• George Mathew

Articles written in Journal of Earth System Science

• Electron spin resonance dating of fault gouge from Desamangalam, Kerala: Evidence for Quaternary movement in Palghat gap shear zone

The field investigations in the epicentral area of the 1994 Wadakkancheri (Desamangalam), Kerala, earthquake (M 4.3) indicate subtle, but clearly recognizable expressions of geologically recent fault zone, consisting of fracture sets showing brittle displacement and a gouge zone. The fracture zone confines to the crystalline basement, and is spatially coincident with the elongation of the isoseismals of the 1994 mainshock and a 10-km-long WNW-ESE trending topographic lineament. The preliminary results from the electron spin resonance (ESR) dating on the quartz grains from the fault gouge indicate that the last major faulting in this site occurred 430 ± 43 ka ago. The experiments on different grain sizes of quartz from the gouge showed consistent decrease in age to a plateau of low values, indicating that ESR signals in finer grains were completely zeroed at the time of faulting due to frictional heat. The results show a relatively young age for displacement on the fault that occurs within a Precambrian shear zone. Discrete reactivated faults in such areas may be characterized by low degree of activity, but considering the ESR age of the last significant faulting event, the structure at Desamangalam may be categorized as a potentially active fault capable of generating moderate earthquakes, separated by very long periods of quiescence.

• Cones and craters on Mount Pavagadh, Deccan Traps: Rootless cones?

Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when a lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice. We report here a group of gentle cones containing nearly circular craters from Mount Pavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similar morphologically to the rootless cones of the type locality of Mývatn in northeastern Iceland. A group of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the Jabalpur craters does not support the possibility that they are located over volcanic vents. They could also be rootless cones. Many more probably exist in the Deccan, and volcanological studies of the Deccan are clearly of value in understanding planetary basaltic volcanism.

• Evidence of paleoearthquakes from trench investigations across Pinjore Garden fault in Pinjore Dun, NW Himalaya

The Pinjore Garden Fault (PGF) striking NNW-SSE is now considered one of the active faults displacing the younger Quaternary surfaces in the piggyback basin of Pinjore Dun. This has displaced the older Kalka and Pinjore surfaces, along with the other younger surfaces giving rise to WSW and SW-facing fault scarps with heights ranging from 2 to 16 m. The PGF represents a younger branch of the Main Boundary Thrust (MBT) system. An ~ 4m wide trench excavated across the PGF has revealed displacement of younger Quaternary deposits along a low angle thrust fault. Either side of the trench-walls reveals contrasting slip-related deformation of lithounits. The northern wall shows displacement of lithounits along a low-angle thrust fault, while the southern wall shows well-developed fault-related folding of thick sand unit. The sudden change in the deformational features on the southern wall is an evidence of the changing fault geometry within a short distance. Out of five prominent lithounits identified in the trench, the lower four units show displacement along a single fault. The basal unit ‘A’ shows maximum displacement of aboutTo = 2.85 m, unit B = 1.8 m and unit C = 1.45 m. The displacement measured between the sedimentary units and retro-deformation of trench log suggests that at least two earthquake events have occurred along the PGF. The units A and D mark the event horizons. Considering the average amount of displacement during one single event (2 m) and the minimum length of the fault trace (~ 45 km), the behaviour of PGF seems similar to that of the Himalayan Frontal Fault (HFF) and appears capable of producing large magnitude earthquakes.

• Laboratory technique for quantitative thermal emissivity measurements of geological samples

Thermal infrared spectroscopy is a powerful technique for the compositional analysis of geological materials. The spectral feature in the mid-IR region is diagnostic of the mineralogy and spectral signatures of mixtures of minerals that add linearly, and therefore, can be used as an important tool to determine the mineralogy of rocks in the laboratory and remotely for planetary exploration. The greatest challenge in the emission measurement lies in the measurement of the weak thermal photons emitted from geological materials in a laboratory setup, and accurately records the temperature of the rock sample. The present work pertains to the details of a new Thermal Emission Spectrometer (TES) laboratory that has been developed under the ISRO Planetary Science and Exploration (PLANEX) programme, for emission related mineralogical investigations of planetary surfaces. The focus of the paper is on the acquisition and calibration technique for obtaining emissivity, and the deconvolution procedure to obtain the modal abundances of the thermal emission spectra in the range of 6–25 𝜇 m using Fourier Transform Infrared (FTIR) spectroscopy. The basic technique is adopted from the work of Ruff et al (1997). This laboratory at the Department of Earth Sciences, IIT-Bombay is currently developing pure end mineral library of mineral particulates (&gt; 65 𝜇m), and adding new end members to the existing ASU spectral library. The paper argues the need for considering Lunar Orbiter Thermal Emission Spectrometer (LOTES) for future Indian Moon mission programme (Chandrayan-II) to determine evidences of varied lithologies on the lunar surface.

• Arc parallel extension in Higher and Lesser Himalayas, evidence from western Arunachal Himalaya, India

The existence of E–W extensional features from northeast (NE) Himalaya is poorly documented. Our investigation in the western part of Arunachal Himalaya provides evidences of active Quaternary E–W arc-parallel extensional features in the Higher and Lesser Himalayas. They are represented by arcperpendicular normal faults and arc-parallel sinistral strike-slip faults. We discuss the occurrences of these arc-parallel extensional features in terms of oblique convergence and radial expansion models. The partitioning of stress due to oblique convergence is argued based on evidences of left-lateral slip in NEHimalaya, right-lateral slip in NW-Himalaya and absence of translation in the central part. The amount of arc-parallel extension in the hinterland regions is correlated to the amount of radial shortening in the foreland. The computation of arc-parallel extension in the NE Himalayan arc is carried out by defining a small-circle centered at 88° 39′ \pm 0.7′E longitude and 33° 40′ \pm 0.6′N latitude having a radius of 770.7 ± 15.1 km, for the segment between 92° 01′ and 95° 16′E longitudes. The amount of arc-parallel extension estimated is ∼110 km for the NE Himalayan segment. Our result agrees closely with the 104 km extension determined based on geodetically computed extension rate and age of initiation of rifting in southern Tibet.

• Phased cooling of the Siang antiform, Eastern Himalaya: Insight from multi-thermochronology and thermal studies

The Siang antiform which forms the southern portion of the Eastern Himalayan Syntaxis is a massive subaerial duplex comprised of Paleogene rocks. The specifics of growth and deformation of the Siang duplex remain ambiguous due to limited studies in the region. Using multi-thermochronometry and Raman spectroscopy of carbonaceous matter (RSCM), this study place temporal depth constraints on the formation of the duplex. Results show that the cooling history of the northern part of the Siang antiform is separate from the central and proximal zone. The study utilises 09 new biotite $^{40}$Ar/$^{39}$Ar and 05 zircon (U–Th)/He (ZHe) ages. The new data is complemented with our earlier published (Salvi et al. in Geomorphology 284:238–249, 2017) 09 ZHe and 11 AFT ages and 02 ZHe ages of Liebke et al. (Geol. Soc. London, Spec. Publ. 353:71–97, 2011). Biotite $^{40}$Ar/$^{39}$Ar cooling ages suggest that the distal end of the MCT zone in the Siang window was active at least till ca. 11 Ma. The ZHe cooling ages ca. 10–8 Ma intimate exhumation due to Lesser Himalayan duplexing on a shallower (${\sim}$7–8 km) MHT. The creation of several duplexed antiforms by the Paleogene rocks on the emplaced MBT thrust sheet led to doming up of the roof sheet. Erosion through the roof sheets exposed the Paleogene rocks presently seen in the Siang window. The peak metamorphic temperatures decrease from 650–400$^{\circ}$C in the lower LHS to 250–300$^{\circ}$C in the upper LHS, and < 200$^{\circ}$C in the sub-Himalaya. The RSCM results corroborate thermochronological ages and inform that the northern part of the metamorphosed lower LHS rocks exhumed from greater depths, >20 km during early-middle Miocene. While in the central and southern regions, the rocks exhumed from comparatively shallower depths of ${\sim}$7–8 km since the late Miocene. We suggest ${\sim}$3–4 km of the cover rocks have been removed since Pleistocene.

$\bf{Highlights}$

$\bullet$ First multi-thermochrometry studies suggest distal end of MCT zone in the Siang window was active till ca. 11 Ma.

$\bullet$ ZHe cooling ages ca. 10–8 Ma intimate exhumation due to Lesser Himalayan Duplexing on a shallower (${\sim}$ 7–8 km) MHT.

$\bullet$ The creation of several duplexed antiforms by the Paleogene rocks on the emplaced MBT thrust sheet led to doming up of the roof sheet.

$\bullet$ RSCM peak metamorphic temperatures furnish 650–400 $^{\circ}$C in the lower LHS to 250–300 $^{\circ}$C in the upper LHS, and < 200 $^{\circ}$C in the sub-Himalaya.

$\bullet$ Average exhumation rate post Late Pliocene is ${\sim}$ 1.6–3.3 mm/a in the Siang window, increases to 4.0–6.6 mm/a in the last 1.0 Ma, north of Tuting.

• Presence of detrital olivine and serpentine minerals in the Dihing unit of upper Assam: Implication towards the source

A significant occurrence of olivine and serpentine (antigorite, lizardite) are reported for the first time in the Dihing sedimentary unit of upper Assam. Petrography of the Dihing Formation documents olivine, serpentine (antigorite, lizardite), pyroxene and major framework grains (quartz, feldspar and variety of lithic fragments). Based on modal composition, the Dihing unit samples are classified as lithic arenite. Heavy mineral analysis in addition to the above, provides evidence of other minerals such as zircon, tourmaline, rutile, garnet, epidote, amphibole, sphene, aluminosilicates, apatite, spinel, staurolite, chloritoid, chlorite and mica. The heavy mineral assemblage is dominated by olivine (36.9%) and pyroxene (16.5%). The low (10.7%) zircon–tourmaline–rutile (ZTR) index infers mineralogically immature nature of sediments. Raman spectroscopy confirms the presence of antigorite and electron microprobe analysis corroborates the existence of olivine, pyroxene, serpentine and chrome-spinel in the analyzed samples. The imbricated clasts and cross-bedding sedimentary structures indicate SSW to SW paleo-current direction. The peridotite and serpentinite rocks of the Tidding suture zone from the Lohit–Dibang valleys are suggested as the likely sediment provenance for olivine and serpentine minerals. We advocate that the Lohit and Dibang rivers earlier flowed close to the Indo-Burman Ranges (IBR) depositing the Plio-Pleistocene Dihing sediments before merging with the Brahmaputra river.

$\bf{Highlights}$

$\bullet$ First report of detrital olivine and serpentine minerals in the Dihing unit of upper Assam.

$\bullet$ Petrography, heavy mineral, XRD and EPMA analyses show the presence of olivine, pyroxene, serpentine and chrome spinel.

$\bullet$ Clast imbrications and current-bedding indicate SSW to SW paleocurrent direction.

$\bullet$ Peridotite and serpentinite rocks of the Tidding suture zone from the Lohit–Dibang valleys are inferred as the source.

$\bullet$ The Lohit and Dibang rivers seem to have carried detritus from source to sink.

• # Journal of Earth System Science

Volume 132, 2023
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Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019