Active tectonics of Himalayan Frontal Fault system

In the Sub-Himalayan zone, the frontal Siwalik range abuts against the alluvial plain with an abrupt physiographic break along the Himalayan Frontal Thrust (HFT), defining the present-day tectonic boundary between the Indian plate and the Himalayan orogenic prism. The frontal Siwalik range is characterized by large active anticline structures, which were developed as fault propagation and fault-bend folds in the hanging wall of the HFT. Fault scarps showing surface ruptures and offsets observed in excavated trenches indicate that the HFT is active. South of the HFT, the piedmont zone shows incipient growth of structures, drainage modification, and 2–3 geomorphic depositional surfaces. In the hinterland between the HFT and the MBT, reactivation and out-of-sequence faulting displace Late Quaternary–Holocene sediments. Geodetic measurements across the Himalaya indicate a ~100-km-wide zone, underlain by the Main Himalayan Thrust (MHT), between the HFT and the main microseismicity belt to north is locked. The bulk of shortening, 15–20 mm/year, is consumed aseismically at mid-crustal depth through ductile by creep. Assuming the wedge model, reactivation of the hinterland faults may represent deformation prior to wedge attaining critical taper. The earthquake surface ruptures, ≥240 km in length, interpreted on the Himalayan mountain front through paleoseismology imply reactivation of the HFT and may suggest foreland propagation of the thrust belt.     

Plate tectonics and volcanism in the gibraltar arc

Late Tertiary and Quaternary volcanism of southeastern Spain can be fitted in a platetectonics model, taking into account the post-Paleozoic evolution of the stable and semimobile Iberian areas and the new orogenic belts bordering the Mediterranean between Africa and the Iberian Peninsula.The occurrence and distribution of calc-alkaline and potassic volcanism suggest an oceanic crust sinking downwards from the Iberian plate. This active margin is causally related to the convergence and collision of Iberia and Africa during Late Cretaceous—Early Miocene time span.A pre-collision distensive phase is inferred from the stratigraphie and tectonic record between the Triassic and Late Cretaceous, while since the Late Miocene another distensive phase is related to the actualistic features.     

Active tectonics in the Assam seismic gap between the meizoseismal zone of AD 1934 and 1950 earthquakes along eastern Himalayan front,India

The Assam Seismic Gap has witnessed a long seismic quiescence since the \({ Mw}{\sim }8.4\) great Assam earthquake of AD 1950. Owing to its improper connectivity over the last decades, this segment of the Himalaya has long remained inadequately explored by geoscientists. Recent geodetic measurements in the eastern Himalaya using GPS document a discrepancy between the geologic and geodetic convergence rates. West to east increase in convergence rate added with shorter time span earthquakes like the 1697 Sadiya, 1714 (\({ Mw}{\sim }8\)) Bhutan and 1950 (\({ Mw}{\sim } 8.4\)) Tibet–Assam, makes this discrepancy more composite and crucial in terms of seismic hazard assessment. To understand the scenario of palaeoearthquake surface rupturing and deformation of youngest landforms between the meizoseismal areas of \({ Mw}{\sim }8.1\) 1934 and 1950 earthquakes, the area between the Manas and Dhanshiri Rivers along the Himalayan Frontal Thrust (HFT) was traversed. The general deformation pattern reflects north-dipping thrust faults. However, back facing scarps were also observed in conjugation to the discontinuous scarps along the frontal thrust. Preliminary mapping along with the published literature suggests that, in the eastern Himalayan front the deformation is taking place largely by the thrust sheet translation without producing a prominent fault-related folds, unlike that of the central and western Himalayas.     

Some elements of continental subduction along the Himalayan front

Detachment tectonics, in which the subducting basement is not internally affected by thrusting; and a steady state system, in which the pattern of deformation and topography are invariant, uplift is balanced by erosion and the material can experience subsequent “phases” of deformation by migrating through the system, are proposed to be the fundamental mechanisms of convergence at the Himalayan arc. Both surface and subsurface data are more consistent with these concepts than with more familiar concepts widely used in current models: intracrustal thrusting and evolutionary tectonics, where superimposed phases of deformation are interpreted as distinct phases. The belt of intermediate-magnitude thrust-earthquakes, the topographic front at the High Himalaya and the Main Central Thrust (MCT) are associated with the same fundamental element, the basement thrust front (BTF), which traces a small circle in the central portion of the Himalayan arc. Fault plane solutions indicate thrusting in the radial direction of this arc. This radial convergence at the BTF implies that Tibet is extending laterally at a rate similar to the rate of convergence across the BTF. This extension cannot be unidirectional and must be parallel to the BTF, if the circular shape of the BTF is invariant.     

Upper crustal shortening and forward modeling of the Himalayan thrust belt along the Budhi-Gandaki River, central Nepal

A balanced cross-section along the Budhi-Gandaki River in central Nepal between the Main Central thrust, including displacement on that fault, and the Main Frontal thrust reveals a minimum total shortening of 400 km. Minimum displacement on major orogen-scale structures include 116 km on the Main Central thrust, 110 km on the Ramgarh thrust, 95 km on the Trishuli thrust, and 56 km in the Lesser Himalayan duplex. The balanced cross-section was also incrementally forward modeled assuming a generally forward-breaking sequence of thrusting, where early faults and hanging-wall structures are passively carried from the hinterland toward the foreland. The approximate correspondence of the forward modeled result to observe present day geometries suggest that the section interpretation is viable and admissible. In the balanced cross-section, the Trishuli thrust is the roof thrust for the Lesser Himalayan duplex. The forward model and reconstruction emphasize that the Lesser Himalayan duplex grew by incorporating rock from the footwall and transferring it to the hanging wall along the Main Himalayan thrust. As the duplex developed, the Lesser Himalayan ramp migrated southward. The movement of Lesser Himalayan thrust sheets over the ramp pushed the Lesser Himalayan rock and the overburdens of the Greater and Tibetan Himalayan rock toward the erosional surface. This vertical structural movement caused by footwall collapse and duplexing, in combination with erosion, exhumed the Lesser Himalaya.     

Expression of active tectonics in erosional landscapes

Understanding the manner and degree to which topography in active mountain ranges reflects deformation of the Earth's surface remains a first order goal of tectonic geomorphology. A substantial body of research in the past decade demonstrates that incising channel systems play a central role in setting relationships among topographic relief, differential rock uplift rate, and climatically modulated erosional efficiency. This review provides an introduction to the analysis and interpretation of channel profiles in erosional mountain ranges. We show that existing data support theoretical expectations of positive, monotonic relationships between channel steepness index, a measure of channel gradient normalized for downstream increases in drainage area, and erosion rate at equilibrium, and that the transient response to perturbations away from equilibrium engenders specific spatial patterns in channel profiles that can be used to infer aspects of the forcing. These aspects of channel behavior lay the foundation for a series of case studies that we use to illustrate how focused, quantitative analysis of channel morphology can provide insight into the spatial and temporal dynamics of active deformation. Although the complexities of river response to climate, lithology, and uplift patterns mean that multiple interpretations of topographic data alone will always possible, we show that application of stream profile analysis can be a powerful reconnaissance tool with which to interrogate the rates and patterns of deformation in active mountain belts.     

活动构造定量研究与应用

自上世纪 70年代末以来 ,活动构造研究已从描述性和定性研究阶段进展到定量研究阶段 ,并主要围绕 3个方面展开工作 :(1)如何刻画活动构造的活动特征 ,即需要得到哪些活动特征定量参数 ;(2 )如何得到这些定量参数 ;(3)如何应用这些参数进行地震危险性评价和工程安全性评价。文中就有关活动构造定量研究的这几个方面进行初步总结和分析 ,并指出在今后一段时间内 ,提高测年精度、减少研究过程各个环节的误差和不确定性 ,将是活动构造定量研究的主要发展方向。     

Normal faulting along the western side of the Matese Mountains: Implications for active tectonics in the Central Apennines (Italy)

We provide new field data from geologic mapping and bedrock structural geology along the western side of the Matese Mts in central Italy, a region of high seismicity, strain rates among the highest of the entire Apennines (4–5 mm/yr GPS-determined extension), and poorly constrained active faults. The existing knowledge on the Aquae Iuliae normal fault (AIF) was implemented with geometric and kinematic data that better constrain its total length (16.5 km), the minimum long-term throw rate (0.3–0.4 mm/yr, post-late glacial maximum, LGM), and the segmentation. For the first time, we provide evidence of post-350 ka and possibly late Quaternary activity of the Ailano – Piedimonte Matese normal fault (APMF). The APMF is 18 km long. It is composed of a main 11 km-long segment striking NW–SE and progressively bending to the E–W in its southern part, and a 7 km-long segment striking E–W to ENE-WSW with very poor evidence of recent activity. The available data suggest a possible post-LGM throw rate of the main segment of ≳0.15 mm/yr. There is no evidence of active linkage in the step-over zone between the AIF and APMF (Prata Sannita step-over).An original tectonic model is proposed by comparing structural and geodetic data. The AIF and APMF belong to two major, nearly parallel fault systems. One system runs at the core of the Matese Mts and is formed by the AIF and the faults of the Gallo-Letino-Matese Lake system. The other system runs along the western side of the Matese Mts and is formed by the APMF, linked to the SE with the Piedimonte Matese – Gioia Sannitica fault. The finite extension of the APMF might be transferred to the NW towards the San Pietro Infine fault. The nearly 2–3 mm/yr GPS-determined extension rate is probably partitioned between the two systems, with a ratio that is difficult to establish due to poor GPS coverage. The proposed model, though incomplete (several faults/transfer zones need further investigations), aids in the seismotectonic interpretation of poorly-known earthquakes (e.g., 346/355 AD earthquake on the Ailano – Piedimonte Matese – Gioia Sannitica fault system), and stimulates and further orients seismotectonic investigations aimed at constraining the segmentation pattern and seismogenic potential of the area.     

Role of transverse tectonics in the Himalayan collision: Further evidences from two contemporary earthquakes

Two contemporary earthquakes originating in the central Himalayan arc and its foredeep (Sikkim earthquake of 18.09.2011, M_w 6.9, h: 10–60 (?) km and Bihar-Nepal earthquake of 20.08.1988, Mw 6.8, h: 57 km) are commonly associated with transverse lineaments/faults traversing the region. Such lineaments/faults form active seismic blocks defining promontories for the advancing Indian Craton. These actually produce conjugate shear faulting pattern suggestive of pervasive crustal interplay deep inside the mountains. Focal mechanism solutions allow inferring that large part of the current convergence across the central Himalayan arc is accommodated by lateral slip. Similar slip also continues unabated in the densely populated foredeep for distances up to several tens of kilometers south of the Main Boundary Thrust (MBT).     

Temporal and spatial evolution of the coastal profiles along the Yellow River Delta over last three decades

The subaerial delta of the Yellow River has undergone a notable reshaping process with drastic changes in erosion and accretion patterns both temporally and spatially. These morphological changes not only have significant impact on the long-term evolution of delta but also pose severe threat to the existing infrastructures adjacent to the eroding delta lobe. This paper presents a data-based study of waves, currents, sediments and subaqueous bedforms including the application of Principal Component Analysis (PCA) to the measured coastal profiles in order to understand qualitative and quantitative processes that are responsible for the observed changes. The results demonstrated that the Yellow River Delta has recently been dominated by recession process due to the reduced sediment supply from the Yellow River in the recent decades. A critical threshold of river sediment discharge was formulated and used to explain the observed morphodynamic processes. It was found that the coast of the Yellow River Delta could be broadly divided into three geomorphic zones: the area near abandoned delta, the estuarine area, and Laizhou Bay. At the nearshore zone of the abandoned delta, waves are the main agent for bed erosion and sediments suspension. The tidal current is effective in transporting suspended sediments and exerts the main control on the depths of the erosion–accretion balance zone on the coastal profiles. The area near the present estuary has seen rapid deposition with the progradation rate being governed by the relative intensity of fluvial and marine processes while the coasts in Laizhou Bay are stable and the particular characteristics of the tidal current field prevent the sediments supplied by the Yellow River from reaching this area.     

The role of transcurrent intra-arc tectonics in the configuration of a volcanic arc

Field investigations show that Plio-Quaternary Mexican and Quaternary Ecuadorian volcanic arcs are coeval with intra-arc transcurrent or transtensional tectonics with motions along faults parallel or subparallel to the arc. This relationship can also be observed in the Quaternary Kamchatka arc. These arcs have an anomalously wide distribution of volcanic centres which cannot be explained simply by a low dip in the subducting lithosphere. Comparison with the intra-arc tectonics and geometry of other Quaternary arcs reveals that strike-slip fault zones, parallel to the arc, favour magma rising in a broad surficial zone and may channel magma feeding mainly huge stratovolcanoes.     

Thrust tectonics along the north-western continental margin of Sabah/Borneo

Widely accepted plate tectonic models suggest that an inactive subduction zone lies along the north-west continental margin of Sabah. In contrast, interpretation of reflection seismic data acquired by BGR shows an autochthonous continental terrane comprising an Oligocene to Early Miocene carbonate platform being progressively overthrust by an allochthonous rock complex. Progressive compression resulted in the development of four structural zones: Imbricated thrust sheets (Zone III); two thrust sheet systems one on top of the other (Zone IV); a complex zone with multiphase deformation (Zone V); and piercement ridges (Zone VI).

---

Zusammenfassung Nach herkömmlichen plattentektonischen Vorstellungen soll eine inaktive Subduktionszone am nordwestlichen Kontinentalrand von Sabah liegen. Reflexionsseismische Meßdaten der BGR zeigen jedoch, daß hier autochthone kontinentale Kruste mit einer oligozänen-frühmiozänen Karbonatplattform progressiv von einem allochthonen Gesteinsverband überschoben wird. Fortschreitender Zusammenschub seit dem frühen Miozän führte zur Anlage von vier Deformationszonen: Tekonische Schuppen (Zone III); zwei übereinander geschobene Verschuppungssysteme (Zone IV); Gürtel mit mehrphasiger Deformation (Zone V) und Durchspießungsstrukturen (Zone VI).

---

Résumé Les modèles géodynamiques que l'on admet habituellement comportent une zone de subduction inactive le long de la marge continentale nord-occidentale de Sabah. Toutefois, des mesures de sismique-réflexion exécutées par le BGR font apparaître qu'à cet endroit, une croûte continentale autochtone, comportant une plateforme carbonatée oligocène à éomiocène, est chevauchée progressivement par un complexe allochtone. La compression, qui s'est manifestée progressivement depuis le Miocène inférieur, a engendré quatre zones structurales: un ensemble d'écailles tectoniques (zone III); deux systèmes de lames tectoniques charriés l'un sur l'autre (zone IV); une zone complexe à déformation multiphasée (zone V); des structures d'extrusion tectonique (zone VI).

---

, - Sabah'a. , - . 4-? : ( 3), ( 4) ( 5) ( 6).

     

Exhumation of subducted continental crust along the arc region

The Kutná Hora crystalline complex (KHCC) in the Bohemian Massif is a HP/HT complex adjacent to the magmatic arc. It is dominated by migmatite, orthogneiss and granulite with bodies of eclogite and peridotite. The KHCC migmatite consists of K-feldspar, plagioclase, quartz, phengite, biotite, garnet and kyanite. Melting conditions were estimated at 780 °C and >16 kbar and inferred melt volume varies between 1 and 4 vol%. Peak temperature is 865 °C at 18–19 kbar followed by decompression in the presence of melt to 12–13 kbar and 770–800 °C. U-Pb monazite geochronology reveals a spread of ages between 550 Ma and 330 Ma. REE patterns show low Yb/Gd for 550–500 Ma, high Yb/Gd for ages at ~480 Ma, and decreasing Yb/Gd towards ~340 Ma. First monazite in equilibrium with garnet constrains the HP metamorphism to ~350 Ma, which is followed by recrystallization of monazite down to 325 Ma. U-Pb zircon geochronology displays an age range from ~670 Ma to ~430 Ma. The broad age range records a span of protolith crystallization and/or old metamorphism. The presence of HP ky + mu migmatite, their P–T path, protolith zircon and monazite metamorphic ages and whole-rock geochemistry are similar to HP migmatites in the Eger crystalline complex (ECC) in the Saxothuringian domain further in the west. We propose the following geodynamic scenario for subduction-relamination-exhumation mechanism: (i) subduction of the Saxothuringian continental lithosphere at 360 Ma related to early stage of trans-lithospheric diapirism triggered by arc-related magma weakening; (ii) large-scale emplacement of relaminant into the upper plate lithosphere at 350–340 Ma; and (iii) return flow of the relaminant along the subduction interface (the ECC) and emplacement of relaminant in the upper–middle crust in the rear part of the arc system (the KHCC) at 340–330 Ma.     

Earthquake source characteristics along the arcuate Himalayan belt: Geodynamic implications

The occurrences of moderate to large magnitude earthquakes and associated subsurface geological processes were critically examined in the backdrop of Indian plate obliquity, stress obliquity, topography, and the late Tertiary regional tectonics for understanding the evolving dynamics and kinematics in the central part of the Himalayas. The higher topographic areas are likely associated with the zones of depressions, and the lower topographic areas are found around the ridges located in the frontal part of the orogen. A positive correlation between plate and stress obliquities is established for this diffuse plate boundary. We propose that the zone of sharp bending of the descending Indian lithosphere is the nodal area of major stress accumulation which is released occasionally in form of earthquakes. The lateral geometry of the Himalayas shows clusters of seismicity at an angle of ～20^° from the centre part of the arc. Such spatial distribution is interpreted in terms of compression across the arc and extension parallel to the arc. This biaxial deformation results in the development of dilational shear fractures, observed along the orogenic belt, at an angle of ～20^° from the principal compressive stress axis.     

Identification of potential areas for the occurrence of strong earthquakes in Himalayan arc region

Morphostructural zoning (MSZ) scheme of the Himalayan arc region as obtained from a joint study of topographic, geological and tectonic maps as well as satellite imagery is analysed. Three types of morphostructures have been determined: territorial units (blocks of different ranks), linear zones limiting these blocks (lineaments) and intersections of the lineaments (knots). Comparison of MSZ scheme with the know seismicity indicates epicenters of strong earthquakes (M≥6·5) clustered around some of these knots. Pattern recognition method is used to determine seismically potential areas for the occurrence of recognition method is used to determine seismically potential, for the occurrence of strong earthquakes of magnitude ≥M ₀. We have carried out two such studies for the Himalayan arc region, one forM ₀=6·5 and the other forM ₀=7·0. Out of a total number of 97 knots, 48 knots are found to be seismically potential for the occurrence of earthquake ofM≥6·5. The results of the study forM ₀=6·5 were presented in the symposium on “Earthquake Prediction” held in Strasbourg, France, March 1991 (Gorshkovet al 1991). The epicenter of Uttarkashi earthquake of magnitude,M _b=6·6 that occurred in the late hours of 19th October 1991 (UTC) lies in the vicinity of one such knot. The second study carried out subsequently shows that only 36, knots are potential for the occurrence of earthquakes ofM≥7·0, which include the knot, associated with theUttarkashi earthquake.     

Development of ecosystem indicators for the Suwannee River estuary: Oyster reef habitat quality along a salinity gradient

The Suwannee River watershed is one of the least developed in the eastern United States, but with increasing urbanization it is facing potential long-term alterations in freshwater flow to its estuary in the Gulf of Mexico. The purpose of this study was to develop biological indicators of oyster reef state along a natural salinity gradient in the Suwannee River estuary in order to allow the rapid assessment of the effect of changing freshwater input to this system. Percent cover and density of three size classes of living oysters, as well as the abundance of several predominant reef-associated invertebrates, were measured along a broad salinity gradient in the estuary and were correlated with salinity estimates from a long-term database for the preceding 12–24 mo. All eastern oyster,Crassostrea virginica, parameters (percent cover and density of three size classes) were significantly and negatively related to salinity. Data from samples collected near the lower intertidal were more closely dependent upon salinity than were samples from the higher intertidal at the same sites. Salinity differences were most closely reflected in differences in total oyster cover. This relationship corresponded with a general decline in oyster habitat with increasing distance from the mouth of the Suwannee River. Species richness was significantly and positively correlated with allC. virginica parameters (percent cover and density of three size classes), but the relationship explained only about half the variability. Density data of the hooked mussel,Ischadium recurvum, and a mud crab,Eurypanopeus depressus, were positively and strongly correlated withC. virginica parameters, likely reflecting the abundance of habitat provided byC. virginica shells. All of the biological indicators measured responded similarly along the salinity gradient, indicating they provide reliable indices of the effect of changing salinities in the Suwannee River estuary over the previous 1 or 2 yr. Some areas of positive relief defined as reefs 30 years ago are no longer oyster habitat, suggesting an ongoing decline, but nearshoreC. virginica were abundant. *** DIRECT SUPPORT *** A02BY003 00002     

Transpressional tectonics and nappe stacking along the Southern Variscan Front of Morocco

The Southern Variscan Front in the Tinerhir area involves Palaeozoic allochthonous units (Ouaklim and Tilouine units) thrust onto the northern edge of the West African Craton during late Carboniferous time. Illite crystallinity data highlight an anchizonal grade for the Ouaklim Unit, and a diagenesis-anchizone transition for the Tilouine Unit during deformation phase D1. The tectonic stack is crosscut by major dextral reverse faults bounding E–W trending domains of dominant shortening deformation (central domain) and strike-slip deformation (northern and southern domains), later segmented by a network of post-Variscan faults. This complex deformation pattern is the result of kinematic partitioning of dextral transpression along the Southern Variscan Front, coeval with the Neovariscan (300–290 Ma) oblique convergence observed at the scale of the whole Moroccan Variscides. Partitioning of dextral transpression described in the Tinerhir area is consistent with dextral wrench faulting along the Tizi n’ Test Fault, and with Appalachian-style south-directed thrusting in the Tinerhir and Bechar-Bou Arfa areas.     

Recent quaternary tectonics in the hellenic arc: Examples of geological observations on land

Upper Pleistocene and Holocene tectonic movements in the Aegean region are analyzed by geological means (deformation of shorelines, faults in Quaternary deposits, historical seismicity). Examples from Crete, Karpathos, Milos, Chios and Samos are presented. While subduction, indicated by geophysical data, occurs beneath the Hellenic Arc, extensional tectonics (i.e., normal faulting) takes place within and behind the arc, resulting in a slight expansion of the Aegean region towards the Eastern Mediterranean.