P. J. Conaghan

Plate tectonics and the Himalayas

Abstract The Himalayas, commonly taken as the type example of continent-continent collision, have developed in two stages. The first stage involves convergence of the northward-drifting Indian subcontinent with a proto-Tibetan landmass during Late Cretaceous and Palaeocene, with collision before Middle Eocene. The second stage involves formation of a fundamental crustal fracture within the Indian block during Late Eocene and Oligocene, and underthrusting of the Indian subcontinent along this fracture from Miocene to Recent. The present elevated Himalayan mountain chain is not a direct result of the continent-continent collision, but of uplift during underthrusting along the deep crustal fracture.

The Himalayan Arc: large-scale continental subduction, oroclinal bending and back-arc spreading

Abstract Palaeomagnetic results from the Himalayan Arc and Southern Tibet compared with simulated apparent polar wander paths for the Indian plate show a consistent pattern of rotations of the Himalayan Arc relative to the Indian Shield, varying gradually from 45° clockwise in the northwestern Himalaya to slightly counterclockwise in the Lhasa region. This pattern is consistent with continental underthrusting of Greater India beneath the Tibetan Plateau since the Early Miocene over at least 650 km at the longitude of western Nepal and oroclinal bending since the latest Miocene. Available palaeomagnetic observations support the steady-state model for the formation of the Himalayan Arc, with refinements as follows: (1) collision between Greater Indias northern boundary and southern Asia occurred at equatorial latitudes, with progressive suturing from Palaeocene in the northwestern Himalaya until Early Eocene in the eastern Himalaya; (2) continuing convergence and indentation of Greater India into southern Asia over about 2000 km up to the Early Miocene resulted in southeastward extrusion of Indochina; and (3) Neogene counterclockwise rotational underthrusting of Greater India along the Main Central Thrust, with Pliocene/Quaternary oroclinal bending of the Himalayan Arc.

Tectonic models of the Tibetan plateau

Tectonic models proposed for the origin of the Tibetan plateau and surrounding regions all have some inadequacies, but models involving underthrusting continental lithosphere of India beneath the whole Tibetan plateau accord best with the geologic and geophysical constraints.

The Hawkesbury sandstone and the Brahmaputra: A depositional model for continental sheet sandstones

Abstract Bedding characteristics of the fluvial Hawkesbury Sandstone (Triassic) of the Sydney Basin are most readily explained in terms of a model of flood‐ and falling‐stage vertical accretion derived from Colemans study of the Brahmaputra. This model suggests inadequacies in present concepts of ‘braided stream’ deposits based on observations made at low stage. A Brahmaputra model may be widely applicable to continental sheet sandstones.

Further palaeomagnetic data from Chitral (Eastern Hindukush): evidence for an early India-Asia contact

The Eastern Hindukush forms part of an elongate belt (“Central Domain”, collage of Cimmerian microcontinents) that encircles the northern part of the Indian subcontinent. A Gondwanan origin is commonly assumed for this belt, but a “Laurasian” origin for the Chitral region has been argued on palaeontological (Talent and Mawson, 1979) and palaeomagnetic (Klootwijk and Conaghan, 1979) grounds. The “Laurasian” view was based on a pilot study we undertook of Upper Devonian pisolitic ironstones from a thrust sheet at Kurāgh Spur in Chitral. Preliminary results showed a characteristic magnetization component [D = 318°, I= −6.5°, N = 7 (block samples), k = 14, α95 = 16.5°] indicating an equatorial palaeoposition. This component was thought to be of primary origin and was interpreted in terms of a Late Devonian “Laurasian” affinity of the Kurāgh Spur rocks. This controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the Reshūn-Kurāgh-Būni region of Chitral and the primary origin of the characteristic magnetization component refuted. Thermal demagnetization of 333 block samples from Middle to Upper Devonian variegated sediments, Permian quartz flysch, Permo-Triassic carbonates, and mid-Cretaceous redbeds showed two interpretable components. A softer component of recent origin (A); and a harder characteristic component (B) of both normal and reverse polarity whose mean direction [D = 314.1°, I = 6.0°, N = 4 (thrust sheets), k = 198.2, α95 = 6.5°] is comparable to the characteristic component observed in our preliminary study. However, the universal presence of this component throughout the thrust pile proves its overprint origin, which we attribute to initial India-Asia contact. Palaeomagnetic information pertinent to the controversy of a “Laurasian” versus a Gondwanan origin of the Chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt. Hence, we retract herewith our original conclusion of a Late Devonian “Laurasian” affinity of the Chitral region on the basis of the palaeomagnetic evidence. The secondary component (B) comprises a suite of secondary magnetizations, acquired at equatorial-to-low-northern palaeolatitudes, and is attributed to initial contact between Greater India and southern Asia. Component B has been observed previously in the Himalayan-Tibetan region, both north and south of the Indus-Tsangpo Suture zone. Identification is herein extended to the Hindukush region north of the Northern Kohistan (or Shyok) Suture zone, which is a western continuation of the Indus-Tsangpo Suture. Comparison of this suite of collision-at-tributed equatorial palaeolatitude data from the India-Asia convergence zone with new palaeolatitude constraints from the Ninetyeast Ridge on the northward movement of the Indian plate, constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of Greater India, indicates that initial contact between northwestern Greater India and southern Asia was established at, or before, the Cretaceous-Tertiary boundary. The overprint origin of component B at about this time is further supported by observations by Zeitler (1985) on rocks from the sampled area in Chitral of partially reset zircon fission-track ages around 68-55 Ma. The NW-SE declination axis of component B indicates a 60–70° counterclockwise rotation of the sampled thrust pile with respect to Eurasia and a counterclockwise rotation between 10 and 30° with respect to India. Some of the recent field components (A) show a comparable rotation and indicate that the tectonic activity that led to the formation of the Hindukush-Pamir-Karakorum syntaxial zone has continued into recent times.

Polyphase Deformation in Phanerozoic Rocks of the Central Himalayan Gneiss, Northwest India: A Reply

Multiple deformation in gneisses and schists of the Chandra Valley, Himachal Pradesh, can be divided into three phases of folding. The earliest recognizable deformation, D₁, is characterized by isoclinal folds, F₁, with a strong axial-surface foliation, S₁, containing a mineral lineation, L₁, which may lie in the kinematic a direction. The F₂ folds deform S₁ and L₁, and vary from open to very tight structures with rounded hinges, crenulation lineations, and a weak axial-surface crenulation cleavage. The consistent sense of asymmetry of F₂ folds southwest of the central crystalline axis indicates strong overthrusting from northeast. The third phase of deformation, D₃, produced the regional broad antiforms and synforms which plunge 5° NW. Calc-schistsnear Tandi contain marine fossils of Jurassic age. These calc-schists are deformed by the three deformations recognized in the gneisses, and are thus part of the central crystalline zone. The fossiliferous Spiti section is a para-autochthon tectonically overlying the central gneiss. Field relations suggest that the metamorphites formed during the Tertiary, and that the central gneiss occupies the root zone of the extensive crystalline thrusts and nappes of the Lesser Himalaya.

A dynamic fluvial model for the Sydney Basin

Abstract Late Permian and Triassic successions of the Sydney Basin, exposed in cliffs of the north coast, south coast and Blue Mountains, include sandstones in which quartz content increases up‐sequence and palaeocurrents swing from southwesterly through southeasterly to northeasterly. These basin‐wide phenomena are interpreted to have resulted from the northeasterly migration of a drainage net, in which northeasterly‐flowing tributaries bearing quartz sand from the craton met southwesterly‐flowing tributaries bearing labile sediment from an arcuate rim‐orogen, and blended in a southeasterly‐flowing trunk stream down the axis of a foredeep. Migration of the drainage net records the retreat of an arc‐derived clastic wedge in the latter part of a megacycle which extends from the top of the mid‐Permian Nowra Sandstone to the top of the mid‐Triassic Hawkesbury Sandstone. The same megacycle occurs in the Bowen Basin, and may also occur in the Nilsen‐Mackay Basin of Antarctica.

The mid‐Palaeozoic turbiditic Mathinna Group, northeast Tasmania

Palaeocurrents, sandstone compositions and sedimentologic profiles in the turbiditic Mathinna Group of northeastern Tasmania have enabled the geometry and dynamics of the Late Silurian‐Early Devonian part of the depositional basin to be defined. The oldest rocks in the Mathinna Group are Early Ordovician or older, and comprise a thick‐ to thin‐bedded turbiditic arenite succession ∼ 1 km thick (Stony Head Sandstone) overlain by a 1–2 km thick pelite (Turquoise Bluff Slate) containing the only known Ordovician fossil in the succession — an Early Ordovician graptolite. This pelite succession passes conformably upwards by influx of gradually increasing proportions of siltstone and fine‐grained sandstone into a thin‐ to thick‐bedded interval of classical turbidites at least several kilometres thick containing Early Devonian fossils in its upper part. The lower part of this upper turbiditic succession (Bellingham Formation) contains the oldest palaeocurrent pattern measured and indicates lobes on a submarine fa...

The extent of Greater India, I. Preliminary palaeomagnetic data from the Upper Devonian of the eastern Hindukush, Chitral (Pakistan)

Abstract Samples of Upper Devonian sedimentary ironstones from the eastern Hindukush, Chitral (Pakistan), give a characteristic palaeomagnetic direction: declination D = 318° , inclination I = −6.5° ; believed to represent the primary magnetization direction. The samples come from an area which lies north of a major ophiolite zone that recent workers suggest is the southwestern continuation of the Indus Suture. As the present palaeomagnetic results are in fair agreement with palaeomagnetic data from the Siberian platform but not with data from Gondwanaland they can be taken as additional evidence that this suture does indeed constitute the main collision zone between the Gondwanic Indian subcontinent and Asia. The palaeomagnetic data presented here from the Devonian of Chitral suggests additionally: (1) in excess of 100° of counterclockwise rotation of the area, associated most likely with the formation of the regional Hindukush-Pamir-Karakoram syntaxial bend; (2) more than 2000 km of crustal shortening between Chitral and the Siberian platform due to the northward indentation of the Indian Gondwanaland fragment subsequent to collision.

Coal measures of an orogenic recess: Late Permian Sydney Basin, Australia

Abstract Comparison of the Late Permian northeastern Sydney Basin with the Pliocene-Quaternary “Purari recess” of the Papuan Basin on the basis of gross lithology and lithologic sequence, sediment provenance, sedimentary environment and basin architecture confirms its tectonic context as an orogenic recess of closely similar character. This comparison, together with comparison to the Late Carboniferous Appalachian Basin of Pennsylvania, suggests that, under favourable climatic circumstances, orogenic recesses are optimum sites for thick coal accumulation.