Horizontal Shortening Rate Research Articles

Disagreements in Geodetically Inferred Strain Rates in the Western US With Stress Orientations and Geologic Moment Rates

AbstractI employ an elasticity‐based method to invert a geodetically derived surface velocity field in the western US using for present‐day surface strain rate fields with uncertainties. The method uses distributed body forces in a thin elastic sheet and allows for discontinuities in velocity across creeping faults using the solution for dislocations in a thin elastic plate. I compare the strain rate fields with previously published stress orientations and moment rates from geological slip rate data and previous geodetic studies. Geologic and geodetic moment rates are calculated using slip rate and off‐fault strain rates from the 2023 US National Seismic Hazard Model (NSHM) deformation models. I find that computed total geodetic moment rates are higher than NSHM summed moment rates on faults for all regions of the western US except the highest deforming rate regions including the Western Transverse Ranges and the northern and southern San Andreas Fault (SAF) system in California. Computed geodetic moment rates are comparable to the moment rates derived from the geodetically based NSHM deformation models in all regions. I find systematic differences in orientations of maximum horizontal shortening rate and maximum horizontal compressive stress in the Pacific Northwest region and along much of the SAF system. In the Pacific Northwest, the maximum horizontal stress orientations are rotated counterclockwise 40–90° relative to the maximum horizontal strain rate directions. Along the SAF system, the maximum horizontal stresses are rotated systematically 25–40° clockwise (closer to fault normal) relative to the strain rates.

Fault geometry and slip rates from the Nullarbor and Roe Plains of south‐central Australia: Insights into the spatial and temporal characteristics of intraplate seismicity

AbstractAnalysis of TanDEM‐X and Shuttle Radar Topography Mission (SRTM) data reveals geomorphic evidence for 292 fault‐propagation fold scarps across the Miocene Nullarbor and Pliocene Roe Plains in south‐central Australia. Vertical displacements (VD) are determined using topographic profiling of a subset (n = 48) of the fold traces. Fault dips (mean = 44 +16/−14° at 1σ) are estimated from seismic reflection data; the mean dip is assigned to faults with unknown dip and combined with VD to estimate net displacements (ND) and average net displacements (AD) for each fault. AD exceeds single‐event displacements estimated from fault‐length scaling regressions, indicating the identified faults have hosted multiple earthquakes. Combining AD with (i) faulted surface ages (Nullarbor ~10–5 Ma, Roe ~2.5 Ma), (ii) ages of faulted erosional–depositional features (e.g. relic Late Miocene dune fields and Pliocene paleochannels), and (iii) onset of the neotectonic regime in Australia at ~10 Ma yields average slip rates from <0.1 m Myr−1to >17 m Myr−1(mean = 1.1 m Myr−1). Summation of displacements across faults yields crustal horizontal shortening rates lower than geodetically detectable resolution (≤0.01 mm yr−1) since the Late Miocene. The ca. 10 Myr‐long record of neotectonic faulting on the Nullarbor Plain provides important insights into earthquake spatial–temporal behaviours in a slowly deforming intraplate continental region.

Late Quaternary Tectonic Activity and Slip Rates of Active Faults in the Western Hexi Corridor, NW China

The Yinwashan and Xinminpu faults are located in the Jiuxi Basin in the western end of the Hexi Corridor. The determination of their activity and slip rates is of great significance for understanding the eastward extension of the Altyn Tagh fault. Based on geological and geomorphologic field survey, trench excavation, optically stimulated luminescence dating, we define the fault geometry and kinematic properties of the two faults. Based on fault scarps measurement using differential GPS and 10Be surface exposure dating, we determined vertical slip rate of 0.09±0.01 mm/yr for the Yinwashan fault and 0.1±0.02 mm/yr for the Xinminpu fault. Using the dips observed in trenches and natural sections, we estimated horizontal shortening rates of 0.05±0.03 and 0.23±0.06 mm/yr, respectively. No significant strike slip motion is observed on these two faults, and we infer that this region was dominated by horizontal shortening in the Late Quaternary. Although the shortening rate is quite low on each individual fault, together with other faults in this area, these two faults have an essential role in transferring slip from the eastern end of the Altyn Tagh fault and in accommodating the northeastward growth of Tibetan Plateau.

Active present faults of the Western segment of the Qilian Mountains (Northern Tiber)

The Qilian Mountains, as the northeastern margin of the Tibetan Plateau, absorbed the crustal shortening and accommodated the left-lateral displacement of the Altun Tagh fault. Detailed geomorphologic study of river valleys on the northern margin of the Qilian Mountains showed that since the late Pleistocene the crustal uplift rate of the northern Qilian Mountains has been greater than the central part. Due to the extension of the Tibetan plateau between the Changma fault and the Yumen fault the latest belt of faults and folds was formed on the northern margin of the Qilian Mountains. The study of the height of river terraces over the past 60 thousand years shows that the rate of vertical displacement along the Changma fault is 0,31±0,06 mm/a and its horizontal crustal shortening rate is 0,11±0,02 mm/a. The rate of vertical displacement along the northernmost Yumen fault is 0,33±0,02 mm/a and its horizontal crustal shortening rate is 0,53±0,03 mm/a. Active faults in the western segment of the northern Qilian Mountains account for 12% of the total crustal shortening in the Qilian Mountains. In addition, the crustal shortening rate of faults in the northern Qilian Mountains is much greater than the crustal shortening rate of faults inside the Qilian Mountains, which further indicates that since the Late Pleistocene the crustal uplift rate of the northern Qilian Mountains has been greater than the central Qilian Mountains.

Regional relative tectonic activity of structures in the Pampean flat slab segment of Argentina from 30 to 32°S

The Pampean flat slab segment of the Central Andes in Argentina is a broad, active deformation zone subject to both plate boundary-related and intraplate deformation. To the west, plate boundary-related deformation is dominated by thin-skinned fold-and-thrusts of the Precordillera, while to the east, intraplate deformation involves thick-skinned reverse faults of the Sierras Pampeanas. GPS data and absolute dating of displaced geomorphic features suggest that most of the active permanent deformation occurs between these two regions at the Andean orogenic front and that there is a west-to-east trend of decreasing shortening rates. However, GPS stations and sites with geomorphic slip rate measurements are currently too sparse and cover too short a time period to make reliable inferences about the longer-term distribution and relative rates of deformation. Mountain range-scale geomorphic indices, on the other hand, reflect tectonic activity since at least the Pleistocene. In this study, we measured geomorphic indices (hypsometric integral and curves, basin elongation ratio, basin volume-to-area-ratio, valley floor width-to-height ratio, mountain front sinuosity, and normalized steepness indices of rivers) from 10 different N-S striking, range-bounding faults that span both the Precordillera and Sierras Pampeanas regions from west to east. We use these data to assess the regional relative tectonic activity of major Quaternary deformation features in the Pampean segment of the Central Andes. Even when variations in climate and geology are considered, mean values for each geomorphic index suggests uplift rates for the 10 mountain-range bounding faults display local trends that are closely associated with the structural style and tectonic evolution of these structures. Although vertical uplift rates vary little, horizontal shortening rates are higher for the Precordillera fold-and-thrusts in the west than the thick-skinned reverse faults of the Sierras Pampeanas in the east, due to shallower fault dips for Precordilleran structures. The similarity with decadal slip rate gradients from GPS studies could possibly indicate that the stress-field has been constant since at least the Pleistocene, but further slip rate studies over broader time intervals in the late Quaternary are necessary to confirm this. Variations observed in relative uplift rates along each fault point to sites that warrant further detailed study of slip rates and evaluation of associated seismic hazards.

Abstract #4325 Anti-inflammatories can prevent but not treat (reverse) tumour-induced cognitive impairment in mice

In this paper, we present the first estimate of the Holocene deformation along the southern front of Gibraltar arc (Morocco) and the first field constraints on the local 1755 CE Fes-Meknes surface rupturing earthquake which could be associated to the “Great Lisbon Earthquake” (M > 8.5) in November 1st, 1755. Using satellite imagery, aerial photographs and field investigations, we carried out a morphotectonic study along the ~ 150 km-long Southern Rif Front (SRF) to identify the most recent evidences of tectonic activity. Analyzed offset alluvial deposits confirm that (i) the last ~ 5 ka cumulative deformation leading to a slip rate of ~ 3.5 ± 1 mm/yr for this segment of the SRF is consistent with the GPS derived horizontal shortening rate of 2–4 mm/yr and (ii) a recent major earthquake ruptured a ~ 30 km-long segment along the SRF. Based on deposits dating and historical seismicity we propose that this seismic event occurred in 1755 as a local earthquake. Even though this 1755 local event cannot be considered as a strong aftershock of the main Lisbon seismic event (M > 8.5), their temporal closeness, their occurrence under the same convergent stress regime (~ NNW-SSE-oriented compression) and the fact that Fes-Meknes area was strongly shaken during the Lisbon earthquake, raises the question of the possible triggering of the Fes earthquake. Anyway, our new results suggest that most of the Nubia-Rif belt convergence is accommodated by the SRF, making it potentially the most destructive structure of the Rif.

A constant slip rate for the western Qilian Shan frontal thrust during the last 200 ka consistent with GPS-derived and geological shortening rates

Active thrust faulting at the front of the Qilian Shan accommodates the northeastward growth of the Tibetan Plateau, however, the lifespan of individual faults and their slip history on different timescales remain largely unknown. Here, we show that the main range-bounding thrust fault of the western Qilian Shan has accrued tectonic slip at an almost constant rate during the last ∼200 ka, and possibly since fault initiation in the mid-Miocene. Our finding is based on 10Be ages from a flight of five deformed fluvial terraces along the Hongshuiba river, which constrain the vertical slip rate of the Qilian Shan frontal thrust to be 1.2±0.1 m/ka during the last 200 ka. With a fault dip of 30±5° constrained by seismic reflection data, we obtain a horizontal shortening rate of 2.0±0.3 m/ka. This value is consistent with both the short-term shortening rate of 1.7±0.3 mm/a derived from GPS data and the long-term shortening rate of 2.1±0.4 m/ka, which is based on a balanced geological cross-section. The latter provides a total shortening estimate of 25±3 km since the thrust fault initiated 12±2 Ma ago. The agreement between the shortening rates on the range of timescales between 100 and 107 years suggests that the western Qilian Shan frontal thrust has slipped at a steady rate since its initiation and implies that this fault is the main structure responsible for the growth of the western Qilian Shan.

Late Quaternary evolution of the Kumkol Basin at the northeastern margin of the Tibetan Plateau revealed by tectonic geomorphology and the analysis of in situ cosmogenic nuclides

The Tibetan Plateau was formed by the collision between the Indian and Eurasian plates, and it continues to grow laterally while keeping its elevation constant. The mechanism of the lateral expansion at the northeastern margin of the plateau, however, is highly debated owing to the scarcity of tectonic research. We conducted tectonic geomorphological analyses in the Kumkol Basin, where lateral growth is ongoing. This basin encloses a large active anticlinorium, the Kumkol Anticlinorium (KA), which would provide a quantitative constraint on the lateral growth of the plateau if the tectonically deformed landforms were dated. Our geomorphological mapping revealed six terrace steps, designated T1 to T6 in descending order, along the Pitileke River. In situ cosmogenic radionuclide concentrations in subsurface and surface samples from T1, T2, T3, T4, and T6 showed their formation ages to be 252 ± 24 ka, 140–160 ka, 103 ± 23 ka, 98 ± 11 ka, and 0–23 ka, respectively. Thus, T1 formed during marine isotope stage (MIS) 8, T2 during MIS 6, and T3 and T4 during MIS 5. Our results are quantitative evidence that degradation–aggradation cycles in the basin are correlated with global climatic fluctuations. To estimate the geometry of faults contributing to the evolution of the anticlinorium, we used a dislocation fault model and the overall surface deformation across the KA. The results suggest that the KA has been produced principally by slip on a detachment fault at 15 to 10 km depth, which is within or just above the brittle–ductile transition zone. We then determined the maximum uplift rate to be ~1 mm/yr and the horizontal shortening rate to be 2.5–3.2 mm/yr from the formation ages of the terraces and their deformation. These results suggest that there is a large-scale detachment fault below a basement-involved fold-and-thrust belt and that the concentration of stress at the plateau margin likely facilitated the development of crustal-scale deformation underlying the Kumkol Basin, and this deformation then accelerated the lateral growth of the Tibetan Plateau.

Latest Pleistocene to Holocene Thrusting Recorded by a Flight of Strath Terraces in the Eastern Qilian Shan, NE Tibetan Plateau

AbstractAt the eastern Qilian Shan mountain front in the NE Tibetan Plateau, the Minle‐Damaying Fault (MDF), the southernmost fault of the North Frontal Thrust (NFT) system, has previously been proposed as an inactive structure during the Holocene. Here we present a detailed record of six strath terraces of the Xie River that document the history of active deformation of the MDF. One optically stimulated luminescence dating sample constrains abandonment of the highest terrace T6at 12.7 ± 1.4 ka. The formation ages of the lower terraces (T4–T1) are dated by AMS14C dating. The cumulative vertical offsets of the MDF recorded by these terraces are determined as 12.2 ± 0.4 m (T6), 8.0 ± 0.4 m (T5), 6.4 ± 0.4 m (T4), 4.6 ± 0.1 m (T3), and 3.2 ± 0.2 m (T1c) by an unmanned aerial vehicle system, respectively. A long‐term vertical slip rate of the MDF of 0.9 ± 0.2 mm/yr is then estimated from the above data of terrace age and vertical offset by a linear regression. Assuming that the fault dip of 35 ± 5° measured at the surface is representative for the depth‐averaged fault dip, horizontal shortening rates of 0.83–1.91 mm/yr are inferred for the MDF. Our new data show that the proximal fault (the MDF) of the NFT system at the eastern Qilian Shan mountain front has remained active when the deformation propagated basinward, a different scenario from that observed at both the western and central Qilian Shan mountain front.

Re-evaluating seismic hazard along the southern Longmen Shan, China: Insights from the 1970 Dayi and 2013 Lushan earthquakes

Competing hypotheses have been proposed to explain the seismic hazard (i.e. whether earthquakes with M≥7 occur) of the southern Longmen Shan (LMS). This region did not rupture during the 2008 Mw 7.9 Wenchuan earthquake, but later generated the 2013 Mw 6.6 Lushan earthquake. Currently, the maximum possible earthquake magnitude, its average recurrence interval, and the seismogenic structure of the southern LMS, remain poorly documented. This study aims to re-evaluate seismogenic structures and seismic hazard along the southern LMS. We first describe the sub-surface structural geometry, as well as the total slip and Quaternary activity of the Range Front blind thrust (RFBT), using high-resolution seismic reflection profiles, borehole data, and intensity-derived macroscopic epicenters. This thrust, which generated the 1970 Ms 6.2 Dayi and 2013 Mw 6.6 Lushan earthquakes, extends for >250km along the LMS range front. Integrating new evidence of active faulting and folding and previous quantitative horizontal shortening rate results, we estimate that the Quaternary slip rate of the RFBT is nearly 1mm/yr, with a minimum total slip of 5km since 8–5Ma. Furthermore, we calculate the accumulation rate of seismic moment, 8.04 (±2.09)×1017N·m/yr, for the main active thrusts on the southern LMS, to compare with the moment release rate for earthquakes in the region. When we combine this with the Gutenberg–Richter distribution obtained from historical and instrumental catalogs, we estimate that the potential maximum moment magnitude of an earthquake in the southern LMS is 7.7. Finally, we conclude that the entire southern LMS is capable of generating much larger earthquakes (Mw 7.3–7.7) than seen recently, with an average frequency of once every 1000–1400years. Therefore, our findings confirm that there is potential for large earthquakes in the southern LMS, especially on the RFBT, which extends beneath the densely populated Chengdu Plain.

Which fault destroyed Fes city (Morocco) in 1755? A new insight from the Holocene deformations observed along the southern border of Gibraltar arc

In this paper, we present the first estimate of the Holocene deformation along the southern front of Gibraltar arc (Morocco) and the first field constraints on the local 1755CE Fes-Meknes surface rupturing earthquake which could be associated to the “Great Lisbon Earthquake” (M>8.5) in November 1st, 1755. Using satellite imagery, aerial photographs and field investigations, we carried out a morphotectonic study along the ~150km-long Southern Rif Front (SRF) to identify the most recent evidences of tectonic activity. Analyzed offset alluvial deposits confirm that (i) the last ~5ka cumulative deformation leading to a slip rate of ~3.5±1mm/yr for this segment of the SRF is consistent with the GPS derived horizontal shortening rate of 2–4mm/yr and (ii) a recent major earthquake ruptured a~30km-long segment along the SRF. Based on deposits dating and historical seismicity we propose that this seismic event occurred in 1755 as a local earthquake. Even though this 1755 local event cannot be considered as a strong aftershock of the main Lisbon seismic event (M>8.5), their temporal closeness, their occurrence under the same convergent stress regime (~NNW-SSE-oriented compression) and the fact that Fes-Meknes area was strongly shaken during the Lisbon earthquake, raises the question of the possible triggering of the Fes earthquake. Anyway, our new results suggest that most of the Nubia-Rif belt convergence is accommodated by the SRF, making it potentially the most destructive structure of the Rif.

Late Pleistocene slip rate of the northern Qilian Shan frontal thrust, western Hexi Corridor, China

AbstractWe derive a slip rate for a frontal thrust in the western Hexi Corridor along the northern Qilian Shan by combining topographic profiling and 10Be exposure dating. The active Yumen‐Beidahe thrust fault offsets late Pleistocene alluvial‐fan deposits, and a prominent north‐facing scarp is well preserved. To quantify the slip rate, we surveyed the uplifted terraces and sampled quartz‐rich pebbles on terrace surfaces and river channels to determine surface exposure ages and pre‐depositional inheritance. The minimum vertical slip rate of the fault is 0.73 ± 0.09 mm a−1. This represents a horizontal shortening rate of 1.26 ± 0.31 mm a−1 for a fault dip of 30 ± 5°. This estimated slip rate supports the inference made from previous geological and GPS constraints that NNE‐directed shortening across the western Qilian Shan and the Hexi Corridor is distributed on several active faults with a total shortening rate of 4–10 mm a−1.

Quaternary activity of the range front thrust system in the Longmen Shan piedmont, China, revealed by seismic imaging and growth strata

Reliable estimates of Quaternary or Cenozoic upper crustal shortening in the Longmen Shan fold-and-thrust belt are rare. In this paper, we report on our use of high-resolution 2-D and 3-D seismic reflection profiles at various scales, together with borehole data, to investigate the structural geometry of the Longmen Shan piedmont. The results reveal a thrust system beneath the Longmen Shan, termed the range front thrust system, which consists of the range front blind thrust and its upward splay faults. Moreover, on these faults we identified growth strata that provide an excellent opportunity for assessing the activity of this thrust system. Analyses of the growth strata reveal early to late Pleistocene activity on the range front blind thrust, with minimum dip slip and horizontal shortening rates of 1.1 ± 0.2 mm/yr and ~1 mm/yr. Accordingly, the maximum accumulated slip on the range front blind thrust is calculated to be 7.5 ± 0.3 km in the Longmen Shan. Using the new horizontal shortening rate and other published data, we also estimated that the long-term shortening rate across the Longmen Shan fold-and-thrust belt is 1–3 mm/yr, which is comparable to the short-term GPS rate. The similarity of these rates suggests that the Longmen Shan attained a steady state condition over the past 2 Myr. An additional highlight of our results is that we show Quaternary activity around the Tibetan Plateau to have been nearly synchronous in different regions, including in the Longmen Shan, the Himalayas, the western Kunlun Shan, and the northern Qilian Shan.

Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin

Research Article| September 01, 2015 Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin Gabriel Veloza; Gabriel Veloza † 1Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA †E-mail: gvelozaf@ku.edu. Search for other works by this author on: GSW Google Scholar Michael Taylor; Michael Taylor 1Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA Search for other works by this author on: GSW Google Scholar Andrés Mora; Andrés Mora 2Ecopetrol–Instituto Colombiano Del Petroleo, Km7 Via Bucaramanga-Piedecuesta, Piedecuesta-Santander, Colombia Search for other works by this author on: GSW Google Scholar John Gosse John Gosse 3Department of Earth Sciences, Dalhousie University, Halifax B3H 4R2, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2015) 127 (9-10): 1155–1173. https://doi.org/10.1130/B31168.1 Article history received: 18 Jul 2014 rev-recd: 18 Dec 2014 accepted: 26 Jan 2015 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gabriel Veloza, Michael Taylor, Andrés Mora, John Gosse; Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin. GSA Bulletin 2015;; 127 (9-10): 1155–1173. doi: https://doi.org/10.1130/B31168.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Active crustal shortening and surface uplift are vividly expressed in the foothills of the Eastern Cordillera of Colombia. Quaternary alluvial-fan and terrace deposits are folded and uplifted tens to hundreds of meters above the local base level, recording deformation and incision of the landscape. We used geomorphic observations and structural analysis from a 300 km2 three-dimensional seismic-reflection survey over an actively growing anticline to investigate the relationship between finite shortening and surface uplift, and its effects on landscape evolution. A quantitative description of finite shortening of the pregrowth strata from excess-area and line-length calculations were compared with uplift and shortening estimates obtained from geomorphic strain markers dated using in situ 10Be cosmogenic nuclide depth profiles. This combined approach provides the first estimates of Quaternary surface uplift and horizontal shortening rates along the foothills of the northeastern Colombian Andes. Finite shortening for the Tame anticline was calculated using two different approaches. Excess-area shortening varies from 530 ± 36 m (2σ) at its southern end, decreasing northward to 404 ± 72 m (2σ), whereas line-length shortening is typically an order of magnitude smaller. Using the maximum horizontal shortening, our results for the Tame anticline reveal a Quaternary uplift rate of 2.8 ± 1.5 mm/yr, and a shortening rate of 2.1 ± 1.2 mm/yr (2σ), with the older terraces located at the highest elevations and recording the highest rotation from the horizontal, indicating growth by limb rotation. Based on these rates, the initiation of folding is estimated to be 0.25 +0.09/–0.05 Ma (2σ). We hypothesize that the discrepancy between shortening estimates from line-length and excess-area calculations can be reconciled by accounting for internal deformation, likely developed during the initial stages of folding, which can be represented by strain hardening, for example. The shortening rates we obtain are comparable to geodetically derived shortening rates but are slightly lower than long-term geologic shortening rates. Slower shortening rates and the presence of a youthful structure at the thrust front suggest that the orogen has been migrating eastward since the late Pleistocene, probably due to reorganization of a supercritical thrust wedge farther west in the hinterland, to attain critical taper. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Drainage migration and out of sequence thrusting in Bhalukpong, Western Arunachal Himalaya, India

In the foreland regions of the Western Arunachal Himalaya (WAH), geological studies along the Kameng river (between Tipi village and the Himalayan Frontal Thrust (HFT)) reveal four levels of unpaired terraces and a paired terrace. In WAH, wrench deformation of HFT zone resulted in a SE propagation of the Balipara anticline and it is suggested that the Mikir high basement controls its orientation. Ages of terrace surfaces from Siwaliks suggest that since the Late Pleistocene, Kameng River migrated at a rate varying between ∼7.5cm/yr in upper reaches and ∼13.5cm/yr towards northeast due to HFT related uplift. In the Brahmaputra plains, luminescence ages of abandoned paleochannel deposits suggest eastward shifting of the Kameng river at an average rate of ∼1m/yr. Field evidences between Bhalukpong and Tipi villages show Pliocene strath and Quaternary terrace surfaces, displaced by faults that do not correspond to the mapped faults in the foreland region. We interpret them as out-of-sequence thrusts (OOSTs). This is the first such report of OOST in the NE Himalaya. Presence of active OOST is inferred by similar age (∼1ka) and differing incision rates of the surface of same terrace (T2b) in adjacent locations. This suggests that OOSTs in the western Arunachal Siwalik are <1ka. Average slip rate and horizontal shortening rate on OOST during the Holocene, are calculated as ∼12mm/yr and 7mm/yr respectively. Thus any estimation of Holocene shortening in the Siwalik therefore, needs to incorporate slip along the OOSTs given that it accommodates a significant amount of N-S compression of the Himalayan fold-and-thrust belt. The reason for OOST in the WAH Siwalik foreland is discussed in terms of the critical wedge dynamics arising from erosion via tectonics-climate interaction. We estimate a minimum slip rate of Siwalik as ∼27mm/yr during the Holocene and suggest acceleration in shortening rates east of Bhutan.

Variable shortening rates in the eastern Himalayan thrust belt, Bhutan: Insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions

We present data on the burial, displacement and exhumation history of the Himalayan fold‐thrust belt in eastern Bhutan. These data document the magnitude and timing of displacement of large, discrete structures and highlight temporal variability in shortening rates. Eight new40Ar/39Ar ages from white mica, 32 new zircon (U‐Th)/He ages, 7 new apatite fission track ages, and 1 new U‐Pb zircon (LA‐MC‐ICP‐MS) metamorphic rim growth age are combined with published cooling ages and deformation temperatures, and incremental shortening magnitudes from restorations of two published balanced cross sections, to illustrate the kinematic and temporal development of the Bhutan thrust belt. Integrating these data from ∼23 Ma to the present illustrates rapid horizontal shortening rates (28–35 mm/yr) between 23–20 Ma and 15–10 Ma, separated by more moderate rates (10–23 mm/yr). Shortening rates decrease significantly to 7–10 mm/yr (and possibly as low as 3–4 mm/yr) from 10 to 0 Ma. This decrease is interpreted to represent the onset of strain partitioning in the eastern part of the Himalayan‐Tibetan orogenic system, between shortening in the Bhutan thrust belt, uplift of the Shillong Plateau, and deformation and outward growth of the northern and eastern Tibetan Plateau. Within estimated error, horizontal shortening rates during emplacement of the Main Central thrust sheet and during construction of the upper Lesser Himalayan duplex approached India‐Asia tectonic velocities. Thus, for periods of time between ∼23–20 Ma and ∼15–10 Ma, the Bhutan thrust belt may have absorbed nearly all India‐Asian convergence at this longitude.

Equivalency of geologic and geodetic rates in contractional orogens: New insights from the Pamir Frontal Thrust

Across contractional orogens, the equivalency between decadal convergence rates from geodetic GPS data and geologic shortening rates at time scales of thousands or millions of years has rarely been documented. Here, we present an example from the northern margin of Chinese Pamir, where the Main Pamir Thrust is tectonically quiescent, and recent deformation is concentrated on the Pamir Frontal Thrust (PFT). Based on dated and faulted fluvial terraces, magnetostratigraphy, and mapping, the horizontal shortening rate of the PFT is ∼6–7 mm/a at time scales of both ∼18.4 ka and ∼0.35 Ma, comparable to the geodetic rate of ∼6–9 mm/a across the same zone, implying that modern geodetic rates are a reasonable proxy for geologic rates since ∼0.35 Ma. Comparing this example with studies in other contractional orogens, we conjecture that a match or mismatch of geologic‐geodetic rates typically depends on the time scale of observation, fault geometry, and fault mechanics.

The age and rate of displacement along the Main Central Thrust in the western Bhutan Himalaya

In western Bhutan, the Main Central Thrust (MCT) is broadly folded, creating multiple exposures of the fault surface over a ~70km across-strike distance. This unusual map pattern presents a unique opportunity to map the MCT and document both the magnitude and age of displacement. In situ Th-Pb (SIMS and LA-ICP-MS) geochronology of metamorphic monazite from the immediate hanging wall of the MCT indicates that prograde monazite growth in Greater Himalayan (GH) rocks continued until 20.8±1.1Ma, whereas crystallization of in situ melts, characterized by high Y monazite overgrowths, occurred during cooling from ca. 15–10Ma. Prograde monazite growth at 15Ma in Lesser Himalaya (LH) rocks in the immediate footwall requires that LH footwall strata began to be buried at this time, and the MCT had reached its southernmost, exposed extent. By combining prograde monazite ages in the immediate hanging wall and footwall, the duration of MCT displacement is bracketed between 20.8±1.1 and 15.0±2.4Ma. Immediately north of our study area, a published estimate of shearing along the outer-South Tibetan detachment (STD) argues for displacement between 20 and 15Ma, coeval with the age range for MCT displacement that we document in this study. However retrograde monazite grains as young as 10Ma suggest that GH rocks were cooling until ~10Ma, 5Myr later than motion on the outer-STD immediately to the north. This cooling was either the result of continued displacement on the MCT, or growth of a duplex that passively folded the MCT. Using a sequential reconstruction, we estimate a total displacement of ~230km, which is the sum of displacements on the MCT and the structurally-lower Paro Thrust, over a duration of 5.8±2.6Myr. This indicates a horizontal shortening rate of 4.0+3.2/−1.3cm/yr, which exceeds present rates estimated from geodetic measurements across the Himalaya, and MCT displacement rates (c. 2cm/yr) inferred from petrologic and thermal models in central Nepal but is indistinguishable from plate convergence rates calculated for eastern Bhutan between 23 and 20Ma (3.3±0.7cm/yr). Our study highlights that displacement on the MCT alone achieved plate velocity rates in western Bhutan, and that the age and rate of MCT displacement varied significantly across the Himalaya.

Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank

The Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region. A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception.

Numerical modeling of neotectonic movements and state of stresses in the central Seismic Gap region, Garhwal Himalaya

This paper presents finite element modeling (FEM) to simulate the present-day stress field and crustal deformation using NE-SW structural section in the central Seismic Gap region of the Garhwal Himalaya. Our study deals with the effect of geometrical characteristics and rock layer parameters on the upper crust. Modeling results show that two types of tectonic regimes developed in the central Seismic Gap region: the geotectonics of the northern part has been controlled by regional compression, whereas southern part is characterized by regional extension. Correspondingly, thrust faults are induced in the northern part and normal faults are extensively developed in the southern front. Those evidences noticeably indicate that the compressive tectonic environment of the Himalaya becomes change into the extensional tectonic regime in its front. The computed shear stress accumulation along the northern flat of Main Himalayan Thrust (MHT) implies that considerable amount of interseismic stress is building up along the MHT system in the Himalaya, which ultimately release through the possible future great Himalayan earthquake (M > 8). The comparison between our modeled stress field, faulting pattern and horizontal shortening rate with the distribution of the microseismic events, focal mechanism solutions, active faulting and GPS data in the central Seismic Gap region shows good agreement.