extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

geomorphically driven late cenozoic rock uplift in the sierra nevada california geologists have long accepted that the sierra nevada california experienced significant late cenozoic tectonically induced uplift a flexural isostatic model presented here shows however that a large fraction of the primary evidence for uplift could be generated by the lithospheric response to coupled erosion of the sierra nevada and deposition in the adjacent central valley and therefore requires less tectonic forcing than previously believed the sum of range wide erosion and the resultant isostatic rock uplift would have lowered sierra mean elevation by 200 to 1000 meters since 10 million years ago and could also have increased summit elevations during the current period of relief production

cenozoic ice volume history development of a composite oxygen isotope record to view cenozoic ice volume and ocean temperature history a composite 18 o time series was constructed for tropical shallow dwelling planktonic foraminifers and for deep water benthic foraminifers we conclude that cenozoic benthic 18 o primarily reflect considerable deep ocean temperature variation and that tropical planktonic 18 o afford the best record of cenozoic ice volume the planktonic composite indicates the presence of a significant ice budget for the past 40 m y the difference between the benthic and planktonic 18 o composites suggests that low latitude production of warm saline bottom water dominated tertiary thermohaline circulation

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

structural evolution of a modern arc continent collision in papua new guinea the finisterre mountains and western solomon sea of northern papua new guinea are the site of an active oblique arc continent collision comparison of structures along the length of the collision zone provides a history of its structural evolution here we compare the results of structural mapping in the finisterre mountains where collision is in an advanced state with side scan sonar images and potential field data from the western solomon sea which illuminate the structure of the juvenile collision zone an accretionary wedge complex composed of a southwestward younging stack of imbricate thrust sheets developed along a westward extension of the modern new britain trench prior to collision this accretionary wedge crops out in the finisterre mountains as the erap structural complex consequences of collision were doubling of the crustal thickness to 50 52 km rapid up lift of the finisterre terrane and initiation of a major out of sequence thrust that displaced the finisterre volcanics over younger sediments of the accretionary wedge this out of sequence thrust is mapped in the finisterre mountains as the wongat thrust and it is recognized in the western solomon sea on side scan sonar images a second generation out of sequence thrust formed after substantial uplift had subaerially exposed the collision zone this thrust crosscuts the wongat thrust forming a window of accretionary wedge sediments bounded to both the ne and sw by finisterre volcanics this younger out of sequence thrust is the gain thrust of the finisterre mountains at least 15 km of crustal shortening has occurred across the wongat thrust at 146 30 e longitude and the gain thrust has accommodated up to 7 4 km of shortening in the same area it is probable that the well lithified finisterre volcanics serve as the backstop for the accretionary wedge collision complex that is forming in the collision zone of the western solomon sea potential field models indicate that this backstop is arcward dipping thrusting of the back stop material over the accretionary wedge as we observe along the wongat thrust is expected for wedges with arcward dipping backstops a series of ne sw trending cross faults perpendicular to the dominant structural trend exist in the collision zone these faults have components of left lateral strike slip and se down dip slip motion it is likely that the finisterre terrane is being segmented along these faults with the segmentation caused by differential resistance to convergence between the downgoing continental crust of the collision zone and the oceanic crust of the subduction zone to the east the origin of the faults is uncertain previous authors believed that they formed during paleogene volcanic activity in the finisterre terrane the colinearity of several of these cross faults with major mapped faults on the downgoing plate raises the possibility that the finisterre range cross faults are upper plate manifestations of lower plate structures and as such reveal strong interplate coupling in the collision zone

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

shallow seismicity of arc continent collision near lae papua new guinea abstract in northeastern new guinea the narrow ramu markham valley rmv separates island arc rocks to the north from those of continental origin to the south and appears to be the western onland extension of the new britain trench to explore the tectonic processes at the leading edge of the island arc during collision we operated a portable seismic network for six weeks near the city of lae at the eastern end of the rmv we observed a narrow near vertical belt of seismicity between 10 and 30 km depth that we call the lae seismic zone lsz starting at the rmv in the southwest and trending northeasterly cutting across surface geologic structure the truncation of the lsz along a steep plane by the ramu markham fault zone rmfz and earthquake first motions suggest that the earthquakes occur in the hanging wall of a steep n dipping fault that crops out at the rmfz we also consider that the lsz is within the lower plate of a gently dipping thrust below 20 km depth the microearthquake zone is truncated by a gently ne dipping plane coinciding in depth and dip with nodal planes of recent large mb 5 6 and 6 0 thrust earthquakes we suggest that the huon peninsula is being emplaced onto the australian plate along a gently 25 dipping thrust fault that is 20 km deep beneath lae the rmfz may be a steeply dipping thrust fault that connects with this gently n dipping thrust but accommodates little convergence at present the lsz trends nearly perpendicular to an anticlinal range which appears to be sheared in a left lateral sense p wave first motions for earthquakes in the lsz with steep 70 to 90 dip nodal planes that strike parallel to the lsz suggest a component of south side up displacement also hence the crustal block south of the lsz may be rising relative to the huon peninsula and the rapid quaternary uplift rates estimated for the lae coastal region may be higher than the uplift rate of the huon peninsula as a whole we suggest that the lsz reveals a tear of small offset in the huon terrane but may be similar to a structure that produced a magnitude 7 earthquake near madang in 1970

quaternary uplift rates at a plate boundary lae urban area papua new guinea abstract uplift along the southern margin of the south bismarck plate in papua new guinea is a result of plate convergence terrane docking and triple junction migration lithofacies data combined with paleontological and 14 c dating yield uplift rates for the markham valley north flank and the lae urban area in the range 2 0 7 6 m 10 3 yr these rates are established for the past 21 000 years and have probably obtained for 10 6 yr depositional surfaces 8100 years old in the lae urban area are tilted the uplift may be occurring by steady creep or in episodic increments of 1 m with recurrence intervals of the order of 10 2 yr as seems to be the case for the northern huon peninsula terraces if episodic uplift is occurring earthquakes of magnitude 7 0 can be anticipated accompanied by slump induced tsunamis

backstacking apatite fission track stratigraphy a method for resolving the erosional and isostatic rebound components of tectonic uplift histories measuring tectonic uplift of the mean surface of earth using thermochronological data to document uplift histories is often plagued by confusion arising from the independent nature of the thermal and sea level frames of reference the technique of backstacking apatite fission track stratigraphy differentiates between vertical movement of rocks toward earth9s surface due to erosional or tectonic denudation and tectonic uplift of earth9s surface apatite fission track analysis can also be used to identify episodes of erosion where no other evidence for the erosional event exists in addition to providing information about the timing magnitude and geographic distribution of erosion

active arc continent collision earthquakes gravity anomalies and fault kinematics in the huon finisterre collision zone papua new guinea the huon finisterre island arc terrane is actively colliding with the north edge of the australian continent the collision provides a rare opportunity to study continental accretion while it occurs we examine the geometry and kinematics of the collision by comparing earthquake source parameters to surface fault geometries and plate motions and we constrain the forces active in the collision by comparing topographic loads to gravity anomalies waveform inversion is used to constrain focal mechanisms for 21 shallow earthquakes that occurred between 1966 and 1992 seismic moment 1017 to 3 1020 n m twelve earthquakes show thrust faulting at 22 37 km depth the largest thrust events are on the north side of the huon peninsula and are consistent with slip on the ramu markham thrust fault zone the northeast dipping thrust fault system that bounds the huon finisterre terrane thus much of the terrane s crust but little of its mantle is presently being added to the australian continent the large thrust earthquakes also reveal a plausible mechanism for the uplift of pleistocene coral terraces on the north side of the huon peninsula bouguer gravity anomalies are too negative to allow simple regional compensation of topography and require large additional downward forces to depress the lower plate beneath the huon peninsula with such forces plate configurations are found that are consistent with observed gravity and basin geometry other earthquakes give evidence of deformation above and below the ramu markham thrust system four thrust events 22 27 km depth directly below the ramu markham fault outcrop are too deep to be part of a planar ramu markham thrust system and may connect to the north dipping highlands thrust system farther south two large strike slip faulting earthquakes and their aftershocks in 1970 and 1987 show faulting within the upper plate of the thrust system the inferred fault planes show slip vectors parallel to those on nearby thrust faults and may represent small offsets in the overriding plate these faults along with small normal faulting earthquakes beneath the huon finisterre ranges and a 25 along strike rotation of slip vectors demonstrate the presence of along strike extension of the accreting terrane and along strike compression of the lower plate

post miocene diagenetic and eustatic history of enewetak atoll model and data comparison the post miocene diagenetic and eustatic history of enewetak atoll was investigated using a one dimensional forward model comparison of model and data suggests that the post miocene history of enewetak atoll was dominated by multiple episodes of meteoric phreatic diagenesis attendant with high frequency 10 sup 4 to 10 sup 5 yr fluctuations in sea level and a subsidence rate of 39 0 m m y sensitivity testing indicates that subaerial erosion results in the preservation of additional subaerial unconformities because stratigraphic shortening permits a succeeding sea level rise to flood the exposure surface and deposit sediment whereas without subaerial erosion this sea level rise would be recorded as a paleophreatic lens model results indicate that less then 10 of lapsed time is recorded by sediment deposition during periods of high frequency changes in sea level incompleteness of the stratigraphic record suggest that magnetostratigraphy may give erroneous ages for shallow marine carbonate sequences deposited during times of high frequency changes in sea level and frequent magnetic polarity reversals

postcollision extension in arc continent collision zones eastern indonesia postcollisional extension superimposed on arc continent collision complexes is a common feature in the orogenic belts of eastern indonesia in the southern banda arc the arc continent collision event is essentially a pliocene and younger feature but already an element of extension is being superimposed on the compressional structures of the collision complex it is thus likely that the extension commences within a very short period 5 m y after the initiation of collision related compressional deformation a similar history of fold belt development immediately followed by extension can be inferred for the lengguru fold belt and the adjacent wandamen wondiwoi terranes of irian jaya and for the gulf of bone in sulawesi it is suggested that the extension results from decoupling of the subducting oceanic lithosphere from the unsubductable continental lithosphere and that the superimposition of extension is a virtually unavoidable consequence of the arc continent collision

measurement of tectonic surface uplift rate in a young collisional mountain belt measurement of the rate of tectonically driven surface uplift is crucial to a complete understanding of mountain building dynamics the lack of a suitable rock record typically prevents determination of this quantity but the unusual geology of papua new guinea s finisterre mountains makes measurement of this rate possible the tectonic surface uplift rate at the finisterre range is 0 8 2 1 mmyr 1 approximately that expected to arise from crustal thickening

age and composition of meta ophiolite from the rhodope middle allochthon satovcha bulgaria a test for the maximum allochthony hypothesis of the hellenides the metamorphosed thrust stack of the rhodopes comprises a level with ophiolites middle allochthon underlain and overlain by continent derived allochthons the upper allochthon represents the european margin but the origin of the lower allochthon remains controversial with suggestions that it may be derived from an inferred microcontinent drama or from the margin of adria trace element compositions and sr and nd isotope ratios of metagabbroic amphibolites and enclosed meta plagiogranites from the satovcha ophiolite middle allochthon show that they are cogenetic and represent suprasubduction zone ophiolites u pb dating using laser ablation sector field inductively coupled plasma mass spectrometry of zircons from two meta plagiogranites and a metagabbro yielded identical jurassic ages 160 1 ma 160 6 1 8 ma and 160 1 ma respectively similar to ophiolites in the eastern vardar zone bordering the rhodopes to the sw the trace element patterns also closely resemble those of the vardar ophiolites the association with late jurassic arc type granitoids is another feature that applies both to eastern vardar and satovcha this strongly suggests that the middle allochthon comprises the metamorphosed northeastward continuation of the vardar zone the jurassic age of the satovcha ophiolite contradicts the hypothesis of early jurassic suturing between europe upper allochthon and the assumed drama microcontinent lower allochthon but is in line with the maximum allochthony hypothesis i e the assumption that the lower allochthon represents adria and that the root of the vardar derived thrust sheets is at the ne boundary of the rhodopes

relation between continental rifting and uplifting evidence from the suez rift and northern red sea abstract the suez rift and the northern red sea are flanked by high standing shoulders with elevations often exceeding 2 km and with a structural relief reaching 4 5 km in this area pre rifting erosion was small not more than a few hundred meters there is no evidence that a precursory uplift formed along the site of future rifting uplifting and erosion began essentially at the same time as rift faulting and magmatism in late oligocene and early miocene limes in the gebel serbal area sinai where the uplifting history is resolved best the tectonic relief reached about 800 m by the end of the early miocene and a similar amount of tectonic uplifting developed in later times other parts of the rift shoulders probably had a similar uplifting history but the total uplifting was often less considerable erosion in places more than 3 km occurred mainly since middle miocene times and caused considerable isostatic rebound this magnified the uplifting and caused the structural and topographic relief to be considerably greater than the tectonic uplifting initial uplifting and contemporaneous magmatic activity were probably caused by rising of deep hot material which thinned the lithosphere and heated it such deep seated processes probably continued to operate also during the later stages of tectonic uplifting though igneous activity no longer occurred on the surface stretching of the lower lithosphere under the rift shoulders could have also contributed to their uplifting the features of uplifting history of the studied region may well be typical of other active rifts

origin of the highly elevated pyrenean peneplain 1 peneplanation of mountain ranges is generally considered the result of long term erosional processes that smooth relief and lower elevation near sea level therefore peneplain remnants at high elevation in mountain ranges are used to infer posttectonic surface uplift such an interpretation has been proposed for the pyrenees where high elevation low relief erosional surfaces rose up to more than 2000 m because the pyrenean foreland basins are filled with very thick continental deposits which have buried the early jagged landscape we challenge this hypothesis by pointing out that relief applanation does not necessarily require elevation lowering we propose an alternative interpretation in which piedmont aggradation of detrital sediment that comes from erosion of the high chain induces the rise of the base level of the range therefore reducing strongly the erosive efficiency of the drainage system and resulting in the progressive smoothing of the relief such a process allows a high elevation low relief erosional surface to develop at the scale of the range in the pyrenees occurrence of high elevation low relief erosional surface remnants does not imply a posttectonic uplift but is instead due to the dissection of the initial miocene high elevation low relief surface by the recent drainage system the erosive activity of which has been enhanced by global climate change from the late pliocene onward

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

the use of gpr to image three dimensional 3 d turbidite channel architecture in the carboniferous ross formation county clare western ireland