Main Boundary Fault Research Articles

The Beatrice Field, Block 11/30a, UK North Sea

Abstract The Beatrice Field was discovered in 1976 in Block 1l/30a within the Inner Moray Firth Basin. The reservoir consists of multilayered Lower and Middle Jurassic sediments containing a STOIIP of 480 MMBBL of high wax crude oil. The reservoir sequence comprises 1100 ft of Hettangian (Lower Jurassic) to Callovian (Middle Jurassic) sandstones, siltstones and claystones. The Beatrice Field is co-sourced by a combination of Devonian and Jurassic rocks. The hydrocarbon trap comprises a tilted faultblock, the top of which is truncated by the main field boundary fault. The field has low energy, and the P i of 2897 psi at 6500 ft TVSS, GOR 126 SCF/STBBL and P h of 635 psi, together with the poor aquifer influx, necessitated the development of water injection from the start of production and use of electrical submersible pumps in all wells. Ultimate oil recovery is expected to be 146 MMBBL. The field has been developed with four platforms at three sites in 133 ft waterdepth. The crude is transported 42 miles by peline to the dedicated oil terminal at Nigg on the Cromarty Firth.

Computer processing of Landsat and spot images for the morpho-structural analysis of the Wei He Graben (Shaanxi-China). Preliminary results

The Cenozoic Wei He Graben is the most southern of the northern China rift system. It is a half-graben, which southern limit are the ESE-WNW main boundary faults of the Qin Ling Shan Mountains. They correspond to the main eastward-trending faults of the Tibet northern border. A first geological, structural and morphological approach was conducted, based on Landsat multispectral data (MSS, 1:500,000 colour composite images). A geological photo-interpretation and the plotting of important faults (plurikilometric) show the geometry and the general structural pattern of the rift. The morphogical study accounts for vertical movements of large areas but major fault kinematics remains unknown. In order to specify the fault kinematics, a more accurate morpho-structural analysis was conducted, based on spot image processing and interpretation. The preliminary results show that during the Recent Quaternary up to Present, no important strike-slip fault movements (more than 20 m) exist along the Wei He Graben faults as suggested by the pull-apart graben model. In return, the vertical component movements (subsidence) prevail and this agrees with field data.

Petrography and rank of the Bhangtar coals, southeastern Bhutan

In Bhutan, a potential coal deposit is exposed at Bhangtar in the “landslide zone”. Nineteen coal seams are encountered in this area, and occur in the Lower Gondwana Supergroup preserved in between the Main Boundary Fault and the Thrust. The coal is low in moisture, < 1.76%, but the coal cores show moisture values of 3.16%. The ash content is up to 48.87% and increases substantially in the younger seams. The volatile content (on a pure coal basis) ranges from 23.38% to 41.02%. The sulphur content is less than 0.61%. The coals are non-coking. The amount of trace elements in the coal is quite low. The average petrographic composition of the Bhangtar coal is vitrinite - 31%, exinite - 2%, inertinite - 31%, and mineral and shaly matter - 36%, the vitrinite proportion decreases from the older to the younger seams, which are shaly. Vitrinite (telinite) is derived mainly from cordaitalean wood, and is fractured or unfractured showing an oil reflectance ( R m) range of 0.71–0.72% and 0.54–0.60%, respectively. The oil reflectance variation is governed by a similar variation in the volatiles of the coal. Exinite is mainly sporinite and cutinite and is constituted of organic matter from Upper Permian mioflora, so that a Late Permian age can be assigned to the Bhangtar coal. Based on oil reflectance, the rank of the coal is metalignitous to hypobituminous. The average microlithotype composition of the coal is vitrite - 30%, clarite - 1%, vitrinertite V - 14%, vitrinertite I - 11%, durite - 3%, fusite - 14%, and carbominerite - 27%. Vitrite decreases in proportion towards the younger seams, “intermediates” show a concomitant increase, while durite and fusite remain constant. Carbonaceous shale contains fragmentary inertinite and vitrinite macerals and is interlayered with micro-bands of shaly coal which is characterised by abundant fragments of fusinite and vitrinite. The coal is very fragile and thus amenable to economic beneficiation. The removal of carbonaceous shale in this process increases the vitrinite content, thereby reducing the inertinite. The coal is used as fuel in electric power plants. The petrographic composition of the Bhangtar coal has been compared with those of the Upper and Lower Permian coals of the Gondwana coalfields of Peninsular India. The Bhangtar coal is characteristically distinct from the Gondwana coals of India in petrography and rank, but correlates petrographically with the Kameng coals of Arunachal Pradesh, India.

Tectonic zonation of the Central Himalaya and the crustal evolution of collision and compressional belts

The Central Himalayan segment is divided from south to north into sub-parallel structural-facies zones. The Main Boundary Fault (MBF) plays the role of an important tectonic element dividing the Siwalik Molasse along the foothills of the Himalaya against the para-autochthonous and allochthonous tectonic units of the Southern Himalaya. On the basis of recent researches I suggest that the Himalayan region should be divided geologically into two major geologic zones. I propose to call the dividing structural line between them, the Main Axial Zone (MAZ) of the Himalaya, which is situated in the crystalline complex of the Higher Himalaya. This deep seated structure in the Himalaya has played a crucial role in the history of geological development as well as incorporating the root zone of southward-pushed thrust sheets. The Main Central Thrust (MCT) tectonically separates the carbonate para-autochthonous zone of Southern Himalaya from the metamorphosed crystalline “Vaikrita” complex. The Vaikrita Central Crystalline Complex in its turn is separated from the huge pile of the sedimentary Tethyan Complex by the “Tethyan Thrust” (TT). Farther north the “Great Himalaya Suture” (GHS) called the Indus-Tsangpo Suture divides the northern extremity of Himalaya from the Karakoram erogenic belt. The GHS has provided excellent conditions for the study of mantle remains on the oceanic crust with the occurrence of ophiolitic melange which was the result of subduction of a continental type plate and obduction of oceanic material. In recent years a large number of data has been recorded especially as a result of Sino-French and other scientific expeditions in the Tibetan region. Reversed critical wide angle reflection profiles of the crust mantle boundary south of the Great Himalaya Suture (GHS) or the Indus-Tsango (Yarlung-Zangbo) Suture in Tibet reveal a deep 70 km Moho extending north of the Higher Himalaya whilst to the south the Moho is 15 km higher. Thus in the southern region of Tibet the crust gets thinner gradually towards the south, and reduces to about 38 km in the Gangetic plain of India. The 300 km of terrain between the GHS and the Gangetic plain represent the transitional collision and compressional belt. The magnetolluric and explosive seismographic data indicate that the layered structures are developed within the crust transisting to the upper mantle. The Moho discontinuity exhibits step terraces. The low velocity and resistance layer within the crust indicates that the crust is overlapping and superimposed. Thus the intense compression from south to north caused by the Indian plate caused the crust to overlap, shorten, thicken and become isostatically adjusted to give rise to the Tibetan plateau. The successive occurrence of nappe-shear zones led to progressive superimposing and thickening of the crust. It also indicates intra-continental subduction and destruction of the lithosphère after plate collision.

Seismicity and the nature of plate movement along the Himalayan arc, Northeast India and Arakan-Yoma: a review

The Himalaya together with Arakan-Yoma form a well defined seismic belt to the north and east of the Indian Peninsula. The Seismicity along this belt is attributed mostly to collision between the Indian and the Eurasian plates. However, the exact nature of activity along the major thrusts and faults is not well understood. The seismicity along the entire Himalaya and Northern Burma has been studied in detail. It has been found that besides the Main Boundary Fault and the Main Central Thrust several transverse features are also very active. Some of these behave like steeply dipping fracture zones. Along the Arakan-Yoma most of the seismicity appears to be due to subduction of the Indian lithosphere to the east. Analysis of focal mechanism solutions for the Himalaya shows that although thrust movements are predominant, normal and strike-slip faulting is taking place along some of the transverse features. In addition to thrusting, strike-slip faulting is also taking place along the Arakan-Yoma. Orientation of P-axes for all thrust solutions show a sharp change from predominantly east-west along the Burmese arc to N-S and NE-SW along the Himalaya. The direction further changes to NW-SE along the Baluchistan arc. It appears that the Indian lithosphere is under compression from practically all sides. The present day seismicity of Northeast India and Northern Burma can be explained in terms of a plate tectonics model after Nandy (1976). No simple model appears to be applicable for the entire Himalaya.

An interpretation of induced magnetic variations near Shillong and Gulmarg

Short-period events such as bays and storm sudden commencements (SSCs) have been analysed to investigate the nature of induced magnetic variations at two Indian magnetic observatories: Shillong and Gulmarg. It seems that near Gulmarg there is obvious connection between the induced magnetic variations and the two large scale features; the main central thrust (MCT) and the main boundary fault (MBF) in the north-west direction. The Dauki fault, an approximately east-west conductor, seems to be responsible for the conductivity anomalies at SHL.

The eastern Jura: Relations between thin-skinned and basement tectonics, local and regional

The thesis area of Hans Cloos in the eastern Jura straddles the boundary between the Paleogene structures of the Tabular Jura, a part of the southeastern end of the Rhine graben, and the Neogene folds and thrusts of the Folded Jura. A recent reflection seismic survey in the area adjacent to the east provides new information on the subsurface structure and the relation between three superposed tectonic events. The main boundary fault zones of a deep and complexly faulted late Paleozoic trough, trending ENE, were reactivated in the early Tertiary. Faulted flexures and small folds, rooted in the basement and Paleozoic, with an amplitude of usually 100 to 300 m were produced, rather than the normal faults typical for the extensional fields of the Rhine-Bresse graben system in the west and the Bavarian Molasse basin in the east. This suggests their being part of a transform zone between these extensional fields. During Jura decollement in the Neogene these inherited, deeply rooted sub-decollement structures remained passive: their small size is kinematically incompatible with the intense subsequent thrusting and folding of the thin Mesozoic-Tertiary skin. However, thrust ramps were apparently nucleated at the respective southern foot of these Paleogene flexures and folds. The best defined and simplest of these composite structures is the Mandach structure within the Tabular Jura, a south-facing flexure of about 300 m relief, developed in the Paleogene by reactivation of the northern border fault zone of the Paleozoic trough. In the Neogene a north-vergent decollement thrust, which is connected with the regional Jura decollement in the middle Triassic evaporites, was initiated at its site.

Crustal magnetisation-model of the Indian subcontinent through inversion of satellite data

An apparent magnetisation-model of the Indian subcontinent and the adjoining regions has been obtained through the inversion of MAGSAT total field (scalar) and vertical field data. A staggered 4° × 4° grid pattern with a constant continental crustal thickness of 40 km yielded magnetisation values over a 2° × 2° grid. Over the major part of the Indian subcontinent, the magnetisation picture brings out a prominent “high” sandwiched between the Eastern Ghats and the main boundary fault of the Himalayas. In contrast, the identity of the Western Ghats, comprising mainly Deccan Traps, is missing. While a distinct magnetisation “low” is observed over the known hot-spot region of the Saurashtra Peninsula, two magnetisation “highs”—one over the Shillong Massif and the other over the Central part of India covering Aravallis and the Bundelkhand Massif—are obtained. Interestingly, even at a height of 400 km, the well known Narmada-Son lineament is clearly reflected in the total intensity map as well as in the corresponding magnetisation map. The Bay of Bengal and the Himalayas ara characterised by magnetisation “lows”.

Extension and rotation of crustal blocks in northern Central America and effect on the volcanic arc

Sinistral displacements across the North American–Caribbean plate boundary in northern Central America are distributed among major arcuate faults that have been active in Neogene time. South of the Jocotan and Motagua faults in Guatemala, extensional tectonics has accompanied rotation of the trailing edge of the Caribbean plate around these faults. Segmentation of the volcanic arc in northern Central America, hitherto attributed to transverse breaks in the subducting Cocos plate, may instead be a result of this block rotation. Accompanying changes along the arc are observed in seismicity, gravity anomaly patterns, volume and composition of volcanic products, and topography. Therefore, the complex volcano-tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding Caribbean plate above the magma production zone along the downgoing Cocos slab.

活断層からみた南部フォッサマグナ地域のネオテクトニクス

The South Fossa Magna in central Japan is thought to be a collisional zone between the Philippine Sea plate and the Asia plate. In this region, intense tectonic movements such as folding, faulting and other crustal movements have been proceeding since the early Quaternary. Several active faults which have an extremely high vertical slip rate are distributed along the north end of the Izu bar.Three tectonic sub-regions can be distinguished on the basis of the sense of fault movement and other tectonic features. That is, the area of the southwestern foot of Mt. Fuji, the area of the south end of the Tanzawa Mts. and the Ashigara-Oiso area. The first area is located in the northern extension of the Suruga trough. This area is divided into three blocks by N-S trending active faults. The easternmost block shows distinct subsidence and the others upheave with a high rate. In the second area, an E-W trending high angle reverse fault bounds the south end of the Tanzawa Mts. The topographic feature of this fault is indistinct. The third area is located in the northwestern extension of the Sagami trough. The NW trending Kozu-Matsuda fault which has a sence of reverse and right lateral slip divides the area into the Oiso hills and the Ashigara plain. The former shows a distinct upheaval which occurred in recent geological ages and the latter suggests the decrease of its subsidence rate.Studies of the crustal movements in each sub-region reveal the following common tectonic features. That is, each crustal block distant from the Izu bar has changed its movement style from the phase of intense subsidence to that of upheaval through the transitional phase. A newly created fault or a subsidence zone has always occurred in a block closer to the Izu bar. As a consequence, the latest tectonic zone in the South Fossa Magna seems to have migrated from a distant position closer to the Izu bar. Fig. 3 shows a concept of the tectonic process across the tectonic zone in the South Fossa Magna. This tectonic process is the process of accretion of the sediment to Honshu in the landward region of the Suruga and Sagami troughs. The active faults in this region are considered to be neither interplate faults nor branches from the main boundary fault but the imbricated thrust faults in the accretional sediments.

Seismicity study in relation to recent earthquakes in Jammu and adjoining Himachal Pradesh

Seismicity variations associated with four recent earthquakes in eastern Kashmir and adjoining Himachal Pradesh have been studied with the help of closely spaced seismological network in the region. It was noticed that earthquakes of August 1980 in Bhaddu and Malar regions of Jammu province and two other earthquakes (1973, 1976) in adjoining western Himachal Pradesh occurred in a region of low seismic activity. These earthquakes of magnitude 4 to 5 were preceded by anomalous seismic activity with the precursory duration ranging from 3 to 7 years. Fault plane solutions of two earthquakes of 23 August 1980 (21 h 36 m 52.3 s) and 7 January 1976 show thrust fault and are associated with the main-boundary fault in the region. However, the second earthquake of 23 August 1980 ( 21 h 50 m 02.90 s) appears to be associated with Surin-Mustgarh anticline.

On the nature of the Main Boundary Fault in Kumaun-Garhwal Himalaya with special reference to measurement of flattening in folds

On the nature of the Main Boundary Fault in Kumaun-Garhwal Himalaya with special reference to measurement of flattening in folds

Seismotectonics of the Hindukush and Baluchistan arc

A seismicity map of that part of the Pakistan-Afghanistan region lying between the latitudes 28° to 38°N and longitudes 66° to 75°E is given using all available data for the period 1890–1970. The earthquakes of magnitude 4.5 and above were considered in the preparation of this map. On the basis of this map, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent, mutually orthogonal, seismicity lineaments, namely the northvestern and the north-eastern trends, characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well-defined zones of intermontane seismicity exhibiting north-south alignment. Thirty-two new focal-mechanism solutions for the above-mentioned region have been determined. These, together with the results obtained by earlier workers, suggest the pre-dominance of strike-slip faulting in the area. The Hazara Mountains, the Sulaiman wrench zone and the Kirthar wrench zone, as well as the supposed extension of the Murray ridge up to the Karachi coast, appear to be mostly undergoing strike-slip movements. In the Hindukush region, thrust and strike-slip faulting are found to be equally prevalent. Almost all the thrust-type mechanisms belonging to the Hindukush area have both the nodal planes in the NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of P-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of the seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast of Uzbekistan on the northwest through Hindukush. Almost orthogonal to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting. The above results are discussed from the point of view of convergence of the Indian and Eurasian plates in the light of plate tectonics theory.

Some geothermal springs of Nepal

Almost all known thermal springs in Nepal are localized close to and south of either the Main central Thrust or the Main Boundary Fault, and therefore they fall broadly into two major groups. The results from four thermal springs — two from each group — are discussed here. The temperature of the spring water is up to 50°C above normal atmospheric temperature. The waters of almost all springs are alkaline. The dissolved silica content and atomic ratio of Na and K do not indicate any high subsurface temperature and therefore the thermal springs are considered to be of tectonic origin.

Seismicity and tectonics in the Hindukush mountains, Sulaiman and Kirthar ranges in the light of plate tectonics theory.

A seismicity map of the Pakistan-Afghanistan region that lies between the latitudes 280 &amp; 380 N and longitudes 660 &amp; 750 E is given using all available data for the period 1890-1972. The earthquakes of magnitude 4.5 and above were considered in preparation of this map. A strain-energy release map for the same period was prepared following the method used by Allen et al. (1965). On the basis of these two maps, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent mutually orthogonal seismicity lineaments, namely, the northwestern and the northeastern trends characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well defined zones of intermontane seismicity exhibiting north-south alignment.&#x0D; Thirty-two new focal mechanism solutions for the above mentioned region have been determined. These, together with the results obtained by earlier workers suggest the prevalence of thrust as well as strike-slip faulting in the area. The Salt range; the Sulaiman wrench zone, the Kirthar wrench zone as well as the supposed extension of the Murray ridge upto the Karachi coast appear to be mostly undergoing strike-slip movements. In the Hindukush region, thrust as well as strike-slip faulting are found to be equally prevalent. Almost all the thrust type mechanisms belonging to the Hindikush area have both the nodal planes in NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of F-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast to Uzbekistan on the northwest through the Hindukush. Almost perpendicular to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting.&#x0D; The above results are discussed in the light of convergence between the Indian and predicted by plate tectonics theory.&#x0D;

Seismotectonics of the himalaya, and the continental plate convergence

The relationship between seismicity and tectonics of the northwestern, central and part of the eastern Himalaya lying between longitudes 74–88°E and latitudes 26–35°N has been studied. A seismicity map of the area for the period 1890–1970 is presented. A strain-energy release map of the Himalaya was prepared following the method discussed by Allen et al. (1965). On the basis of this map it is suggested that the Himalaya does not behave as a single unit so far as the seismic activity is concerned. It could be divided appropriately into three different units; 1. (1) the Panjab Himalaya; 2. (2) the Kumaon Himalaya; and 3. (3) the Nepal Himalaya. It is suggested that the extension of Aravalli structures into the Himalayan regions has played a role in the tectonics of the Kumaon Himalaya, and probably is the cause of the complex nature of seismicity of the region. In the Kumaon Himalaya the maximum strain-energy release is related to the Main Central Thrust (MCT), whereas in the Panjab as well as in Nepal Himalaya it is related to the activity of the Main Boundary Fault (MBF). The strain-energy release characteristics of the two thrusts show that their mechanisms of storage and release of energy are different. Strain-energy release through the MCT seems to be more uniform as compared to the MBF along which the release of energy has been mostly abrupt, through large magnitude earthquakes. Focal-mechanism solutions of Himalayan earthquakes located north as well as south of the Indus Suture Zone indicate that the Indian plate is underthrusting the Tibetan plate towards the north, wehereas the latter is underthrusting the Indian plate towards the south. P-axes lying to the north as well as south of the Indus Suture Line are shallow dipping. This confirms the view that the present-day seismicity of the area is a result of continental-continental collision and not of lithospheric subduction.

Notes on the Glen Coe Cauldron Subsidence, Argyllshire, Scotland

General inward dip of the main boundary fault surrounding the Glen Coe cauldron subsidence indicates that the ring fault has the over-all form of an upward-opening cone. The cone shape is substantiated by peripheral upturning of down-dropped volcanic rocks against the encircling ring fault. Some rocks are even overturned. A peculiar zone of rhyolite and associated breccia that puzzled the pioneer geologists (Clough and others, 1909) contains rhyolitic dikes and intrusive breccias. Two rhyolitic dikes, composed of welded pyroclastic rocks, qualify as possible feeders for welded tuffs. Ignimbrites in the volcanic pile confirm repeated explosive activity during the evolution of the cauldron. Marked subsidence occurred after major episodes of explosive activity, as indicated by lake deposits and coarse sediments that overlie the ignimbrites. Flinty crush rock (Clough and others, 1909) is a fine-grained product of fluidization, as suggested by Reynolds (1956). Transported inclusions of volcanic rocks prove that the flinty crush rock is a fine-grained intrusive breccia. Most breccias near the main boundary fault are intrusive or explosion breccias, but the breccia (Hardie, 1963)south of Stob Mhic Mhartuin is probably of sedimentary origin. Fault intrusion (Clough and others, 1909) is a catchall name for a wide variety of plutonic and hypabyssal bodies. The common irregularity of many intrusions is a typical of ring dikes. The classification of igneous ring complexes is discussed. In Britain, there is no unequivocal example of an outward-dipping ring fracture at Ardnamurchan, Mull, or Slieve Gullion, but outward-dipping ring faults are present in the Mourne Mountains. The ring fracture that encircles the downdropped volcanic rocks on Ben Nevis is essentially vertical, as seen in exposures along the north and east sides of the annular fault. At Glen Coe, the writer did not recognize any evidence for resurgent doming.

The tectonics of the sub-Himalayan fault zone in the northern Potwar region and in the Kangra district of the Punjab

Summary The paper defines three major zones of deformation across the Nimadric (Tertiary freshwater) geosyncline between the foreland and the autochthonous folded belt in the North-West Himalaya:— (1) A zone of open folding, affected to a varying degree by fold-faulting. (2) A fault zone, displaying a number of reversed faults of steep northerly hade and great lateral extent with open folds between them in the west and, in places, gently dipping monoclines in the east. (3) A zone with closely spaced strike-faults and severely compressed folds, referred to in the Potwar area as the isoclinal zone, terminating at the main boundary fault which forms the general northern limit of Nimadric rocks. This succession of structural zones has a superficial resemblance to progressive deformation of the Jura type towards the nappe zone, and previous workers have regarded the tectonic pattern as reflecting tangential pressures from the north. The author seeks to demonstrate that such an interpretation is denied by the evidence of the structures themselves. Two areas are chosen to illustrate in adequate detail the tectonics of all three zones, and in particular the crucial fault zone. In the west, the Jhamat-Khushalgar area of western Potwar typifies the fault zone of the area between the Indus and the Jhelum rivers. In this area there is clear evidence of the steep hade of the great strike-faults at the surface, and also important evidence of their continued steep hade in depth from the results of boreholes drilled by the Attock Oil Company. Analysis of the relationship of the faults to the fold structures leads to the conclusion that the faults have no genetic relationship to the fold axes. They are considered to be fractures due to shearing stresses principally with a vertical orientation, whilst the folding reflects the subsidiary compressive force. The Nadaun Dun area of Kangra district is some 200 miles to the southeast of the Potwar area, but lies across similar structural zones and typifies the relationships of the anticlinal zone and the fault zone in the region east of the Jhelum River. In this area the Gumber fault is seen to present all the features of the great strike-faults of the fault zone in the Potwar and is an upthrust structure produced by quasi-vertical movement. A feature not present in the Potwar is the marked contrast between the gently dipping monocline to the north of the fault and the strongly folded country to the south, providing a clearer distinction between the influence of tangential and vertical forces. It appears that the fault movement provided a block-line against which tangential forces from the south were directed. Important evidence of tangential pressure from the south is provided by the Barsa fault, which typifies the most common form of fold-fault in the anticlinal zone throughout the region. Gravity collapse structures are recorded in the form of severely overturned “ flaps ” of sandstone on the south side of the Sola Singhi ridge.