György Hetényi

Underplating in the Himalaya-Tibet Collision Zone Revealed by the Hi-CLIMB Experiment

Himalayan-Tibetan Underplate The Himalayas formed from the collision of India with Eurasia beginning about 50 million years ago, but the fate and position of the subducted Indian crust was not well defined until the Hi-CLIMB seismic experiment was initiated. The centerpiece of the project is an 800-kilometer-long, closely spaced, linear array of broadband seismographs, extending from the Ganges lowland, across the Himalayas, and onto the central Tibetan plateau. Nábělek et al. (p. 1371) present images of the crust and upper mantle of the Southern Tibetan plateau underthrust northward by the Indian plate, in which they trace the base of the Indian plate to 31°N. The character of the crust-mantle interface in this region suggests that the Indian crust is at least partly decoupled from the mantle beneath. A seismic study delineates the position and local thickening of the Indian plate underlying the Himalayas and southern Tibet. We studied the formation of the Himalayan mountain range and the Tibetan Plateau by investigating their lithospheric structure. Using an 800-kilometer-long, densely spaced seismic array, we have constructed an image of the crust and upper mantle beneath the Himalayas and the southern Tibetan Plateau. The image reveals in a continuous fashion the Main Himalayan thrust fault as it extends from a shallow depth under Nepal to the mid-crust under southern Tibet. Indian crust can be traced to 31°N. The crust/mantle interface beneath Tibet is anisotropic, indicating shearing during its formation. The dipping mantle fabric suggests that the Indian mantle is subducting in a diffuse fashion along several evolving subparallel structures.

The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault

The 2015 Gorkha earthquake sequence provides an outstanding opportunity to better characterize the geometry of the Main Himalayan Thrust (MHT). To overcome limitations due to unaccounted lateral heterogeneities, we perform Centroid Moment Tensor inversions in a 3-D Earth model for the main shock and largest aftershocks. In parallel, we recompute S-toP and P-to-S receiver functions from the Hi-CLIMB data set. Inverted centroid locations fall within a low-velocity zone at 10–15 km depth and corresponding to the subhorizontal portion of the MHT that ruptured during the Gorkha earthquake. North of the main shock hypocenter, receiver functions indicate a north dipping feature that likely corresponds to the midcrustal ramp connecting the flat portion to the deep part of the MHT. Our analysis of the main shock indicates that long-period energy emanated updip of high-frequency radiation sources previously inferred. This frequency-dependent rupture process might be explained by different factors such as fault geometry and the presence of fluids.

Initiation of crustal‐scale thrusts triggered by metamorphic reactions at depth: Insights from a comparison between the Himalayas and Scandinavian Caledonides

Active eclogitization has recently been inferred at depth beneath the Himalaya from geophysical observations, and the mechanical consequences of eclogitization can be observed in the field in the eroded and extended nappe stack of the Scandinavian Caledonides. There, Proterozoic metastable granulites and igneous protoliths underwent partial eclogitization during the collision of Baltica with Laurentia. The reaction began in pseudotachylites and veins and eventually formed a connected network of eclogite-facies shear zones that localized deformation and weakened the lower crust of Baltica during the collision with Laurentia. All these features can be compared with the seismic activity of the Indian Lower Crust, its strength loss beneath the Himalayan ranges, and its delayed density increase regarding its penetration in the eclogite facies. The Caledonian Bergen Arc eclogites and the Himalayan Ama Drime eclogites are both derived from continental crust. In both cases, these eclogites were formed contemporaneously with the activation of the main thrusts responsible for the construction of the orogenic wedges, the Main Central Thrust in the Himalayas, and the main thrust below the Jotun Nappe Complex in the Caledonides. The similarities in these two orogens, which compare both in size and structure, highlight the importance of eclogitization at depth as a mechanism for weakening of the lower crust and for decoupling of the crust and lithospheric mantle in collision zones.

Geophysical applicability of atomic clocks: direct continental geoid mapping

SUMMARY The geoid is the true physical figure of the Earth, a particular equipotential surface of the Earth’s gravity field that accounts for the effect of all subsurface density variations. Its shape approximatesbest(inthesenseofleastsquares)themeanlevelofoceans,butthegeoidismore difficult to determine over continents. Satellite missions carry out distance measurements and derive the gravity field to provide geoid maps over the entire globe. However, they require calibrationandextensive computations includingintegration,whichisanon-unique operation. Hereweproposeadirectmethodandanewtoolthatdirectlymeasuresgeopotentialdifferences on continents using atomic clocks. General relativity theory predicts constant clock rate at sea level, and faster (slower) clock rate above (below) sea level. The technology of atomic clocks is on the doorstep of reaching an accuracy level in clock rate (frequency ratio inaccuracy of 10 −18 ), which is equivalent to 1 cm in determining equipotential surface (including geoid) height. We discuss the value and future applicability of such measurements including direct geoid mapping on continents, and joint gravity–geopotential surveying to invert for subsurface density anomalies. Our synthetic calculations show that the geoid perturbation caused by a 1.5km radius sphere with 20 per cent density anomaly buried at 2km depth in the Earth’s crust is already detectable by atomic clocks of achievable accuracy. Therefore atomic clock geopotential surveys, used together with relative gravity data to benefit from their different depth sensitivities, can become a useful tool in mapping density anomalies within the Earth.

Discontinuous low-velocity zones in southern Tibet question the viability of the channel flow model

Abstract Low-velocity zones (‘bright spots’) imaged by the INDEPTH seismic experiment in southern Tibet are extensively interpreted as widespread partial melt within the crust, which has given a strong support for the channel flow model. These suggest that a continuous seismic low-velocity zone underlies Tibet on the large scale. Here we take advantage of the Hi-CLIMB seismic experiment which includes a dense south–north profile and a lateral 2D seismic network to assess the vertical and the horizontal extension of low-velocity zones in southern Tibet. Several approaches including migration, amplitude analysis and waveform inversion of receiver functions are performed to detect crustal low-velocity zones using this new seismological dataset. Our results reveal localized and discontinuous low-velocity zones in Tibet. They indicate that the vertical extension of the low-velocity zones is about 10 km, and their maximum horizontal length appears to be c. 50 km. Our study suggests a partial correlation between the location of these low-velocity zones and the spatial distribution of Tibetan grabens. These results, especially the non-continuity of low-velocity zones, together with the observed regular value of mean crustal VP/VS ratio, question the existence of widespread partial melt of the southern Tibetan crust and, therefore, the viability of the channel flow model.

Melt migration in basalt columns driven by crystallization-induced pressure gradients

The structure of columnar-jointed lava flows and intrusions has fascinated people for centuries and numerous hypotheses on the mechanisms of formation of columnar jointing have been proposed. In cross-section, weakly developed semicircular internal structures are a near ubiquitous feature of basalt columns. Here we propose a melt-migration model, driven by crystallization and a coeval specific volume decrease inside cooling and solidifying columns, which can explain the observed macroscopic features in columnar-jointed basalts. We study basalts from Hrepphólar (Iceland), combining macroscopic observations, detailed petrography, thermodynamic and rheological modelling of crystallization sequences, and Anisotropy of Magnetic Susceptibility (AMS) of late crystallizing phases (that is, titanomagnetite). These are all consistent with our proposed model, which also suggests that melt-migration features are more likely to develop in certain evolved basaltic lava flows (with early saturation of titanomagnetite), and that the redistribution of melt within individual columns can modify cooling processes.

Segmentation of the Himalayas as revealed by arc-parallel gravity anomalies.

Lateral variations along the Himalayan arc are suggested by an increasing number of studies and carry important information about the orogen’s segmentation. Here we compile the hitherto most complete land gravity dataset in the region which enables the currently highest resolution plausible analysis. To study lateral variations in collisional structure we compute arc-parallel gravity anomalies (APaGA) by subtracting the average arc-perpendicular profile from our dataset; we compute likewise for topography (APaTA). We find no direct correlation between APaGA, APaTA and background seismicity, as suggested in oceanic subduction context. In the Himalayas APaTA mainly reflect relief and erosional effects, whereas APaGA reflect the deep structure of the orogen with clear lateral boundaries. Four segments are outlined and have disparate flexural geometry: NE India, Bhutan, Nepal & India until Dehradun, and NW India. The segment boundaries in the India plate are related to inherited structures, and the boundaries of the Shillong block are highlighted by seismic activity. We find that large earthquakes of the past millennium do not propagate across the segment boundaries defined by APaGA, therefore these seem to set limits for potential rupture of megathrust earthquakes.

Ground-based optical atomic clocks as a tool to monitor vertical surface motion

SUMMARY According to general relativity, a clock experiencing a shift in the gravitational potential �U will measure a frequency change given byf/f ≈ �U/c 2 . The best clocks are optical clocks. After about 7 hr of integration they reach stabilities off/f ∼ 10 −18 and can be used to detect changes in the gravitational potential that correspond to vertical displacements of the centimetre level. At this level of performance, ground-based atomic clock networks emerge as a tool that is complementary to existing technology for monitoring a wide range of geophysical processes by directly measuring changes in the gravitational potential. Vertical changes of the clocks position due to magmatic, post-seismic or tidal deformations can result in measurable variations in the clock tick rate. We illustrate the geopotential change arising due to an inflating magma chamber using the Mogi model and apply it to the Etna volcano. Its effect on an observer on the Earths surface can be divided into two different terms: one purely due to uplift (free-air gradient) and one due to the redistribution of matter. Thus, with the centimetre-level precision of current clocks it is already possible to monitor volcanoes. The matter redistribution term is estimated to be 3 orders of magnitude smaller than the uplift term. Additionally, clocks can be compared over distances of thousands of kilometres over short periods of time, which improves our ability to monitor periodic effects with long wavelength like the solid Earth tide.

Internal flow structures in columnar jointed basalt from Hrepphólar, Iceland: II. Magnetic anisotropy and rock magnetic properties

The anisotropy of magnetic susceptibility (AMS) and rock magnetic properties were measured on specimens from a basalt plate that was cut from a vertical section of a basalt column from Hrepphólar, Iceland. Macroscopic structures are clearly distinguishable in the plate, including banding inferred to represent viscous fingering parallel to the vertical axis of the column. Rock magnetic experiments indicate that the dominant ferromagnetic (sensu lato) mineral is titanomagnetite, Fe3−xTixO4, with a Ti-composition of x = ~0.6. Magnetic properties are related to the position within the plate and reveal a dominant volume fraction of single domain titanomagnetite in the center of the basalt column, with multidomain titanomagnetite away from the center. The AMS determined by low-field measurements shows an inconclusive relationship with the visual structures, which arises from variation of the grain size (i.e., single domain versus multidomain) across the column. In contrast, the AMS measured with a high-field torsion magnetometer avoids the complication of magnetic domain state, as is demonstrated in this contribution, and additionally allows for the separation of ferrimagnetic from paramagnetic sub-fabrics. Both sub-fabrics display a clear relationship with the macroscopic structures and support the hypothesis that vertical flow of melt took place during development of the Hrepphólar columnar basalt. Maximum susceptibility axes of the ferrimagnetic sub-fabric are grouped near the vertical axis of the column. The paramagnetic sub-fabric varies systematically across the column in coincidence with internal structure. The shape of the magnetic susceptibility ellipsoid varies across the basalt column, showing an increasingly prolate fabric toward its center.

Quantifying the impact of mechanical layering and underthrusting on the dynamics of the modern India‐Asia collisional system with 3‐D numerical models

The impact of mechanical layering and the strength of the Indian lower crust on the dynamics of the modern India-Asia collisional system are studied using 3-D thermomechanical modeling. The model includes an Indian oceanic domain, Indian continental domain, and an Asian continental domain. Each domain consists of four layers: upper/lower crust, and upper/lower lithospheric mantle. The Tarim and Sichuan Basins are modeled as effectively rigid blocks and the Quetta-Chaman and Sagaing strike-slip faults as vertical weak zones. The geometry, densities, and viscosities are constrained by geophysical data sets (CRUST2.0, gravity, and seismology). Both static (no horizontal movement of model boundaries) and dynamic scenarios (indentation) are modeled. It is demonstrated that 3-D viscosity distributions resulting from typical creep flow laws and temperature fields generate realistic surface velocities. Lateral variations in the gravitational potential energy cause locally significant tectonic overpressure (i.e., difference between pressure and lithostatic pressure) in a mechanically strong Indian lower crust (up to ~500 MPa for the static scenario and ~800 MPa for the dynamic scenario). Different density distributions in the lithosphere as well as different viscosities (3 orders of magnitude) in the Indian lower crust cause only minor differences in the surface velocity field. This result suggests that surface velocities alone are insufficient to infer the state of mechanical coupling of the lithosphere. Model results are in agreement with GPS velocities for Indian lower crustal viscosities of 1021–1024 Pa s, for a strong Quetta-Chaman Fault (1022 Pa s) and a weak Sagaing Fault (1020 Pa s).