strain measurement (axial strain ∼0.3) in a strongly deformed quartzite by the available 3.5 m.y. deformation interval. We estimate a maximum strain rate of 7.0 × 10^(−14) s^(−1) by distributing the geodetic convergence rate throughout a region homogeneously deformed under horizontal compression. These stress, strain rate and temperature estimates are consistent with the predictions of widely applied dislocation creep flow laws for quartzite. The samples record stress levels at the brittle-plastic transition, indicating a coefficient of friction (μ) of 0.37 in the upper crust consistent with results based on critical taper. Integrated crustal strength of the Hsüehshan range amounts to 1.7 × 10^(12) N/m based on our analysis, consistent with potential energy constraints based on topography. Other strength profiles are considered, however high crustal stresses (>300 MPa) conflict with our analysis. The study supports the use of the recrystallized grain size piezometer in quartz as a quick and inexpensive method for resolving stress histories in greenschist facies rocks. For consistency with the independent constraints presented here, we find it accurate to within +20%/−40%, significantly better than previously recognized

Thermal structure and exhumation history of the Lesser Himalaya in central Nepal

The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ~20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°–550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°–50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3–4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m^2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units

Spatially variable fault friction derived from dynamic modeling of aseismic afterslip due to the 2004 Parkfield earthquake

We investigate fault friction from dynamic modeling of fault slip prior to and following the Mw 6.0 earthquake which ruptured the Parkfield segment of the San Andreas Fault in 2004. The dynamic modeling assumes a purely rate-strengthening friction law, with a logarithmic dependency on sliding rate: μ=μ *+a-blnvv*. The initial state of stress is explicitly taken into account, and afterslip is triggered by the stress change induced by the earthquake source model given a priori. We consider different initial stress states and two coseismic models, and invert for the other model parameters using a nonlinear inversion scheme. The model parameters include the reference friction μ*, the friction rate dependency characterized by the quantity a-b, assumed to be either uniform or depth dependent. The model parameters are determined from fitting the transient postseismic geodetic signal measured at continuous GPS stations. Our study provides a view of frictional properties at the kilometers scale over the 0-15 km depth illuminated by the coseismic stress change induced by the Parkfield earthquake. The reference friction is estimated to be between 0.1 and 0.5. With independent a priori constraints on the amplitude of differential stress, the range of possible values narrows down to 0.1-0.17. The friction rate coefficient a-b is estimated to be � 10- 3 - 10- 2 with a hint that it increases upward from about 1-3 � 10-3 at 3-7 km depth to about 4-7 � 10-3 at 0-1 km depth. It is remarkable that our results are consistent with frictional properties measured on rock samples recovered from the fault zone thanks to the San Andreas Fault Observatory at Depth experiment. ©2013. American Geophysical Union. All Rights Reserved

Superficial simplicity of the 2010 El Mayor–Cucapah earthquake of Baja California in Mexico

The geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation, propagation and arrest of seismic ruptures. The fault system that runs from southern California into Mexico is a simple strike-slip boundary: the west side of California and Mexico moves northwards with respect to the east. However, the M_w 7.2 2010 El Mayor–Cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike-slip fault. Here we use geodetic, remote-sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake. We find that the earthquake produced a straight 120-km-long fault trace that cut through the Cucapah mountain range and across the Colorado River delta. However, at depth, the fault is made up of two different segments connected by a small extensional fault. Both segments strike N130° E, but dip in opposite directions. The earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike-slip motion. We show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves. We conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone

Safety of paracetamol in osteoarthritis: What does the literature say?

Osteoarthritis (OA) is a major cause of pain and physical disability in adults, and an increasingly common disease given its associations with aging and a growing obese/overweight population. Paracetamol is widely recommended for analgesia at an early stage in the management of OA, and, although frequently prescribed, evidence suggests the efficacy of paracetamol for OA pain is low. Furthermore, there have been recent concerns over the safety profile of paracetamol, with reports of gastrointestinal, cardiovascular, hepatic and renal adverse events. This narrative review summarizes recent literature on the benefits and harms of paracetamol for OA pain. Data on long-term paracetamol safety are derived largely from observational evidence, and are difficult to interpret given the potential biases of such data. Nonetheless, a considerable degree of toxicity is associated with paracetamol use among the general population, especially at the upper end of standard analgesic doses. Paracetamol is linked to liver function abnormalities and there is evidence for liver failure associated with non-intentional paracetamol overdose. Safety data for paracetamol use in the older population (aged >65 years) are sparse; however, there is some evidence that frail elderly people may have impaired paracetamol clearance. Given that the analgesic benefit of paracetamol in OA joint pain is uncertain and potential safety issues have been raised, more careful consideration of its use is required

Decreased expression of the endothelial cell-derived factor EGFL7 in systemic sclerosis: potential contribution to impaired angiogenesis and vasculogenesis

INTRODUCTION: Microvascular damage and defective angiogenesis and vasculogenesis have a major role in the pathogenesis of systemic sclerosis (SSc). Epidermal growth factor-like domain 7 (EGFL7) is a proangiogenic molecule which is predominantly expressed and secreted by endothelial cells and their progenitors and controls vascular development and integrity. In this study, we investigated the possible involvement of EGFL7 in SSc. METHODS: Serum EGFL7 levels from 60 patients with SSc and 35 age- and sex-matched healthy controls were examined by colorimetric sandwich enzyme-linked immunosorbent assay. The expression of EGFL7 in forearm skin biopsies (n = 16 SSc, n = 10 controls), cultured dermal microvascular endothelial cells (MVECs) (n = 3 SSc, n = 3 controls) and late-outgrowth peripheral blood endothelial progenitor cell (EPC)-derived endothelial cells (n = 15 SSc, n = 8 controls) was investigated by immunofluorescence and Western blotting. RESULTS: Serum EGFL7 levels were detectable in 68.6% of healthy controls and 45% of SSc cases (P < 0.05). Circulating levels of EGFL7 were significantly decreased in SSc patients compared with healthy controls (P = 0.01). Serum levels of EGFL7 were significantly lower in both limited cutaneous SSc and diffuse cutaneous SSc patients than in controls (P = 0.02 and P = 0.04, respectively). In SSc, decreased serum EGFL7 levels were significantly correlated with the severity of nailfold capillary abnormalities. Patients with the most severe capillary changes and digital ulcers had serum EGFL7 levels significantly lower than healthy controls, while the EGFL7 levels did not differ significantly between controls and SSc patients with less capillary damage and lack of digital ulcers. Endothelial EGFL7 expression was strongly downregulated or even almost completely undetectable in SSc-affected dermis compared with controls (P < 0.001). In cultured SSc dermal MVECs and late-outgrowth peripheral blood EPC-derived endothelial cells, EGFL7 was significantly downregulated compared with cells obtained from healthy subjects (P < 0.01 and P < 0.001, respectively). CONCLUSIONS: Our findings suggest that the loss of EGFL7 expression in endothelial cells and their progenitors might play a role in the development and progression of peripheral microvascular damage and the defective vascular repair process characteristic of SSc