Michael R. W. Johnson

Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution

In India, the Dagshai and overlying Kasauli Formations represent the oldest exposed continental foredeep sediments eroded from the Himalayan orogen. 40Ar/39Ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of <28 Ma for the Dagshai Formation at one locality and <25 Ma at a second locality, whereas deposition of the Kasauli Formation occurred after 28 Ma at two localities and after 22 Ma at a third locality. This timing suggests that, in India, the start of substantial exhumation and erosion from the rising Himalayan orogen was delayed until 28 Ma.

Early stages of foreland basin evolution in the Lesser Himalaya, N India

Abstract In this paper we report preliminary results of an integrated sedimentological and structural study of the Late Cretaceous-Early Tertiary facies exposed in the Lesser Himalaya. The Precambrian-Palaeozoic basement of the Indian plate was initially transgressed in Late Cretaceous times (after c. 100 Ma), with the deposition of shallow shelf carbonates, (50 m thick Singtali Formation). These are depositionally overlain by green ferruginous mudstones and thin limestones of the 150 m thick Palaeocene-Eocene (c. 65–40 Ma) Subathu Formation. During Late Eocene-Oligocene times (c. 40–30 Ma), clastic sediment input increased and red lenticular sandstones, thinner planar-bedded sandstones, mudstones and caliche of the 350 m thick Dagshai Formation were deposited. A semi-arid, meandering fluvial/floodplain setting is envisaged, with thick channelised and overbank sands. Palaeocurrent evidence indicates a general SE progradation. Our detailed field mapping shows that where the entire Subathu and Dagshai Formations are intercalated, this is the result of post-depositional tectonics, rather than primary intertonguing as described in some previous reports. The relative abundance of sandstones greatly increased in upper Dagshai times, continuing into the Early-Mid-Miocene (c. 30–10 Ma) when the 250 m thick Kasauli Formation was laid down. This formation is characterized by lenticular and planar-bedded grey sandstones, rich in plant material including occasional logs (but without caliche) interbedded with minor grey mudstones. A rapidly prograding, braided fluvial environment is proposed, with the petrography (e.g. presence of garnets) suggesting erosion and derivation from deeper levels of the Himalayan mountain belt to the north. The climate had by then changed from semi-arid to humid, possibly in response to the onset of the monsoon, initiated when the mountain belt had reached sufficient height to interfere with the jet-stream. During Late Miocene-Quaternary times (younger than c. 15 Ma), the Tertiary foreland basin was deformed and incorporated into three structural levels of a southward migrating thrust stack. At the lowest level, thick successions of the Tertiary Subathu, Dagshai and Kasauli Formations are structurally underlain by fluvial sediments (Siwalik Gp.) and overlain by the Krol Precambrian-Palaeozoic sedimentary nappe. At the intermediate structural level, Early Tertiary sediments unconformably overlie the stratigraphy within this nappe and comprise, often thin (<50 m), successions of mainly Subathu and Dagshai Formation sediments. The highest structural level sediments form small imbricates, highly dismembered units and melange (including evaporites) beneath the major metamorphic Jutogh Nappe. We interpret the Late Cretaceous-Palaeocene Singtali Formation as pre-collisional transgressive sediments related either to flexural downwarping of the Indian plate due to the obduction of the Spontang ophiolite onto the north Indian plate margin, or to extensional tectonics related to the detachment and drift of India from Gondwana and Neo-Tethyan subduction beneath Asia. This would have been coupled with the Late Cretaceous global eustatic sea-level high stand. The marine Subathu Formation is interpreted as having been deposited between initial and terminal India-Eurasia continental collision, with the overlying fluvial/floodplain Late Eocene-Oligocene Dagshai and Miocene Kasauli Formations as southward prograding foreland basin successions related to progressive stages of India-Eurasia continental collision.

Palaeomagnetic dating of the earliest continental Himalayan foredeep sediments: Implications for Himalayan evolution

Abstract Between the time of the India-Eurasia collision (50–45 Ma) [1] and the climax of crustal shortening and thrust stacking in the Himalaya when the Main Central Thrust (MCT) was active (21 Ma) [1,2] there is a ca. 30 My gap about which little is known. This paper aims to shed light on this period by dating the initiation of major erosion from the rising Himalaya and the probable start of uplift, a significant event in the orogens history. This was achieved by accurately dating, for the first time, the Dagshai Formation sediments of northern India, which are interpreted as early Himalayan foreland basin deposits that record initial large-scale erosion of the orogen [3]. Oriented hand samples were collected from six sites and analysed, using palaeomagnetic techniques. Both polarities are represented and the remanence passes a fold test. Fitting the measured palaeolatitude to that expected for the Indian plate dates the Dagshai Formation at 35.5 Ma ± 6.7 Ma, and this is taken as the time when the embryonic Himalaya began to be strongly eroded and regionally uplifted.

Culminations and domal uplifts in the Himalaya

Abstract Culminations and depressions affected the thrust sheets in the Himalaya at different times in the evolution of the orogen. Most culminations appear to reflect the influence of lateral ramps which caused hanging-wall thickening and the formation of horses, thereby resulting in folding of higher thrusts. The pattern of culminations and depressions shows that erosion and uplift have been markedly variable along the length of the orogen. In the High Himalaya as much as 35 km may have been removed from culminations, but in depressions erosional loss is much less.

Orogenesis: The Making of mountains

Preface 1. Major features of the Earth and plate tectonics 2. Driving mechanisms for plates, slab retreat and advance, a reason for orogenesis 3. Physical and chemical principles: rock deformation and heat production in the lithosphere 4. Large scale features of orogenic belts: thrusts, folds, orogenic wedges 5. Evolution of orogens 6. Lateral spreading of orogens: foreland propagation, channel flow and weak zones in the crust 7. Orogenic metamorphism 8. The erosion, uplift and exhumation of orogens 9. The sedimentary history of foreland basins 10. Deep structure: the support of mountains and the importance of phase changes 11. Mountains and climate 12. Precambrian orogenesis References Index.

A re-interpretation of the mechanics of the ben sgriol fold, loch hourn (northwest scotland): A discussion

Abstract J.G. Ramsay has suggested that shear folding has operated on all observed scales in the Arnisdale area, the argument being based on a wealth of geometrical data. It is suggested that the geometrical evidence is an ambiquous guide to fold mechanics. An alternative hypothesis, based on models of buckle folding, is presented. It is believed that this hypothesis satisfactorily explains not only the geometrical data but also the evidence of tectonic style.

An improved method for calibrating time-of-flight Laue single-crystal neutron diffractometers

An improved method of calibrating neutron time-of-flight single-crystal instruments is described. The calibration method has led to improved lattice parameter determination and ability of the orientation matrix to describe the reflection positions on the detector surface.

Orogenesis: The Making of Mountains: Index

Preface 1. Major features of the Earth and plate tectonics 2. Driving mechanisms for plates, slab retreat and advance, a reason for orogenesis 3. Physical and chemical principles: rock deformation and heat production in the lithosphere 4. Large scale features of orogenic belts: thrusts, folds, orogenic wedges 5. Evolution of orogens 6. Lateral spreading of orogens: foreland propagation, channel flow and weak zones in the crust 7. Orogenic metamorphism 8. The erosion, uplift and exhumation of orogens 9. The sedimentary history of foreland basins 10. Deep structure: the support of mountains and the importance of phase changes 11. Mountains and climate 12. Precambrian orogenesis References Index.

Orogenesis: The Making of Mountains: Contents

Preface 1. Major features of the Earth and plate tectonics 2. Driving mechanisms for plates, slab retreat and advance, a reason for orogenesis 3. Physical and chemical principles: rock deformation and heat production in the lithosphere 4. Large scale features of orogenic belts: thrusts, folds, orogenic wedges 5. Evolution of orogens 6. Lateral spreading of orogens: foreland propagation, channel flow and weak zones in the crust 7. Orogenic metamorphism 8. The erosion, uplift and exhumation of orogens 9. The sedimentary history of foreland basins 10. Deep structure: the support of mountains and the importance of phase changes 11. Mountains and climate 12. Precambrian orogenesis References Index.

Orogenesis: The Making of Mountains: Acknowledgments

Preface 1. Major features of the Earth and plate tectonics 2. Driving mechanisms for plates, slab retreat and advance, a reason for orogenesis 3. Physical and chemical principles: rock deformation and heat production in the lithosphere 4. Large scale features of orogenic belts: thrusts, folds, orogenic wedges 5. Evolution of orogens 6. Lateral spreading of orogens: foreland propagation, channel flow and weak zones in the crust 7. Orogenic metamorphism 8. The erosion, uplift and exhumation of orogens 9. The sedimentary history of foreland basins 10. Deep structure: the support of mountains and the importance of phase changes 11. Mountains and climate 12. Precambrian orogenesis References Index.