metamorphism melting and extension age constraints from the high himalayan slab of southeast zanskar and northwest lahaul abstract we present evidence for two distinct stages of tertiary metamorphism m1 and m2 in the high himalayan slab of southeast zanskar and northwest lahaul as well as evidence for an older pre himalayan metamorphism pre m1 the m1 was a regional barrovian type event related to crustal shortening and thickening of the indian plate margin while m2 was associated with crustal melting and the emplacement of the gumburanjon leucogranite into the zanskar shear zone at the top of the slab u pb dating of metamorphic and magmatic accessory phases constrains the timing of m1 between 30 and 37 ma and the crystallization and emplacement age of the gumburanjon leucogranite at 21 22 ma inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least lower paleozoic in age and that they experienced a major pre m1 thermal perturbation at ca 450 500 ma whether this was associated with a regional barrovian type metamorphism or whether it was a thermal event rel

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

higher himalayan shear zone zanskar indian himalaya microstructural studies and extrusion mechanism by a combination of simple shear and channel flow thin section studies of the zanskar shear zone zsz rocks reveal a top to sw and subsequent primary and secondary top to ne ductile shearing brittle ductile and brittle extensions top to sw brittle shear steep normal faulting and fracturing in the proposed two phase model of ductile extrusion of the higher himalayan shear zone hhsz a top to sw simple shearing during 22 18 ma was followed by a combination of top to sw simple shear and channel flow at 18 16 ma the second phase simulates a thin zsz characterized by a top to ne shearing the channel flow component ceased around 16 ma the extruding hhsz entered the brittle regime but the top to sw shearing continued until perturbed by faults and fractures variation in the extrusion parameters led to variable thickness of the zsz shear strain after the extrusion is presumably maximum at the boundaries of the hhsz and falls towards the base of the zsz which crudely matches with the existing data the other predictions 1 spatially uniform shear strain after the first stage 2 fastest extrusion rate at the base of the zsz and 3 a lack of continuation of the zsz along the himalayan trend are not possible to validate due to paucity of suitable data non parabolic shear fabrics of the zsz indicate their heterogeneous deformation

lithospheric structure of tibet and western north america mechanisms of uplift and a comparative study we seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the cenozoic uplift of the tibetan plateau located behind the himalaya collision zone and the western united states located behind the san andreas transform convergent plate boundary selected first p arrivals of international seismological centre isc data at distances less than 22 are analyzed by the two station method to constrain the upper mantle structure of the basin and range br colorado plateau cp great plains gp and tibetan plateau tp the data are presented as plots of apparent velocity versus distance to the second station we show that these observations of apparent velocities versus distance as well as those that concern the attenuation of sn provide estimates of the mantle lid thicknesses we obtain the following mantle lid thicknesses br 20 40 km cp 35 50 km gp 150 195 km and tp 135 180 km sn is not observed for paths beneath the br and cp while sn efficiently propagates beneath the gp sn is also observed beneath most of tibet except in the north central part of the tp a possible interpretation of our observations is that indian continental lithosphere underthrusts only about the southern 2 3 of tibet

characterization of sediment trapped by macroalgae on a hawaiian reef flat abstract reef researchers studying community shifts in the balance between corals and fleshy macroalgae have noted that algae are often covered with sediment this study characterizes sediment trapping by macroalgae within a hawaiian reef habitat and constrains the controls on this process sediment laden macroalgae were sampled and macroalgal cover was assessed on a wide 1 km reef flat off south central molokai macroalgae trapped a mean of 1 26 0 91 sd grams of sediment per gram of dry weight biomass and that sediment was dominantly terrigenous mud 59 by weight it was determined that biomass as a proxy for algal size and morphology were not strict controls on the sediment trapping process over 300 metric tons of sediment were estimated to be retained by macroalgae across 5 75 km 2 of reef flat 54 g m 2 suggesting that this process is an important component of sediment budgets in addition understanding the character of sediment trapped by macroalgae may help constrain suspended sediment flux and has implications for nutrient dynamics in reef flat environments

a quantitative three dimensional depositional model of gravelly braided rivers a quantitative three dimensional depositional model of gravelly braided rivers has been developed based largely on the deposits of the sagavanirktok river in northern alaska these deposits were described using cores wireline logs trenches and ground penetrating radar profiles the origin of the deposits was inferred from observations of 1 channel and bar formation and migration and channel filling interpreted from aerial photographs 2 water flow during floods and 3 the topography and texture of the river bed at low flow stage this depositional model quantitatively represents the geometry of the different scales of strataset the spatial relationships among them and their sediment texture distribution porosity and permeability in the model are related to sediment texture the geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type e g ripples dunes bedload sheets bars associated with deposition of the strataset in particular the length to thickness ratio of stratasets is similar to the wavelength to height ratio of associated bedforms furthermore the wavelength and height of bedforms such as dunes and bars are related to channel depth and width therefore the thickness of a particular scale of strataset i e medium scale cross sets and large scale sets of inclined strata will vary with river dimensions these relationships between the dimensions of stratasets bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits the depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs

where does the time go assessing the chronostratigraphic fidelity of sedimentary geological outcrops in the pliocene pleistocene red crag formation eastern england it is widely understood that earth9s stratigraphic record is an incomplete record of time but the implications that this has for interpreting sedimentary outcrop have received little attention here we consider how time is preserved at outcrop using the neogene quaternary red crag formation england the red crag formation hosts sedimentological and ichnological proxies that can be used to assess the time taken to accumulate outcrop expressions of strata as ancient depositional environments fluctuated between states of deposition erosion and stasis we use these to estimate how much time is preserved at outcrop scale and find that every outcrop provides only a vanishingly small window onto unanchored weeks to months within the 600 800 kyr of crag time much of the apparently missing time may be accounted for by the parts of the formation at subcrop rather than outcrop stratigraphic time has not been lost but is hidden the time completeness of the red crag formation at outcrop appears analogous to that recorded in much older rock units implying that direct comparison between strata of all ages is valid and that perceived stratigraphic incompleteness is an inconsequential barrier to viewing the outcrop sedimentary stratigraphic record as a truthful chronicle of earth history supplementary material further details of the regional geology and specific information on outcrops are available at https doi org 10 6084 m9 figshare c 4561001

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

sedimentary history of the tethyan basin in the tibetan himalayas after an epicontinental phase the sedimentary rocks in the tibetan himalayas document a complete wilson cycle of the neo tethyan tethys ill evolution between the gondwana supercontinent and its northward drifting margin lhasa block from the late permian to the eocene during the triassic rift stage the basin was filled with a huge clastic dominated sediment wedge with up to 5 000 m of flysch in the northern zone widespread deltaic clastics and shallow water carbonates of late norian to earliest jurassic age in the southern zone mark in conjunction with decreasing tectonic subsidence the transition to the drift stage some 4 500 m of jurassic and early cretaceous shallow water carbonates and siliciclastics accumulated on the tethyan indian passive margin deepening upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments extensive abyssal sediments with basaltic volcanics in the northern deep water zone reflect continued ocean spreading and thermal subsidence paleomagnetic data gained separately for the northern indian plate and the lhasa block indicate that the neo tethys reached its maximum width about 110 ma ago with a spreading rate of 4 8 cm year before it commenced to close again during the remnant basin stage in the late cretaceous and paleogene a shallowing upward megasequence capped by a carbonate platform developed in the southern inner shelf realm in the northern slope basin plain zone turbidites and chaotic sediments derived from both the acretionary wedge and the steepening slope of the passive margin accumulated the depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting the sediments from the triassic to the paleogene are tentatively subdivided into five mega sequences which are controlled mainly by regional tectonics climatic influence e g carbonate deposition due to northward plate motion is partially subdued by terrigenous input and or increased water depth during the oligocene and miocene crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

constraints to the timing of india eurasia collision a re evaluation of evidence from the indus basin sedimentary rocks of the indus tsangpo suture zone ladakh india deposited within the indus tsangpo suture zone the cenozoic indus basin sedimentary rocks have been interpreted to hold evidence that may constrain the timing of india eurasia collision a conclusion challenged by data presented here the eurasian derived 50 8 51 ma chogdo formation was previously considered to overlie indian plate marine sedimentary rocks in sedimentary contact thus constraining the timing of collision as having occurred by this time using isotopic analysis u pb dating on detrital zircons ar ar dating on detrital white mica sm nd analyses on detrital apatite sandstone and conglomerate petrography mudstone geochemistry facies analysis and geological mapping to characterize and correlate the formations of the indus basin sedimentary rocks we review the nature of these contacts and the identification and correlation of the formations our results reveal that previously interpreted stratigraphic contacts identifying chogdo formation unconformably overlying indian plate sedimentary rocks are incorrect rather we suggest that the inaccuracy of previous interpretations is most likely a result of formation misidentification and thus cannot be used to constrain the timing of india asia collision

landforms and landscape evolution in the skardu shigar and braldu valleys central karakoram abstract the central karakoram which includes k2 in pakistan is one of the most rapidly rising areas on earth and exhibits complex topography and extreme relief impressive valley fills and glacial landforms are present throughout the valleys the dynamics of landscape evolution of the region are currently not well understood consequently the landforms were mapped and assessed in the skardu shigar and braldu valleys to elucidate the spatio temporal scale dependencies of surface processes active in the region these valleys were examined using geomorphic field methods remote sensing geomorphometry and terrestrial cosmogenic nuclides tcns surface exposure dating the glaciers in this region have oscillated considerably throughout the late quaternary and four glacial stages have been recognized including at least six glacial advances surface processes readjusted after glacier retreat and ubiquitous mass movements and catastrophic landsliding transported material from steep slopes to valley bottoms while glaciofluvial meltwater and glacier outburst floods redistributed sediment down valley glacier geochronology and late holocene ages of the outburst flood deposits indicate that landscape evolution has been dominated by glaciation and paraglaciation during the late quaternary

temporal and spatial variability in the flow and dispersal of suspended sediment on a fringing reef flat molokai hawaii abstract a multi year study was conducted on a shallow fringing reef flat on molokai hawaii to determine the temporal and spatial dispersal patterns of terrigenous suspended sediment during this study trade wind conditions existed for the majority of the year on the reef flat the trade wind conditions produced strong currents and resuspended moderate amounts of sediment on the reef flat on a daily basis during the year of study resulting in an overwhelming contribution to the total sediment flux the magnitude and direction of the trade winds relative to the orientation of the coastline the shallow relief and broad morphology and tidal elevation provided the primary control of the physical processes that resuspended and transported sediment on the reef flat over the period of record spatial data indicate that much of the terrigenous sediment resuspended on the reef flat is transported predominantly alongshore and is confined to the inner to mid reef flat evidence for the limited across shore mixing and transport is provided by the dominantly alongshore wind driven currents during trade wind conditions and the well defined across shore gradient in percentage calcium carbonate of the suspended sediment regions of slightly offshore suspended sediment transport along the reef flat can be attributed to the circulation pattern set up by the interaction between the trade winds coastal morphology and anthropogenic coastal structures i e fish ponds and wharf the regions in which sediment were seen to move offshore provide the strongest link between the sediment dynamics on reef flat and fore reef and qualitatively appears to be correlated with low coral coverage on the fore reef

the bangong lake ophiolite nw tibet and its bearing on the tectonic evolution of the bangong nujiang suture zone abstract the jurassic bangong lake ophiolite nw tibet is a key element within the western part of the bangong nujiang suture zone which marks the boundary between the lhasa and qiangtang blocks it is a tectonic melange consisting of numerous blocks of peridotite mafic lavas and dikes the mantle peridotites include both clinopyroxene bearing and clinopyroxene free harzburgites the cpx bearing harzburgite contains al rich spinel with low cr s 20 25 resembling peridotites formed in mid ocean ridge settings on the other hand the cpx free harzburgite is highly depleted with cr rich spinel cr 69 73 typical of peridotites formed in subduction zone environments mafic rocks include lavas of n morb and e morb affinity and boninites the n morb rocks consist of pillow lavas and mafic dikes whereas the e morb rocks are brecciated basalts the boninites have high sio 2 53 2 57 9 wt mgo 6 5 12 5 wt cr 166 752 ppm and ni 63 213 ppm and low tio 2 0 22 0 37 wt and y 5 34 8 10 ppm and are characterized by having u shaped chondrite normalized ree patterns the n morb and e morb lavas probably formed by different degrees of partial melting of primitive mantle whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment the geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid ocean ridge mor and then trapped above an intraoceanic subduction zone ssz where the mantle peridotites were modified by boninitic melts the bangong nujiang suture zone is believed to mark the boundary between two blocks within gondwanaland rather than to separate gondwanaland from eurasia

reappraisal of the sequence boundary in time and space case and considerations for an su subaerial unconformity that is not a sediment bypass surface a time barrier or an unconformity abstract the sequence bounding unconformity bears the key defining traits of being a surface separating younger from older strata along which there is evidence of subaerial erosional truncation or subaerial exposure with a significant hiatus indicated van wagoner et al 1988 this subaerial component of sequence boundaries subaerial unconformity su is also broadly considered to form as a topographic surface of sediment bypass carved during relative sea level fall and buried by backfilling during relative sea level rise accordingly the su is commonly presumed to record an approximate time barrier which separates older from younger strata along its full length in this paper we show that regional composite scour rcs surfaces that are traditionally mapped as an integral component of the su were never a single subaerial topographic surface characterized by sediment bypass are not unconformities do not record an effective time barrier and form diachronously at the channel belt scale over the entire fall to rise of a base level cycle these rcs surfaces and by inference the su surfaces they comprise thus do not fully fit key defining characteristics embodied in the conceptual sequence boundary flume observations and field data show that the rcs is buried by fluvial sediment simultaneously as it is scoured accordingly the rcs is perennially covered with stored sediment during formation is only exposed as a subaerial topographic surface at the local place and time where it is undergoing active growth and forms over the duration of local marine drainage during a relative sea level cycle this cut and cover model differs greatly from more established bypass models which assume that the rcs was roughly sediment free and subaerially exposed for long durations of incision during regression and thus preserves a significant depositional hiatus upon later burial instead the rcs may commonly and locally record a hiatus more typical of a facies bounding diastem without a lacuna significantly greater than that of surfaces within the strata it binds fragments of fluvial strata may commonly and sporadically be preserved above the rcs that are older than underlying marine units overrun by this surface consequently the rcs is not an effective time barrier lateral planation by migrating and avulsing channels as the rcs expands laterally after nucleation can place younger fluvial strata over much older units which means that this surface is also composite and highly diachronous laterally at the scale of channel belts the cut and cover model has additional implications not captured by the bypass model first significant sediments may be stored within fluvial strata above the rcs during regression that are not available for contemporary falling stage and lowstand marine shorelines this can result in marine sediment starvation particularly of the sand fraction and in extreme cases can result in sand autodetachment and an absence of regressive marine reservoir sandstones second cutting of the rcs co generates a suprafluvial surface above the covering fluvial strata during regression that may be used as a mappable proxy for the conceptual maximum regressive surface mrs the mrs may be raised above this surface locally by low accommodation aggradation during lowstand normal regression but in either case preserves an approximate time line where not reworked during later transgression third valley development across the rcs does not exclusively form by landward knickpoint growth and may include complexly formed and potentially cross cutting buffer valleys su valley incision can be divided into four modes which include denudation structural buttress and buffer valley components which may work together locally and tend to have variable importance along the shore to hinterland profile although the rcs is not a good rock proxy for the conceptual sequence boundary it remains a very mappable surface which may separate facies of potentially very different origin and reservoir quality the rcs is also inseparable from the su and typically the only terrestrial erosional surface of extent in most short duration sequences its nullification as a sequence boundary would mean abandonment of depositional sequence stratigraphy as a correlation and interpretive tool within these sections an alternative to abandonment of the su as a sequence boundary is to loosen the definition of a sequence boundary to a discrete surface of erosional truncation carved landward of contemporary shorelines that is traceable beyond the scale of a single valley or comparable local system and its correlative surfaces of conformity and or non deposition and continue its use as before

fractal nature of stratigraphic sequences orders of stratigraphic sequences are being used loosely and with widely varying definitions the orders seem to be subdivisions of convenience rather than an indication of natural structure it is proposed that at least at time scales of 103 106 yr sequences and systems tracts are scale invariant fractal features in which units bounded by exposure surfaces and units bounded by flooding surfaces are about equally likely support for this conceptual model derives from the well known facts that 1 sediment architecture is largely scale invariant over a wide range of scales in time and space 2 first order trends of sea level movements and sedimentation rates are fractal on all geologically relevant time scales and 3 detailed studies in carbonates indicate approximately equal abundance of flooding surfaces and demonstrable exposure surfaces among unit boundaries finally traces of prograding shelf edges that step up and step down in response to sea level changes are demonstrated in this study to have a fractal nature

incomplete but intricately detailed the inevitable preservation of true substrates in a time deficient stratigraphic record true substrates are defined as sedimentary bedding planes that demonstrably existed at the sediment water or sediment air interface at the time of deposition as evidenced by features such as ripple marks or trace fossils here we describe true substrates from the silurian tumblagooda sandstone of western australia which have been identified by the presence of the surficial trace fossil psammichnites the examples are unexpected because they have developed along erosional internal bounding surfaces within a succession of cross bedded sandstones however their seemingly counterintuitive preservation can be explained with reference to recent advances in our understanding of the time incomplete sedimentary stratigraphic record ssr the preservation of true substrates seems to be an inevitable and ordinary result of deposition in environments where sedimentary stasis and spatial variability play important roles we show that the true substrates developed during high frequency allogenic disturbance of migrating bedforms forcing a redistribution of the loci of sedimentation within an estuarine setting and subsequently permitting an interval of sedimentary stasis during which the erosional bounding surfaces could be colonized these observations provide physical evidence that supports recent contentions of how sedimentary stasis and the interplay of allogenic and autogenic processes impart a traditionally underestimated complexity to the chronostratigraphic record of geological outcrop

diurnal variability in turbidity and coral fluorescence on a fringing reef flat southern molokai hawaii terrigenous sediment in the nearshore environment can pose both acute and chronic stresses to coral reefs the reef flat off southern molokai hawaii typically experiences daily turbidity events in which trade winds and tides combine to resuspend terrigenous sediment and transport it alongshore these chronic turbidity events could play a role in restricting coral distribution on the reef flat by reducing the light available for photosynthesis this study describes the effects of these turbidity events on the hawaiian reef coral montipora capitata using in situ diurnal measurements of turbidity light levels and chlorophyll fluorescence yield via pulse amplitude modulated pam fluorometry average surface irradiance was similar in the morning and the afternoon while increased afternoon turbidity resulted in lower subsurface irradiance higher fluorescence yield f fm and lower relative electron transport rates retr model calculations based on observed light extinction coeffecients suggest that in the absence of turbidity events afternoon subsurface irradiances would be 1 43 times higher than observed resulting in retr for m capitata that are 1 40 times higher

quantitative morphology of a fringing reef tract from high resolution laser bathymetry southern molokai hawaii high resolution scanning hydrographic operational airborne lidar survey shoals laser determined bathymetric data were used to define the morphology of spur and groove structures on the fringing reef off the south coast of molokai hawaii these data provide a basis for mapping and analyzing morphology of the reef with a level of precision and spatial coverage never before attained an extensive fringing coral reef stretches along the central two thirds of molokai9s south shore 40 km along the east and west ends there is only a thin veneer of living coral with no developed reef complex in total 4800 measurements of spur and groove height and the distance between adjacent spur crests wavelength were obtained along four isobaths between the 5m and 15m isobaths the mean spur height increased from 0 7 m to 1 6 m whereas the mean wavelength increased from 71 m to 104 m reef flat width was found to exponentially decrease with increasing wave energy overall mean spur and groove height and wavelength were shown to be inversely proportional to wave energy in high energy environments spur and groove morphology remains relatively constant across all water depths in low energy environments however spur and groove structures display much greater variation they are relatively small and narrow in shallow depths and develop into much larger and broader features in deeper water therefore it appears that waves exert a primary control on both the small and large scale morphology of the reef off south molokai

tectonic evolution of the himalayas and tibet shackleton r m dewey j f and windley b f editors 1988 philosophical transactions of the royal society of london series a 325 pp price 65

magnetic anomaly modeling at the indo eurasian collision zone abstract one of the consistent features of satellite crustal magnetic anomaly maps is the large negative anomaly in the indo eurasian collision zone this region is modeled by the introduction of magnetic blocks representing the tibetan asian and himalayan mountain regions the crustal thickness of these blocks is based on the principle of isostasy it has been shown that the removal of the core field from the magsat data removes most of the long wavelength anomalies associated with the ocean continent susceptibility contrast and that some of the observed anomalies after processing of the satellite data are a manifestation of this contrast to account for this the final model is a superposition of the above block model on a global model of typical continent and ocean susceptibility called a standard earth magnetization model semm a combination of the three blocks superposed on the semm reproduces the major features of the observed magsat anomalies block model susceptibilities of the asian and himalayan regions are the same as for the continental portion of the semm albeit with a greater crustal thickness thus the crust in these regions is typical by contrast the satellite magnetic data require an integrated susceptibility in the tibetan region which is considerably lower than typical two possible explanations are suggested one that the origin and tectonic history of the tibetan region differs from the neighboring regions in such a way as to result in lower susceptibility this is consistent with current thinking that the tibetan plateau is made up of several fragments or micro continents which were attached to asia prior to the collision of india with asia the other possible explanation is that the curie isotherm in the tibetan region is elevated compared to the asian and himalayan regions

importance of wave induced bed liquefaction in the fine sediment budget of cleveland bay great barrier reef data from a three year long field study of fine sediment dynamics in cleveland bay show that wave induced liquefaction of the fine sediment bed on the seafloor in shallow water was the main process causing bed erosion under small waves during tradewinds and that shear induced erosion prevailed during cyclonic conditions these data were used to verify a model of fine sediment dynamics that calculates sediment resuspension by both excess shear stress and wave induced liquefaction of the bed for present land use conditions the amount of riverine sediments settling on the bay may exceed by 50 75 the amount of sediment exported from the bay sediment is thus accumulating in the bay on an annual basis which in turn may degrade the fringing coral reefs for those years when a tropical cyclone impacted the bay there may be a net sediment outflow from the bay during the dry tradewind season fine sediment was progressively winnowed out of the shallow reefal waters

gssps global stratigraphy and correlation procedures used to define an international chronostratigraphic stage boundary and to locate and ratify a global boundary stratotype section and point gssp are outlined a majority of current gssps use biostratigraphic data as primary markers with no reference to any physico chemical markers despite the international subcommission on stratigraphic classification issc suggestion that such markers should be included if possible it is argued that such definitions will not produce the high precision phanerozoic time scale necessary to understand such phenomena as pre pleistocene ice ages and global climate change it is strongly recommended that all gssps should have physico chemical markers as an integral part of their guiding criteria and where such markers cannot be found the gssp should be relocated the methods and approach embodied in oceanic stratigraphy coring logging analysing and archiving of drill sites by numerous experts using a wide range of methods could usefully serve as a scientific model for the analysis and archiving of gssps all of which are on the present day continents the incorporation of many more stratigraphic sections into gssp studies the application of physico chemical methods and the replacement of old u pb dates by newer ca tims u pb dates together with the use of constrained optimization conop programs that produce a calendar of events from many sections should lead to much more precise timescales for pre cenozoic time than are currently available

fine sediment budget on an inner shelf coral fringed island great barrier reef of australia abstract a field study was undertaken to determine the deposition and removal budget of fine sediments on the windward and leeward sides of an inner shelf coral fringed island of the central great barrier reef total sedimentation rates were about 2000 mg cm 2 yr 1 at all depths with 30 60 imported from the reef surrounding waters during calm periods and the remaining material locally resuspended during storms storms resuspended fine sediment at depths less than 5 5 m on the leeward reef side and 12 m on the windward side in these shallow waters there appeared to be a net annual sediment balance between import and export by resuspension events below these depths there was no resuspension during storms further there was a tenfold increase of the sedimentation rate during storms and most of this additional mud originated from resuspended material from shallower waters on the island slopes the mud had accumulated on these deeper coral reefs to levels of about 9000 mg dw cm 2 equivalent to 4 years of sediment deposition that is presumably flushed out only during tropical cyclones the data show that increased rates of advection of fine suspended sediments from soil erosion on land can result in increased sediment accumulation on inshore coral reefs below the depth of storm resuspension with negative effects on the coral communities

isolated highstand shelf sandstone body of turbiditic origin lower kenilworth member cretaceous western interior book cliffs utah usa abstract an isolated highstand shelf sandstone body of turbiditic origin has been identified in the lower kenilworth member hatch mesa area book cliffs utah relative to most other marine mudstone encased isolated sandstone bodies the hatch mesa succession has an abundance of bouma like turbidite beds a greater proportion of mudstone and siltstone is thinner 6 20 m and is weakly coarsening upward these attributes in combination with locally abundant hummocky cross stratified sandstones a paucity of wave ripples in the background fair weather deposits carbonaceous matter in all facies a lack of clinoforms and a low diversity abundance trace fossil suite suggest deposition in a prodelta environment between fair weather and storm wave base a three component shoreface to shelf model consisting of delta front subaqueous channel and prodelta turbidite deposits is proposed to explain the depositional environment and setting of the hatch mesa succession regional correlation has demonstrated that the hatch mesa succession is time equivalent with the second parasequence in the kenilworth kps2 which is characterized by inner shelf turbidite bodies at a number of other localities paleogeographic reconstructions reveal that the hatch mesa succession was deposited at least 16 km basinward of the kps2 lower shoreface in approximately 20 m of water the kps2 hatch mesa sandstone body package is part of the early highstand systems tract indicating that isolated or stray sandstone bodies are not solely restricted to the falling stage or lowstand systems tracts

basic building blocks and process variability of a cretaceous delta internal facies architecture reveals a more dynamic interaction of river wave and tidal processes than is indicated by external shape abstract although delta building processes and the resultant facies and facies associations have been studied for over a century there remains a lack of well documented facies architectural studies of ancient deltas in this study architectural element analysis is applied to determine the basic building blocks of a cretaceous delta at the top of the upper turonian wall creek member exposed along the western flanks of the powder river basin wyoming we document the bed scale basic building blocks of this ancient delta and show them in the context of their facies and stratal variability and their hierarchical spatial arrangement within a single deltaic parasequence we also test the idea that external sand body shape may not reflect the internal facies complexity as has recently been suggested in studies of several modern deltas including the burdekin in australia the brazos in the gulf of mexico and the baram trusan in borneo five orders of bounding surfaces separate six facies architectural elements in the prodelta and delta front deposits these elements are prodelta fines pf frontal splay fs channel ch storm sheet ss tidally modulated deposit tm and bar accretion ba seasonal to decadal river floods are thought to represent the main building phases of the delta producing channels bars and frontal splay elements during intervening periods the delta was reworked by waves storms and tides producing tidally modulated and storm sheet elements due to the complex interactions of river effluents with waves storms and tides the delta is interpreted as mixed influenced regional sandstone isolith facies and paleocurrent maps help to reconstruct the paleogeography and show how the various architectural elements varied across the delta front the channels bars frontal splays and tidally modulated elements are found only near the distributary mouth whereas the storm sheets occur extensively away from the distributary mouth the plan view morphology of the studied delta shows a smooth fronted arcuate to cuspate shape which should indicate wave dominance in contrast our analysis of the internal facies architecture shows that the delta was constructed rapidly during major river floods with significant tidal reworking additional outcrop mapping shows that storm wave reworked sands are attached to the flanks of the system previously published delta models predict incorrectly that the tidal reworking of a river flood dominated system should result in a more bird foot shape versus the lobate geometry that is actually mapped our analysis of internal facies versus external shape matches similar observations on several modern deltas and suggests that external shape is a poor indicator of internal facies complexity

shortening budgets and the role of continental subduction during the india asia collision abstract the india asia collision provides a remarkable example of diffused deformation because the zone of compressive strain extends far into the hinterland of the himalaya molnar and tapponnier s science 189 1975 419 concept of india as a rigid indenter responsible for the diffused deformation has been widely accepted perhaps uncritically this review gives a reappraisal of landmark papers by dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 which discussed the shortening budget in se asia and the evidence for the amount and the rate of the convergence between india and asia taking place after the initial collision of these continents at roughly 55 50 ma evidence for the total convergence comes from magnetic anomaly palaeomagnetic and volumetric balancing studies the results are consistent in showing about 1800 2100 km convergence between india and asia in the western sector 2475 km in the central and 2750 2800 km in the eastern sector the rate of convergence between india and asia over the period since initial collision is roughly 5 cm year however the possible long term rate of convergence within the himalaya is considerably less i e 1 5 2 0 cm year a fraction of the total convergence this shortfall can be made up by adding to the rate for the himalaya alone the convergence rates in other parts of the diffused zone of compressive strain i e tibet tien shan altai this works well for the post oligocene but not for the earlier post collisional history when the areal extent of thrust fold tectonics was much more limited possibly the apparent shortening deficit before the miocene was made up by strike slip faulting but with the notable exception of the red river fault evidence for widespread pre miocene strike slip faults is not convincing summation of the amount of shortening in thrust fold regimes in the himalaya and its hinterland falls short of that required by the total convergence figures quoted above the deficit arises firstly because of the likelihood that pre collisional crustal shortening of uncertain areal extent in tibet reduces tibet s contribution 40 according to dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 to the shortening budget for the cenezoic revised shortening budgets are presented on the assumption that most of the lhasa block southern tibet was already thickened prior to collision this thickening is difficult to quantify secondly balanced cross sections from various parts of the himalaya give 60 70 shortening that is only about 500 km roughly half of the shortening proposed by dewey et al the conclusion of le pichon et al tectonics 11 1992 1085 that only indian mid upper crust is involved in the himalayan thrust sheets is supported but it needs to be reassessed in the light of the recognition of eocene oligocene thickening in the himalaya this early crustal thickening antedates the sequence of thrusts which started with the main central thrust and therefore the shortening estimates based on balanced cross sections probably underestimate the total shortening in the himalaya the prospect of substantial underthrusting of india beneath tibet minimizes the amount of horizontal shortening of tibetan lithosphere during the cenozoic the conclusion is that either the total convergence deduced from geophysical and volumetric work is too large or the deficit must be made up by shortening mechanisms other than thrust fold regimes i e strike slip faulting which serves to move rocks out of the way of india

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

fluid enhanced melting during prograde metamorphism anatexis is a commonly recognized feature of high grade metamorphism but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path a suite of small volume deformed two mica leucogranites has been recognized within the high himalayan crystalline series of the garhwal himalaya these granites are consistently more siliceous than minimum melt granite compositions and are characterized by low rb sr ratios high ba low abundances of hfs elements and positive eu anomalies such trace element characteristics contrast strongly with the geochemistry of the well studied early miocene leucogranites of the high himalaya derived from fluid absent melting sm nd garnet dating of one deformed granite indicates a crystallization age of 39 3 ma c 15 ma before the emplacement of the more voluminous high himalayan leucogranites whilst some entrainment of restitic phases cannot be excluded trace element signatures suggest a low temperature 2 o activity positive eu anomalies and unusually low rb sr ratios are indicative of rapid disequilibrium melting fluid enhanced melting may be a common feature of prograde upper amphibolite facies metamorphism of orogenic belts predating peak metamorphism by at least 15 ma these melts will only crystallize within this period if they segregate from their protoliths subsequent dating of long lived melts would indicate erroneously young ages for the prograde melting events however melts formed in this way may be recognized by their distinctive trace element chemistry the persistence of early formed melts within an orogen provides insights into the prograde heating path and may be critical in controlling the rheology of the middle crust and hence its deformational history

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

new cretaceous paleomagnetic poles from the tarim basin northwestern china most paleomagnetic studies of the tarim have been focused on the paleozoic with little data published for mesozoic rocks however the ancient configuration of this part of asia has been affected by the indian eurasian collision during the second french chinese karakorum expedition from kashkar to kunjerab in the autumn of 1990 we sampled about 500 cores from 53 sites of upper and lower cretaceous red sandstones from three areas near uytak yingjisha and wuqia 38 5 39 5 n 75 76 5 8 isothermal remanent magnetization acquisition indicates the presence of minerals with high magnetic coercivity stepwise thermal demagnetization confirms that hematite is the dominant carrier and allowed the separation of low temperature components ltc 600 c in most samples the mean direction of ltcs is aligned with that of the present earth field normal and reversed polarities have been found for the htcs of each age group htcs show positive fold tests for at least one sub study where the attitudes of bedding were sufficiently different to allow the test to be performed more important the four sub studies from yingjisha and wuqia give fully compatible results results from the uytak area show a wide dispersion in declinations as could be expected from the rather severe deformation and proximity to major thrust and strike slip faults data obtained in the kuche area from northern tarim by li and co workers and in southern junggar by chen and co workers are statistically identical to the new data from yingjisha and wuqia data from paicheng in northern tarim appear to have been subject to local rotation excluding those sites which appear from paleomagnetic study to have been significantly rotated we calculated a mean cretaceous pole for the tarim basin at 69 1 n 223 88 ass 3 5 n 5 this result indicates that the total post cretaceous rotation of tarim relative to junggar and to eurasia has been negligible 2 6 6 1 and 0 4 4 6 respectively continental shortening and rotation between tarim and junggar within the tien shan are not resolved by paleomagnetism although the values obtained 400 t 600 km 3 6 are compatible with tectonic models of the range a major discrepancy warranting further geological investigation is that between the tarim and north china block poles which would imply some 1400 1060 km of north south relative displacement

multi stage development of the southern tibet detachment system near khula kangri new data from gonto la abstract field observations from gonto la southern tibet a pass through the high himalaya reveal a continuous planar 10 n dipping detachment horizon the gonto la detachment the detachment juxtaposes tethyan dark slates over a footwall of extensive leucogranite of the khula kangri pluton intruded into an injection complex layer regarded as an early southern tibet detachment system stds horizon the leucogranite emplacement is protracted and overlaps the stds development it is observed to intrude the earlier horizon of the stds which is deformed partially cut by the pluton and in the southern part rotated to a present south dip evidence for large scale folding and plutonism is also found east of khula kangri at lhozag la kang where the earlier stds horizon is inferred here it excises the entire palaeozoic tethyan sedimentary sequence and is similarly folded the gonto la detachment which cuts the middle upper miocene khula kangri pluton is in turn cut by the more steeply n dipping dzong chu fault demonstrating later n s extension in this area we interpret known outliers of tethyan sequences in bhutan as klippen underlain by an early stds horizon which provide a more regional illustration of early extension on the stds in the khula kangri area early extension was followed by plutonism and local relative uplift and further n s extension

crustal volume balance during the india eurasia collision and altitude of the tibetan plateau a working hypothesis a working hypothesis is developed for the two stages of evolution of the tibetan plateau during the continental collision between india and eurasia the first phase was dominated by crustal shortening and thickening in what is now tibet while indochina was expelled southeastward away from eurasia at the end of this phase at 17 ma the estimated tibetan crustal thickness was 51 km the second phase has been dominated by thickening of tibet by the influx of crustal material from india with no associated crustal shortening the crustal thickening that is inferred to have occurred by this process is only feasible because leakage of crust from beneath the plateau is restricted by neighboring archean cratons that block lower crustal flow this leakage to the east and south from tibet to beneath yunnan is estimated as 20 30 of the influx at present but was much smaller at the start of this second phase of deformation it is suggested that at 11 ma the force exerted against the tibetan upper crust by this flowing material first exceeded the threshold that this brittle layer could resist and tibet began to extend eastward any satisfactory working hypothesis for the tibetan plateau must explain its dimensions its 17 ma ending of shortening and 11 ma start of extension and volcanism the eastward slip sense on its active normal and strike slip faults and the ratios of their displacements to present day slip rates which imply that their slip rates have gradually increased this working hypothesis accounts for these features and predicts a smooth and continuing increase in altitude