Robert D. Lawrence

Strike-slip faulting terminates the Basin and Range province in Oregon

The pattern of faulting in southeastern Oregon is interpreted in terms of four major zones of right-lateral strike-slip faulting that separate blocks broken by normal faulting. The total amount of east-west extension is considered to decrease in the block north of each strike-slip fault zone. The right-lateral offset results from the decrease in extension. Extension essentially dies out across the northern two fault zones, which are thus considered the northern limit of the Basin and Range province. The greatest offset is apparently recorded on the next zone to the south by the displacement of the eastern edges of the Sierra Nevada and Idaho batholiths. The two southern zones offset the Pleistocene to Holocene trend of the High Cascades by 10 to 20 km in a right-lateral sense. The Brothers fault zone, one of the northern zones, is regarded as of special interest because both ends of the fault are interpreted to be exposed at the surface.

Thrust and strike slip fault interaction along the Chaman transform zone, Pakistan

Summary The interaction between thrust and strike slip fault systems is well detailed in Pakistan where the Chaman transform zone connects the Makran and Himalayan convergence zones and contains an internal convergence zone in the Zhob district. The transform zone contains numerous strike slip faults of which the Chaman fault proper is the westernmost. We can demonstrate at least 200 km of left lateral displacement along the Chaman fault alone. In the Zhob belt N-S shortening by folds and a major thrust fault amounts to several dozen kilometres. The 400 km wide Makran convergence zone is now being shortened by E-W oriented folds, thrust faults, and reverse faults. As these faults in the Makran zone approach the transform zone, their traces bend to the N and motion on each of them becomes oblique, combining reverse and left lateral slip. They merge continuously with the strike slip faults of the Chaman transform zone. The Makran thrust system and the Chaman transform zone first became active in the late Oligocene or early Miocene. Later (Pliocene?), a component of left lateral shear occurred across the entire Makran Zone in association with the opening of the newly identified Haman-i-Mashkel fault trough S of the Chagai Hills and W of the Ras Koh. The total displacement and displacement rate across the Chaman transform zone varies in response to the rates of convergence in the plates E and W of the zone.

Late Cretaceous ophiolite obduction and Paleocene India-Asia collision in the westernmost Himalaya

AbstractCollision of the Kohistan island arc with Asia at ~100 Ma resulted in N-S compression within the Neo-Tethys at a spreading center north of the Indo-Pakistani craton. Subsequent India-Asia convergence converted the Neo-Tethyan spreading center into a short-lived subduction zone. The hanging wall of the subduction zone became the Waziristan, Khost and Jalalabad igneous complexes. During the Santonian- Campanian (late Cretaceous), thrusting of the NW IndoPakistani craton beneath Albian oceanic crust and a Cenomanian volcano-sedimentary complex, generated an ophiolite-radiolarite belt. Ophiolite obduction resulted in tectonic loading and flexural subsidence of the NW Indian margin and sub-CCD deposition of shelf-derived olistostromes and turbidites in the foredeep. Campanian-Maastriehtian calci- clastic and siliciclastic sediment gravity flows derived from both margins filled the foredeep as a huge allochthon of Triassic-Jurassic rise and slope strata was thrust ahead of the ophiolites onto the Indo-P...

Detrital modes and provenance of the Paleogene Khojak Formation in Pakistan: Implications for early Himalayan orogeny and unroofing

Different tectonic settings have characteristic detrital modes and sediment-dispersal patterns. Detrital modes and sediment-dispersal patterns of the siliciclastic Khojak Formation in the Katawaz basin, Pakistan, suggest that its sand was derived from the early Himalayan orogen and longitudinally transported to the Katawaz remnant ocean, where it was deposited as a delta−submarine-fan complex. Modal analysis of the Khojak Formation suggests composition that is dominated by subangular quartz with abundant lithic fragments and minor feldspar, i.e., Qt 60 F 9 L 31 (Qt, total quartz; F, feldspar; L, lithic fragments). The predominance of quartz, sedimentary, and low-grade metamorphic lithic fragments suggests early derivation from a collision orogen; scarcity of detrital feldspar and volcanic lithic fragments precludes a magmatic arc as the main source. The decrease in monocrystalline quartz, concomitant increase in total lithic percentages, and relative abundance of low-grade metamorphic lithic fragments from the bottom to the top of the Khojak Formation reflect progressive erosional history of the early Himalaya. This history is part of a previously known major unroofing trend collectively depicted by the detrital modes of the Murree Formation, Siwalik Group, and the modern Indus fan in the Indian Ocean. These detrital modes are also related in time and space.

Discovery of the palaeo-Indus delta-fan complex

The Indus River carries sediments from the western Himalaya and deposits some of these as channel and floodplain sediments or molasse. The rest of its load forms the Indus delta at the margin of the Indian Ocean. The Indus delta passes some of its sediments to be deposited as Indus submarine fan turbidites. Thus, as elsewhere, Himalayan molasse, delta, and fan deposition are related hi tune and space. However, when we examine fluvial and marine age-range data of the older Indus deposits, in terms of this sedimentary assemblage, a major portion of its marine record is missing. The oldest known molasse along the Indus suture zone, and hi the foredeep are middle Eocene and late Palaeocene hi age, respectively. A recent synthesis of sedimentation in the northern Indian Ocean, however, shows that turbidite sedimentation started around early Miocene in the modern Indus fan, and even later in the Bengal fan. Where are the Palaeogene Indus delta and fan sediments? We suggest herein that these are preserved as the Palaeogene siliciclastic Khojak Formation hi the Katawaz Basin and eastern Makran. The modern Indus River is 2900 km long (Coumes & Kolla 1984) and has 30 000 m3 s − 1 discharge (Milliman et al. 1984) in summer (Fig. 1), twice that of the Mississippi River. The annual suspended sediment load (Wells & Coleman 1984) varies between 395 and 435 x 103 t, almost equal to that of the Mississippi. The existence of the middle Eocene to middle Miocene Indus molasse along the

Structure and Evolution of the Northern Potwar Deformed Zone, Pakistan

The northern Potwar deformed zone (NPDZ) is part of the active foreland fold and thrust belt of the Salt Range and Potwar Plateau in northern Pakistan. About 500 km of seismic reflection profiles are integrated with surface geologic and drilling data to examine the deformation style and structure of the NPDZ with particular emphasis on history of deformation of the Dhurnal oil field. The seismic lines suggest that the overall structure of the eastern NPDZ is a duplex structure developed beneath a passive roof thrust. The roof thrust is generated from a tipline in the Miocene Murree Formation, and the sole thrust is initiated from the same Eocambrian evaporite zone that extends 80 km southward beneath the Soan syncline and Salt Range. The Dhurnal oil field structure is a pop-up at the southern margin of the NPDZ, and developed beneath the passive roof thrust. The passive roof thrust crops out just north of Dhurnal on the steep, northern limb of the Soan syncline. An overstep passive roof thrust (Sakhwal fault) is interpreted west of Dhurnal; this fault developed due to

Newly discovered Paleogene deltaic sequence in Katawaz basin, Pakistan, and its tectonic implications

In the Katawaz basin, Pakistan, the deltaic and turbidite facies of the Khojak Formation are the Paleogene analogue of the modern Indus River emptying into the Arabian Sea to form the Indus delta-fan system. Facies identified include upper continental slope to prodelta, distal to proximal distributary mouth bar, distributary channel, interdistributary bay, estuary, fluvial channel, natural levee, and flood plains of lower and upper delta plains. The proposed model is a fluvial-dominated, wave-modified delta that axially fed Khojak submarine-fan turbidites, exposed in the southwestern part of the basin. These sediments were eroded from the early Himalayan orogenic highlands and transported southwestward down the axis of the Katawaz remnant ocean basin.

Stratigraphy south of the Main Mantle Thrust, Lower Swat, Pakistan

Abstract The metamorphic sequence in Lower Swat is described and placed into the geological framework of Pakistan. The stratigraphic sequence consists of the Precambrian-Cambrian(?) Manglaur formation unconformably overlain by the Alpurai group which is subdivided into the Carboniferous or younger Marghazar formation, and the Triassic or younger Kashala, Saidu, and Nikanai Ghar formations. A third unit, the Jobra formation of unknown age, is present as discontinuous lenses unconformably below the Alpurai group. Type sections are indicated for each rock unit. Comparison with the stratigraphy in the Peshawar Basin indicates that the Lower Swat area existed as a highland with active normal faulting during and perhaps before deposition of the Marghazar formation. The Marghazar formation appears to be a rift facies that correlates with the Panjal Traps of western India. The Kashala and Nikanai Ghar formations represent a transition to a stable shelf environment. These units may correlate, in part, with the Zanskar Supergroup of western India. The Saidu formation may represent drowning of the shelf as it was pulled down and overridden by the Main Mantle Thrust suture melange. It may correlate with the Lamayuru Formation of western India. The Alpurai group thus records a depositional history from Late Paleozoic breakup of Gondwana, to development of a passive Mesozoic shelf, to drowning of the shelf at the onset of Himalayan orogeny.

Late Paleozoic Rifting in northern Pakistan

Metasedimentary rocks exposed in the eastern Peshawar basin and the southern Swat region of northern Pakistan provide evidence for late Paleozoic continental rifting. The onset of extensional tectonics in the Early Carboniferous is indicated by north derived clasts in the Jafar Kandao Formation eroded from thermally induced uplifts of parts of the formerly passive margin of Gondwana. Rift highlands were eroded until they were inundated during the Middle Carboniferous. Renewed uplift accompanied the eruption of basaltic lava flows during the Early Permian. Uplift along south dipping, northeast striking normal faults during the Carboniferous was accompanied by alkaline magmatism represented by the Shewa-Shahbazgarhi and Warsak porphyries and Koga syenite. Geochemistry of basaltic flows (now amphibolites) and intrusions associated with Permian uplift is similar to the coeval Panjal volcanics of northwestern India and indicates rift zone magmatism. Postrifting thermal subsidence led to the deposition of Upper Triassic marine carbonate rocks which unconformably overlie the rift basalts. A similar tectonic history in central Afghanistan suggests continuity between the two regions prior to the opening of the Neo-Tethys.

Locating earliest records of orogenesis in western Himalaya: evidence from Paleogene sediments in the Iranian Makran region and Pakistan Katawaz basin

A combination of sediment petrography, detrital zircon U-Pb, and fission-track dating is used to show that provenance of the Paleogene sedimentary rocks exposed in the Makran region of southern Iran and the Katawaz basin of Pakistan is consistent with a source from the nascent western Himalaya and associated magmatic arc. Results from this important archive show that Paleogene erosion was focused mainly on the arc and northern Indian margin, and, in contrast to the east-central Himalaya, we have not detected widespread rapid exhumation indicative of strong forcing by climate-driven erosion at that time.