Crustal deformation and exhumation within the India-Eurasia oblique convergence zone: New insights from the Ailao Shan-Red River shear zone

During the collision of India and Eurasia, regional-scale strike-slip shear zones played a key role in accommodating lateral extrusion of blocks, block rotation, and vertical exhumation of metamorphic rocks as presented by deformation on the Ailao Shan-Red River shear zone (ARSZ) in the Eastern Himalayan Syntaxis region and western Yunnan, China. We report structural, mica Ar/Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) data from the Diancangshan massif in the middle segment of the ARSZ. These structural data reveal that the massif forms a region-scale antiform, bordered by two branches of the ARSZ along its eastern and western margins. Structural evidence for partial melting in the horizontal mylonites in the gneiss core document that the gneiss experienced a horizontal shear deformation in the middle crust. Muscovite Ar/Ar ages of 36−29 Ma from the core represent cooling ages. Muscovite Ar/Ar ages of 25 and 17 Ma from greenschist-facies mylonites along the western and southern shear zones, respectively, are interpreted as recording deformation in the ARSZ. The AFT ages, ranging from 15 to 5 Ma, represent a quiescent gap with a slow cooling/exhumation in the massif. AHe results suggest that a rapid cooling and final exhumation episode of the massif could have started before 3.2 Ma, or likely ca. 5 Ma, and continue to the present. The high-temperature horizontal shearing layers of the core were first formed across the Indochina Block, locally antiformed along the tectonic boundaries, and then cooled through the mica Ar-Ar closure temperature during Eocene or early Oligocene, subsequently reworked and further exhumed by sinistral strike-slip movement along the ARSZ during the early Oligocene (ca. 29 Ma), lasting until ca. 17 Ma, then final exhumation of the massif occurred by dextral normal faulting on the Weixi-Qiaohou and Red River faults along the limbs of the ARSZ since ca. 5 Ma. The formation of the antiform could indicate local crustal thickening in an early transpressional setting corresponding to India-Asia convergence. Large-scale sinistral ductile shear along the ARSZ in the shallow crust accommodated lateral extrusion of the Indochina Block, and further contributed to the vertical exhumation of the metamorphic massif from the late Oligocene to the middle Miocene. Furthermore, the change of kinematic reversal and associated cooling episodes along the ARSZ since the middle Miocene or early Pliocene imply a tectonic transfer from strain localization along the major tectonic boundaries to continuous deformation corresponding to plateau growth and expansion.

Petrogenesis of the peralkaline Flowers River Igneous Suite and its significance to the development of the southern Nain Batholith

The Flowers River Igneous Suite of north-central Labrador comprises several discrete peralkaline granite ring intrusions and their coeval volcanic succession. The Flowers River Granite was emplaced into Mesoproterozoic-age anorthosite–mangerite–charnockite–granite (AMCG) -affinity rocks at the southernmost extent of the Nain Plutonic Suite coastal lineament batholith. New U–Pb zircon geochronology is presented to clarify the timing and relationships among the igneous associations exposed in the region. Fayalite-bearing AMCG granitoids in the region record ages of 1290 ± 3 Ma, whereas the Flowers River Granite yields an age of 1281 ± 3 Ma. Volcanism occurred in three discrete events, two of which coincided with emplacement of the AMCG and Flowers River suites, respectively. Shared geochemical affinities suggest that each generation of volcanic rocks was derived from its coeval intrusive suite. The third volcanic event occurred at 1271 ± 3 Ma, and its products bear a broad geochemical resemblance to the second phase of volcanism. The surrounding AMCG-affinity ferrodiorites and fayalite-bearing granitoids display moderately enriched major- and trace-element signatures relative to equivalent lithologies found elsewhere in the Nain Plutonic Suite. Trace-element compositions also support a relationship between the Flowers River Granite and its AMCG-affinity host rocks, most likely via delayed partial melting of residual parental material in the lower crust. Enrichment manifested only in the southernmost part of the Nain Plutonic Suite as a result of its relative proximity to multiple Palaeoproterozoic tectonic boundaries. Repeated exposure to subduction-derived metasomatic fluids created a persistent region of enrichment in the underlying lithospheric mantle that was tapped during later melt generation, producing multiple successive moderately to strongly enriched magmatic episodes.

Petrogenesis of the 91-Mile peridotite in the Grand Canyon: Ophiolite or deep-arc fragment?

Recognition of fundamental tectonic boundaries has been extremely difficult in the (>1000-km-wide) Proterozoic accretionary orogen of southwestern North America, where the main rock types are similar over large areas, and where the region has experienced multiple postaccretionary deformation events. Discrete ultramafic bodies are present in a number of areas that may mark important boundaries, especially if they can be shown to represent tectonic fragments of ophiolite complexes. However, most ultramafic bodies are small and intensely altered, precluding petrogenetic analysis. The 91-Mile peridotite in the Grand Canyon is the largest and best preserved ultramafic body known in the southwest United States. It presents a special opportunity for tectonic analysis that may illuminate the significance of ultramafic rocks in other parts of the orogen. The 91-Mile peridotite exhibits spectacular cumulate layering. Contacts with the surrounding Vishnu Schist are interpreted to be tectonic, except along one margin, where intrusive relations have been interpreted. Assemblages include olivine, clinopyroxene, orthopyroxene, magnetite, and phlogopite, with very rare plagioclase. Textures suggest that phlogopite is the result of late intercumulus crystallization. Whole-rock compositions and especially mineral modes and compositions support derivation from an arc-related mafic magma. K-enriched subduction-related fluid in the mantle wedge is interpreted to have given rise to a K-rich, hydrous, high-pressure partial melt that produced early magnetite, Al-rich diopside, and primary phlogopite. The modes of silicate minerals, all with high Mg#, the sequence of crystallization, and the lack of early plagioclase are all consistent with crystallization at relatively high pressures. Thus, the 91-Mile peridotite body is not an ophiolite fragment that represents the closure of a former ocean basin. It does, however, mark a significant tectonic boundary where lower-crustal arc cumulates have been juxtaposed against middle-crustal schists and granitoids.

Geophysical research at the Revda intersection of the Serov-Mauk regional fault

Research aim is to study the characteristic features of geophysical fields over the main geological structures in the zone of influence of the Serov-Mauk regional fault in the Middle Urals. Methodology. Electromagnetic studies included an express version of audiomagnetotelluric sounding (AMT) with a broadband OMAR-2m receiver (Institute of Geophysics UB RAS, Ekaterinburg). Office processing is based on obtaining frequency spectra of impedance using fast Fourier transform, and their transformation into deep sections of electrophysical parameters of the medium. Magnetic prospecting was carried out using GEM GSM-19T proton magnetometer (GEM Systems, Canada). Gamma-field survey was carried out with a survey radiometer SRP-68-01 (Electron, Zhovti Vody). Results. Based on observation processing results, high-quality sections of electrical resistivity and effective longitudinal conductivity were constructed on the parametric profile, as well as graphs of magnetic and radiation fields. The studies revealed features of change in the electrophysical parameters and potential fields over various geological structures of the near-contact fault zone. Summary. The signs of the main geologic features border lines were identified by changes in physical properties. The lithological and tectonic boundaries have been identified of a complex rock assemblage adjacent to the regional fault according to the characteristic anomalies of geophysical parameters. Geophysical survey results comply with the real geological conditions of the study area.

Climate as the great equalizer of continental-scale erosion

<p>Central Asia is one of the most tectonically active and orographically diverse regions in the world and is the location of the highest topography on Earth resulting from major plate tectonic collisional events. Yet the role of tectonics versus climate on erosion remains one of the greatest debates of our time. We present the first regional scale analysis of 2526 published low-temperature thermochronometric dates from Central Asia spanning the Altai-Sayan, Tian Shan, Tibet, Pamir, and Himalaya. We compare these dates to tectonic processes (proximity to tectonic boundaries, crustal thickness, seismicity) and state-of-the-art paleoclimate simulations in order to constrain the relative influences of climate and tectonics on the topographic architecture and erosion of Central Asia. Predominance of pre-Cenozoic ages in much of the interior of central Asia suggests that significant topography was created prior to the India-Eurasia collision and implies limited subsequent erosion. Increasingly young cooling ages are associated with increasing proximity to active tectonic boundaries, suggesting a first-order control of tectonics on erosion. However, areas that have been sheltered from significant precipitation for extensive periods of time retain old cooling ages. This suggests that ultimately climate is the great equalizer of erosion. Climate plays a key role by enhancing erosion in areas with developed topography and high precipitation such as the Tian Shan and Altai-Sayan during the Mesozoic and the Himalaya during the Cenozoic. Older thermochronometric dates are associated with sustained aridity following more humid periods.</p>

A Geomorphological Approach to Geodiversity and Geotourism in Uttarakhand Himalaya, India: A Pilot Survey

<p>Himalaya is the greatest heritage of India. The objective of this paper is to present a view of the geomorphological heritage of the Himalaya.Uttarakhand<strong> </strong>state (77°35’5”-81°2’25” E and 28°43’45”-31°8’18’’N, Area: 53,066 sq.km.)<strong> </strong>lies almost wholly within the realm of the Himalaya and is a distinct geographical entity. The state is a land of vast geological and topographic diversities and a realm with rich geo-wealth and geoheritage. Geological and geomorphological features occurring in different parts of Uttarakhand Himalaya are part of the natural assets and are precious state heritage (geoheritage), worthy of conservation. Apart from rock monuments and fossil parks, geomorphological features or geomorphosites have great potential to exert a pull on tourists. These sites have noteworthy impact on the geoscience education and research. Geotourism is growing rapidly all over the world and Himalaya region is no exception to this. To promote geotourism in the Himalayan State of Uttarakhand, comprehensive information about geomorphosites should be made available to the tourists by way of websites. For this, first a peer-reviewed state inventory of geomorphosites and their classification, mapping and assessment is required. Geodiversity in Uttarakhand State can best be understood in the form of the rise of Himalayan mountains from the bed of Tethys Sea which gave rise to four distinct tectonic units largely varying in lithology and structure. The relief was fragmented into four major morphosculptural units which signify the mountainous part of the state: viz. i. the Tethys zone or the Trans-Himalaya ii. the Greater Himalaya iii. the Lesser Himalaya and iv. the Siwalik. Apart from this mountainous region of the State, there is  outlying region of the state, which incompasses : iv. Bhabhar and Tarai (a sub-montane tract) - a landscape feature along the foothills, v. Dun Valleys – valleys of tectonic origin and vi. Plains of North India - the lowest part in Uttarakhand with an altitude of 200 m. These geological units recognised on the basis of evolutionary history, stratigraphic sequences and component rock units and reveal identical topographic and climatic characteristics. These units are separated by various tectonic boundaries. Apart from geodiversity, the geomorphological diversity can be assessed in the form of towering snow peaks, awe-inspiring horned peaks with natural grandeur, widely distributed stretches of wide and fertile valleys, valleys of tectonic origin-canoe shaped longitudinal valleys, lofty snow capped peak surrounded by several small and big snowfields, glaciers and lakes, mountain passes and  elevated zones packed in a series of multi-level distinctive waterfalls. Thus, being the youngest mountain of the world, this Himalayan State has geotouristic potential from the point of view of its geomorphological heritage.</p><p><strong> </strong><strong>Keywords: </strong>Himalaya<strong>, </strong>geodiversity , geomorphological heritage, geomorphosites, geotourism.  </p>

Recent measurements of subsidence in the Ganges-Brahmaputra Delta, Bangladesh

<p>Deltas, the low-lying land at rivers mouths, are sensitive to the delicate balance between sea level rise, land subsidence and sedimentation. Bangladesh and the Ganges-Brahmaputra Delta (GBD) have been highlighted as a region at risk from sea level rise, but reliable estimates of land subsidence have been limited. While early studies in the GBD suggested high rates of relative sea level rise, recent papers estimate more modest rates. Our objective is to better quantify the magnitude, spatial variability, and depth variation of compaction and subsidence in the GBD in order to better evaluate the processes controlling it and the pattern of relative sea level rise in this vulnerable region.</p><p>With support from the Bangladesh Water Development Board, we have rehabilitated previously installed GNSS and installed new GNSS co-located with Rod Surface Elevation Tables (RSET) to better understand the balance of subsidence and sedimentation in the coastal zone in SW Bangladesh, which is less affected by the active tectonic boundaries to the north and the east. The continuous GNSSs installed in 2003 and 2012 were mounted on reinforced concrete building roofs. GPS stations in the area yield subsidence rate estimates of 3-7 mm/y.  To densify the subsidence data, in early 2020 we resurveyed 48 concrete Survey of Bangladesh geodetic monuments in SW Bangladesh that were installed in 2002. Although only measured at the start and end of the period, the time span between the two measurements is ~18 years enabling us to estimate subsidence over this timespan.</p><p>Preliminary results show that about ½ the sites yielded very high subsidence rates; repeat measurements confirm the suspicion that the monuments at these sites are unstable and have undergone localized subsidence from settling or anthropogenic activity. The remaining sites show an increase in subsidence from the NW to the SE, consistent with estimates of average Holocene subsidence (Grall et al., 2018). However, rates from the campaign stations are much higher than those from continuous GNSS sites, but only slightly higher than an RSET site. We interpret that the continuous building GNSS omit very shallow compaction-related subsidence, while RSETs neglect deep subsidence. This is further reinforced by results from a compaction meter consisting of 6 wells from 20 to 300 m depth with vertical optical fiber strainmeters in each well. They show a decrease in compaction with depth. While initial results require further investigation, we highlight the importance of multiple methodologies for interpreting subsidence rates--deep, shallow, natural, anthropogenic--in vulnerable delta regions.</p>

An edge-assisted smooth method for potential field data

Edge detection is one of the most commonly used methods for the interpretation of potential field data, because it can highlight the horizontal inhomogeneous of underground geological bodies (faults, tectonic boundaries, etc.). A variety of edge detection methods have been reported in the literature, most of which are based on the combined transformation results of horizontal and vertical derivatives of the observations. Consequently, these edge detection methods are sensitive to noise. Therefore, noise reduction is desirable ahead of applying edge detection methods. However, the application of conventional filters smears discontinuities in the data to a certain extent, which would inevitably induce unfavourable influence on subsequent edge detection. To solve this problem, a novel edge-preserving smooth method for potential field data is proposed, which is based on the concept of guided filter developed for image processing. The new method substitutes each data point by a combination of a series of coefficients of linear functions. It was tested on synthetic model and real data, and the results showed that it can effectively smooth potential field data while preserving major structural and stratigraphic discontinuities. The obtained data from the new filter contain more obvious features of existing faults, which brings advantageous to further geological interpretations.

Applications and Evaluation of the IRIS Earthquake Browser: A Web-Based Tool That Enables Multidimensional Earthquake Visualization

The Incorporated Research Institutions for Seismology Earthquake Browser (IEB; see Data and Resources) is a web-based tool that enables anyone to query an earthquake database composed of over five million events recorded over the past 50 yr. The IEB visitor can query on earthquake magnitude, depth, timing, and location and can visually display the results in 2D map view or as an interactive pseudo 3D view. The user can toggle features such as plate tectonic boundaries, terrain or satellite mapping, and zoom to place the results in a geologic or geopolitical context and add visual appeal. To better understand who visits the IEB and why, to include information on demographics and how users perceive the IEB functionality and ability to meet their needs, we conducted a pop-up user survey on the IEB from 25 January to 21 February 2018. We received 495 useable responses from 58 countries, with 40% of the total respondents from the United States. The largest demographic consists of interested citizens who are 55 yr of age or older and have a high school education. We also find that visitors come to the IEB to learn about earthquakes for two main reasons: for their own personal knowledge or because expanding their knowledge is important to their research or work in a professional context. We also find a dramatic increase in survey respondent activity following the 16 February 2018 M 7.2 Oaxaca, Mexico, earthquake, with many respondents interested in finding more information about recent earthquake events that affect them or their family. Our observations indicate that users are successful and satisfied with the ease of use and amount of time spent on the IEB to find answers to their questions about earthquakes. The most beneficial feature of the IEB as identified by survey respondents is the spatiotemporal visual display of real earthquake data.

3D geological modelling of the western Aar Massif (external Central Alps, Switzerland)

<p>3D modelling of complex and irregular geological bodies is an expanding discipline that combines two-dimensional cartographic and structural data managed with GIS technology. This study presents a complete workflow developed to process geological information to build a 3D model of major stratigraphic, structural and tectonic boundaries. The investigated area is located in the western part of the Aar Massif (external Central Alps, Switzerland) characterized by pronounced topographic (600–<4000 m) relief, making it prone for surface based 3D depth constructions. The workflow comprises four major steps:</p><p>(1)  <strong>Generation of 2D polylines in a map view</strong>: a two-dimensional dataset of sequences of polylines has been generated in ArcGIS (10.3.1) defining the starting dataset for the major stratigraphic and tectonic boundaries of the bedrock units. This dataset has been compiled and integrated by using: (i) GeoCover vector datasets 1:25 000 of the Swiss Geological Survey; (ii) The Geological Special Map 1:100 000 of the Aar Massif and the Tavetsch and Gotthard Nappes of the Swiss Geological Survey; (iii) data from literature; and (iv) additional field work conducted for this study in key-locations.</p><p>(2) <strong>Projection of 2D information onto 3D digital elevation model</strong>: with the 3D structural modelling software Move (Petex/Midland Valley; 2019.1) the boundaries have then been projected on a digital elevation model (swissALTI3D) with 2 m resolution.</p><p>(3) <strong>Construction of tectonic cross sections</strong>: the use of geometric arguments as well as structural measurements allows for projection of these boundaries into a dense regularly spaced network of 2D cross-sections.</p><p>(4) <strong>Interpolation of 3D surfaces</strong>: the surface and cross-sections boundaries can be interpolated by applying 3D projection and meshing techniques resulting in a final 3D structural model.</p><p>Generally, steps (2–4) require iterative adaptations particularly in the case of surface areas being covered by glaciers or unconsolidated Quaternary sediments. In the model, special emphasis is given to visualize the current structural disposition of the western Aar Massif as well as the relative geometric and overprinting relationships of the deformation sequence that shaped the investigated area throughout the Alpine deformation. Finally, since in the investigated area underground data are scarce, an assessment of the relative uncertainties related to input data and is intended to be performed following the approach proposed by Baumberger (2014) and Ferńandez (2005). The workflow presented here offers the chance to gain validation approaches for domains only weakly constrained or with no surface data available, by generating a 3D model that integrates all accessible geological information and background knowledge.</p><p> </p><p>REFERENCES</p><p>Baumberger, R. (2014): Quantification of Lineaments: Link between internal 3D structure and surface evolution 328 of the Hasli valley (Aar massif, central alps, Switzerland), University of Bern, PhD Thesis, unpublished.</p><p>Ferńandez, O. (2005): Obtaining a best fitting plane through 3D georeferenced data, Journal of Structural Geology 27, pp. 855–858</p>