Michael G. Petterson

RbSr dating of the Kohistan arc-batholith in the Trans-Himalaya of north Pakistan, and tectonic implications

The Kohistan arc-batholith in northern Pakistan is situated between the Indus Suture and the Northern Suture. It belongs to the Trans-Himalayan belt which continues eastwards as the Ladakh arc-batholith in northwest India and the Kangdese batholith in southern Tibet. In Kohistan the island arc consists (upward sequence) of the Chilas stratiform complex of norites, noritic gabbros and chromite-layered dunites which formed in the sub-arc magma chamber, plutons of tonalite and diorite, one of which has a RbSr isochron age of 102 ± 12 Ma, the Chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites, and the Yasin Group sediments which formed in overlying intra-arc basins and which contain Albian-Aptian faunas. All these rocks were deformed by major fold structures which are correlated with the formation of the Northern Suture. NE-SW trending basic Jutal-Nomal dykes (north of Gilgit) cross-cut all the above structures and have a39Ar/40Ar hornblende age of 75 Ma. Thus the Northern Suture must have formed in the period 102 ± 12 to 75 Ma. With the island arc attached to the Eurasian plate, further northward subduction of the Tethyan plate gave rise to an Andean-type batholith, two plutons of which have RbSr isochron ages of 54 ± 4 Ma and 40 ± 6 Ma. The Dir-Utror Group of calc-alkaline lavas (and sediments with Eocene fossils) are remnants of the volcanic cover of the Andean-type batholith. We suggest that late Cretaceous blueschists formed during subduction under the active continental margin, and that continental collision and formation of the Indus Suture was in the Eocene. The batholith was intruded by layered aplite-pegmatite sheets at 34 ± 14 Ma and 29 ± 8 Ma (RbSr ages) in post-collisional times. The87Sr/86Sr initial ratios of all the dated rocks range between 0.7039 and 0.7052.

Collision tectonics in the NW Himalayas

Summary West Himalayan tectonics involve the collision of microplates between the Indian and Asian Plates. The Kohistan Complex consists largely of tightly folded basic volcanics and sediments generated as Late Jurassic to Late Cretaceous island arcs. These were intruded by post-folding Mid-Cretaceous — Eocene plutonics produced from continued subduction of the Indian Plate after closure of a suture between Kohistan and the Karakorum. The Himalayan structures show major thrust sheets and the Kohistan Arc is essentially a crustal ‘pop-up’ with southward-upright and northward-verging structures developed above a thick ductile decoupling zone (the Indus Suture), which can be traced for >100 km beneath Kohistan on large reentrants. This pop-up formed by a two stage process, closure of the Northern Suture followed by closure of the southern Indus Suture. Granitic rocks of the Kohistan-Ladakh Batholith (dated at ≅ 100-40 Ma) post-date most of the structures related to the Northern Suture but were deformed and carried southwards on shear structures related to the Indus Suture. Post-collisional deformation carried this Kohistan Complex on deep decoupling zones over the Indian Plate on a series of imbricated gneiss sheets, the thrusts climbing up section in the movement direction so that in the far S some override their own molasse debris. Folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms—i.e. the Nanga Parbat and Hazara Syntaxes. The Nanga Parbat Syntaxis probably formed due to a shear couple near a branch line where one of the main Himalayan thrusts joined the Indus Suture beneath Kohistan. Crustal delamination, to produce the imbricated gneiss sheets, could not account for all the displacement of India into Asia, suggested by palaeomagnetic data. There must also have been lateral displacement as demonstrated by the large oblique-slip shear zone in the Hunza Valley, N of Kohistan.

A re-evaluation of the stratigraphy and evolution of the Kohistan arc sequence, Pakistan Himalaya: implications for magmatic and tectonic arc-building processes

New field mapping and structural data, combined with published geochemical data, from the Kohistan arc in the NW Himalaya, enable a re-evaluation of the arc stratigraphy. Key lithological units and their relationships are more clearly defined, permitting the construction of a revised magmatic-tectonic history for the arc. The oldest units are transitional oceanic-type basalts, which form the basement to the subduction related sequence. Arc-type gabbroic sheets and plutons intrude the oceanic basalts: together these form the Kamila Amphibolite Belt. Metasediments and basaltic lavas were deposited, within an extensional basin, onto the Kamila Amphibolite Belt basement. This sequence, exposed across the arc, forms a distinct stratigraphic unit which is formally defined here as the Jaglot Group. Sediment-charged turbidity currents transported material into the basin, whilst submarine eruptions contributed the basaltic component. This period of extension culminated in the eruption of high-Mg boninites of the Chalt Volcanic Group which overlie the rocks of the Jaglot Group. The earliest granitoids of the Kohistan Batholith predate suturing and intrude the Jaglot and Chalt sequences. At c. 100 Ma Kohistan sutured to Asia. suturing being accompanied by thickening of the arc with the development of major intra-arc shear zones and a penetrative, regionally developed steep cleavage. At c. 85 Ma intra-arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the Chilas Complex which clearly intrudes the Kamila Amphibolite Belt to the south and the Jaglot Group to the north. The Chilas Complex has been regarded as part of the pre-suturing, juvenile arc sequence. Field evidence summarized here show this to be not so. Heat advection associated with emplacement of the Complex caused amphibolite facies regional metamorphism, melting of the lower arc and plutonism. Some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 Ma, before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 Ma. At least three phases of extension and rifting, each separated by short lived phases of compression, characterized arc evolution. Much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone.

Structure and deformation of north and central Malaita, Solomon Islands: tectonic implications for the Ontong Java Plateau-Solomon arc collision, and for the fate of oceanic plateaus

Abstract The island of Malaita, Solomon Islands, represents the obducted southern margin of the Ontong Java Plateau (OJP). The basement of Malaita formed during the first and possibly largest plateau-building magmatic event at ∼122 ± 3 Ma. It subsequently drifted passively northwards amassing a 1–2 km thickness of pelagic sediment overburden. A major change in OJP tectonics occurred during the Eocene, possibly initiated by the OJP passing over the Samoan or Raratongan hotspot. Extension facilitated increased sedimentation and basin formation (e.g., the Faufaumela basin) and provided readily available deep-crustal pathways for alkali basalt and subsequent Oligocene alnoite magmas, with related hydrothermal activity producing limited Ag + Pb mineralisation. Eocene to Mid-Miocene sediments record the input of arc-derived turbiditic volcaniclastic sediment indicating the relative closeness of the OJP to the Solomon arc. The initial collision of the OJP and Solomon arc at 25-20 Ma was of a ‘soft docking’ variety and did not result in major compressive deformation on Malaita. South-directed subduction of the Pacific Plate briefly ceased at this time but resumed intermittently on a local scale from ∼15 Ma. Subduction of the Australian Plate beneath the Solomon arc commenced at ∼8-7 Ma. Increased coupling between the Solomon arc and the OJP led to the gradual emergence of the OJP at 6-5 through to 4 Ma. The most intense period of compressive to transpressive deformation recorded on Malaita is stratigraphically bracketed at between 4 and 2 Ma, resulting in estimated crustal shortening of between 24 and 46%, and the inclusion of between 1 and 4 km of basement OJP basalts within the larger anticlines. Basement and cover sequences are deformed together in a coherent geometry and there are no major decollement surfaces; the large asymmetrical fold structures of Malaita are likely to be the tip regions of blind thrusts with detachment surfaces between 1 and 4 km beneath the cover sequence. Mid-Pliocene deformation records the detachment of the upper parts of the OJP, with initial material movement direction towards the northeast and later obduction of an upper allochthonous block of the OJP southwestwards over the Solomon arc. A model is presented whereby an upper 5–10-km-thick flake of the OJP is obducted over the Solomon arc to form the Malaita

Changing source regions of magmas and crustal growth in the Trans-Himalayas: evidence from the Chalt volcanics and Kohistan batholith, Kohistan, northern Pakistan

The Kohistan batholith and Chalt volcanics form a major part of the Kohistan island arc terrane in northern Pakistan. They record some 70–80 Ma of magmatism within the northwestern Himalayan orogen; the Chalt volcanics are Albian-Aptian (mid-Cretaceous), and currently available RbSr age data for the batholith indicate an intrusive age span of 102 to 30 Ma. The volcanics are composed of medium-K, calc-alkaline andesites and low-K, high-Mg, tholeiites, some of which have boninitic and basaltic komatiitic chemistry. Three intrusive stages are recognised in the batholith: (1) (110–90 Ma) comprises low-K trondhjemites and medium-high-K calc-alkaline gabbro-diorites with associated hornblendite cumulates; (2) (85–40 Ma) comprises low-high-K, calc-alkaline gabbro-diorites (with hornblendite cumulates, granodiorites and granites; and (3) (circa 30 Ma) comprises biotite ± muscovite ± garnet leucogranites. 87Sr/86Sr0 for the batholith range between 0.7039 and 0.7052. Four magmatic source regions (Sce 1–Sce 4) have been identified. Sce 1 is a variably metasomatised mantle wedge situated above an active subduction zone during stages 1 and 2. Sce 2 is a harzburgite depleted with respect to major elements and incompatibles, whilst retaining “subduction-related” trace element ratios. Sce 2 melted during stage 1 within a fore-arc position during the subduction of young, hot oceanic crust to provide the necessary extra thermal input for harzburgite anatexis. Melts from Sce 2 produced the high-Mg tholeiitic volcanics. Sce 3 melted during stage 1 to produce the low-K trondhjemites, and it is envisaged as incompatible element-depleted primitive arc crust with a similar composition to the depleted Chalt volcanics. Sce 4 melted at 30 Ma during stage 3 as India underthrust Eurasia: frictional heating and dehydration of the Indian plate caused crustal anatexis within the deep Kohistan arc and these melts formed the stage 3 leucogranites. The low 87Sr/86Sr initial ratios for the great bulk ( > 95%) of the batholith indicate that it represents a major juvenile addition to the continental crust. This conclusion has important implications for crustal growth theories of island arc accretion and batholith underplating.

Maximising Multi-Stakeholder Participation in Government and Community Volcanic Hazard Management Programs; A Case Study from Savo, Solomon Islands

Participatory rural appraisal (PRA) methods and philosophies were trialed in a volcanic risk management planning and awareness activity for Savo Island, a historically highly destructive volcano in the Solomon Islands. Through a combination of methods we tried to combine the roles of facilitators and educators, and to involve the input of all stakeholders (from community to national government) in the process of volcanic risk management. The PRA approach was an ideal way to address the fundamental differences in outlook, education, needs, and roles of individuals and groups involved or affected. It was also an important catalyst to Savo island- or community-based planning initiatives, which are arguably the most important step toward the preparedness of the 2500 inhabitants of the island for any future destructive volcanic activity. We adapted almost every tenet of the PRA philosophy through inexperience, self-perceived importance and desire to combine both scientific and traditional views for Savo volcanic risk management planning. Nevertheless, what emerged from our experiences was an idea of how fundamentally well suited many PRA approaches are to initiating dialogue within diverse stakeholder groups, and deriving combined scientific/geologic and local/community risk assessments and mitigation action plans. The main challenge remaining includes increasing the involvement or voice of less powerful community members (women, youth, non-landowners) in risk management decision-making in such male-dominated hierarchical societies.

Volcanostratigraphy of arc volcanic sequences in the Kohistan arc, North Pakistan: volcanism within island arc, back-arc-basin, and intra-continental tectonic settings

Abstract The Kohistan arc was initiated, offshore of Asia, during the mid-Cretaceous above northward subducting, Tethyan oceanic crust. The arc sutured to Asia c. 90 Ma ago. Subduction of oceanic crust beneath the arc continued until Indian Plate continental rocks began to underthrust the arc c. 50 Ma ago. The arc shows an evolutionary history from the juvenile stages of an intra-oceanic island arc, through a thickened Andean-style volcanic arc accreted to a continental margin, to an arc underplated by older continental crust. Extrusive volcanic activity spanned the mid-Cretaceous to Oligocene. This paper presents new and detailed lithostratigraphic data relating to two volcanic groups. The mid-Cretaceous Chalt Volcanic Group (CVG) documents volcanism in the last stages of the island arc phase. The Eocene-Oligocene Shamran Volcanic Group (SVG) documents Andean margin to post-Himalayan collision volcanism. The CVG comprises two formations, formally defined here. The back-arc Hunza Formation is dominated by subaqueous back-arc effusive basalt, andesite and boninite volcanism with a brief phase of subaerial silicic volcanism. The intra-arc Ghizar Formation comprises basalt and andesite-dominated crystalline and volcaniclastic rocks produced by subaerial and subaqueous calc-alkaline arc stratovolcano and shield eruptions. Two facies are present: a basalt and andesite lava flow-dominated sequence and a volcaniclastic-dominated sequence with characteristics that indicate effusive-explosive volcanism and subsequent volcanic sediment reworking and deposition within both subaqueous and subaerial settings. A stratovolcanic centre in the Ishkoman Valley contains abundant proximal volcanic lithofacies suggestive of Strombolian–Vulcanian explosive eruptive activity. The SVG, which unconformably overlies deformed rocks of the CVG, crops out in relatively small, high-altitude outliers. Previous suggestions that it has a large outcrop area in western Kohistan are unfounded. The SVG is an undeformed sequence of reddened, dominantly silicic volcanic rocks comprising mainly andesitic to dacitic and rhyolitic lavas, parataxitic and eutaxitic welded silicic ignimbrites, poorly sorted volcaniclastic sandstones, conglomerates and tuffs, and well-sorted, very fine-grained vitric tuffs. The SVG records highly evolved explosive and effusive volcanism within a mature Andean continental margin to post-Himalayan collisional environment. Primary magmas were probably generated at c. 40–30 Ma within relict metasomatised Tethyan mantle wedged between the Kohistan arc above and the underplating Indian Plate below.

Petrogenetic implications of neodymium isotope data from the Kohistan batholith, North Pakistan

Neodymium data are presented for five granitoid (trondhjemite, granite and leucogranite) plutonic units from the Kohistan batholith aged 102 Ma to 29 Ma. These have present day l43Nd/144Nd ratios of between 0.512980 and 0.512734, calculated initial 143Nd/144Nd ratios of between 0.512861 and 0.512705 and Nd values of between 6.91 and 2.04. There is a decrease in ɛNd with time which is inversely correlative with a similar increase in ɛSr. Three plutonic units (Matum Das, Gilgit and Shirot) formed from a source enriched in Sm and depleted in Rb or radiogenic Sr relative to bulk earth, whilst a fourth unit (the Indus Confluence acid sheets), is only slightly enriched in radiogenic Sr. These four units define a temporal trend from the least evolved Matum Das pluton to the most evolved Indus Confluence acid sheets. This trend was produced as a result of subducting oceanic sediment or seawater-altered oceanic crust which melted or dehydrated and increasingly modified the isotopic composition of an original mantle protolith situated above the subducting plate. The data preclude any significant input to the magmatism from ancient crust. New data presented here, together with other published data, indicate an immature metavolcanic crustal source for the Matum Das pluton, and a plutonic basement crustal source for the Indus Confluence acid sheets. These crustally derived units retain a mantle isotopic signature indicating that the geochemical signature of Kohistan evolved by remobilization of recently formed arc crust in addition to new inputs of mantle-derived magma. A fifth plutonic unit (the Parri acid sheets) shows a clear compositional break from the other units, being significantly enriched in radiogenic Sr and Nd with respect to bulk earth. The source to the Parri acid sheets is interpreted as metasedimentary rocks with a high Rb/Sr ratio and a crustal residence time of c. 70 Ma.

The Oyut Ulaan Volcanic Group: Stratigraphy, magmatic evolution and timing of Carboniferous arc development in SE Mongolia

Abstract: The Palaeozoic–Mesozoic tectonic evolution of Central Asia, including the vast terrane collage that makes up Mongolia, has been a topic of considerable debate. The Oyut Ulaan Volcanic Group is a sequence of volcanic and sedimentary rocks in SE Mongolia that forms the southern part of the Devonian–Permian Saykhandulaan Inlier. Fieldwork traverses and mapping have established four distinct formations in the Oyut Ulaan Volcanic Group that record the nature of arc activity in part of the Central Asian Orogenic Belt during the Carboniferous. Physical volcanological and sedimentological characteristics of the four formations suggest three clear eruptive styles: (1) periodic andesite volcanism in an actively eroding arc setting that also contained large rivers and swamps; (2) highly effusive plateau andesite volcanism; (3) explosive rhyolitic effusion. Geochemical analyses of volcanic lithologies suggest that the group represents subduction-related, mature, continental arc volcanism. Geochemical results document an evolving magma system to which surface processes of the volcano-sedimentary model may be linked. Magma pulses and replenishments are identified from variations in chemostratigraphy. Newly obtained zircon ages from the volcanic succession fix its emplacement (eruption) at 323.0 ± 0.7 Ma (mid-Carboniferous or late Mississippian). A granite cobble from the lower part of the Oyut Ulaan Volcanic Group gives a U–Pb zircon age of 338.9 ± 0.4 Ma indicating that arc plutons were emplaced 10 Ma prior to the Oyut Ulaan volcanism and were eroded soon after. Our work provides timing constraints for final accretion and continental assembly in SE Mongolia, and also sheds light on the petrological development of a magmatic arc system within an evolving accretionary orogen.

A Review of the geology and tectonics of the Kohistan island arc, north Pakistan

Abstract This paper summarizes some 30 years of more intense recent work and almost 100 years of geological observations in Kohistan. The paper is divided into two section: an earlier factual-based section with minimal interpretation, and a later section summarizing a range of ideas based on the data as well as presenting new thoughts and interpretations. Kohistan is a c. 30 000 km2 terrane situated in northern Pakistan. The great bulk of Kohistan represents growth and crustal accretion during the Cretaceous at an intra-oceanic island arc dating from c. 134 Ma to c. 90 Ma (Early to Late Cretaceous). This period saw the extrusion of c. 15–20 km of arc volcanic and related sedimentary rocks as well as the intrusion of the oldest parts of the Kohistan batholith, lower crustal pluton intrusion, crustal melting and the accretion of an ultramafic mantle–lower crust sequence. The crust had thickened sufficiently by c. 95 Ma to allow widespread granulite-facies metamorphism to take place within the lower arc. At around 90 Ma Kohistan underwent a c. 5 Ma high-intensity deformation caused by the collision with Eurasia. The collision created crustal-scale folds and shears in the ductile zone and large-scale faults and thrusts in the brittle zone. The whole terrane acquired a strong penetrative foliation fabric. Kohistan, now an Andean margin, was extended and intruded by a diapiric-generated crustal-scale mafic–ultramafic intrusion (the Chilas Complex) with a volume of 0.2×106 km3 that now occupies much of the mid–lower crust of Kohistan and had a profound impact on its thermal structure. The Andean–post-collisonal (c. 90–26 Ma) period also saw the intrusion of the stage 2 and 3 components of the batholith and the extrusion of the Dir Group and Shamran/Teru volcanic rocks. Collision with India at c. 55–45 Ma saw the rotation, upturning, underplating and whole-scale preservation of the terrane. The seismic structure of Kohistan has some similarities to that of mature arcs such as the Lesser and Greater Antilles and Japan, although Kohistan has a higher proportion of high-velocity granulites in the lower crust. The chemical composition of Kohistan is very different from that of average continental crust, although it is similar to an analogue obducted arc within Alaska (Talkeetna), suggesting that ‘mature’ continental crust undergoes a series of geochemical processes and reworking to transform an initial stage 1 ‘primitive arc crust’. Most of Kohistan is gabbroic in composition, particularly within the lower and middle crust. A high proportion of the ‘basement’ volcanic units is also basaltic to basaltic andesite with smaller proportions of boninite, andesite to rhyolite, ignimbrite and volcaniclastic material. Post Eurasian-collision ‘cover’ volcanic rocks are highly evolved, comprising predominant rhyolites, ignimbrites and related volcaniclastic rocks. Most lithological units throughout the crustal section have an arc-like geochemical composition (e.g. high LREE/HREE and LFSE/HFSE ratios) although some have oceanic (main ocean and back-arc) characteristics. Isotopic compositions indicate that the great bulk of igneous rocks have an ultimate sub-arc mantle source. In broad terms the Kohistan terrane represents a juvenile mantle extract addition to the Phanerozoic continental crust with a total volume of c. 1.2×106 km3 (equivalent to c. 1/50 the volume of the Ontong–Java Plateau or Alaska).