Continental India Research Articles

Tectonics of the Himalayan collision between the Indian Plate and the Afghan Block: a synthesis

Abstract A synthesis of the available tectonic, structural, magmatic and geochronologic data from Afghanistan and western Pakistan suggests that collision between the Indian Plate and the Afghan Block (a collage of blocks assembled during the Cimmeride orogeny), took place during the Pliocene. Prior to this, the thinned western margin of the Indian Plate, together with the transitional to oceanic crust which separated the Indian Plate from the Afghan Block, was overlain by a wedge of Tertiary clastic sediments deposited in a large flexural basin that included what is now the Katawaz basin as well as the Sulaiman foredeep. At least three phases of collision can thus be recognised in the northwest Himalaya: the first, at about 100 Ma involved the suturing of the Kohistan-Ladakh-East Nuristan island arc to southern Asia; the second, at about 55 Ma involved India-Asia collision and the third, during the Pliocene, involved India-Afghanistan collision. During the Eocene and Miocene, the Afghan block was displaced southwards in response to Indian indentation into Asia. Simultaneous counterclockwise rotation of the Afghan Block resulted in intracontinental extension causing alkaline magmatism and right lateral displacements along the Herat Fault. As the Indian Plate moved northward with respect to Afghanistan, some of the wedge of clastic sediments deposited on the Indian transitional crust were accreted to the leading edge of the Afghan block and displaced sinistrally with respect to the rest of the sediments in the wedge. These displaced sediments now form the Makran sequence. Collisional structures on both sides of the India-Afghan collision zone in the Helmand block and the Sulaiman-Kirthar Ranges probably represent the inversion of continental margin basins rather than thrust belts with large shortening amounts. After initial inversion, the dominant feature of this highly oblique collision has been transpressional tectonics. Prior to its final docking with the Afghan Block, the Kabul block was a continental slice, completely surrounded by oceanic crust, and located northwest of continental India. Palaeocene collision between India and the Kabul block could have led to thickening of the Indian Plate prior to initiation of India-Asia collision.

Tectonic stress in the plates

The state of stress in the lithosphere provides strong constraints on the forces acting on the plates. The directions of principal stresses in the plates as indicated by midplate earthquake mechanisms, in situ stress measurements, and stress‐sensitive geological features are used to test plate tectonic driving force models, under the premises that enough data exist in selected areas to define a regionally consistent stress field and that for most such areas the dominant forces producing such stresses are plate tectonic in origin. Force models include buoyancy forces at ridges, subduction zones, and continental convergence zones and variously parameterized viscous shear between the lithosphere and the asthenosphere. A linear finite element method, based on the wave front solution technique, is used to predict the intraplate stress for each force model. Several long‐wavelength patterns for the orientation of horizontal principal deviatoric stresses are observable in the stress data. Maximum compressive stresses trend E‐W to NE‐SW for much of stable North America and E‐W to NW‐SE for continental South America. In western Europe the maximum compressive stresses trend NW‐SE, while in Asia the trend is more nearly N‐S, especially near the Himalayan front. In the Indian plate the trend varies from nearly N‐S in continental India to more nearly E‐W in Australia. Horizontal stresses are variable in Africa but tend to indicate a NW‐SE trend for the maximum compressive stress in west Africa and an E‐W trend for the minimum compressive stress in east Africa. Oceanic lithosphere away from plate boundaries is generally in a state of deviatoric compression, although few focal mechanisms can be constrained to define the orientation of the principal stresses.Comparison of stress orientations predicted for a wide range of driving force models to these regional stress observations provides a powerful test of the models. Ridge pushing forces are required in all models that match the stress orientation field. The net pulling force of subducted lithosphere, if such a force acts approximately symmetrically about the plate boundary, is at most a few times larger than other forces acting on the plates. Resistive forces associated with trench thrust faults and motion of the slab with respect to the mantle must therefore nearly balance the large gravitational potential of the slab. The upper limit on the ratio of net slab to ridge forces may be increased by less than a factor of 2 if net slab forces are reduced for the fastest moving plates by assuming that the resistance to subduction increases with convergence rate. Forces acting to resist further continental convergence along the Alpine‐Himalayan belt are important for models of the intraplate stress field in Europe, Asia, and the Indian plate. Inclusion of continental convergence zone forces, however, does not affect the upper bound on the ratio of net slab to ridge forces. A variety of possible lithosphere‐asthenosphere interactions have been tested. Resistive viscous drag forces acting on the base of the lithosphere improve the fit between calculated and observed stresses in the Nazca and South American plates as long as the drag coefficient is nonzero beneath oceanic lithosphere. The calculated intraplate stress field is not very sensitive to an increased drag coefficient beneath old oceanic lithosphere compared to young oceanic or continental lithosphere. Increasing the drag coefficient beneath continents compared to oceanic lithosphere by a factor of 5 or 10 has little effect on the overall fit of calculated stresses to observed stresses. Models in which viscous drag forces drive, rather than resist, plate motions are in poor agreement with intraplate stress data, although this lack of fit may depend on the oversimplified model of mantle flow patterns that has been assumed. A model of the driving mechanism in which slab forces act only on the plate with subducted lithosphere and where viscous drag forces are assumed to balance the torque on each plate produced by ridge and trench forces predicts stresses in good agreement with the data for most continental regions. The fit between calculated and observed stresses for this model is relatively poor for most oceanic regions. This model suggests, however, that it is probably an oversimplification to assume that the net force exerted by the slab acts symmetrically about the plate boundary, though some pulling force on the overthrust plate appears necessary. While the role of drag forces in the driving mechanism remains poorly constrained, the finite element technique that has been developed may be applied in the future to any specific and realizable flow pattern that is proposed for the mantle.

Seismicity, gravity, and tectonics of northeast India and northern Burma

abstract Practically the whole of northeastern India and northern Burma is characterized as an anomalous gravity field as well as an area of high seismicity. The Bouguer anomaly in the region varies from +44 mgals over Shillong Plateau to −255 mgals near North Lakhimpur in Assam Valley. Isostatic anomaly (Hayford) varies from +100 to −130 mgals in these areas. Over Arakan-Yoma and the Burmese plains, the isostatic anomalies vary from −20 mgals to −100 mgals. Regions of high seismicity in the area include the eastern Himalaya (including Assam syntaxis), Arakan-Yoma including the folded belt of Tripura, Irrawaddy basin, Shillong Plateau, Dauki fault and the northern part of Bengal basin. The abnormal gravity and seismicity are related to large scale tectonic movements that have taken place in the area mostly during the Cretaceous and Cenozoic times, due to interaction of the Indian, Tibetan, and Burmese plates. The high seismicity indicates that the movements are continuing. The seismic zone underlying Burma is approximately V shaped and dips toward the east underneath Arakan-Yoma. Most of the intermediate-focus earthquakes in Burma underlie the area characterized by negative isostatic anomalies, indicating the probable existence of a subduction zone underneath the Arakan-Yoma and the Burmese plains. The Shillong Plateau has a history of vertical uplift since Cretaceous times. Provided this statement is true, the uplift of the plateau preceded Himalayan tectonics starting 20 to 30 m.y. before continental India made solid contact with the Eurasian plate. The plateau is characterized by large positive isostatic anomalies as well as high seismicity. The positive isostatic anomalies may be due to intrusion or incorporation of basic material from the mantle into the crust underlying the Plateau. These intrusions may have taken place through deep seated faults such as the Dauki and could be responsible for its uplift as well.

The European Mercantile Community in Singapore, 1819–1867

In continental India the East India Company subjected European immigration to tight restrictions. Private Europeans had to obtain a licence to live in India and were forbidden to buy land. When the trade of India was thrown open to private merchants in 1813, the restriction on settlement was retained. The same licensing system applied in theory to European immigrants in the Straits Settlements, but in practice it was not enforced. In 1797 when Calcutta first enquired by what authority the European inhabitants had settled in Penang, few of them could produce licences, but no steps were taken to evict them, and in 1823 the Recorder ruled that the possession of land entitled them legally to remain in Penang.

A short description of the Culicidæ of India, with descriptions of new species of anopheles

The following account has been drawn up at Professor Lankester's request with a view to facilitating the work of investigation of malaria in British India. It includes all the known species of Culicidæ found in continental India. Singalese and Malayan species are not recorded, but probably several of them will be found in India.

The Hemiptera Heteroptera of the British Islands

IT is now nearly thirty years since Douglas and Scott first made the study of the British Hemiptera Heteroptera possible to ordinary students by the publication of a description of these insects in a volume issued by the Ray Society. The difficulties were then very great, for purely insular ideas in entomology were prevalent, and our hemipterous insects had not been sufficiently compared with continental species. Douglas and Scott did all that was possible at that time and produced a good work that has held the ground as the best published authority on the subject. Very much, however, has been done since that period, and restricted specialists in entomology, as in most other branches of natural science, have exercised unlimited time and patience in studying the classificatory problems of a single family or even of a large genus. Hence in a monograph of to-day the standard of advanced classification and descriptive facility is considerably raised from that which dominated the writings of the earlier authors. Mr. Saunders has not only aimed at this perfection, but has sought to place in the hands of the British student and collector a thoroughly trustworthy handbook by which he may understand and identify his collection, and in this we think the author has altogether succeeded. We must not look for bibliographical references or synonymical notes, the names of the describers of families, genera, and species being only indicated, while the habitats of the species are confined to such localities in the British Islands as are recorded by collectors; and this is perhaps all that can be expected in a local monograph. It is therefore in no spirit of criticism we express a regret that in all faunistic writings the complete recorded distribution of the species is not given. Thus even the purely British collector would not be the worse for learning that Zicrona cærulea, to be found in the suburbs of London, is not only widely distributed throughout the Palæarctic region, but is also found in Continental India and in the Malay Peninsula and Archipelago; or that Ischnorhynchus resedæ, to be taken even at Hampstead, is common throughout Europe and Siberia, and is also neither scarce in North nor in Central America. The Hemiptera Heteroptera of the British Islands. By Edward Saunders (London: L. Reeve and Co., 1892.)

A Monograph of the Alcedinidæ, or Family of Kingfishers

A Monograph of the Alcedinidæ, or Family of Kingfishers