Abstract. Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.
Phase delays at high frequencies are observed in body waves that travel in the Alaska slab, along its strike at 100–150 km depth. The delays, between 2–6 Hz energy and the direct 0.5–1 Hz arrival, are 0.5–1.5 s for P waves and 1.5–4 s for S waves. Such dispersion suggests a waveguide structure that parallels the slab, perhaps near its top. A channel that is 2–6 km thick and 2.5–5% slower than surrounding mantle can explain the observations. The thickness of the layer is comparable to that of subducted oceanic crust or somewhat thinner. The layer may be crust that is slow at these depths. The required velocity anomaly is too small to be due to a continuous layer of metastable gabbro yet too large to represent an eclogite layer. It may indicate a mixture of the two, or persistence of hydrated mineral assemblages to depth.
Earthquakes recorded by a dense seismic array at Nanga Parbat, Pakistan, provide new insight into synorogenic metamorphism and mass flow during mountain building. Microseismicity beneath the massif drops off sharply with depth and defines a shallow transition between brittle failure and ductile flow. The base of seismicity bows upward, mapping a thermal boundary with 3 km of structural relief over a lateral distance of 12 km. Anomalously low seismic velocities are observed at the core of the massif and extend to depth through the crust. The main locus of seismicity and low velocities correlates with a region of high topography, rapid exhumation, high geothermal gradients, young metamorphic and igneous ages, and crustal fluid flow. We suggest a genetic link between these phenomena in which hot rocks, rapidly advected from depth, are pervasively modified at relatively shallow levels in the crust.
Seismic attenuation (Q−1 ) of P and S waves shows a major discontinuity from the Russian platform to the tectonically active Greater Caucasus. Broadband records show this boundary over paths ≤4° long, as revealed by the decay of amplitude spectra from a digital seismic network flanking the Greater Caucasus. We measure attenuation from individual spectra, using a non‐linear least‐squares procedure to determine an attenuation parameter (t* ) simultaneously with source parameters at frequencies between 1 and 15 Hz. The t* measurements are then inverted for spatial variations of Q−1 , with parametrizations of varying complexity. Model variance for heterogeneous structures improves by more than 30 per cent compared with homogeneous parametrizations. Site corrections also significantly improve the fit. In these inversion results, mountainous regions exhibit Q values 2–3 times lower (QS = 775 ± 75) than do the adjacent shields (QS = 2060 ± 315), showing that the discontinuity is large. For both regions, QP is roughly equal to QS . Comparison of body‐wave to coda spectra indicates that intrinsic absorption rather than scattering dominates the Q−1 measurements, at least beneath the mountains. Hence Q−1 variations may give a reasonable proxy for temperature; if so, then temperature beneath the mountains exceeds that beneath the shield by 70°–400 °C. These temperature increases may not be high enough to generate widespread partial melting beneath the mountains, but could produce regional metamorphism and could contribute substantially to isostatic compensation of the mountains. Whatever their origin, the boundary in seismic attenuation is abrupt and large between stable craton and an adjacent mountain belt, demonstrating that Q−1 is a sensitive indicator of tectonic process.
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