The Nanga Parbat Haramosh massif (NPHM) is located in the western syntaxis of the India-Eurasia collision zone and is subject to erosion rates that are so extreme as to impact the isostatic ...equilibrium of the massif. In order to investigate the interaction between large scale tectonic forces and local isostatic processes, we employ a Rayleigh wave tomography method to measure phase velocities within the massif and surrounding region at crust and mantle depths. Our inversion solves for phase velocity anomalies by representing perturbations in the wavefield as the interference of two plane waves. Our data set was obtained from a temporary seismic array deployed in 1996 and includes 53 teleseismic events with M
w ≥ 5.0, at periods from 20 to 79 s. Phase velocities at short periods are low, ranging from 3.2 km s−1 at 20 s, and increasing gradually to 3.5 km s−1 at 40 s. These velocities are 11 per cent lower than velocities observed in the Indian continental Plate at periods below 45 s. Above 50 s, phase velocities in the Nanga Parbat region are significantly higher, ranging from 3.7 km s−1 at 45 s to 4.0 km s−1 at 79 s. These high phase velocities above 60 s are consistent with average velocities measured within the Indian Plate. Comparison of these results with surface wave studies in other regions of the Tibetan plateau including the eastern syntaxis and central Tibet show a similar low velocity anomaly below 45 s. Phase velocities above 55 s, however, are significantly higher in the Nanga Parbat region compared to velocities reported for all other regions of the plateau. Shear wave inversions produce significantly low velocities in the upper crust of the NPHM but exceed average lithospheric velocities below the Moho. We suggest the combination of anomalously low velocities in the upper crust and high velocities at lithospheric depths is due to rapid exhumation of deep crustal material causing elevated geothermal gradients. Azimuthal anisotropy shows a NNW-SSE fast direction at 1.5 per cent peak to peak for periods from 22 to 80 s. This suggests deformation occurs at lithospheric depths, that is driven by large scale stresses of the India-Eurasia collision zone, along an axis parallel to the least principal stress direction.
The topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion. Dating of abandoned river-cut surfaces in the northwestern Himalayas ...reveals that the Indus river incises through the bedrock at extremely high rates. An equilibrium is maintained between bedrock uplift and river incision.
The mechanics and petrological signature of a collisional mountain belt can be significantly influenced by topographic and erosional effects at the scale of large river gorges. The geomorphic ...influence on crustal scale processes arises from the effects of both stress localization due to existing topography, and also erosional removal of advected crustal mass. The shear stress concentration and normal stress amplification due to topographic gradients and loads divert strain away from existing topographic loads, while concentrating strain into topographic gaps. Efficient erosional removal of material within topographic gaps with widths of at least the thickness of the brittle crustal layer results in differential advection of crustal material. Concentrated exhumation within a gap leads to thermal thinning of the upper brittle layer of the crust, removing the highest strength part of the continental crust and significantly reducing the integrated crustal strength beneath the topographic gap. A rheological weak spot, triggered by efficient incision, grows in intensity as strain becomes increasingly concentrated within the weak region. The growth of extreme topography of an isolated massif requires that the process of creation of the massif is related to the weakening process and can result from the velocity pattern produced by erosional-rheological coupling. As a result, distinctive thermal/mechanical regions develop within the crust in response to these river-influenced velocity patterns and these regions impose a characteristic signature on material advecting through. The signal is one in which the region of highest topography is bracketed by two high-strain zones between which concentrated advection produces lozenges of sillimanite and dry melt stability approximately 20 kilometers beneath the summit. Above these lozenges is a thermal/mechanical boundary layer containing an active hydrothermal system driven by steep thermal, topographic and mechanical gradients. These thermal mechanical regions are fixed with respect to a crustal reference frame. Passage of rock beneath and through these regions under these conditions produces the distinctive petrology and structure of mantled gneiss domes and is recorded within the moving petrological reference frame. Such erosional-rheological coupling can explain the occurrence of some high-grade gneiss domes in ancient collisional belts as well as the presence of active metamorphic massifs at both ends of the Himalayan orogen.
Puccinia pygmaea var. angusta and P. urticata var. urticata were collected in Fairy Meadows and are new records for Pakistan. Similarly the aecidial stages of Uromyces hedysari-obscuri and U. ...polygoni-avicularis are an addition to the rust flora
of this country. Puccinia alpina, P. leveillei, and P. ribis are redescribed and illustrated from Fairy Meadows.
We have conducted a systematic inversion of striated fault planes throughout northern Pakistan in order to better depict the temporal and spatial variations in stress patterns. Two domains are ...evidenced at a regional scale, separated by the active Raikhot fault, the western boundary of the Nanga Parbat spur. West of this fault, a wrench‐type stress field with σ1 axis oriented around N–S predominates in the Karakorum and in Kohistan. It predates Pliocene‐Quaternary exhumation of Nanga Parbat and corresponds to the Miocene or earlier regional stress field related to Indian‐Asian convergence. East of the Raikhot fault, compression parallel to the belt accounts for initiation of the Nanga Parbat anticlinorium after 5 Ma. It is followed by predominant post‐2 Ma extension, both parallel to the belt and NNE–SSW oriented. Thus, in the N–W Himalayan syntaxis, multidirectional extension is juxtaposed on short timescales to shortening either parallel or perpendicular to the belt. Such juxtaposition could be characteristic of strain and stress partitioning during oblique convergence.
The Indus River system is the only major drainage system in the western Himalaya, and erodes not only the High Himalaya, but also topographically high regions within and north of the Indus Suture ...Zone, most notably the Karakoram. Ion microprobe analysis of Pb isotopes in detrital K-feldspar grains taken from the tributaries of the Indus, together with bulk Nd isotope analysis of those same sediments, is here used to identify distinct sediment source regions. These span the very radiogenic Nanga Parbat and associated Lesser Himalaya, the relatively radiogenic-intermediate High Himalaya, the unradiogenic Ladakh and Kohistan Batholiths and intermediate values in the Hindu Kush, Karakoram and Lhasa Block. The range of compositions reflects differing degrees of recycling of older continental crust during petrogenesis. K-feldspars from the Ladakh and Kohistan Batholiths are less radiogenic than the laterally equivalent Gangdese granite of Tibet, interpreted to reflect the preferential recycling of accreted oceanic arc units within the western Transhimalaya prior to India–Asia collision. Similarly the Zanskar High Himalaya are less radiogenic than their equivalents in Nepal. Isotope values from Pleistocene Indus Fan sediment are compatible with a dominant source in the Karakoram, with additional important contributions from the arc batholiths and High Himalaya, reflecting both the area and modern rates of tectonic uplift within the drainage basin. In contrast, radiogenic grains are common in the lower reaches of the modern Indus River, possibly as a result of the damming of the main river channel where it reaches the foreland.
The Nanga Parbat‐Haramosh massif (NPHM; western Himalayan syntaxis) requires an influx of mass exceeding that in the adjacent Himalayan arc to sustain high topography and rapid erosional exhumation ...rates. What supplies this mass flux and feeds this “tectonic aneurysm?” We show, using a simple 3‐D model of oblique orogen convergence, that velocity/strain partitioning results in horizontal orogen‐parallel (OP) crustal transport, and the same behavior is inferred for the Himalaya, with OP transport diverting converging crust toward the syntaxis. Model results also show that the OP flow rate decreases in the syntaxis, thereby thickening the crust and forming a structure like the NPHM. The additional crustal thickening, over and above that elsewhere in the Himalayan arc, sustains the rapid exhumation of this “aneurysm.” Normally, velocity/strain partitioning would be minimal for the Himalayan arc where the convergence obliquity is no greater than ~40°. However, we show analytically that the Himalayan system can act both as a critical wedge and exhibit strain partitioning if both the detachment beneath the wedge and the bounding rear shear zone, which accommodates OP transport, are very weak. Corresponding numerical results confirm this requirement and demonstrate that a Nanga Parbat‐type shortening structure can develop spontaneously if the orogenic wedge and bounding rear shear zone can strain rate soften while active. These results lead us to question whether the position of NPHM aneurysm is localized by river incision, as previously suggested, or by a priori focused tectonic shortening of the crust in the syntaxis region as demonstrated by our models.
Key Points
Three‐dimensional analytical model predicts strain partitioning in Himalayan‐type orogen
Three‐dimensional numerical models demonstrate partitioning and orogen‐parallel mass flux
Orogen‐parallel mass flux produces localized rapid uplift in model syntaxis
Low-temperature apatite (U–Th)/He (AHe) thermochronology on vertical transects of leucogranite stocks and
10Be terrestrial cosmogenic nuclide (TCN) surface exposure dating on strath terraces in the ...Lahul Himalaya provide a first approximation of long-term (10
4–10
6 years) exhumation rates for the High Himalayan Crystalline Series (HHCS) for northern India. The AHe ages show that exhumation of the HHCS in Lahul from shallow crustal levels to the surface was ~
1–2 mm/a and occurred during the past ~
2.5 Ma. Bedrock exhumation in Lahul fits into a regional pattern in the HHCS of low-temperature thermochronometers yielding Plio-Pleistocene ages. Surface exposure ages of strath terraces along the Chandra River range from ~
3.5 to 0.2 ka. Two sites along the Chandra River show a correlation between TCN age and height above the river level yielding maximum incision rates of 12 and 5.5 mm/a. Comparison of our AHe and surface exposure ages from Lahul with thermochronometry data from the fastest uplifting region at the western end of the Himalaya, the Nanga Parbat syntaxis, illustrates that there are contrasting regions in the High Himalaya where longer term (10
5–10
7 years) erosion and exhumation of bedrock substantially differ even though Holocene rates of fluvial incision are comparable. These data imply that the orogen's indenting corners are regions where focused denudation has been stable since the mid-Pliocene. However, away from these localized areas where there is a potent coupling of tectonic and surface processes that produce rapid uplift and denudation, Plio-Pleistocene erosion and exhumation can be characterized by disequilibrium, where longer term rates are relatively slower and shorter term fluvial erosion is highly variable over time and distance. The surface exposure age data reflect differential incision along the length of the Chandra River over millennial time frames, illustrate the variances that are possible in Himalayan river incision, and highlight the complexity of Himalayan environments.
The newly developed laser microprobe (U‐Th)/He thermochronometer permits, for the first time, the ability to generate precise (U‐Th)/He cooling ages for even very young (<1 Ma) samples with a spatial ...resolution on the order of tens of micrometers. This makes it possible to test the reproducibility of independent (U‐Th)/He age determinations within individual crystals, further increasing the reliability of the method. As an example, we apply it here to a Pleistocene granite from Nanga Parbat, Pakistan, where previous constraints on the thermal history are consistent with rapid exhumation and cooling. Twenty‐one (U‐Th)/He dates determined on two monazite crystals from a single granite sample yield a mean of 748,000 years with a ∼95% confidence level of ±19,000 years. There is no discernible variation in the distribution of (U‐Th)/He ages in the cores of these crystals and therefore no evidence for the development of substantial diffusive‐loss 4He zoning over 80% of the interior of the monazite crystals during postcrystallization cooling of the granite. Modeling of these data suggests that cooling at a mean rate of ∼300 K/Ma would be necessary to produce the observed ages and the lack of a 4He gradient, which is consistent with preexisting constraints for Nanga Parbat. Increased precision in thermochronology permits more tightly constrained exhumation models, which should aid geologic interpretation.
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