a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

stratigraphic development of synkinematic deposits in a large growth fault system onshore brunei darussalam kilometre scale synsedimentary faults associated with well preserved sedimentary successions on both hanging wall and footwall sides are rarely seen in outcrop however one such example is found in a middle miocene deltaic sand shale sequence exposed along the jerudong anticline in onshore brunei darussalam integration of detailed outcrop information with regional geophysical subsurface data documents that 1 initiation of the jalan tutong growth fault was related to delta front instability and failure 2 sediment loading under shallow marine conditions maintained syndepositional faulting 3 growth fault abandonment coincided with a significant basinward shift of deposition and subaerial exposure high resolution analysis of facies characteristics stratal expansion and throw development within the studied growth sequence indicates that differential fault movement strongly influenced facies composition sedimentary geometry and bedding characteristics sandstone poor shelf to lower shoreface successions developed during periods of rapidly increasing fault movement whereas sandstone rich amalgamated upper shoreface deposits preferentially formed during intervals of constant or decreasing throw development the data and interpretations presented may help to focus attention on the complex interplay between tectonics and sedimentation associated with the growth and decay of deltaic faults by providing for the first time an outcrop based view on a feature documented in similar scale only by geophysical subsurface data

strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

eocene oligocene syn rift deposition in the northern gulf of tonkin vietnam abstract reconstruction of non marine syn rift deposition in ancient basins is challenged by intense deformation rapid lateral facies variation and limited biostratigraphic resolution this affects petroleum resource assessments that relies on prediction of basin fill the northern gulf of tonkin contains such non marine dominated rift system the area is in an early stage of exploration depositional facies analysis based on all existing 2d seismic and well data suggests that eocene oligocene deposition in grabens and half grabens occurred in three steps this resulted in formation of three successive packages namely the rift initiation the rift development and the rift termination succession rift initiation deposits are mainly composed by alluvial fluvial strata deposited in segmented and often isolated grabens and half grabens as rifting progressed faults joined and rift depressions merged into larger coherent areas of deposition faulting peaked which resulted in rapid creation of accommodation space relief and the development of deep lakes alluvial fans and fan deltas elsewhere floodplain deltaic and shallow lake deposits comprise most of the rift development succession as rift subsidence declined floodplain deltaic and shallow lake deposits filled accommodation space and therefore dominate the rift termination succession structuring by strike slip faulting and inversion complicates sequence stratigraphic analysis moreover diachronic rift culmination caused the rift initiation development and termination successions to vary laterally in age further obscuring sequence stratigraphic analysis even so tectonostratigraphic grouping into these three units allows a meaningful mapping of gross depositional facies the locally developed deep lake successions potentially containing petroleum source rocks and seals together with areas including potential deltaic fan delta fluvial and turbidite reservoir sand suggest play types similar to those working in the chinese part of the beibuwan basin the most prospective areas are least explored suggesting an overlooked petroleum potential in the northern gulf of tonkin

basin superposition on the northern margin of the south china sea abstract basin superposition on the northern margin of the south china sea is a product of pulsed basin development with geometries which changed through space and time of changing strain fields and of tectonically inherited pre existing zones of crustal weakness this region of the south china sea was welded together or assembled prior to the cretaceous three distinct episodes of continental disassembly and accompanying basin evolution ensued each episode commencing with rifting and the formation of grabens or half grabens and each terminating with a gentle subsidence phase the major trends of the faults and basins varied from predominantly ne for the first episode through ne ew for the second to ew nw for the third the first episode occurred during the late cretaceous to paleocene as a back arc extensional domain induced by the dip steepening of a subducted oceanic slab the second basin episode of the late eocene to early oligocene was initiated by the collision of the indian plate with the eurasian plate the continued penetration of the former into the latter led to the tectonic escape and subsequent clockwise rotation of the indochina block creating regional transtension and left lateral motion along the red river fault the third episode of basin development occurred during the middle miocene as a reactivated product of this collision owing to the clockwise rotation of indochina the approximately e w trend of the simple shear principal displacement zone changed to northwest by the pliocene the eastward extrusion of south china had exceeded that of indochina reversing the sense of red river fault displacement to right lateral a clockwise rotation of the red river geosuture together with considerations of the influence of zones of inherited crustal weakness provides a model for the observed episodic changes in regional strain such a model best explains the post cretaceous evolution of superimposed basins on the northern margin of the south china sea

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

folding of mylonitic zones in cordilleran metamorphic core complexes evidence from near the mylonitic front metamorphic core complexes of north america are asymmetric when viewed perpendicular to the direction of tectonic transport on one side mylonitic fabrics and the overlying detachment fault dip gently in the direction of upper plate transport and the mylonitic rocks have a down dip sense of shear on the opposite side the mylonitic zone rolls over to a dip opposed to the direction of upper plate transport diverges downward from the detachment fault and dies out upward across a mylonitic front new kinematic data show that the backdipping mylonitic zone in most complexes is a downfolded continuation of the main mylonitic zone folding of the mylonitic zone was accompanied by the formation of antithetic shear zones and occurred during the late stages of mylonitization

the strength of the continental crust detachment zones and the development of plastic instabilities abstract the maximum strength of the continental crust is constrained by the geothermal gradient the lithological make up of the crust and whether or not the byerlee relation holds to the base of the continental crust if this linear byerlee relation holds then maximum shear stress levels sustained at low geothermal gradients 10 20 c km 1 could be as high as 300 mpa in the upper part of the mantle for strike slip regimes and up to 700 mpa in thrust regimes towards the base of the crust however if byerlee s relation breaks down at moderate temperatures and pressures and if such breakdown is associated with the transition from unstable to stable sliding on pre existing faults then the maximum stress levels in the crust being set by the shear stress at the breakdown depth are much lower ca 300 mpa for low geothermal gradients and thrust regimes the lithosphere is quite weak below the crust for geothermal gradients greater than 20 c km 1 so attempts to treat the lithosphere as an elastic slab for time scales for which plastic flow contributes significantly to the deformation should be questioned for a geothermal gradient of 10 c km 1 the entire continental crust and part of the upper mantle behave as an elastic brittle slab at higher geothermal gradients the distribution of strength is variable vertically according to the magnitude of the geothermal gradient and the lithological make up large contrasts in strength occur across rheologically defined boundaries as well as across lithological boundaries at these higher geothermal gradients such contrasts in strength could act as nuclei for detachment zones within the continental crust and upper mantle detachment at or near the moho is to be expected only for low geothermal gradients lateral variations in the geothermal gradient produce shallowly dipping zones of highly contrasting strength throughout the continental crust and uppermost mantle possibly resulting in flat lying thrusts and normal faults which may ramp upwards through the crust instabilities leading to seismic events which occur solely during plastic shearing should be common in thrust terranes where the stresses are relatively high ca 100 200 mpa for geothermal gradients ranging from 10 to 50 c km 1 and for quartz rich felsic mafic and peridotitic rock types thrust terranes are therefore predicted to be characterized by coseismic plastic instabilities on all detachment zones throughout the crust and into the uppermost mantle but embryonic rift zones should be aseismic at depth ca 10 km

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

the generation of ductile and brittle shear bands in a low angle mylonite zone in the saint barthelemy massif french pyrenees a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies the last major structures formed in the zone are isolated shear bands divided into three types anastomosing inhomogenous ultramylonitic shear bands type i are subparallel to the mylonitic foliation in the gneiss sg most of these bands developed by ductile deformation processes only planar homogeneous ultramylonite bands type ii are usually oblique to sg they generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation type iii shear bands are planar and oblique to sg they consist of pseudotachylyte weakly affected by ductile deformation type i ii and iii bands seem to represent progressively younger structures on a local scale linked to falling p t conditions the systematic variation in orientation of the different shear bands with respect to sg is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

large scale erosional response of se asia to monsoon evolution reconstructed from sedimentary records of the song hong yinggehai and qiongdongnan basins south china sea

carbonate platform growth and demise offshore central vietnam effects of early miocene transgression and subsequent onshore uplift abstract miocene carbonate platforms cover a large part of the central vietnamese south china sea margin early carbonate deposition took place on two regional platforms separated by a narrow depression developed along the trace of the east vietnam boundary fault zone west of the east vietnam boundary fault zone the tuy hoa carbonate platform fringes the continental margin between da nang and nha trang here platform growth initiated during the early miocene and continued until middle miocene time when regional uplift led to subaerial exposure termination of platform growth and karstification east of the fault zone the triton carbonate platform was also initiated during the early miocene carbonate growth thrived during early and part of middle miocene time and a thick clean lower and middle miocene carbonate succession cover the triton horst and the qui nhon ridge during the middle miocene partial drowning resulted in the split up of the triton carbonate platform repeated partial drowning events throughout the middle and late miocene resulted in westwards retreat of platform growth and eventual platform drowning and termination of carbonate deposition modern carbonate growth continues on isolated platforms hosting the paracel islands farther seawards the onset of widespread carbonate deposition largely reflects the early miocene transgression of the area linked with early post rift subsidence and the opening of the south china sea the mid neogene shift in carbonate deposition is interpreted as a consequence of regional uplift and denudation of central and south indochina starting during middle miocene time when the tuy hoa carbonate platform became subaerially exposed stressed carbonate growth conditions on the triton carbonate platform probably resulted from increased inorganic nutrient input derived from the uplifted mainland possibly enhanced by deteriorated climatic conditions and rapid sea level fluctuations promoting platform drowning

growth demise and dolomitization of miocene carbonate platforms on the marion plateau offshore ne australia abstract strontium isotope stratigraphy has been used to examine the timing of depositional events and dolomitization in two miocene carbonate platforms cored by ocean drilling program odp leg 194 just seaward of the great barrier reef the results provide firm constraints for correlating surfaces and depositional stages between the two platforms and thereby relating seismic sequences previously defined in the off platform sediments to the lithostratigraphic units described from cores in the seismically transparent platform top sites oyster bearing beds at the base of both platform successions yield early oligocene ages 29 31 ma thus dating initial transgression of the marion plateau s volcanic basement there followed a period of slow accumulation of shallow water grainstones rich in quartz and phosphate grains in late oligocene time 29 23 ma seismic megasequence a the main growth of the carbonate platforms took place in early to late miocene time 23 7 ma comprising five depositional sequences the first four of these seismic megasequence b are common to both platforms and terminated with a possible karst surface at 10 7 ma different sedimentologic expression of this megasequence in the two platforms reflects contrasting progradational versus aggradational geometries in the locations studied the final growth stage seismic megasequence c occurred only in the southern platform and terminated at 6 9 ma both platform demise events 10 7 and 6 9 ma approximately coincide with falls in global sea level combined with longer term trends of decreasing water temperature sr isotope ages of dolostones increase with depositional age and older dolostones in the southern platform have more coarsely crystalline and fabric destructive textures than overlying younger dolostones these relationships are consistent with dolomitization by normal seawater shortly after deposition and overprinting of multiple times of dolomite recrystallization and cementation in the deeper strata

early miocene mylonitization and detachment faulting south mountains central arizona the south mountains of central arizona are one of the geologically simplest metamorphic core complexes of the north american cordillera an early miocene age of mylonitization is indicated by crosscutting relationships between mylonitic fabric and a composite pluton dated at 22 25 ma by rb sr u th pb and k ar techniques the kinematic agreement and close temporal association of mylonitization and detachment faulting support models in which the two processes are related to an evolving crustal shear zone that accommodated mid tertiary continental extension 19 references 3 figures 2 tables

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

neogene carbonate platform development in the nw south china sea litho bio and chemo stratigraphic evidence abstract the variations of lithostratigraphy foraminifera geochemical elements and organic biomarkers in well xk1 from the xisha paracel islands reveal the history of carbonate platform development since the early miocene in the south china sea our results indicate that reef growth was more active in the middle miocene than in any earlier periods decreased in the late miocene revitalized in the pliocene and active again in the pleistocene interglacial periods unlike those limestone rich intervals the late miocene succession in well xk1 is characterized by dolomitized limestones with a low deposition rate 30 60 m ma post compaction these late miocene rocks also contain relative high mgo 20 more ree abundant algae and some small sized foraminifera but rare larger benthic foraminifera very low land sourced brgdgts 0 0 5 ng 20 g low bit index 0 1 0 5 and low tex86 derived sea temperature 20 25 c indicating limited active reef growth in relative cooler mesophotic environments at 30 m water depths similar dolomitized limestones of late miocene age also occur in other carbonate platform areas including the nansha spratly islands in the southern south china sea together these south china sea records provide regional evidence of a recession in global carbonate platform development starting from the late miocene while carbonate platform reduction in other regions may have been triggered by global cooling and changing oceanic chemistry accelerated basin subsidence coupled with monsoon cooling are considered as the main factors causing the reef decline in the late miocene south china sea

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

tectonic stratigraphic division and blind fold structures in nansha waters south china sea extensive multiple channel reflection seismic data have been collected in the nansha spratley islands waters the southern margin of the south china sea stratigraphic correlation is shown with focus on a comprehensive geophysixcal survey line run front offshore nw sabah to offshore se vietnam according to the varying tectono stratigraphy from southeast to northwest five tectonic belts can be determined i e the palawan borneo nappe nansha trough nanwei liyue compressive belt zheng he extensional belt and the circum southwest subbasin belt in the palawan borneo nappe neogene quaternary deposits were highly upthrust northwestwards resulting in a series of moderately to tightly folded anticlines separated by open synclines the nansha trough was a narrow deep water belt filled with thick undisturbed neogene quaternary deposits the nanwei liyue compressive bell was dominated by strongly folded paleo anticlines overlain by an undeformed sedimentary cap with a pronounced hiatus of paleogene sediments the zheng he extensional belt consisted of a rugged topography and paleogene half grabens bounded by listic faults the major extensional faults were reactivated to cut through the overlying neogene quaternary deposits over the circum southwest subbasin belt neogene quaternary deposits draped on the largely subsided fault blocks related to the late oligocene mid miocene seafloor spreading based on the regional stratigraphic correlation the prominent paleo anticlines found within the nanwei liyue compressive belt are deduced to consist of mainly mesozoic marine sediments that were compressed in the late mesozoic era therefore the nansha microcontinent block is shown to be a collision complex assembled during late mesozoic era c 2004 elsevier ltd all rights reserved

cenozoic tectonic and stratigraphic development of the central vietnamese continental margin abstract the east vietnam boundary fault zone evbfz forms the seaward extension of the red river shear zone and interacted with the extensional rift systems in basins along the central vietnamese continental margin the structural outline of the central vietnamese margin and the timing of deformation are therefore fundamental to understanding the development of the south china sea and its relation to indochinese escape tectonism and the india eurasia collision this study investigates the structural and stratigraphic evolution of the central vietnamese margin in a regional tectonic perspective based on new 2 d seismic and well data the basin fill is divided into five major oligocene to recent sequences separated by unconformities deposition and the formation of unconformities were closely linked with transtension rifting the opening of the south china sea and late neogene uplift and denudation of the eastern flank of indochina the structural outline of the central vietnamese margin favors a hybrid tectonic model involving both escape and slab pull tectonics paleogene left lateral transtension over the nnw striking evbfz occurred within the song hong basin and the quang ngai graben and over the da nang shelf western phu khanh basin related to the escape of indochina east of the evbfz paleogene ne striking rifting prevailed in the outer phu khanh basin and the hoang sa graben fitting best with a prevailing stress derived from a coeval slab pull from a subducting proto south china sea beneath the southwest borneo palawan region major rifting terminated near the end of the oligocene however late stage rifting lasted to the early miocene when continental break up and seafloor spreading commenced along the edge of the outer phu khanh basin the resulting transgression promoted lower and middle miocene carbonate platform growth on the da nang shelf and the tri ton high whereas deeper marine conditions prevailed in the central part of the basins partial drowning and platform retreat occurred after the middle miocene due to increased siliciclastic input from the vietnamese mainland as a result siliciclastic marine deposition prevailed offshore central vietnam during the pliocene and pleistocene

the nature and significance of pseudotachylite from the nason terrane north cascade mountains washington abstract pseudotachylite has been recently discovered at numerous localities in the nason terrane of the north cascade mountains washington the pseudotachylite is widespread not associated with any single major fault or shear zone and tends to occur in narrow foliation parallel zones single pseudotachylite veins range in thickness from less than 0 1 to 10 cm veins and networks of veins occur in six distinct types and these form the basis of a suggested classification scheme multiple generations of pseudotachylite are present in some samples host lithologies include quartz diorite semipelitic schist tonalite amphibolite and especially graphitic pelitic schist glassy veins are rare most pseudotachylite is apparently holocrystalline spherulites vesicles and dendritic microphenocrysts are present and indicate a frictional melt origin the ubiquitous occurrence of cataclasite xenoliths within the pseudotachylite along with other textural observations indicates that in the nason terrane the development of cataclasite zones was the precursor to pseudotachylite generation it is inferred that cataclastic deformation of the schist provided the strain softening which resulted in faulting and pseudotachylite generation the water rich and ultra fine grained matrix of the cataclasite was an ideal starting material for pseudotachylite generation based upon bulk chemistry of host lithologies and microprobe analyses of the pseudotachylite it appears that the pseudotachylite formed as a result of a virtually complete melting of the pelitic i e non quartz vein component of the schist where pseudotachylite occurs within pelitic schist alone the composition of the pseudotachylite is very similar to the pelitic component of the schist while the composition of veins cutting tonalite gneiss in addition to pelitic schist appears to define a mixing line between these two lithologies this suggests either participation of the tonalite during initial melting or subsequent assimilation of gneiss into the melt

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

an early miocene transition in deformation regime within the red river fault zone yunnan and its significance for indo asian tectonics recent work has tended to support the view that lateral extrusion of indochina along the red river fault zone was a significant factor in accommodating the convergence of india with asia to better understand the nature and timing of this feature 40ar 39 ar age spectrum analyses were undertaken on minerals from the ailao shan red river metamorphic belt in southern yunnan china interpreted using diffusion domain theory these results yield thermal histories in the temperature range 550 150 c discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early miocene at least in the vicinity of our samples left lateral strike slip ductile deformation appears to have ceased by about 20 ma the mylonitic gneisses in the shear zone were subsequently obliquely unroofed probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief locally 100 c m y cooling episode this transition may have resulted from a component of northeast southwest extension attributable to greater clockwise rotation of indochina relative to south china after 20 ma than that consistent with pure strike slip along the fault k ar ages on clay minerals from gouge within the normal faults bounding the ailao shan and diancang shan yield a range of ages between 20 and 185 ma but show a significant clustering at 20 25 ma these results suggest that early miocene movement on a precursor to the range front normal fault in the ailao shan caused oblique unroofing of the shear zone this early miocene event corresponds in time with a transition from low to very high denudation rates throughout the himalaya and southern tibet this coincidence may reflect that once left lateral strike slip motion on the red river fault zone slowed or was terminated accommodation of much of the continued northward convergence of india occurred on the main central thrust resulting in rapid uplift in the tethyan himalaya and gangdese belt

development of cenozoic basins in thailand abstract there are over 60 cenozoic basins distributed in various parts of thailand both onshore and offshore these basins are mainly n s trending half grabens or grabens and most were initiated in the late oligocene the structural framework of the basins is generally governed by n s trending extensional faults which are spatially related to the movement of the nw se and ne sw trending strike slip faults that have been active since the oligocene the nw se faults are the principal dextral strike slip faults whereas the ne sw faults are the conjugate sinistral strike slip faults the collision of the india craton with southern asia since 40 50 ma caused clockwise rotation of southeast asia and movements on the strike slip faults with the associated development of cenozoic basins in this region four major cenozoic basins mergui pattani malay and phitsanulok contain up to 8000 m of sediments generally the cenozoic fills are continental sediments except in the mergui basin in the andaman sea where the sedimentary fill is mainly marine sediments fluviatile with occasional lacustrine deposits were prominent in the gulf of thailand and onshore basins during the oligocene to early miocene from early to middle miocene widespread perennial lake conditions were well developed in the onshore basins and in the western graben area of the gulf of thailand a change in tectonic and or climatic conditions took place in the middle to late miocene times resulting in the cessation of perennial lake conditions and development of a regional unconformity

development and controlling factors of miocene carbonate platform in the nam con son basin southwestern south china sea miocene carbonate platforms are located on intrabasinal structural highs and along the southern margin of the nam con son basin also referred to as the wanan basin by chinese southwestern south china sea these isolated platforms together cover a gross area of over 25 000 km 2 we map the platforms reconstruct their evolution and evaluate the controls on platform growth and demise within the regional geological setting using available seismic and well data the platforms initiated in the late early miocene and reached their widest extent during the middle miocene the platform margins were controlled by synsedimentary faults associated with carbonate slope failure deposits and or reef talus the platforms formed as isolated platform platform growth history involves six phases start up second start up keep up exposure renewal and drowning phases quantitative analysis of the basin evolution indicates that tectonism eustasy the supply of both terrigenous sediment and nutrients were the dominant controls on platform development and demise c 2013 elsevier ltd all rights reserved

finite element modelling of the structure and evolution of the south kenya rift east africa abstract a finite element fe model was used in this study to investigate the intraplate rifting process in south kenya east africa the rifting model shows that the important factors influencing the amount of shoulder uplift and rift subsidence include the horizontal deviatoric extensional stresses the viscosity of the lower crust and the dimension and density contrast of the low velocity upper mantle anomaly thus for instance a reduced lower crust viscosity as well as an increased extensional far field stress favours the subsidence of the rift basin in turn for a reduced lower crust viscosity the magnitude of the applied extensional stress should be reduced in order to end up with the same topography of the rifted area as can be obtained with a more viscous lower crust combined with a relatively high extensional far field stress in addition it also became obvious that the uplift of the crust mantle boundary beneath the rift depends predominantly on the size and density contrast of the upper mantle anomaly an increased density contrast between the upper mantle anomaly and normal mantle results in increased buoyancy forces thus producing an increased uplift of the crust mantle boundary in turn in order to avoid too large values for the surface topography a reduced lower crust viscosity in the order of 1021 pa s is required models without an upper mantle low density anomaly have resulted in crustal thickening beneath the rift and not in crustal thinning in accordance with the geological field evidence we have modelled the evolution of the south kenya rift in two stages in the first stage from 16 to 10 ma ago a broad regional uplift of a few hundred metres produced by a relatively small but broad low density upper mantle anomaly was accompanied by an insignificant amount of faulting in the second stage from 10 ma ago to the present day significant faulting accompanied by the buoyancy effects of a larger low density low velocity upper mantle anomaly concentrated more beneath the rift itself have quite successfully reproduced the rift subsidence shoulder topography and crust mantle boundary uplift observed at the present day

different expressions of rifting on the south china sea margins abstract rifting of continental margins is generally diachronous along the zones where continents break due to various factors including the boundary conditions which trigger the extensional forces but also the internal physical boundaries which are inherent to the composition and thus the geological history of the continental margin being opened quite recently in the tertiary in a scissor shape manner the south china sea scs offers an image of the rifting structures which varies along strike the basin margins the scs has a long history of extension which dates back from the late cretaceous and allows us to observe an early stretching on the northern margin onshore and offshore south china with large low angle faults which detach the mesozoic sediments either over triassic to early cretaceous granites or along the short limbs of broad folds affecting palaeozoic to early cretaceous series these early faults create narrow troughs filled with coarse polygenic conglomerate grading upward to coarse sandstone because these low angle faults reactivate older trends they vary in geometry according to the direction of the folds or the granite boundaries a later set of faults characterized by generally e w low and high angle normal faults was dominant during the eocene associated half graben basement deepened as the basins were filling with continental or very shallow marine sediments this subsequent direction is well expressed both in the north and the sw of the south china sea and often reactivated earlier detachments at places the intersection of these two fault sets resulting in extreme stretching with crustal boudinage and mantle exhumation such as in the phu khanh basin east of the vietnam fault a third direction of faults which rarely reactivates the detachments is ne sw and well developed near the oceanic crust in the southern and southwestern part of the basin this direction which intersects the previous ones was active although sea floor spreading was largely developed in the northern part and ended by the late miocene after the onset of the regional mid miocene unconformity known as mmu and dated around 15 5 ma latest miocene is marked by a regional basement drop and localized normal faults on the shelf closer to the coast the se margin of the south china sea does not show the extensional features as well as the northern margin detachments are common in the dangerous grounds and reed bank area and may occasionally lead to mantle exhumation the sedimentary environment on the extended crust remained shallow all along the rifting and a large part of the spreading until the late miocene when it suddenly deepened this period also corresponds to the cessation of the shortening of the nw borneo wedge in palawan sabah and sarawak we correlate the variation of margin structure and composition of the margin mainly the occurrence of granitic batholiths and mesozoic broad folds with the location of the detachments and major normal faults which condition the style of rifting the crustal boudinage and therefore the crustal thickness

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

pseudotachylyte generated in the semi brittle and brittle regimes bench canyon shear zone central sierra nevada pseudotachylyte cuts cretaceous plutonic and volcanic rocks in the bench canyon shear zone central sierra nevada california as documented by optical and scanning electron microscopy the pseudotachylyte displays evidence for a friction melt origin including vesicles amygdules crystallites flow fabrics and embayed crystal fragments based on fault related rock association vein morphology and inferred crustal level of generation two distinct types of pseudotachylyte are documented in the shear zone pseudotachylyte associated with cataclasite was generated at shallow crustal levels 1 10 km via unstable velocity weakening abrasive wear friction pseudotachylyte associated and interlayered with mylonite was generated at deeper crustal levels 10 15 km most likely via stable velocity strengthening adhesive wear friction this pseudotachylyte underwent ductile deformation in the solid state interlayered pseudotachylyte and mylonite records cyclic aseismic seismic slip at a transitional crustal level between the semi brittle field of quartzofeldspathic rocks and the base of the seismogenic zone the cyclicity is attributed to strain rate fluctuation effected by strain resistant phacoids pore fluid pressure variation ductile instabilities or downward fault propagation the mode of deformation within the shear zone progressed from i production of mylonite during amphibolite to greenschist facies conditions to ii generation of pseudotachylyte by intermittent seismic faulting within a background of aseismic shear in the upper realm of the semi brittle field 300 400 c to iii generation of pseudotachylyte through seismic events in the brittle 300 c during uplift exhumation and cooling some of the latter may have been generated at shallow enough crustal levels 4 km to allow vesicle formation the exhumation of a crustal level exposing such an assemblage provides a rare opportunity to study the enigmatic nature of the transitional change from semi brittle flow to seismogenic rupturing

episodic rifting and subsidence in the south china sea the south china sea experienced at least three stages of rifting and two intervening stages of sea floor spreading since the early cretaceous its evolution can be described by an episodic model of tectonism one of thermal cooling and subsidence pulsed by temporally and spatially confined heating events analysis of regional geologic and geophysical data suggests episodes of rifting and associated thermal activities initiated during the late cretaceous the late eocene and the late early miocene the rift system corresponding to the first episode trends northeast southwest whereas those of the second and third trend east west these two trends coincide with the orientations of the major tectonic lineations within the basin age estimates from heat flow and bathymetric data suggest the oceanic crust in the southwest subbasin is considerably older 55 ma than that in the northwest 35 36 ma or east 32 ma subbasins the episodic tectonic model is supported by basin subsidence analysis subsidence especially subsidence rate curves derived from the regional well data demonstrates that unlike a classic atlantic passive margin thermal subsidence in the south china sea was punctuated by rapid subsidence events which are chronologically consistent with the rifting episodes in terms of hydrocarbon potential the episodes of rifting and drifting would be conducive to the development of overprinted structures and the deposition of several discrete transgressive packages of source rocks and reservoirs separated by widespread unconformities the thermal maturity of sedimentary organic matter affected by episodic rifting and subsidence may be greater than expected on a purely passive margin of equivalent age that had not experienced repeated heating

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

spreading direction in the central south china sea recent work1 2 indicates that the south china sea is an atlantictype marginal basin of late tertiary age magnetic anomalies in the eastern part of the sea are consistent with seafloor spreading directed approximately north south1 2 we present here a new morphostructural study based on coupled seabeam mapping and single channel seismic reflection profiling which reveals dominant normal fault scarps striking n50 e between 113 and 119 e longitude near the axis of this basin such a structural fabric implies a nw se spreading direction at least in the 150 200 km wide axial region of the south china sea and places new constraints on geodynamic models for the formation of this basin in the tectonic and palaeogeographic framework of south east asia and the south west pacific

the origin of ridge in groove slickenside striae and associated steps in an s c mylonite abstract a three dimensional study of an s c mylonite reveals that the s surfaces anastomose around porphyro clasts and c surfaces are not truly planar but curvi planar with the linear generator parallel to the movement direction it is suggested that it may be common for shear surfaces at all scales to take this form in the material studied surfaces exposed by parting primarily along c surfaces which are a product of ductile deformation and are penetrative have all the characteristics of classic slickensides on the slickensides striae of the ridge in groove type and steps of both congruous and incongruous types are developed the ridge in groove shape of the striae is directly related to the curvi planar shape of the c surfaces incongruous steps are smooth they are the result of parting along s surfaces congruous steps are rough first order congruous steps are formed by fracturing between individual c surfaces whereas second order congruous steps form by breaking off the tips of the dihedra between s and c surfaces thus the smoothness roughness technique can be used to determine the sense of shear in strongly deformed rocks where the angle between s and c surfaces is too small to be recognized in the field this may prove to be a useful shear sense indicator

north palawan block philippines its relation to asian mainland and role in evolution of south china sea the island of mindoro the northern part of palawan island and the reed bank area southwestern philippines together constitute a continental fragment the north palawan block lying within an island arc oceanic setting the permian to paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the philippine archipelago these rocks also suggest that the north palawan block once occupied a pre drift position contiguous with the south china mainland four prominent pre neogene regional unconformities are recognized both onshore and offshore the china mainland in taiwan and in the north palawan block the synchrony of these unconformities and the facies relation of the unconformity bounded sedimentary units strongly suggest a common pre neogene history for all these areas by contrast an important regional unconformity occurring at the top of the middle miocene throughout the palawan area is absent from the asian mainland extension in the south china sea basin since the mesozoic which has separated the north palawan block from the asian mainland has been approximately uniform from west to east however there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea floor spreading from the foregoing observations and using the most recent magnetic spreading anomaly data for the south china sea a suite of palinspastic reconstructions has been prepared which shows the evolution of the south china sea area from the late triassic to the pliocene the reconstructions illustrate 1 the convergent continental margin setting of the north palawan block during much of the jurassic and cretaceous 2 the late cretaceous inception of the philippine island arc system 3 the subsequent counter clockwise rotation of the arc system from the late eocene onward 4 the paleocene to middle miocene opening of the south china sea and 5 the early to middle miocene collision between the north palawan block and the palawan subduction system

glassy pseudotachylyte veins from the fuyun fault zone northwest china glassy pseudotachylytes occur in granitic mylonites from the fuyun fault zone northwest china powder x ray diffraction data indicate that the fine grained matrices of the fuyun pseudotachylytes vary from almost crystalline to complete glass a few to 90 wt glass powder x ray diffraction patterns also indicate that clasts in the glass type pseudotachylyte veins mostly consist of quartz grains microlites displaying quenched or rapid cooling textures vesicles and amygdules flow structures and rounded fragments are also common in the fuyun pseudotachylytes the average bulk chemical compositions of these pseudotachylytes are similar to those of the host rocks but the glass matrix has a low sio2 component which is 5 15 wt lower than that of the host rocks the pseudotachylytes contain 2 3 wt water h2o using experimental data which relate the solubility of water to pressure approximately 400 bars lithostatic pressure corresponding to approximately 1 5 km depth of pseudotachylyte formation is estimated a minimum temperature estimate of 1450 c can be estimated due to the presence of the pure sio2 glass the x ray diffraction and chemical data indicate that the fuyun pseudotachylytes formed during seismic faulting at shallow depths by selective melting under water saturated conditions rather than by total melting or by crushing of the host rocks along fault surfaces

cenozoic plate reconstruction of the south china sea region abstract reconstructions of the south china sea region at 60 ma 40 ma 30 ma 20 ma 10 ma and 5 ma are presented we have attempted to place the south china sea basin in a regional tectonic framework the tectonic evolution of the major blocks surrounding the south china sea were analyzed as well as the relative motions of the indian and australian plates we have tried to correct the tectonic models available in this region a 3 d graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self consistency when the model conflicted with previous interpretations the input data were evaluated for alternative explanations at least two and possibly three stages of extension can be recognized in this region the earliest one active in the late cretaceous to eocene involved nw se extension the second one active from the late eocene to early miocene involved north south extension the third stage of extension which probably trended nw se can be dated as post oligocene the first extensional event produced the ne sw trending proto south china sea and a series of sedimentary basins along the south china margin following the southeastward extrusion of indochina the proto south china sea was mostly consumed at the palawan trough renewed north south extension in the south china continental margin started the present day south china sea spreading in the oligocene the southeastward extrusion of indochina blocked by sundaland resulted in the nw se trending opening of the south china sea basin in the early miocene collision of the north palawan microcontinental block with the west philippines block stopped the opening of the south china sea at the end of early miocene spreading activity switched to the sulu sea basin in the middle miocene but collision between the sulu ridge and the west philippines at mindanao halted the opening of the sulu sea at the end of the middle miocene in the late miocene greater india continued its northward path and seems to have ripped open the andaman sea in the pliocene subduction along the northern manila trench placed the north luzon arc on a collision path with the east asia continental margin at taiwan our reconstructions along with detailed geological and geophysical information may be used as a predictive tool for basin evolution models and block interactions in this region the development of the south china sea basin the gulf of thailand the malay basin and the central thailand basins are the result of collision induced extensional forces the sulu celebes and sumatra basins were formed as a consequence of prolonged subduction the opening of the pearl river mouth west natuna south china sea sulu and possibly celebes basins were terminated by various plate collisions during the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution

major unconformities termination of extension events and associated surfaces in the south china seas review and implications for tectonic development abstract the distribution of unconformities and end of cenozoic rifting events in the south china seas scs reflects both the modes of rift development and the effects of driving mechanisms continental rifting began in the eastern basins during the paleocene and propagated westwards to the vietnam basin margin in the late eocene continental breakup around 32 28 ma caused a regional reduction or cessation in extensional activity particularly affecting basins furthest from the spreading centre basins in the slope and deepwater area north of the spreading centre exhibit reduced fault activity until 21 20 ma propagation of oceanic crust westwards between 25 and 23 ma and termination of seafloor spreading sometime between 20 5 and 16 ma affected fault activity in the qiongdongnan and nam con song basins in the phu khanh basin and south in the dangerous grounds area extension continued until about 16 ma ending at the red unconformity the end of seafloor spreading around 20 5 ma reflects loss of extensional driving force as thinned continental crust entered the nw borneo subduction zone controversially a key component of the driving force maybe attributed to slab pull a transitional period of about 5 7 my between the onset of subduction of continental crust and final jamming of the subduction zone deep regional unconformity dru is inferred the last pulse of extension was focussed in the western scs and terminated around 10 5 ma detailed understanding of proto south china seas development remains uncertain and controversial

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

constraints of sea beam data on crustal fabrics and seafloor spreading in the south china sea abstract the south china sea is a mid late tertiary marginal basin the magnetic anomaly lineations in the eastern part of the basin trend approximately east west 1 2 suggesting a north south direction of spreading in the spring of 1985 two cruises on the french research vessel jean charcot provided sea beam coverage seismic reflection magnetic and gravity profiles the sea beam data exhibit two major structural trends scarps striking n50 e 15 interpreted as normal faults and scarps striking n140 e 15 interpreted as fracture zones this fabric implies a northwest southeast direction of spreading up to about 100 km north and south of the inferred spreading axis 3 dense sea beam coverage of a roughly 1 square area northwest of the scarborough seamounts chain shows that the emplacement of these seamounts was at least in its initial stage controlled by faulting in two orthogonal directions n50 e and n140 e magnetic and gravimetric maps of the same area also reveal anomalies trending roughly n50 e which are disrupted by transform zones striking n140 e this detailed study indicates that the fracture zones may be closely spaced less than 20 30 km east of macclesfield bank furthermore magnetic anomalies identified as 6 and 6a 20 myr along two north south profiles located at both edges to the north of this detailed study area may be correlated with the n80 e trend characteristic of such anomalies in the eastern part of the south china sea the east west trend of magnetic anomalies 6 and 6a south of scarborough seamount chain recognised by taylor and hayes 1983 1 is incompatible with the trend of the fault scarps observed on sea beam data we infer that progressive right lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east west average trend of some of the magnetic anomalies and of the scarborough seamount chain which represents the location of the relict spreading axis in spite of a n50 e spreading direction the whole central part of the basin east of 115 e may have formed in this way since n130 140 e striking fracture zones are observed on the sea beam swaths this model may correspond to the second of two successive spreading phases with n s and nw se directions of extension respectively as was presented by the authors in a previous paper pautot et al 1986 3 alternatively we may assume that the direction of extension did not change radically during the opening history 3 implying that oblique spreading occurred along n80 e trending ridge segments creating the n80 e trending scarps and magnetic anomalies observed to the north and to the south of the study area such a direction of extension is compatible with mid late tertiary left lateral movements along large n130 150 e strike slip faults such as the red river and wang chao faults in south china and sundaland

development of phyllonite from granodiorite mechanisms of grain size reduction in the santa rosa mylonite zone california abstract field and laboratory investigations indicate that phyllonite within the santa rosa mylonite zone developed from granodiorite without hydrous alteration of the original mineral assemblage initial grain size reduction within zones of phyllonite was accomplished through whole rock cataclasis subsequent deformation occurred through ductile flow the original cataclastic failure is recorded by sharp boundaries with the surrounding rock which are retained through the ductile overprint biotite in phyllonite has a mean grain size of 3 m and exhibits straight boundaries and a strong crystallographic preferred orientation trace analyses of hvem images indicate that the largest biotite grains are most strongly oriented and the grains are elongate parallel to lineation tem observations indicate that typical grains have irregularly spaced stacking faults and twins evidence for intracrystalline folding and cataclasis present in protomylonite and mylonite is absent bending or kinking of grains is rare these observations suggest that biotite in phyllonite accommodated deformation through dynamic recrystallization in concert with intracrystalline slip and mechanical rotation grain size reduction to form phyllonite is accompanied by macroscopically and microscopically visible changes in the character of foliations in the rock we suggest that these changes are linked to the deformation mechanisms operating within the fine grained phyllonite

al rich calcic pyroxene in pseudotachylite an indicator of high pressure and high temperature abstract an aluminous pyroxene with an approximate formula ca 0 60 na 0 05 0 20 mg 0 58 fe 0 11 2 al 0 60 vi si 1 68 a1 0 32 o 6 or jd 0 05 ts 0 32 es 0 20 wo 0 10 en 0 27 fs 0 06 has been found in a pseudotachylite vein from gabbros in the musgrave ranges central australia textural evidence indicates that it formed by breakdown of calcic plagioclase augite and olivine which constitute the isochemical country rock there is a large amount of m 2 vacancy compared to most pyroxenes the vein mineral is most similar in composition to natural pyroxenes in kimberlites and eclogites and especially to synthetic pyroxenes which formed at pressures in excess of 3 gpa 30 kb we suggest that these high pressures were not lithostatic but represent dynamic stresses released during catastrophic brittle rupture either at tips of propagating fractures or spallation of unsupported crackwalls such transient shear stresses may reach the shock range we propose such a fracture mechanism as an alternative to frictional melting for the productoin of pseudotachylite veins

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

relationship between plagioclase crystallization and cooling rate in basaltic melts rock textures commonly preserve a record of the near surface crystallization history of volcanic rocks under conditions of simple cooling without convection or mixing textures will reflect sample cooling rate the temperature at which crystallization was initiated and the distribution of mineral phase precipitation across the crystallization interval compilation of plagioclase size and number density data on natural dike sill and lava lake and experimental samples suggests that 1 growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate and 2 the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization evaluation of textural data in the context of recent numerical models of crystallization in simple one and two component systems provides new insight into regularities in the crystallization behavior of basaltic magmas for example the rate of change in crystal size and number density as dictated by mass balance has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems in natural samples plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width solidification time comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes distributions broaden and the grain size at peak frequency increases with increasing crystallization times decreasing cooling rates in contrast partially crystallized makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size measured size distributions in dikes can be explained by late stage modification of makaopuhi type distributions through loss of small crystals possibly the consequence of growth without nucleation finally this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near surface basaltic systems predicted growth rates will be slow and relatively constant 10 10 10 11 cm s for crystallization times expected in most shallow volcanic systems 1000 years

deep seated fluid involvement in ductile brittle deformation and mineralization south mountains metamorphic core complex arizona in the south mountains metamorphic core complex of central arizona a middle tertiary pluton exhibits the entire spectrum of ductile to brittle structures characteristic of metamorphic core complexes apart from minor cu enrichments in brecciated samples original major and minor element abundances of the deformed igneous rocks are remarkably undisturbed and there is little correspondence between chemical composition and deformational state similarly biotite and feldspar behaved almost isochemically during deformation except in the most severely deformed samples where plagioclase became somewhat more sodic and where biotite lost fe mg relative to al ti and was partly converted to vermiculite the oxygen and hydrogen isotope compositions of the samples are also quite uniform and are typical for continental granitoid rocks the range in whole rock 18 o values is extremely limited for the entire suite 8 3 to 9 9 versus v smow from undeformed granodiorite to mylonite green breccia and microbreccia although 18 o values for quartz 9 4 to 11 2 and feldspar 7 8 to 9 2 are similarly restricted the highest values occur in the most strongly deformed samples variably vermiculitic biotite has mildly and apparently randomly disturbed 18 o values 4 9 to 6 0 but d values 75 4 are remarkably uniform and are typical for unaltered plutonic rocks the 18 o values of barren ductilely deformed quartz veins 9 1 to 9 7 and of late brittlely deformed gangue quartz from the max delta gold mine 12 1 are consistent with the persistent circulation of a single isotopically uniform fluid reservoir at temperatures from 600 c ductile deformation barren quartz veins to 400 c brittle deformation mineralized quartz veins the inferred fluid 18 o 7 and d 35 values imply a deep seated igneous or high t metamorphic origin for the fluids fluids present during the entire history of ductile to brittle deformation were apparently close to chemical and oxygen hydrogen isotope equilibrium with the rocks the temporal association of deformation with tertiary plutonism and the evidence for high temperature fluid rock near equilibrium support the interpretation that the fluids were expelled from differentiated late crystallizing portions of the south mountains plutonic complex

an argument against the crush origin of pseudotachylytes based on the analysis of clast size distribution abstract the origin of pseudotachylytes has been controversial since wenk cast doubt on the melt origin of the matrix of pseudotachylytes in 1978 the matrix of this rock is so fine grained that the crush origin of pseudotachylytes revived by wenk cannot easily be denied this paper presents a new line of argument based on the size analysis of clasts contained in pseudotachylytes in felsic granulite from the musgrave range central australia these clasts are definitely crush products produced during the pseudotachylyte generation their sizes as measured on photomicrographs of thin sections obey the size distribution n n r d where n is the cumulative number of clasts with sizes greater than r d is the fractal dimension and n is a constant that depends on the number of measurements d was found to be 1 5 0 05 for the size ranges of 10 2000 m if the matrix of the pseudotachylytes consists mostly of ultrafine crush products they must have formed simultaneously with those coarse crush products the proportion of fine products relative to the coarse clasts can be estimated assuming that a similar size distribution also holds for the fine products the estimated area occupied by fine products in a thin section is of the order of only several percent whereas the measured area of the matix is about 60 thus the major part of the matrix of the pseudotachylytes cannot be regarded as crush products it is also shown that the number of clasts smaller than about 5 m becomes very small perhaps as a result of nearly complete dissolution of fine clasts in a melt however if the ultrafine grained matrix of the pseudotachylytes had formed by crushing during seismogenic fault motion the grain size refinement during the crushing should have occurred jumping the size range of at least 1 5 m this is quite unreasonable and disproves the crush origin for the matrix of pseudotachylytes

a tectonic model for the tertiary evolution of strike slip faults and rift basins in se asia abstract models for the tertiary evolution of se asia fall into two main types a pure escape tectonics model with no proto south china sea and subduction of proto south china sea oceanic crust beneath borneo a related problem is which if any of the main strike slip faults mae ping three pagodas and aliao shan red river asrr cross sundaland to the nw borneo margin to facilitate continental extrusion recent results investigating strike slip faults rift basins and metamorphic core complexes are reviewed and a revised tectonic model for se asia proposed key points of the new model include 1 the asrr shear zone was mainly active in the eocene oligocene in order to link with extension in the south china sea the asrr was less active during the miocene tens of kilometres of sinistral displacement with minor amounts of south china sea spreading centre extension transferred to the asrr shear zone 2 at least three important regions of metamorphic core complex development affected indochina from the oligocene miocene mogok gneiss belt doi inthanon and doi suthep around the asrr shear zone hence paleogene crustal thickening buoyancy driven crustal collapse and lower crustal flow are important elements of the tertiary evolution of indochina 3 subduction of a proto south china sea oceanic crust during the eocene early miocene is necessary to explain the geological evolution of nw borneo and must be built into any model for the region 4 the eocene oligocene collision of ne india with burma activated extrusion tectonics along the three pagodas mae ping ranong and klong marui faults and right lateral motion along the sumatran subduction zone 5 the only strike slip fault link to the nw borneo margin occurred along the trend of the asrr fault system which passes along strike into a right lateral transform system including the baram line

high resolution palaeogene sequence stratigraphic framework for the cuu long basin offshore vietnam driven by climate change and tectonics established from sequence biostratigraphy abstract the chronostratigraphy of the cuu long basin offshore vietnam to the south of the mekong delta is poorly understood especially for the palaeogene succession this paper utilises a comprehensive biostratigraphic database from 46 exploration wells to establish a high resolution sequence biostratigraphic framework for the oligocene and upper eocene based on the identification of 36 transgressive regressive depositional cycles they are shown to be climate driven and correlate to 406 kyr heartbeat of the oligocene eccentricity cycles their occurrence has been rigorously cross checked against seismic across the basin this re evaluation requires the repositioning of the oligo miocene boundary in the cuu long basin from the c seismic pick to the bi 1 seismic pick by integrating sequence biostratigraphic and seismic datasets the oligocene can be seen as characterised by two types of depositional sequence firstly thin and repetitive depositional packages here termed sequences but perhaps better visualised as parasequences the deposition of which was controlled by cyclically changing climate secondly much thicker packages each comprising a succession of parasequences separated by unconformities and which tie to the main seismic divisions within the oligocene upper eocene and were driven by tectonics the latter are visualised as 3rd order sequences with the parasequences forming 4th order sequences correlating the succession of parasequences to the 406 kyr heartbeat of the oligocene isotope cycles allows accurate ages to be proposed for each parasequence boundary and in doing so an accurate time framework is established for the basin the first sediments may have been deposited during the latest middle eocene seismic group f g at the same time as or just prior to the initial stage of faulting prior to the opening of the south china east vietnam sea with deposition of seismic group e commencing at about 36 6 ma during the late eocene unconformities reflecting 3rd order sequence boundaries are dated at 33 4 ma 29 8 ma and 27 4 ma

petroleum potential of oligocene lacustrine mudstones and coals at dong ho vietnam an outcrop analogue to terrestrial source rocks in the greater song hong basin the outcrop of oligocene age at dong ho northern vietnam may constitute an immature analogue to offshore terrestrial source rocks in the greater song hong basin the outcrop includes an interval with two source rocks 1 highly oil prone carbonaceous mudstones containing kerogen types iia and iia i and with toc contents from 6 48 to 16 89 wt and hi values from 472 to 690 and 2 oilprone humic coals kerogen type iii with hi values from 200 to 242 the mudstones were deposited in oxygen deficient lakes which on occasion were subject to marine influence and the coals accumulated in freshwater peat forming mires the coals have broad activation energy ea distributions while the mudstones have ea distributions characterised by a pronounced principal ea during artificial maturation about 16 17 of the organic carbon in the coals and 45 50 of the organic carbon in the mudstones participated in petroleum formation the two source rocks primary generate oil and secondary generate gas however the mudstones realised the majority of their potential over a more narrow temperature range than the coals the excellent generative potential of the terrestrial source rocks at dong ho is encouraging for offshore exploration for reservoirs charged by cenozoic rift lake successions q 2001 elsevier science ltd all rights reserved

late cretaceous and cenozoic tectono stratigraphic development of the east java sea basin indonesia abstract the east java sea basin is underlain by a metamorphic basement complex subsidence of this basement during the late cretaceous resulted in accumulation of up to 3 km of marine upper cretaceous sediments megasequence 1 contraction and near peneplanation of the upper cretaceous sediments and underlying basement occurred before the middle early eocene producing a regional unconformity which defines the base of megasequence 2 the cenozoic east java sea basin started to form during the early eocene by crustal extension on both planar normal faults and extensionally reactivated cretaceous thrusts normal faulting was pulsed from the early eocene to early oligocene and affected a progressively larger area with time complex structural geometries evolved in response to local extensional reactivation of obliquely orientated pre existing structures the resultant palaeogene fault controlled sub basins were filled with fluvial coastal plain and shelf clastic and carbonate sediments recording an overall transgressive evolution regional subsidence became dominant over fault controlled subsidence during the early oligocene basin fill during this time was dominated by deep marine fine grained clastic sediments a regional unconformity of intra oligocene age is recognized and is overlain by oligocene to lowermost miocene deep water calcareous mudrocks and limestones which locally onlap erosionally truncated eocene rocks the lower eocene to lowermost miocene sediment package comprises megasequence 2 regional inversion of palaeogene sub basins commenced in the early miocene and continues to the present day the syn inversion lower miocene to recent sediments comprise megasequence 3 inversion has resulted from regional contraction and resultant reverse reactivation of palaeogene normal faults and cretaceous thrusts regional subsidence has been continuous during the inversion history resulting in a gradual reversal of depocentre location palaeogene depocentres became neogene highs whereas palaeogene platforms became neogene depocentres miocene deposition during inversion was dominated by the alternation of fine grained carbonate dominated and clastic dominated cycles subsequent pliocene sandstone deposition followed fluvial incision during an early pliocene lowstand the depositional history during the pleistocene to recent records rapid relative sea level changes progradation of clastic and carbonate sediments erosional truncation channelling and slumping the tertiary structural geometrical evolution and preserved sediment distribution can be explained by dominantly dip slip fault movement during the extensional and contractional phases of basin development and deformation basin scale cross sectional geometries similar to classical positive flower structures have evolved by the reverse reactivation of a geometrically complex extensional fault system

denudation history of onshore central vietnam constraints on the cenozoic evolution of the western margin of the south china sea abstract apatite fission track analysis is used to monitor the timing and rate of upper crustal cooling and denudation across central vietnam in response to rifting of the south china sea results show regional denudation in the syn and post rift phases of the south china sea occurred at a similar rate 40 10 m myr consistent with the sedimentation record in adjacent offshore basins this suggests that extension was not the dominant factor controlling cenozoic denudation probably because the study area was sufficiently distant from the actively rifted continental margin a significant change in regional cooling rate during the upper miocene inferred denudation rates also shift from 34 m myr to 390 500 m myr is linked to enhanced erosion and deposition of prograding sediments in adjacent offshore basins this shift in denudation is associated and contemporaneous with initiation of regional basaltic magmatism and development of topography

tectonic evolution and regional setting of the cuu long basin vietnam abstract the cuu long basin is situated off the southeastern coast of vietnam this study utilizes all available seismic data in the cuu long basin vinh chau basin and coastal mekong delta to constrain the tectonic history of the area the linkage between the mae ping fault zone and deformation in the cuu long basin is also examined in detail two major deformational events are observed a transtensional rifting event from the mid eocene to mid early oligocene 40 31 ma followed by transpression from mid early oligocene to mid late oligocene 31 25 ma the data strongly suggest that all the very straight major tributaries of the mekong river delta follow northwest trending faults that link into to the southwestern end of the cuu long basin fault system and that the basin opened between where the mekong delta faults enter and a point of zero extension at the ne termination of the basin the mekong delta fault zone exhibited left lateral displacement during extension in the cuu long basin and right lateral displacement during younger transpression the orientation age and sense of displacement of the mekong delta fault zone are consistent with slip on the mae ping fault zone in thailand and cambodia and hence are considered to be part of the same regional fault system based on a chainat duplex analogue and other data the mekong delta and mae ping fault zones appear to be linked by a nnw striking step over zone referred to here as the mekong ton le sap step over zone this zone follows the eastern boundary of the paleocene to early eocene kampot fold belt based on structural and stratigraphic similarities it appears the adjacent vinh chau basin shares a common tectonic history with the cuu long basin but the linkage to the mekong delta fault zone is less clear

petrology of pseudotachylytes from the alpine fault of new zealand abstract the petrology of pseudotachylytes derived from contrasting host rocks within the alpine fault of new zealand a well known dextral shear zone is presented the structural observations indicate that parallel synthetic shears formed first in a non dilational system at low angles to the alpine fault then between these shears fracturing and frictional heating occured according to a new dilational riedel system accompanied by listric faults the observations suggest that pseudotachylyte melt is mainly formed in the listric faults and channelled into neighbouring areas of extension pseudotachylyte results from in situ melting and its nature depend strictly on the parent rock mineralogy the microlitic nature of the pseudotachylites is confirmed and the variability of the textures explained in terms of degrees of undercooling the morphology and composition of the minerals in the pseudotachylyte plagioclase biotite and amphibole are documented they crystallized at a higher temperature and pressure than their parent rock minerals the melting process is believed to be governed by the mechanical properties of mafic minerals in a fluid free environment at the beginning of the melting mafic minerals provide water which allows the development of the pseudotachylyte

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

the pearl river mouth basin a rift basin and its geodynamic relationship with the southeastern eurasian margin abstract based on multichannel seismic reflection profiles exploration well data and gravity studies the tectonic development of the pearl river mouth basin is discussed in terms of the geodynamic evolution of the southeastern eurasian margin the area around the pearl river mouth basin has been dominated by two episodes of rifting of the attenuated continental crust along the south china margin from the late cretaceous to the early oligocene the rifting brought about ruptures of various magnitudes in various segments of the basin regional normal faults have been produced and resulted in the formation of three major subbasins zhu 1 zhu 2 and zhu 3 which are separated by local uplifts and are characterized by horst and graben structures regional subsidence prevailed throughout the late oligocene recent period and resulted in a tilted continental shelf upon which rests a unified rift basin the pearl river mouth basin a three stage model involving doming rifting and drifting is proposed for the tectonic evolution of the pearl river mouth basin the uplifting and rifting of the continental crust along the south china margin are related to the late episodes of the yanshan orogeny during the late cretaceous early oligocene period when the intensity of rifting decreased the thermal subsidence of the continental margin of south china prevailed during the late oligocene recent period of the himalaya orogeny

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

geological development of the central and south vietnamese margin implications for the establishment of the south china sea indochinese escape tectonics and cenozoic volcanism abstract the vietnamese margin forms a key region to the understanding of escape tectonics and the development of the south china sea scs the existing geological reconstructions of the region are restricted to studies of single basins are based on limited amounts of geophysical data or analysis of onshore geological features these models are critically assessed on the basis of interpretation of the most comprehensive 2 d digital seismic database published to date within the area combined with a thorough analysis of existing literature and a new model is presented the vietnamese margin is underlain by a series of paleogene rift basins established through southeastward extrusion of indochina the east vietnam boundary fault evbf forms the almost 1000 km long seaward continuation of the left lateral ailao shan red river shear zone asrrsz toward the southern half of the phu khanh basin the evbf breaks up into discrete segments and splays into the se directed tua hoa fault zone paleogene faults splayed from the evbf and the mae ping shear zone and accommodated the coeval motion of these two major left lateral structural lineaments during the late oligocene basin inversions offshore occurred contemporaneously with initial right lateral inversion along the mae ping shear zone and the onset of major uplift of the metamorphic core complexes along the asrrsz it is suggested that a dramatic change of the regional stress pattern occurred in response to the northward movement of india and the effective coupling of the west burma block and india the later resulting in broadening of the indenting continental mass after the mid oligocene left lateral movements across the offshore evbf decreased and eventually ceased later onshore sinistral movements were accommodated by internal shortening and local clockwise block rotations within the shan thai terrain renewed rifting offshore south vietnam resulted from the jump of the scs spreading axis and subsequent neogene southwestward propagation of continental break up the neogene opening of the scs is viewed as a consequence of a slab pull associated with subduction of the proto scs underneath borneo as offshore neogene escape tectonism had virtually ceased during this phase of rifting right lateral transtension is inferred along n to nw oriented fault zones in the phu khanh and the nam con son basins rifting and by inference seafloor spreading continued until the end of middle miocene time although at significantly reduced rates during the final 5 10 ma termination of seafloor spreading is marked by a distinct latest middle miocene unconformity in the nam con son and the southern phu khanh basins neogene volcanism resulted in the build up of several magmatic structures across the central and south vietnamese margin and induced reactivation of older faults in the phu khanh basin the offshore magmatism heralded widespread volcanism onshore southern indochina associated with a latest neogene uplift and denudation event the uplift led to significantly increased siliciclastic accumulation rates offshore which repressed widespread carbonate deposition

semi brittle deformation within the alpine fault zone new zealand abstract this paper reports the results of an attempt to identify the deformation micromechanisms in the brittle ductile transition zone of the alpine fault new zealand characterization of sequentially developed microstructures was carried out using optical microscopy hvem and stem analysis it was found that the transition zone corresponds to a broad zone of retrogression produced initially by microcracking and fluid infiltration the non catastropic nature of the fracturing indicates that fluid enhanced sub critical cracking is a significant crustal deformation mechanism after the initial phase of retrogression both ductile and brittle processes coexist and cyclically alternate in response to stress cycling the relative importance of macroscopic brittle processes may decrease with time as more ductile minerals especially phyllosilicates develop during continued retrogression

the geology of northern sabah malaysia its relationship to the opening of the south china sea basin abstract the northern part of sabah consisting of sedimentary and igneous rocks of early cretaceous to pliocene age has experienced three major episodes of deformation associated with nw se and n s oriented compressions the earliest episode deformed and uplifted an oceanic basement chert spilite formation to form an elongate basin trending approximately ne sw during the late cretaceous to early eocene this elongate basin became the site for the deposition of middle eocene to early miocene quartzose sediments of the crocker and kudat formations sourced from continental basement towards the southwest and north respectively these sediments were subsequently deformed by a second episode of deformation associated with nw se and n s oriented compressions during the latter part of the late oligocene and the early middle miocene to form a series of imbricate thrust slices the n s trending compressive direction controlled the development of approximately e w trending basins during the deposition of the upper miocene sediments of the south banggi and bongaya formations the continuation of n s compression which represents the third episode of deformation gently deformed these sediments the three episodes of deformation were related to the differential southward movements of continental blocks separated from the southern margin of china during the intermittent opening of the south china sea subbasins the first episode was related to the opening of the southwest subbasin while the second episode was related to both the opening of the southwest and east subbasins the third episode was related to continued opening in the east subbasins

structural aspects of fluid rock interactions in detachment zones metamorphic core complexes and associated detachment faults of the north american cordillera represent gently dipping normal displacement shear zones detachment zones along which hot deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate structures that formed during the ductile to brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones mylonitization which occurred along deeper segments of detachment zones below the brittle ductile transition was locally accompanied by tensile failure indicative of low deviatoric stress and high fluid pressure this condition persisted into the earliest phases of brittle deformation after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress brittle deformation of upper plate rocks occurred largely under conditions of low fluid pressure structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems 1 an upper plate system driven by convection and dominated by meteoric and connate fluids at near hydrostatic pressures and 2 a system within deeper levels of the shear zone where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping the late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear zone rocks to be overprinted by mineralization related to the upper plate fluid system

depositional environment and reservoir quality of miocene sediments in the central part of the nam con son basin southern vietnam shelf abstract the nam con son basin is a rift basin that formed offshore southern vietnam at the time of opening of the east vietnam sea south china sea the cenozoic stratigraphy in the basin is subdivided into four megasequences based on the main tectonic stages of basin evolution syn rift 1 inter rift syn rift 2 and post rift this study investigates the depositional environment and reservoir quality of the miocene sediments in the central part of the basin and thus discusses the later part of the inter rift the second syn rift and the post rift megasequences we focus on seismic interpretation sediment core analysis biostratigraphy and well log analysis the lower miocene depositional succession is dominated by stacked coarsening upward progradational packages possibly related to the interplay between high sediment supply and fluctuating sea levels the depositional environment is fluvio deltaic to paralic the middle miocene depositional succession is characterized by an upward deepening trend which seems related to northeast southwest rift propagation during this time several intra rift packages demonstrate increasing rift forming activities that resulted in deposition of footwall derived turbidite deposits in the central grabens half grabens and non deposition erosion on structural highs shallow marine facies dominated in the west while the deep marine settings occurred in most of the eastern part of study area carbonate platforms were initiated and widely developed on the structural highs in the southeast and north as a result of overall marine transgression and backstepping of the clastic shoreline toward the west the upper bounding surface of this succession is a regional unconformity marking the boundary between the syn rift 2 and the post rift megasequences during the late miocene deep marine facies extended across the entire study area with extensive submarine fan deposits and submarine channels downcutting w e and s ne which extended across the shelf and slope the miocene sandstones comprise mostly fine to very fine grained feldspathic litharenite to lithic arkose deep marine turbidite sandstones of the upper miocene succession exhibit the best reservoir qualities both porosity and permeability of all the sedimentary facies examined paralic sandstones of the lower miocene succession and footwall derived turbidite sandstones of the middle miocene succession have similar reservoir qualities relatively good reservoir quality sandstones of the middle and upper miocene successions were deposited as thick massive turbidites however parallel laminated turbidites and slumps debris flows often show poor to moderate reservoir qualities precipitation of carbonate cement during burial diagenesis has destroyed the reservoir quality of the miocene sandstones

geological evolution regional perspectives and hydrocarbon potential of the northwest phu khanh basin offshore central vietnam abstract seismic stratigraphic and structural analyses of the northwest phu khanh basin offshore central vietnam based on 2 d seismic data indicate that the initial rifting began during the latest cretaceous or palaeogene controlled by left lateral transtension along the east vietnam boundary fault zone evbfz and northwest southeast directed extension east of the evbfz rifting stopped due to transpression during middle oligocene times but resumed by left lateral transtension during the late oligocene thick sequences of lacustrine and alluvial sediments were deposited during the palaeogene rift periods the late oligocene rifting ended due to inversion triggered by right lateral wrenching near the palaeogene neogene boundary following the onset of this inversion regional uplift and volcanism took place in the southern half of the study area and contemporaneous subsidence and transgression took place farther north leading to widespread carbonate deposition as the right lateral wrenching decreased during the early neogene thermal subsidence and siliciclastic sedimentation became dominant resulting in the buildup and southward propagation of the shelf slope sediment accumulation and subsidence rates increased after the middle miocene times due to eastward tilting of central vietnam and the adjacent offshore area potential direct hydrocarbon indicators dhis in the phu khanh basin include common amplitude anomalies gas chimney like features and seafloor gas seeps in addition oil seeps are found at dam thi nai immediately landward of the basin geochemical analyses of the oil seeps indicate the existence of at least two early to peak mature source rocks maturation modelling combined with the seismic analysis suggests the likely presence of oil kitchens 40 50 km downdip in the basin and a fairly simple oil migration route the basin is probably charged from lacustrine syn rift mudstones humic coals and fore reef marls potential reservoirs are turbidite lowstand delta shelf and coastal sandstones as well as platform and reef carbonates and fractured basement various structural and stratigraphic traps formed during the palaeogene and early neogene prior to the main stage of petroleum generation

pseudotachylyte and the development of ultramylonite bands in the saint barthelemy massif french pyrenees abstract in mylonitic gneisses of the saint barthelemy massif in the french pyrenees evidence was found that ultramylonite bands developed by ductile deformation of pseudotachylyte in the gneiss a mylonitic shape fabric was produced by a continuous structural event during retrogression from amphibolite to upper greenschist facies decrease in temperature caused gradual hardening in the gneiss which led to the development of pseudotachylyte bands by intermittent seismic slip these acted as weak zones in which ductile deformation was concentrated a microstructure typical of ultramylonite was produced in these deformed pseudotachylyte bands

intracrystalline folding and cataclasis in biotite of the santa rosa mylonite zone hvem and tem observations abstract high voltage electron microscope hvem and transmission electron microscope tem analyses indicate that grain size reduction of biotite in deformed granodiorite of the santa rosa mylonite zone southern california was in part accomplished by intracrystalline folding and cataclasis through this process deformation takes place within discrete parallel sided or lens shaped zones up to 6 m wide within the zones deformation begins with the opening of cavities along cleavage planes further deformation causes layers parallel to 001 to become detached from the surrounding crystal along cleavage planes or ruptured along micro shear zones at an angle to 001 the layers may be strongly folded branching or completely fragmented the latter suggesting cataclastic behavior the smallest measured lens of cataclasite is 0 05 m in width high resolution tem images show that 001 lattice fringes fold into these regions zones of intracrystalline folding and cataclasis are visible petrographically because of the many orientations of biotite within these regions the optical properties are effectively averaged the zones are characterized by little or no change in color with rotation of the optical microscope stage and in most orientations by higher refractive indices than the surrounding crystal