Malay Mukul

The geometry and kinematics of the Main Boundary Thrust and related neotectonics in the Darjiling Himalayan fold-and-thrust belt, West Bengal, India

Abstract The Main Boundary Thrust (MBT) is well-exposed in a section of the Tista River valley in the Darjiling Himalayan fold-and-thrust belt. The fault trace trends E–W across the N–S flowing Tista and thrusts Gondwana (Permian) age sandstones over Lower Siwalik (Miocene) age sandstones. The MBT has been folded into a fault propagation synform–antiform pair by tectonic activity along a younger, South Kalijhora Thrust (SKT) in its footwall that transports Lower Siwalik rocks over Middle Siwalik sandstones; the SKT cuts through the fault propagation structure preserving the MBT fault–propagation fold pair in its hanging wall. The motion along the SKT rotated the bedding and the limbs of the folded MBT in its hanging wall. Footwall imbrication in front of the SKT further rotated the folded MBT. Footwall imbrication continued to about 30 km south of the study area in the Middle Siwalik and younger section. The region has been subsequently reactivated as evident from uplifted gravel beds and strath terraces along the folded trace of the MBT fault zone and also in the footwall of the SKT. The reactivation resulted in the formation of connective splays off the existing faults, probably forming horses of a connecting splay duplex in the sub-surface within the Siwalik section. The motion along these splays passively folded overlying beds, and formed uplifted gravel beds and strath terraces along the local drainage. The rotations resulting from footwall imbrication and reactivated splays probably caused the southern limb of the fault propagation synform to become overturned. The duplexing also raised the river bed and arrested the flow of the River Tista causing the river to ‘pond’ and deposit its load, resulting in the formation of a bar upstream of the MBT trace. The tectonic reactivation of the area points to taper-building activities in a subcritical wedge probably in response to monsoon-induced excessive erosion in the middle of the Darjiling–Sikkim–Tibet Himalayan thrust wedge.

Timing of recent out-of-sequence active deformation in the frontal Himalayan wedge: Insights from the Darjiling sub-Himalaya, India

Recent studies of India-Eurasia convergence suggest that the entire convergence in the Himalayan wedge is almost exclusively accommodated along its basal detachment fault (Main Himalayan thrust, MHT) and its near-surface equivalent (Main Frontal thrust, MFT). Using direct dating of fault-zone gouge and strath terrace deposits, we conclude the following. (1) The present mountain front in the Darjiling sub-Himalaya was emplaced by ca. 40 ka. (2) Out-of-sequence deformation on surface-breaking faults north of the MFT in the Darjiling sub-Himalaya began ca. 20 ka and has probably continued since. (3) The Tista River responded to the ca. 20 ka deformation by migrating 150 m eastward (average rate ∼13 mm yr −1 ) and by incising 48 m vertically (average rate ∼4.4 mm yr −1 ), creating unpaired, disjointed strath terraces between 11.3 ± 1.3 ka and 1.4 ± 0.3 ka. Out-of-sequence, surface-breaking faults in the Himalaya indicate partial accommodation of active convergence within the Himalayan wedge. Using the results from the Bhuj earthquake of 2001, we suggest that active deformation along the out-of-sequence faults is a potential seismic hazard in the Himalaya, and Himalayan seismic hazard models must account for this. We also propose a conceptual model for active deformation in the Himalaya.

Finite strain and strain variation analysis in the Sheeprock Thrust Sheet: an internal thrust sheet in the Provo salient of the Sevier Fold-and-Thrust belt, Central Utah

Abstract The Sheeprock thrust sheet in west-central Utah is an internal thrust sheet in the Provo salient of the Sevier fold-and-thrust belt. We have measured finite strain in quartzites (the dominant lithology), sampled along a square grid within the thrust sheet, using the modified normalized Fry method (McNaught M.A. (1994) Modifying the normalized Fry method for aggregates of non-elliptical grains. Journal of Structural Geology16 493–503). The X Y and X Z axial ratios from unsampled locations within the sample area were estimated using the spatial statistics approach. The strain ellipsoids exhibit a variable three-dimensional orientation pattern resulting from modification of the initial layer parallel shortening (LPS) strain ellipsoid by fault parallel shear in conjunction with vertical flattening and/or horizontal stretching indicating that the thrust sheet did not undergo plane strain deformation in the transport plane. This suggests that the plane strain assumption used in drawing restorable balanced cross-sections breaks down for internal thrust sheets with more than one penetrative-strain producing deformation event. The X Z strain axial ratios decrease away from the thrust towards the middle of the sheet. The X Y strain axial ratios from interpolated image diagrams indicate transport-parallel stretching at the front end of the sheet and strike-parallel stretching at the back end of the sheet. The footwall and hanging wall finite strain patterns are similar indicating that most of the strain in the Sheeprock thrust sheet developed early in the deformation history of the thrust sheet before and perhaps during the growth of a large fault propagation fold pair.

Pre-seismic, co-seismic and post-seismic displacements associated with the Bhuj 2001 earthquake derived from recent and historic geodetic data

The 26th January 2001 Bhuj earthquake occurred in the Kachchh Rift Basin which has a long history of major earthquakes. Great Triangulation Survey points (GTS) were first installed in the area in 1856–60 and some of these were measured using Global Positioning System (GPS) in the months of February and July 2001. Despite uncertainties associated with repairs and possible reconstruction of points in the past century, the re-measurements reveal pre-seismic, co-seismic and post-seismic deformation related to Bhuj earthquake. More than 25 Μ-strain contraction north of the epicenter appears to have occurred in the past 140 years corresponding to a linear convergence rate of approximately 10 mm/yr across the Rann of Kachchh. Motion of a single point at Jamnagar 150 km south of the epicenter in the 4 years prior to the earthquake, and GTS-GPS displacements in Kathiawar suggests that pre-seismic strain south of the epicenter was small and differs insignificantly from that measured elsewhere in India. Of the 20 points measured within 150 km of the epicenter, 12 were made at existing GTS points which revealed epicentral displacements of up to 1 m, and strain changes exceeding 30 Μ-strain. Observed displacements are consistent with reverse co-seismic slip. Re-measurements in July 2001 of one GTS point (Hathria) and eight new points established in February reveal post-seismic deformation consistent with continued slip on the Bhuj rupture zone.

A SPATIAL STATISTICS APPROACH TO THE QUANTIFICATION OF FINITE STRAIN VARIATION IN PENETRATIVELY DEFORMED THRUST SHEETS : AN EXAMPLE FROM THE SHEEPROCK THRUST SHEET, SEVIER FOLD-AND-THRUST BELT, UTAH

Abstract Strain is an important component of the total displacement field in the emplacement of a thrust sheet. The finite strain tensor in a penetratively deformed thrust sheet is a spatial variable. I describe a method for quantitative estimation of the finite strain variation in thrust sheets by applying spatial statistics analysis on strain data collected from a part of the Sheeprock thrust sheet, in the southern Sheeprock Mountains and the West Tintic Mountains, north-central Utah. Strain was measured in the quartzites of the Sheeprock thrust sheet and the spatial statistics method is illustrated using the X Z strain axial ratios. The Sheeprock thrust sheet was penetratively deformed during Sevier-age fault propagation folding. I quantified finite strain from quartzites using the modified normalized Fry method and calculated the three-dimensional strain ellipsoid from the quartzites using three orthogonal thin-sections from each oriented field sample. The variation of finite strain in the Sheeprock thrust sheet was best represented by an exponential semivariogram model, which I used to predict values of strain from unsampled locations by ordinary kriging. Cross-validation showed that, in general, the predicted and measured values show good agreement (within 1% of each other). The sampled space was contoured using the measured and predicted strain values to obtain a detailed finite strain variation pattern in a part of the Sheeprock thrust sheet.

Analysis of the accuracy of Shuttle Radar Topography Mission (SRTM) height models using International Global Navigation Satellite System Service (IGS) Network

The Shuttle Radar Topography Mission (SRTM) carried out in February 2000 has provided near global topographic data that has been widely used in many fields of earth sciences. The mission goal of an absolute vertical accuracy within 16 m (with 90% confidence)/RMSE ∼10 m was achieved based on ground validation of SRTM data through various studies using global positioning system (GPS). We present a new and independent assessment of the vertical accuracy of both the X- and C-band SRTM datasets using data from the International GNSS Service (IGS) network of high-precision static GPS stations. These stations exist worldwide, have better spatial distribution than previous studies, have a vertical accuracy of 6 mm and constitute the most accurate ground control points (GCPs) possible on earth; these stations are used as fiducial stations to define the International Terrestrial Reference Frame (ITRF). Globally, for outlier-filtered data (135 X-band stations and 290 C-band stations), the error or difference between IGS and SRTM heights exhibits a non-normal distribution with a mean and standard error of 8.2 ± 0.7 and 6.9 ± 0.5 m for X- and C-band data, respectively. Continent-wise, Africa, Australia and North America comply with the SRTM mission absolute vertical accuracy of 16 m (with 90% confidence)/RMSE ∼10 m. However, Asia, Europe and South America have vertical errors higher than the SRTM mission goal. At stations where both the X- and C-band SRTM data were present, the root mean square error (RMSE) of both the X- and C-bands was identical at 11.5 m, indicating similar quality of both the X- and C-band SRTM data.

Accuracy analysis of the 2014–2015 Global Shuttle Radar Topography Mission (SRTM) 1 arc-sec C-Band height model using International Global Navigation Satellite System Service (IGS) Network

Global Shuttle Radar Topography Mission (SRTM) data products have been widely used in Earth Sciences without an estimation of their accuracy and reliability even though large outliers exist in them. The global 1 arc-sec, 30 m resolution, SRTM C-Band (C-30) data collected in February 2000 has been recently released (2014–2015) outside North America. We present the first global assessment of the vertical accuracy of C-30 data using Ground Control Points (GCPs) from the International GNSS Service (IGS) Network of high-precision static fiducial stations that define the International Terrestrial Reference Frame (ITRF). Large outliers (height error ranging from –1285 to 2306 m) were present in the C-30 dataset and 14% of the data were removed to reduce the root mean square error (RMSE) of the dataset from ∼187 to 10.3 m which is close to the SRTM goal of an absolute vertical accuracy of RMSE ∼10 m. Globally, for outlier-filtered data from 287 GCPs, the error or difference between IGS and SRTM heights exhibited a non-normal distribution with a mean and standard error of 6.5 ± 0.5 m. Continent-wise, only Australia, North and South America complied with the SRTM goal. At stations where all the X- and C-Band SRTM data were present, the RMSE of the outlier-filtered C-30 data was 11.7 m. However, the RMSE of outlier-included dataset where C- and X-Band data were present was ∼233 m. The results suggest that the SRTM data must only be used after regional accuracy analysis and removal of outliers. If used raw, they may produce results that are statistically insignificant with RMSE in 100s of meters.

Uncertainties in the Shuttle Radar Topography Mission (SRTM) Heights: Insights from the Indian Himalaya and Peninsula

The Shuttle Radar Topography Mission (SRTM) Digital Terrain Elevation Data (DTED) are used with the consensus view that it has a minimum vertical accuracy of 16 m absolute error at 90% confidence (Root Mean Square Error (RMSE) of 9.73 m) world-wide. However, vertical accuracy of the data decreases with increase in slope and elevation due to presence of large outliers and voids. Therefore, studies using SRTM data “as is”, especially in regions like the Himalaya, are not statistically meaningful. New data from ~200 high-precision static Global Position System (GPS) Independent Check Points (ICPs) in the Himalaya and Peninsular India indicate that only 1-arc X-Band data are usable “as is” in the Himalaya as it has height accuracy of 9.18 m (RMSE). In contrast, recently released (2014–2015) “as-is” 1-arc and widely used 3-arc C-Band data have a height accuracy of RMSE 23.53 m and 47.24 m and need to be corrected before use. Outlier and void filtering improves the height accuracy to RMSE 8 m, 10.14 m, 14.38 m for 1-arc X and C-Band and 3-arc C-Band data respectively. Our study indicates that the C-Band 90 m and 30 m DEMs are well-aligned and without any significant horizontal offset implying that area and length computations using both the datasets have identical values.

Strain variation in Fold-and-Thrust belts implications for construction of retrodeformable models

Deformation in fold-and-thrust belts such as the Himalayas can be represented by the displacement vector field. The strain component of the displacement vector field across the fold-and-thrust belt varies from near zero in external thrust sheets to a significant part of the field in internal thrust sheets. In addition, strain exhibits three-dimensional patterns in parts of internal sheets, near fault zones, and in the overturned limbs of fault-related folds due to superposition of penetrative-strain producing deformation events. This paper examines superposition of these strain producing deformation events in some detail and points out situations in fold-and-thrust belts wherein the finite strain becomes three-dimensional. This suggests that the plane-strain assumption used in the construction of retrodeformable models of fold-and-thrust belt evolution breaks down in these situations and the models lose their validity. Therefore, current techniques used for construction of retrodeformable models in fold-and-thrust belts need to be modified and three-dimensional models which include three-dimensional finite and incremental strain data need to be constructed for an accurate study of the evolution of geometry and kinematics in fold-and-thrust belts.