Chris Klootwijk

Early palaeozoic palaeomagnetism in Australia I. Cambrian results from the Flinders Ranges, South Australia II. Late Early Cambrian results from Kangaroo Island, South Australia III. Middle to early-Late Cambrian results from the Amadeus Basin, Northern Territory

Abstract I. Cambrian results from the Flinders Ranges, South Australia A total of 460 samples from six sequences spanning the Cambrian succession of the Flinders Ranges (Adelaide “Geosyncline”, South Australia) has been analyzed through thermal demagnetization studies. All samples showed a recent field component, generally constituting more than 50% of the initial intensity, which in most cases was removed by 200–400°C. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Cambro-Ordovician age (S-pole at 75.3°E 26.0°N, α 95 = 7.4°, N = 5 localities) interpreted as having been induced by thermochemical activity during a period of enhanced heat flux prior to the Late Cambrian-Early Ordovician diastrophistic phases of the Delamarian Orogeny. 2. (B) A primary magnetic component, which indicates rapid polar motion during the Early Cambrian and a much reduced polar motion during the Middle Cambrian. Representative palaeomagnetic pole positions for the primary component are: 1. (1) Basal Hawker Group (earliest Cambrian): S-pole at 2.3°E 26.7°S, d p = 8.1°, d m = 14.3°, N = 10 (sites). 2. (2) Billy Creek Formation — Wirrealpa Limestone — Aroona Creek Limestone (late Early Cambrian to early Middle Cambrian): S-pole at 20.1°E 37.4°S, d p = 7.2°, dm = 14.4°, N = 11(sites). 3. (3) Basal Lake Frame Group (Middle Cambrian): S-pole at 26.1°E 29.3°S, d p = 6.6°, d m = 13.1°, N =10 (sites). 4. (4) Pantapinna Formation (late Middle Cambrian?): S-pole at 29.2°E 36.4°S, d p = 8.4°, dm = 16.7°, N = 4 (sites). Available data suggest that deposition of the Lake Frome Group beds probably did not continue into the Late Cambrian. II. Late Early Cambrian results from Kangaroo Island, South Australia A total of 108 block samples from a late Early Cambrian red-bed sequence on Kangaroo Island (Adelaide “Geosyncline”, South Australia) has been analysed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Cambrian—Early Ordovician age (S-pole at: 75.8°E 17.4°N, d p = 4.2°, d m = 1.9°, N = 54 specimens), attributed to thermochemical activity predating the main folding phases of the Delamarian Orogeny. 2. (B) A primary magnetic component corresponding to a S-pole position at 15.1°E 33.8°S (d p = 6.2°, d m = 12.3°, N = 16 sites). Both the primary and the secondary magnetic component are in very good directional agreement with the magnetization pattern from the correlated Billy Creek Formation of the Flinders Ranges (I). Consequently, noticeable rotational movement since late-Early Cambrian times between Kangaroo Island and the northwestern part of the Adelaide “Geosyncline” can be ruled out. III. Middle to early-Late Cambrian results from the Amadeus Basin (Northern Territory) A total of 328 samples from a Middle Cambrian red-bed succession and a Middle to early-Late Cambrian carbonate succession in the Amadeus Basin (Central Australia) have been analyzed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity which became broken down, respectively below 200°C in the carbonate samples and between 300°C and 500°C in the red-bed samples. Another recent field component, broken down between 600°C and 675°C, was noted in some of the red-bed samples. Three characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Devonian—Early Carboniferous age (S-pole at 110.5°E 46.9°S, N = 2 localities) which predates the main folding phase of the Early Carboniferous Alice Springs Orogeny. 2. (B) Another secondary magnetic component (S-pole at 60.8°E 33.8°N, N = 2 localities) which is very similar to a thermo-chemically induced Cambro-Ordovician magnetic component, noted in rocks from the Adelaide “Geosyncline”. 3

The extent of greater India, III. Palaeomagnetic data from the Tibetan Sedimentary series, Thakkhola region, Nepal Himalaya

Abstract The palaeomagnetism of 750 carbonate and sandstone samples from the Tibetan Sedimentary Series (TSS) of the Thakkhola region (north central Nepal) has been studied. This region forms part of Indias former leading edge, underthrust by India along the Main Central Thrust (MCT). Samples of Devono-Carboniferous to Early Cretaceous age were collected from 14 localities. Five magnetic components have been distinguished: (1) a predominant component of recent origin, (2) a secondary component probably acquired during early Tertiary collision of India with Asia or offshore island arcs, (3) primary magnetic components of Middle? Permian to Early Cretaceous age, and another two secondary components either tentatively related with the predominant b-axes pattern of the TSS (4), or attributed to MCT-related metamorphism of the lower TSS (5). Declinations of the secondary collision component and the primary components mismatch declination data from the Indian subcontinent by 10–15°. A testable model is proposed, which related this mismatch to rotational underthrusting of India along the MCT beneath its former leading edge and beneath the Tibetan Plateau. The model accounts for underthrusting of continental lithosphere over 200–350 km at the longitude of the sampled region in central Nepal. A minimum estimate for the northern extent of India (post-Neotethys formation) is proposed, based on this magnitude of rotational underthrusting and on generally accepted values for crustal shortening across the Himalaya. Within the modified Smith and Hallam reconstruction of Gondwanaland, this estimate satisfies constraints imposed by seafloor spreading data from the eastern Indian Ocean and facies analyses of the western Australian continental margin. The palaeomagnetic results herein presented as well as those obtained elsewhere in the Himalayan region can be interpreted in support of Molnar and Tapponniers model for deformation of southern Asia.

The extent of Greater India, II. Palaeomagnetic data from the Ladakh Intrusives at Kargil, northwestern Himalayas

The Ladakh Intrusives at Kargil (central Ladakh, northwestern Himalayas), immediately north of the Indus-Tsangpo suture zone, have been studied palaeomagnetically. These intrusives were emplaced before and during the early Tertiary with final closure of the isotopic systems between 49 and 45 m.y. ago. Six different magnetization components have been identified, on basis of which a two-stage collision model is proposed. Two magnetically very “soft” components are present in most of the samples examined. They represent (1) a recent field component, and (2) an eastwards directed component streaking towards the plane of the east-west-oriented foliation pattern. Two other magnetically rather “soft” components are present in some of the samples only. One of these components (3) indicates a palaeolatitude of the sampled area at about 23°N and suggests crustal shortening in south-central Asia over more than 10° of latitude after final collision of the Ladakh island arc, then already united with the Indian subcontinent. The other soft component (4) may be related to younger phases of the Himalayan Orogeny (Mio-Pliocene or younger). Two magnetically very “hard” components (5 and 6) indicate a palaeolatitude of the sampled area at 7–10°N. These components were acquired shortly before final closure of the isotopic systems, but after initial collision (pre-Middle Eocene) of the Indian subcontinent with the Ladakh island arc along the Indus-Tsangpo suture zone. This palaeolatitude of the sampled area is in good agreement with palaeolatitudinal control of the Indian plate from DSDP core data and indicates that after their initial collision, this combined Indian subcontinent—Ladakh island arc block moved northwards over a total distance of about 25° of latitude. Declination control of the various magnetization components suggests that after initial collision the sampled area followed the counterclockwise rotation of the Indian plate, which subsequently reversed to a clockwise rotation after final collision with Asia.

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.

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.

Palaeomagnetic constraints on formation of the Mianwali reentrant, Trans-Indus and western Salt Range, Pakistan

Abstract Successions of Lower to lower Middle Cambrian, Upper Permian to Upper Triassic and Lower Tertiary carbonates and arenites have been sampled in five sections, representative of the three main segments of the Mianwali reentrant in the (Trans-Indus) Salt Range (northern Pakistan), i.e.: the southern Khisor Range, the northern Surghar Range and the western Salt Range. Comparison of primary and secondary magnetization directions with the Indian APWP demonstrates the secondary origin of the Mianwali reentrant and shows a pattern of rotations which varies in sense and magnitude along the reentrant with the main structural trends. Data from the Trans-Indus and western Salt Range and published Early Cambrian, Early Permian and Late Tertiary palaeomagnetic results from the southern Salt Range and the Potwar Plateau show that the Hazara Arc underwent a 20–45° counterclockwise rotation relative to the Indian Shield. A contrasting clockwise rotation over about 45° has recently been established for thrust sheets in the opposing eastern limb of the Western Himalayan Syntaxis, i.e. for the Panjal Nappe [1] and the Riasi thrust sheet [2]. These palaeomagnetically established rotations conform with the about 75° azimuthal change in structural trend along the Syntaxis, and support Crawfords [3] suggestion that the Salt Range was originally in line with the northwestern Himalaya. The Salt Range front prograded and moved southwards as part of the Hazara Arc thrust sheet, detached from basement along the evaporitic Salt Range Formation. The Mianwali reentrant originated through obstruction of the southwards advancing thrust sheet by moulding around basement topography of the northwest oriented Sarghoda Ridge.

A palaeomagnetic reconnaissance of Kashmir, northwestern Himalaya, India

Abstract Thermal demagnetization studies of 800 Palaeozoic and Lower Mesozoic samples, mainly carbonates, from 14 localities in Kashmir, showed four magnetic components: (A)A recent field component of normal polarity and large intensity. (B)A Middle to Late Tertiary secondary component of large intensity and of reversed and normal polarity, which indicates clockwise rotation of central and eastern Kashmir over approximately 45° with respect to peninsular Indo-Pakistan. (C)Primary magnetic components of Triassic, Late Permian, Early Carboniferous, Siluro-Ordovician and Middle to Early Ordovician age. The Triassic and Late Permian components show a 20–30° cumulative counterclockwise offset with respect to the Indo-Pakistan apparent polar wander path (APWP). This indicates a post-Middle to -Late Triassic counterclockwise rotational movement of Kashmir over 65–75° with respect to peninsular Indo-Pakistan. (D)A component of low inclination and predominantly east-west declination, which is observed only in rocks of pre-Panjal Traps age. A secondary origin associated with extrusion of the Panjal Traps flows is surmised, but could not be established beyond doubt. Alternatively, this component may be related to the regional foliation. The Triassic and Late Permian primary component directions are predominantly of normal polarity. This suggests a pre-Punjabian (Late Permian) upper age limit for the Permo-Carboniferous (Kiaman) reversed polarity interval, which is earlier than the Early to Late Tatarian limit observed in the U.S.S.R.

Rotational overthrusting of the northwestern Himalaya: further palaeomagnetic evidence from the Riasi thrust sheet, Jammu foothills, India

Abstract Thermal demagnetization results (316 samples) are presented for the Tertiary succession of the Riasi thrust sheet (Jammu foothills, northwestern Himalaya). Primary and secondary magnetization directions of Murree Group red beds (Miocene to Upper Eocene) sampled northeast of Jammu indicate, for this part of the Riasi thrust sheet, a clockwise rotation over about 45° with respect to the Indian shield since Late Eocene/Early Miocene time. This accords with clockwise rotations of similar magnitude observed in the Panjal Nappe and the Krol Belt, and is interpreted as representative for the northwestern Himalaya. Results from the western part of the Kalakot inlier, sampled northwest of Jammu, i.e. basal Murree claystone (Middle Eocene) and carbonate from the Subathu Group (lower Middle to Lower Eocene), indicate an aberrant 20–25° counterclockwise rotation which is of local importance only. Available observations on rotation of Himalayan thrust sheets with respect to the Indian shield, indicate that the Himalayan Arc has formed through oroclinal bending. This supports Powell and Conaghans and Veevers et al.s model of Greater India with large-scale intracontinental underthrusting along the Main Central Thrust beneath the Tibetan Plateau. Minimal magnitudes of underthrusting of 550 km in the Krol Belt and 650 km in the Thakkhola region are concluded. Palaeolatitude observations (herein and in [1[) agree with absolute positioning of the Indian plate based on India-Africa relative movement data fixed to a hotspot frame in the Atlantic Ocean, and with palaeolatitude observations from DSDP cores on the Indian plate. Collision-related secondary magnetic components observed both to the north and to the south of the Indus-Tsangpo Suture zone show palaeolatitudes between the equator and 7°N. Comparison of both datasets indicates that initial contact between Greater India and south-central Asia had been established in the Hindu Kush—Karakorum region by about 60 Ma ago whereas eastwards progressive suturing had advanced to the Lhasa Block segment of the Indus-Tsangpo Suture zone before 50 Ma ago.

Sedimentary basins of eastern Australia: paleomagnetic constraints on geodynamic evolution in a global context

Changes in plate movements cause intraplate deformation and lead to basin development, fluid flow and mineralisation phases. Movement changes are detailed by seafloor-spreading data, back to the Oxfordian, and by paleomagnetic data before that time. Paleomagnetism records and interprets plate movement changes as pole path features—loops, bends, overprints—and these are applicable as tectonic and stratigraphic baselines at continental and global scales. Australian pole path features, from Late Paleozoic to Holocene, are interpreted in terms of regional evolution of eastern Australian basins and in relation to global plate movements. Emphasis is on the Late Paleozoic pole path of Australia. Two opposing views, the SLP path and the KG path, are discussed. Discussed also is an emerging pole path for the New England Orogen (NEO path), representing a potentially more detailed version of the KG path. The Mesozoic pole path of Australia is understudied and poorly defined. The Cenozoic path is better defined, but disputed in detail. The Late Paleozoic to Holocene pole path shows four major loops, L2 to L5, and several bends and lesser features. The L2 loop (mid-Carboniferous apex) relates to formation of the Westralian Superbasin; the L3 loop (Late Carboniferous–Early Permian) relates to formation of the Bowen–Gunnedah–Sydney basin system and oroclinal deformation of the Southern New England Orogen; the poorly defined L4 loop (Late Triassic–Early Jurassic) relates to formation of basins in eastern Queensland and rifting along the New Guinean margin of the Australian Plate; the poorly defined L5 loop (Late Jurassic–Early Cretaceous) relates to rifting along the western, northeastern and southern margins of the Australian Plate. Other features—minor Early Permian and Late Permian loops, bends of mid–Late Triassic, Early–Late Cretaceous and latest Cretaceous–earliest Tertiary age, and a Mio-Pliocene excursion—likewise are interpreted in terms of tectonostratigraphic and basin-forming phases. The L2 and L3 loops in particular are interpreted also within a global context. The L2 loop indicates a Late Devonian to mid-Carboniferous northward excursion of northeastern Gondwanaland, interpreted as leading to contact with the central Asian Altaids. This identifies the Central Asian Orogenic Belt and the Kanimblan and Alice Springs Orogenies as Pangea-forming, Variscan orogens on conjugate, northeastern Gondwanaland and southeastern Laurasian, margins of the Paleoasian Ocean. Alice Springs Orogeny-related deformation may have led to eastward extrusion of the Thomson Orogen, a Variscan equivalent to Cenozoic India–Asia deformation. The succeeding early-Late Carboniferous southward movement of northeastern Gondwanaland was extremely fast and created an extensional environment, initiating the Westralian Superbasin. The L3 loop reflects a fundamental change in rotation of Gondwanaland from counterclockwise (Late Carboniferous) to clockwise (Early Permian), leading to Stephanian initiation of the Bowen–Gunnedah–Sydney basin system and Early Permian oroclinal deformation of the Southern New England Orogen. The Texas, Coffs Harbour and Manning Oroclines are interpreted as telescoped mega dragfolds related to a major dextral shear system along the Protopacific margin of Gondwanaland and Alice Springs Orogeny-related dextral shear along the Darling River and Cobar–Inglewood Lineaments. A relationship between magnetic overprint phases and the younger limb and/or apex of pole path loops is emerging. Practical application in identifying and dating fluid flow and mineralisation phases has been demonstrated for the Paleoproterozoic–Mesoproterozoic pole path of Australia, but not yet so for the Phanerozoic pole path.

Carboniferous palaeomagnetism of the Werrie Block, northwestern Tamworth Belt, and the New England pole path*

Palaeomagnetic, rock‐magnetic and magnetic‐fabric results are presented for Carboniferous and Lower Permian volcanic and volcaniclastic rocks (Merlewood Formation, Currabubula Formation, Werrie Basalt and Boggabri Volcanics),mainly from the Werrie Block of the northwestern Tamworth Belt, southern New England Orogen. Detailed thermal demagnetisation results (91 sites, 998 samples) show two groups of magnetic components with low (<400°C; LT) and high (500–700°C; HT) unblocking temperature ranges. Rock magnetic tests indicate the HT components to reside in magnetite, with single or pseudo‐single domain and multidomain characteristics, and in hematite. Detailed demagnetisation up to 700°C demonstrates subtle directional differences between primary components, optimally cleaned and derived from the magnetite‐hematite carrier (HT‐P2), and pseudocomponents, incompletely cleaned and derived from the mainly magnetite carrier (HT‐P1). Directional results evidence three phases of magnetic overprinting which arose from: (i) a possible Middle‐Late Cenozoic regional weathering event (LT); (ii) fluid movements associated with the Permo‐Triassic Hunter‐Bowen Orogeny (HT); and (iii) formation of the Bowen‐Gunnedah‐Sydney Basin system in the latest Carboniferous ‐ Early Permian (HT). Magnetic fabric results show the Kmax and Kmin axes of the susceptibility anisotropy ellipsoids to better concentrate in stratigraphic than in geographical (in situ) coordinates, suggesting a primary depositional pattern. The prevailing north‐south alignment of Kmax axes changes to east‐west towards the top of the Currabubula Formation. The sense of transport could not be established in absence of evidence for imbrication. Well‐defined primary magnetisation components (HT‐P2) have been determined for 39 of the 69 Currabubula Formation sites and for all sites of the Merlewood Formation (7), Werrie Basalt (9) and Boggabri Volcanics (6). Currabubula Formation site results have been combined into three mean‐site results, all with positive fold tests at the 99% confidence level. They show good agreement with correlatable results from the northward adjacent Rocky Creek Block. Integration of the palaeomagnetic poles for the Currabubula Formation and a virtual geomagnetic pole for the Merlewood Formation with earlier determined Visean to Westphalian poles for the Rocky Creek Block outlines a Visean (Tournaisian?) to Stephanian pole path for the northwestern Tamworth Belt. Comparison with a preliminary pole path for the Rouchel and western Gresford Blocks shows no evidence for significant rotational deformation between the northwestern and southwestern Tamworth Belt. Comparison with the Northern Hastings Block indicates counterclockwise rotation of the latter relative to the northwestern Tamworth Belt over about 150°, in agreement with structural estimates.