boiling fluids in a region of rapid uplift nanga parbat massif pakistan abstract the nanga parbat massif of northern pakistan is currently undergoing rapid uplift 5 10 mm a 1 resulting in near surface elevated temperatures numerous quartz veins cut geologically young structures 100 c km 1 and the isotherms describe a giant pillar of heat above about 4 km fluid temperature is greater than conductive rock temperature in permeable zones which carry convecting boiling meteoric fluid

geomagnetic paleosecular variation in the brunhes period from the island of el hierro canary islands we have sampled a sequence of 69 flows in the island of el hierro canary islands 46 of them belong to the el golfo formation and 23 to the most recent rift formation radiometric dating brackets this section between 442 ka and 134 ka with six intermediate datings all the samples were stepwise demagnetized partly with alternative field partly thermally with very similar results the large majority of the samples are characterized by a single stable component of magnetization chrm isolated after the first steps of demagnetization are all of normal polarity inclinations are on average shallower than expected from a geocentered dipole field but consistent with results from other areas at the same latitude mean declinations of the rift and el golfo sequences are both easterly deviated about 5 and 15 respectively and significantly different suggesting that the section has undergone a progressive clockwise rotation vgp scatter and directional scatter calculated for the rift sequence are consistent with available data from other geographical areas and also with recent models of paleosecular variation on the other hand a significantly smaller directional scatter is obtained for the lower el golfo sequence suggesting that a period of low secular variation occurred in the canary islands area between approximately 440 and 260 ka

isotope studies reveal a complete himalayan section in the nanga parbat syntaxis many models of orogenesis invoke simple anatomies for mountain belts comprising a small number of major tectonic provinces separated by major faults in the himalayan arc three main tectonic provinces the lesser himalayan high himalayan and tethyan series have been recognized over 2500 km along strike from bhutan to kashmir however their extension westward to the nanga parbat syntaxis remains equivocal we have supplemented detailed field work in the area with isotopic analysis aimed at revealing distinct signatures for each of the three main provinces using sr isotopic data to refine previous nd based discrimination we demonstrate that the three main tectonic provinces of the central himalaya also occur in the western syntaxis of northern pakistan these three units are thus continuous along the entire orogenic arc however their metamorphic grade is generally higher in the syntaxis than in the central himalaya challenging the validity of distinctions commonly drawn on this basis elsewhere in the mountain belt the corollary is that these high grade units probably continue beyond the syntaxis into northwestern pakistan which suggests that although the precollisional materials may be identical the western syntaxis marks a change in tectonic style from the main orogen this conclusion in turn requires that the burial and exhumation history in the western himalaya be radically different from that in the central himalaya and thus necessitates a reexamination of models for india asia collision

reprocessing and reinterpretation of cocorp southern appalachian profiles abstract reprocessing of cocorp southern appalachian data was focused on basic seismic evidence for continuation of sediments beneath a master decollement the most important evidence is a nearly continuous series of subhorizontal reflections extending from the valley and ridge province into the piedmont province continuity of subhorizontal reflections becomes tenuous in the inner piedmont careful reprocessing has yielded evidence for termination of strong reflections beneath the allochthon and the beginning of a relatively weak and complex series of events termination of sedimentary rocks beneath the piedmont is interpreted from true amplitude seismic data a zone of detachment continues southeast of the sediment termination as far as the master decollement root zone research on stacking velocities has indicated that complex velocity structures could create apparent low stacking velocities this phenomenon may occur in the charlotte belt of georgia bouguer gravity can be modeled as a former craton of normal density with an accreted margin of very slightly higher density variation in crustal thickness also contributes to the bouguer gravity gradient no continuous large scale overthrust is needed southeast of the interpreted master decollement root zone located beneath the kings mountain belt and charlotte belt

collision tectonics in the nw himalayas west himalayan tectonics involve the collision of microplates between the indian and asian plates the kohistan complex consists largely of tightly folded basic volcanics and sediments generated as late jurassic to late cretaceous island arcs these were intruded by post folding mid cretaceous eocene plutonics produced from continued subduction of the indian plate after closure of a suture between kohistan and the karakorum the himalayan structures show major thrust sheets and the kohistan arc is essentially a crustal pop up with southward upright and northward verging structures developed above a thick ductile decoupling zone the indus suture which can be traced for 100 km beneath kohistan on large reentrants this pop up formed by a two stage process closure of the northern suture followed by closure of the southern indus suture granitic rocks of the kohis tan ladakh batholith dated at 100 40 ma post date most of the structures related to the northern suture but were deformed and carried southwards on shear structures related to the indus suture postcollisional deformation carried this kohistan complex on deep decoupling zones over the indian plate on a series of imbricated gneiss sheets the thrusts climbing up section in the movement direction so that in the far s some override their own molasse debris folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms i e the nanga parbat and hazara syntaxes the nanga parbat syntaxis probably formed due to a shear couple near a branch line where one of the main himalayan thrusts joined the indus suture beneath kohistan crustal delamination to produce the imbricated gneiss sheets could not account for all the displacement of india into asia suggested by palaeomagnetic data there must also have been lateral displacement as demonstrated by the large oblique slip shear zone in the hunza valley n of kohistan

results of recent cocorp profiling in the southeastern united states summary recently acquired cocorp profiles in the southeastern united states show that 1 reflections associated with the appalachian detachment are prominent beneath the inner piedmont of western georgia but do not extend further southeast beneath the pine mountain belt 2 the brunswick magnetic low is associated with a broad zone of crustal penetrating dipping reflections that probably marks the alleghanian suture in the southeastern u s 3 the south georgia basin is a composite feature consisting of several half graben locally containing 5 km of triassic e jurassic basin fill these basins occur within the interior of the alleghanian orogen but are not specifically associated with alleghanian suture 4 across strike crustal thickness variation and distribution and character of lower crust and moho reflections in the southern appalachians is grossly similar to that observed in other parts of the appalachiadhercynian orogenic belt global comparisons suggest that these regional variations are a consequence of post collisional extensional tectonics rather than a primary palaeozoic or older feature of the orogenic belt

neotectonics of the nanga parbat syntaxis pakistan and crustal stacking in the northwest himalayas the northwest termination of the himalayan arc is marked by two large antiformal structures termed syntaxes and a nw se zone of intermediate depth earthquakes which have yielded both strike slip and thrust sense fault plane solutions this contribution presents new structural data from the nanga parbat area the northern syntaxis one of the fastest rising portions of the orogen 7 mm yr 1 our studies show uplift related to syntaxis growth to be accommodated by structures located along its western margin fault kinematics in the southern part of the margin indicate nw directed thrusting along a shear zone and high level cataclastic faults termed the liachar thrust zone this carries amphibolite facies basement rocks out onto quaternary sediments further north along the western margin active faults are dominantly dextral strike slip oriented north south e g the shahbatot fault overprinting relationships suggest that the strike slip fault zone has migrated southwards into the nw directed thrust zone these relationships are consistent with the northwest termination of the arcuate himalayan thrust belt at a lateral tip generating folding and radial thrust directions faulting patterns at nanga parbat suggest that this tip zone has migrated southwards active faulting is now concentrated in the northwest continuation of the hazara southern syntaxis along a seismogenic strike slip and thrust zone these deep level tips lie on the crustal stacking thrusts which cut through the higher level sse directed thrusts of pakistan

badrinath gangotri plutons garhwal india petrological and geochemical evidence for fractionation processes in a high himalayan leucogranite the gangotri leucogranite is the western end of the badrinath granite one of the largest bodies of the high himalayan leucogranite belt hhl it is a typical fine grained tourmaline muscovite biotite leucogranite the petrography shows a lack of restitic phases the inferred crystallization sequence is characterized by the early appearance of plagioclase quartz and biotite and by the late crystallization of the k feldspar this suggests that in spite of being of near minimum melt composition the granite probably had long crystallization or melting interval in agreement with previous experimental studies tourmaline and muscovite have a mainly magmatic origin even though the major element composition is homogeneous there are several geochemical trends when cao decreases there is an increase in na2o rb sn u b f and a decrease in k2o fe2o3 tio2 sr ba zr ree th which are best explained by a fractionation process with early crystallizing phases experimental solubility models for zircon and monazite in felsic melt support a magmatic origin for these two accessory phases as well rb sr isotope data show this granite to have like other hhl heterogeneous isotopic values for sr initial 87sr 86sr ratios calculated at 20 ma range between 0 765 and 0 785 therefore no mixing i e no convection occurred between the different batches of magma in contrast 18o data show little variation 13 04 0 25 implying a source with homogeneous 18o values differences in timing between fluid infiltration and the onset of melting related to differences in temperature of the source could explain why source homogenization occurred for the gangotri and not for the manaslu granite the use of experimental results for solubility and the position of the accessory minerals during melting predict a low viscosity for the melt during its extraction this in turn explains the lack of restitic phases major and accessory in the granite as well as some field features lensoid shape pronounced magmatic layering based on the petrographic and isotopic studies it is suggested that the mechanism of ascent was not diapiric but rather that the melt ascended along several fractures and the level of emplacement was partialy controlled by the density contrast between the melt and host rocks

tectonic implications of metamorphism and gravity field in the penokean orogen of northern michigan published geologic data together with new metamorphic pressure and temperature data and gravity data provide strong evidence for northward directed thrusting of lower proterozoic rocks in northern michigan and wisconsin metamorphic studies indicate pressures of 7 kbar at watersmeet node and 4 kbar at the peavy node the republic metamorphic node located 30 km to the north of peavy attained pressures of 2 3 kbar all three metamorphic highs reached temperatures in the range of 600 650 c and are correlated with areas of negative gravity anomalies relative to surrounding regions in the peavy high the center of the gravity anomaly lies on the sillimanite isograd and we suggest that the source of this negative anomaly may be remobilized underlying archean gneiss the highest grade rocks of the republic node lie well within a residual gravity anomaly which is interpreted to have a very shallow source and the watersmeet high lies above gneiss domes which show negative gravity anomalies a north south geologic cross section constructed from a two dimensional gravity model which extends northward from the dunbar dome in the magmatic arc terrane of wisconsin across a terrane boundary interpreted as a suture and marked by the niagara fault and across the peavy metamorphic node suggests that the dunbar dome was transported northward onto the continental foreland during the penokean orogenic event these north verging fold thrust sheets or nappes produced prominent tectonic thickening at the peavy and watersmeet as indicated by the relatively high metamorphic pressures there the thrust sheets evidently thinned to the north as suggested by the relatively low pressures at republic we suggest that the archean continental edge defined by the steep gradient in the regional northeast trending gravity high lies beneath the magmatic rocks of wisconsin and thus extends south of the niagara fault suture

structural evolution and asymmetric uplift of the nanga parbat syntaxis pakistan himalya the nanga parbat syntaxis in the nw himalaya is a still growing crustal scale north trending antiformal structure in the core of which indian plate gneisses have been uplifted from beneath the overthrust rocks of the kohistan island arc isotopic and fission track geochronology show that uplift rates within the syntaxis have increased to present day rates of 6 mm yr uplift has been accommodated by a combination of initial northwest verging thrusting on the western margin of the syntaxis followed by crustal scale folding within the syntaxis and latterly by dextral reverse faulting on the western margin this thrusting folding and faulting is the effect of deformation at the north western lateral tips of the main himalayan thrusts where they interfere with the south southeast verging thrusts of the northwest himalaya

contrasting accreted terranes in the southern appalachian orogen basement beneath the atlantic and gulf coastal plains and west african orogens abstract a variety of lithostratigraphic units may be outlined within the pre mesozoic crystalline basement of the atlantic and gulf coastal plains these include 1 a group of metamorphic rocks of variable grade together with deformed and retrogressed granite in southwestern alabama and southeastern mississippi wiggins uplift hornblende and biotite within higher grade units record 40ar 39ar post metamorphic cooling ages of 310 300 ma 2 a suite of contrasting igneous rocks granite basalt and agglomerate and serpentinite which occurs along the brunswick altamaha magnetic anomaly in southwestern alabama and 3 an extensive apparently coherent tectonic element the suwannee terrane comprised of undeformed granite in which biotite records 40ar 39ar post magmatic cooling ages of 530 525 ma low grade felsic metavolcanic rocks a suite of high grade metamorphic lithologies st lucie metamorphic complex in which hornblende yields 40ar 39ar post metamorphic cooling ages of 515 510 ma and a succession of lower ordovician middle devonian sedimentary rocks characterized by gondwanan fauna the west african orogens mauritanides bassarides and rokelides record a locally complex polyphase tectonothermal evolution the earliest event corresponded to westward rifting of a continental fragment from the west african craton at 700 ma this led to development of a rift facies lithotectonic succession which included sedimentary units and intracontinental igneous sequences rifting was limited and a western ensialic arc began to develop by 680 ma the associated convergence culminated in an episode of folding and metamorphism at 650 ma pan african i orogenesis following widespread deposition of late paleozoic glacial and flyschoid sediments a second tectonothermal event occurred between 550 and 500 ma pan african ii orogenesis the west african orogens were marked by general tectonic quiescence throughout most of the early and middle paleozoic late paleozoic 300 ma collision of gondwana and laurentia resulted in eastward translation of previously tectonized mauritanide units over their foreland and emplacement at highest structural levels of previously imbricated nappes which include sequences with uncertain palinspastic origins the late paleozoic hercynian transport was largely intracontinental and only westernmost portions of the exposed mauritanides record an associated penetrative tectonothermal overprint characteristics of pre mesozoic basement units which comprise the suwannee terrane in the florida subsurface suggest they represent extensions of the bassaride rokelide orogen there is no apparent record of paleozoic tectonothermal activity in the suwannee terrane this contrasts markedly with penetrative late paleozoic reworking of basement units in the wiggins uplift this suggests that a major dextral transcurrent fault was active during the late paleozoic and that proximal basement units were directly involved in collisional aspects of pangea assembly the various basement units of the coastal plain are not correlative with any of the non laurentian terranes exposed in the southern appalachian orogen e g carolina terrane of the eastern piedmont which accreted in the ordovician devonian to exterior positions along the eastern margin of the north american craton these exotic appalachian terranes were transported into their present structural positions during late paleozoic collision of gondwana and laurentia they are separated from gondwanan elements of the suwannee terrane by a suture approximately marked by the brunswick altamaha magnetic anomaly

tectonics of nanga parbat western himalaya synkinematic plutonism within the doubly vergent shear zones of a crustal scale pop up structure detailed mapping and geochronologic investigations from the eastern southern and western nanga parbat haramosh massif reveal two thrust displacement shear zones that have a spatial and temporal link with granite plutonism from ca 10 to 1 ma the shear zones define a crustal scale antiformal pop up structure with dominant west northwest vergent and subordinate east southeast vergent thrusting this is substantially different than the surrounding area where the main exposed himalayan structures are oriented parallel to the orogenic trend and are early to middle miocene or older structures mapped throughout nanga parbat demonstrate that its rapid and young exhumation is not due to orogen scale structural unroofing and that sustained high erosion rates are required the observed west northwest directed shortening is proposed to be a result of differential arc parallel motion accommodated at the syntaxial bend of the northwest himalaya

geology geochemistry and ar ar geochronology of the nangimali ruby deposit nanga parbat himalaya azad kashmir pakistan abstract the nangimali ruby deposit in the southern part of the nanga parbat himalaya has been investigated through field work geochemistry stable and radiogenic isotopes it outcrops in the shontar valley in a large north vergent syncline consisting of high grade metamorphic gneisses capped by a metasedimentary series dominated by marbles and amphibolites the ore body is stratiform ruby is found within 0 1 2 cm thick shear veinlets and gash veins cutting dolomitic marbles and carbonate bearing bands the marbles of the nangimali formation display restricted ranges in 18o from 23 6 to 27 6 relative to smow and in 13c from 1 9 to 2 6 relative to pdb fluid infiltration along the shear zone in the marble has no effect on the isotopic signatures of the carbonates fluids are metamorphic and co2 is derived from the decarbonation of marbles mass balance and geochemical analyses suggest that the mobilisation by the fluids of aluminium and chromium in the marbles is sufficient to enable the formation of ruby in the shear zone rubies have been indirectly dated using a stepwise 40ar 39ar laser heating technique on syngenetic phlogopites the miocene age records a neogene cooling in the south of the nanga parbat massif and a minimum formation age for ruby of 16 ma

stratigraphic and palaeoenvironmental summary of the south east georgia embayment a correlation of exploratory wells a continental offshore stratigraphic test cost well and six exploratory wells have been drilled in the south east georgia embayment the oldest rocks penetrated are weakly metamorphosed lower ordovician quartz arenites and silurian shales and argillites in the transco 1005 1 well and upper devonian argillites in the cost ge 1 well these marine strata which are equivalent to the tippecanoe sequence in florida underlie the post rift unconformity and represent part of a disjunct fragment of gondwana that was sutured to the north american craton during the late palaeozoic alleghanian orogeny the palaeozoic strata are unconformably overlain by interbedded non marine jurassic bajocian and younger sandstones and shales and marginal marine lower cretaceous sandstones calcareous shales and carbonates which contain scattered beds of coal and evaporite together these rocks are stratigraphically equivalent to the onshore fort pierce and cotton valley formations and rocks of the lower cretaceous comanchean provincial series the abundance of carbonates and evaporites in this interval which reflects marine influences within the embayment increases upwards eastwards and southwards the upper cretaceous part of the section is composed mainly of neritic calcareous shales and shaley limestones stratigraphically equivalent to the primarily marginal marine facies of the onshore atkinson cape fear and middendorf formations and black creek group and to limestones and shales of the lawson limestone and peedee formations cenozoic strata are primarily semiconsolidated marine carbonates palaeocene to middle eocene strata are commonly cherty middle miocene to pliocene strata are massive and locally phosphatic and glauconitic quaternary sediments are dominated by unconsolidated carbonate sands the effects of eustatic changes and shifts in the palaeocirculation are recorded in the upper cretaceous and tertiary strata

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

crustal profile of mountain belt cocorp deep seismic reflection profiling in new england appalachians and implications for architecture of convergent mountain chains relatively shallow reflections from beneath the taconic allochthon and green mountain anticlinorium are suggested to mark a zone of detachment that carries precambrian grenville basement of the green mountains in its upper plate the data suggest that tectonic thickening of the shelf sedimentary and synorogenic flysch sequence beneath the taconic allochthon has occurred and that major thrust fault systems champlain thrust of western vermont which imbricate shelf lithologies project downplunge into the seismic profile eastward dipping reflections beneath the east flank of the green mountains appear to project across the connecticut river valley into new hampshire because some of these reflections can be projected into an exposed zone of probable thrust imbricated precambrian grenville basement and younger metasediments there is a strong implication that similar rock types exist in the deeper crust beneath new hampshire the gross geometry of the eastward dipping zone is a large thrust ramp that facilitated faulting of continental basement in the green mountains to higher structural levels thus suggesting the anticlinorium is a large hanging wall structure or ramp anticlinorium highly deformed satellitic massifs of grenville basement such as the chester dome and sub silurian and silurian devonian rocks of the bronson hill anticlinorium and merrimack synclinorium are probably transported along intracrustal detachments as well data suggest the possiblity of a tectonically buried continental margin transition zone much like that interpreted for the southern appalachians this zone would have been preserved from prior rifting of the proto atlantic ocean and its inherited geometry could have controlled subsequent convergent deformation during appalachian orogenesis although regional deep seismic reflection profiling has not been carried out across the alps existing data imply lateral changes in crustal reflection character similar to those observed in the appalachians deep seismic reflection data from convergent mountain belts raises the possibility of such preserved transition zones beneath transported rocks and provides an alternative interpretation to lithosphere penetrating sutures in explaining lateral geologic and geophysical contrasts in the crust modified journal abstract

transects of the ancient and modern continental margins of eastern canada abstract rocks of the west flank of the northern appalachian orogen miogeocline record the history of the late precambrian early paleozoic passive continental margin of eastern north america the ancient margin was destroyed by ophiolite obduction and arc collision during the ordovician taconic orogeny the present sinuous form of the miogeocline is interpreted to reflect ancient promontories and re entrants of a previous orthogonal margin bounded by rifts and transforms four major terranes are recognized east of the miogeocline in newfoundland and nova scotia from west to east these are the dunnage gander avalon and meguma the dunnage and gander terranes were linked to the miogeocline during the middle ordovician taconian orogeny the avalon terrane arrived later possibly during the mid paleozoic acadian orogeny the meguma terrane of southern nova scotia had docked with the avalon terrane by carboniferous time the dunnage terrane contains arc volcanics which lie above an ophiolitic substrate the gander terrane comprises a thick sequence of clastic sedimentary rocks underlain by basement rocks with continental affinities it has been interpreted as a continental margin perhaps once on the eastern side of the paleozoic iapetus ocean the avalon terrane consists of belts of sedimentary and volcanic rocks which are probably underlain by grenvillian basement its tectonic affinities are unclear the meguma terrane comprises a thick sequence of sediments derived from the south east it is found only in southeastern atlantic canada the boundaries between terranes are compressional in the west and steep transcurrent faults in the east the surface extent of the geological terranes is grossly correlative with deep structural zones although no direct evidence exists for linking the two because most surface structures can be traced geophysically to only a few kilometres depth a striking feature of the deep crustal structure is a lower high velocity crustal layer beneath the dunnage and gander terranes the modern margin of atlantic canada developed by rifting and by transform motion between adjacent continents stretching and thinning of the lithosphere and the consequent production of basaltic magma that in places intrudes or underplates the thinned continental crust are the most likely processes responsible for the evolution of the modern margin these processes predict the observed deep sedimentary basins along the margin the thinning of continental crust and the high seismic velocities found within the ocean continent transition zones rifting adjacent to nova scotia began in late triassic early jurassic time between the present african and north american plates these plate motions are also responsible for the major transform margin south of the grand banks separation between iberia and the eastern grand banks occurred in mid cretaceous time before the late cretaceous opening of the labrador sea while the rifted segments of the margin exhibit deep sedimentary basins and thinned continental crust the grand banks transform segment is characterized by a sharp transition zone and a relatively thin sediment cover numerous volcanic seamounts are built on the ocean crust adjacent to this transform segment mimicry of paleozoic promontories and re entrants by modern rift and transform margin segments the location of mesozoic sedimentary basins on ancestral appalachian structures and the reactivation and propagation of major precambrian and paleozoic structural boundaries during the latest phase of ocean opening attest to ancestral controls of the modern margins the rift phase of both the ancient and modern passive margins is characterized by volcanism mafic dike intrusion and by the development of basins filled with clastic sediments the drift phase of both the ancient margin and the present nova scotia margin is marked by a change in sedimentary environment such that carbonates replaced the rift phase clastic sediments two of the markers used to delineate the ancient ocean continent transition zone carbonate banks and steep gravity anomaly gradients should be used with caution as the modern analogs of these markers may lie 100 km or more of this transition zone furthermore it is naive to view the ancient transition as simple and narrow for the modern margins exhibits complex transition zones between 30 and 300 km wide in general the evolution of the ancient and modern passive margins appear to be remarkably similar predictably closing the present atlantic will mimic the evolution of the appalachian orogen

towards an understanding of the southern appalachian piedmont crustal transition a multidisciplinary approach abstract the piedmont of the southern appalachians is characterized by significant geophysical and geological anomalies which indicate there is a major crustal transition multiple hypotheses including a suture zone and a subducted continental margin have been presented to explain the variations although crustal seismic reflection data have provided significant constraints there are ambiguities inherent in the interpretation of such data the ambiguities can be reduced by careful consideration of related geophysical and geological observations although the importance of correlating crustal reflection data with known geologic features by tracing reflections to drill holes or to the surface cannot be overestimated only rarely are such correlations possible in almost all interpretations of crustal reflection structure it is necessary to constrain the model with methods such as seismic refraction gravity magnetics conductivity and surface geology including palinspastic reconstructions when information from these techniques is incorporated into interpretations of the piedmont crustal structure the model which appears to be most consistent with the observations is one in which the upper crust of the piedmont is decoupled from the lower crust and in which the lower crust thins eastward the lower crust may be a subducted precambrian continental margin and its associated transition toward thinner basinal crust

cocorp and the continental crust this is a speculative paper on the continental crust a major frontier of modern earth science seismic reflection profiling data from the cocorp project coupled with other kinds of geophysical and geological observations suggest some fresh perspectives on the crust for example there is evidence for great differences within the basement at a single site and from one cocorp site to another a simple layered model is surely not an adequate representation of the crust in most areas there is however the puzzling and rather common occurrence of limited zones of parallel though not always flat layered features at various depths in certain parts of the crust are the corresponding rocks metasedimentary or igneous or both there are transparent zones from which little or no coherent reflected seismic energy is detected are the corresponding rocks intrusions or highly deformed metasediments can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust there is evidence for large scale thrusting and related features that emphasizes the great importance of the wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences in some places the modern moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting the widely held concept of the moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust

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

the extension of grenville basement beneath the northern appalachians results from the quebec maine seismic reflection and refraction surveys from 1983 to 1984 the united states geological survey and the geological survey of canada undertook a series of seismic experiments to determine the crustal structure of the appalachian mountain belt in maine and southeast quebec these experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high resolution refraction wide angle reflection profiles both parallel and perpendicular to strike here the data recorded in quebec and the northwest part of maine are interpreted in conjunction with existing reflection profiles from quebec synthetic seismogram calculations in laterally varying media are used to model the refraction wide angle reflection data while reprocessing migration and attribute plotting are used in the interpretation of the reflection data a major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the st lawrence lowlands southeast to about 25 km depth beneath the southeast edge of the chain lakes massif this zone of reflections is interpreted as a master decollement separating autochthonous grenville basement from overlying allochthonous rocks of the appalachian orogen including foreshortened miogeoclinal rocks the remnants of one or more magmatic arcs the connecticut valley gaspe synclinorium and the chain lakes massif in the vicinity of the chain lakes massif extension has caused grenville basement to be thinned by as much as 40 reflections from the decollement show strong layering and evidence for basement faulting associated with the guadeloupe and other faults normal incidence and wide angle reflections are observed to be almost continuous beneath the baie verte brompton line and connecticut valley gaspe synclinorium indicating that the baie verte brompton line is confined to the upper crust the connecticut valley gaspe synclinorium is characterized by p wave velocities of about 5 3 km s 1 and is shown to be a shallow structure with a maximum depth of about 3 km rock velocities immediately below the connecticut valley gaspe synclinorium are typical of chain lakes material 6 1 km s 1 a new model for the tectonic development of the region is proposed in which all pre silurian units between the baie verte brompton line and the southeastern edge of the chain lakes massif are allochthonous and confined to the upper crust we infer that the chain lakes massif volcanics of the ascot weedon formation and associated deeper crustal rocks were thrust over grenvillian basement as a composite unit and may now underlie parts of the connecticut valley gaspe synclinorium

geochronologic constraints on syntaxial development in the nanga parbat region pakistan 40ar 39ar data hornblende biotite muscovite and k feldspar and u pb data zircons were obtained from the nanga parbat haramosh massif nphm nw pakistan along three transects in the southern regions of the nphm we have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif geochronologic data show that the nphm has experienced exceptionally high denudation and cooling rates over the past 10 m y u pb ages determined through sensitive high resolution ion microprobe shrimp depth profiling experiments on metamorphic zircons and conventional u pb monazite dates suggest that the timing of metamorphism varied across the massif in addition we have documented that the massif has experienced postmetamorphic differential cooling both along and across strike thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rates in the south central regions of the massif representing deeper crustal levels than along the margins and northern regions of the massif within the tato region cooling following peak metamorphic temperatures of 600 700 c was as high as 140 c m y following partial melting of pelitic units biotites from this area record plateau ages of 0 9 0 1 ma along the astor and indus gorges cooling was less rapid approximately 70 80 c m y following peak metamorphism as indicated by u pb monazite ages of 6 8 ma and 40ar 39ar muscovite cooling ages of 2 2 3 4 ma cooling over the last 3 m y occurred at rates of 100 140 c m y the overall cooling age pattern within the massif is interpreted syntaxial growth through the development of north plunging antiforms prior to 3 ma followed by reverse faulting along east dipping fault zones along the raikot river transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surfaces the age pattern was disrupted at 1 ma due to faulting along the raikot and tato faults an electronic supplement of tables a1 a2 and a3 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type guest and the name of the file to get it finally type exit to leave the system paper 95tc00032 geochronologic constraints on syntaxial development in the nanga parbat region pakistan david m winslow peter k zeitler c page chamberlain and ian s williams diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

evolution of gravity anomalies across collisional mountain belts clues to the amount of continental convergence and underthrusting a series of density models illustrates the gross form of free air and bouguer gravity anomalies anticipated during ocean basin closure and consequent development of collisional orogens when compared to gravity anomalies observed across some mountain belts the hypothetical anomalies provide a clue to the degree of under thrusting of crust associated with one lithospheric plate beneath crust of the opposing plate margin the results of the study suggest very early stage collision in the ouachita mountains of arkansas and the sulaiman range of pakistan with thin transitional or oceanic crust still intact on the lower plate in contrast the himalaya of pakistan represent a much more advanced stage of collision where continental crust may have underthrust the mountains for 600 km

isostatic compensation for a portion of the southern appalachians evidence from a reconnaissance study using wide angle three component seismic soundings wide angle reflections generated by five controlled blasts and over 110 timed quarry blasts in the southern appalachians were used to test models for isostatic compensation of topography the profiles cross the appalachian gravity gradient and gravity low and sample the highest elevations within the orogen migration of p sv and sh reflections suggests that crustal thickness varies from 35 to 39 km within the coastal plain and 37 39 km within the carolina terrane it increases northwestward from 40 to 45 km across the inner piedmont and then thickens to 50 52 km along the southeastern flank of the blue ridge mountains crustal thickness within the blue ridge mountains ranges from 47 to 56 km receiver functions for broadband stations goga and mync show a similar pattern in crustal thickness assuming airy compensation the correlation between elevation and moho depth suggests a range of 50 150 kg m 3 for the density contrast between the crustal root and mantle the greatest moho depths are associated not with the tallest peaks but rather with the broadest portions of the mountain chain this observation is consistent with regional bending of the lithosphere however the planar basement surface suggests that the root either predates alleghanian thrusting and therefore is unrelated to the present topography or formed in response to some other mechanism bounds on curvature of the basement surface suggest a lower bound of 30 40 km for the effective elastic thickness of the lithosphere this is consistent with previous estimates for the southern appalachians based on analysis of gravity data

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

orogen scale decollements the continental lithosphere responds to stress by deforming as a generally layered medium deep seismic reflection data coupled with a variety of ancillary geological and geophysical data are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers and flat detachments in mechanically weak layers this geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen scale decollements in a manner that is analogous to so called thin skin deformation in sedimentary rocks decollements may occur within the crust for example near the base of a sedimentary section or in the middle crust near the moho in the subcrustal lithosphere or in the asthenosphere even intracratonic basement cored thick skin uplifts that occasionally occur in foreland regions such as the wyoming province are probably large crustal scale versions of ramp flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tectonic processes

proterozoic links between northeastern brazil and west africa a plate tectonic model based on gravity data abstract using the bullard predrift reconstruction for the southern atlantic analysis of the major gravimetric trends of the west african craton 2000 ma and the surrounding mobile belts allows us to delineate the upper proterozoic shield boundary and to put forward an evolutionary model for late precambrian time over the cratonic area gravimetric anomalies are related to a basement framework or to sedimentary basins whose geometry is linked to reactivation of ancient faults gravity anomalies and landsat imagery show a ne sw lineament in west africa which can be correlated with a variety of geologic features the largest being the late precambrian gourma aulacogen at the margins of the craton the deformation patterns and gravity signatures associated with the 600 ma orogeny were largely controlled by the geometry of the stable craton itself on the eastern edge pan african belt and brazilian belt east of the sao luis and amazonian cratons the distribution of the anomalies and deduced structures can be explained by a continental collision process on the western and southern edge brazilian belt between the sao luis and amazonian cratons rokelide belt mauritanide belt the gravity signature is different suggesting a large amount of intensely tectonized basic rocks a limited spreading process followed by an intense compressional episode is suggested

anatomy of a continental subduction zone the main mantle thrust in northern pakistan the himalayas form an ideal natural laboratory to study the deformation processes of continental crust during collision orogeny new information is presented concerning the structural evolution of the main mantle thrust zone in the himalayas of n pakistan in the region around nanga parbat the hanging wall lies at relatively high levels within the kohistan arc terrane which has been emplaced onto indian continental rocks this thrust probably originated as a breakback structure in the hanging wall to the pre collisional oceanic subduction zone the present hanging wall contains a shear zone of about 1 km width developed under amphibolite facies conditions simple shear dominant strains have developed new fabrics parallel to the main thrust zone however these structures are redeformed by discrete extensional and compressional shears within about 100 m of the thrust contact again developed under amphibolite facies conditions the footwall consists of an old basement complex the nanga parbat gneisses overlain by a distinct suite of metasediments now at amphibolite facies this cover assemblage of psammites pelites and marbles with local metabasites consistently lies directly against rocks derived from the kohistan arc along the mmt the absence of structures suggestive of consistent rheological contrasts within the cover assemblages suggests that the vast majority of the deformation features they contain were developed only once they experienced substantial tectonic overburdens prior to this the indian cover rocks will have been passively subducted beneath the kohistan arc until into amphibolite facies we discuss these inferences in terms of evolving shear zone width with time and comment on the implications for predicting the character of mid deep crustal shear zones particularly from seismic reflection profiles

mafic sheets from indian plate gneisses in the nanga parbat syntaxis their significance in dating crustal growth and metamorphic and deformation events

the 1886 charleston south carolina earthquake and the appalachian detachment several lines of evidence from new and existing data are consistent with an extended fault slip associated with the 1886 earthquake in south carolina back slip over a portion of the appalachian detachment approximately corresponding to the coastal plain of south carolina fits some of the data including coseismic and post seismic strain indicators a small seismic source for the august 31 1886 main shock cannot be ruled out and a large but only partially seismic fault slip is possible during this event intensities of mm viii occurred over a wide area of the coastal plain and are not limited to the charleston summerville area where the traditional small source for this earthquake is located near source effects such as ground fissuring liquefaction changes in line of sight foreshocks and aftershocks occurred over a wide area of the coastal plain and neighboring inner piedmont which we tentatively refer to as the meizoseismal area this area is sharply bounded by the savannah river toward the southwest and approximately by the north carolina south carolina state border toward the northeast these boundaries are not associated with any known structure transverse to the strike of the appalachians during the 14 years prior to 1886 seismicity was concentrated around the extended meizoseismal area while the seismicity within that area was unusually low suggesting a premonitory doughnutlike pattern the intensity distribution in the far and near field of the 1886 event and of the great indian detachment earthquakes e g the 1905 m 8 and the 1934 m 8 3 events are qualitatively and quantitatively similar this is consistent with comparable source dimensions since well known intermediate magnitude earthquakes in india and in eastern north america exhibit similar intensity patterns and indicate similar seismic attenuation in the two cratonic environments several meters of coseismic 1886 northwest compressive strain near summerville are indicated by buckling of rails a large postseismic normal displacement is suggested by releveling data 1935 1968 across the brevard fault zone updip northwest of the presumed 1886 detachment slip these strains are consistent with a southeastward slip of the detached crystalline sheet associated with the 1886 earthquake available data indicate that in the southern appalachians the updip transition from maximum horizontal to least horizontal compressive stress in the dip direction perpendicular to structural trends occurs somewhere in the piedmont and may coincide with the steepest portion of the detachment the stress field may also contain a time dependent component in the sediments of the coastal plain orthogonal sets of subvertical clastic dikes oriented along and across the appalachian strike direction suggest one or more interchange of the principal stress axes we infer from coseismic dry fissuring and liquefaction that clastic dikes filled from above and from below were also generated during the 1886 event prehistoric clastic dikes along with other structures in the sediments may have been generated during prehistoric earthquakes similar to the 1886 event stress reversals corresponding to earthquake cycles may characterize the stress field within detached sheets or wedges that move primarily by great detachment earthquake ruptures appendices are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 008 1 00 payment must accompany order

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

strain partitioning along the himalayan arc and the nanga parbat antiform shortening along the himalayan arc of continental convergence is approximately in the radial direction if the underthrusting foot wall block india is not deformed the hanging wall block tibet needs to stretch along the arc as suggested by radial grabens in southern tibet in contrast the nanga parbat haramosh massif and the western himalayan syntaxis are part of a 250 km long antiform that strikes in the radial direction northeast and verges northwest the nanga parbat antiform is the structural and topographic expression of arc parallel shortening that compensates for arc parallel extension in southern tibet this shortening is predicted to be as high as 12 mm yr

tectonic implications of subthrust structures revealed by seismic profiling of appalachian ouachita orogenic belt a growing body of deep seismic reflection profiles across the appalachian ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting of later thrusting or of some combination of rifting and thrusting the subthrust features can be placed into three general categories 1 small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading perhaps reactivating earlier rifting faults 2 moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland interpreted in part as strata deposited during the earlier rifting and 3 hinterland dipping reflection sequences beneath more interior portions of the mountain belt previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin these interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings the comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process these dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the appalachian foreland the comparisons also suggest an alternative to interpretations of the hinterland dipping sequences as thrust related in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins the implication of this new interpretation is that the actual continent ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the appalachians and ouachitas

results from regional vibroseis profiling appalachian ultra deep core hole site study summary in 1985 180 km of regional vibroseis profiles were acquired in the carolinas and georgia southeastern united states as part of the appalachian ultra deep core hole adcoh site study the data quality is excellent with large amplitude reflections from faults and crystalline rocks lower palaeozoic shelf strata and from within autochthonous grenville basement the profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern appalachians the major points in the interpretation are 1 the blue ridge master decollement is at a depth of 2 3 km beneath the blue ridge this thrust increases in dip just nw of the brevard fault zone 2 the brevard fault zone appears to splay from the master decollement at 6 km 2 2 s near westminster s c and defines the base of the crystalline inner piedmont allochthon 3 below the blue ridge thrust sheet are images of duplex and imbricate structures duplex tuning wedges connected by other thrust faults that duplicate shelf strata to a thickness of 4 5 km 4 subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower palaeozoic strata as suggested by others 5 eocambrian cambrian rift basins in the grenville basement are also imaged the adcoh data were originally recorded with 14 56 hz bandwidth and 8 s length but an extended vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust below 8 s reflections from within the grenville basement become weak but are observable as late as 13 s however these moho reflections are generally short segments

isotopic constraints on the cooling history of the nanga parbat haramosh massif and kohistan arc western himalaya nineteen 40ar 39ar and 22 rb sr mineral ages are presented for metamorphic rocks and granites collected from the northern part of the nanga parbat haramosh massif nphm and adjacent parts of the kohistan arc western himalaya the majority of these ages are interpreted as exhumation related cooling ages and they therefore place important constraints on the exhumation history of the region the 40ar 39ar and rb sr mica cooling ages for areas of the nphm to the north of the indus river lie in the range 2 8 ma and are similar to previously published ages from the indus gorge itself cooling histories for these areas show little systematic variation from east to west suggesting that the basement may have been exhumed as a relatively rigid block after the main stages of folding mica ages increase dramatically from the nphm westward into the kohistan arc where ages lie in the range 13 30 ma the contrast in both cooling ages and the age differential between mineral pairs between the two terranes indicates rapid cooling of the nphm in the last 10 m y compared with the kohistan arc the relationship between rates of cooling and exhumation is poorly constrained in regions of rapid uplift where the characteristics of the evolving geotherm are not well understood however the results are consistent with an anomalously rapid exhumation history of the nphm during the last 10 m y compared with the adjacent kohistan arc the interpretation of some of the ages documented in this study is problematical two biotite samples from contrasting lithologies within the nphm have yielded higher 87sr 86sr ratios than the corresponding whole rocks but the cause of this isotopic disequilibrium remains unclear other biotite samples from the region have consistently yielded younger rb sr ages than 40ar 39ar ages which may be a reflection of either preferential leaching of 87sr during fluid infiltration on a regional scale or the presence of excess argon alternatively the relative ages indicate slightly lower closure temperature for strontium diffusion in biotite than for argon diffusion in biotite where samples have undergone rapid cooling

tectonic evolution of the himalayan syntaxes the view from nanga parbat acknowledgements i was introduced to the himalayas by mike coward in the mid 1980s and remain indebted to him for inspiring a career studying orogenic tectonics i also thank the numerous colleagues both from the uk and from peshawar university who were companions on many field campaigns especially around nanga parbat this contribution has been greatly improved by incisive reviews by tom argles and phaedra upton although of course the author takes full responsibility for the views presented here finally i thank peter treloar for inviting this review and for his patience while i put it together funding over the years this research has been variously supported by the uk s natural environment research council and the royal society published in himalayan tectonics a modern synthesis

constraints from deep crustal structure on long term landform development of the british isles and eastern united states abstract it is assumed that some major landforms not easily explained by surface geology could be controlled by deep crustal properties the concept was first applied in the british isles and then to the u s atlantic provinces both regions were affected by similar orogenies in late precambrian paleozoic times and presently belong to the passive margins of the north atlantic ocean in both areas the pattern of rifting was greatly influenced by preexisting tectonic fabric of the continental crust in the british isles pre mesozoic structures are cut obliquely by the atlantic margin while in the u s border the present margin is nearly parallel to the paleozoic sutures the british isles were affected by differential block tectonics during the tertiary examples are given of regional scale landforms linked with crustal hinges recent data confirm the great heterogeneity of the crust beneath the british isles each crustal unit has its own isostatic response according to the state of stress and its own dynamic equilibrium between rock resistance erosion processes and uplift rate in the atlantic provinces of the united states the regional physiographic framework is simpler but allochthonous terranes thrust westward over the grenville basement and shelf sediments have buried some major discontinuities in the deep crust reverse faults associated with the nw se compressional stressfield are numerous but cannot explain regional scale landforms like the blue ridge escarpment above the piedmont rock control is not the direct cause of the escarpment the blue ridge from reaches its highest relief where discontinuities in the deep crust interfere with surficial geological features ocean to continent creep processes are suggested to explain the sustained uplift of the appalachian highlands and the relatively stable position of the mountainous front since middle mesozoic times

wave gradiometry and its link with helmholtz equation solutions applied to usarray in the eastern u s wave gradiometry is an array processing technique utilizing the shape of seismic wavefields captured by usarray ta stations to determine fundamental wave propagation characteristics we first explore a compatibility relation that links the displacement spatial gradients to seismogram displacements and velocities through two unknown coefficients a and b these coefficients are solved for through iterative damped least squares inversion to provide estimates of phase velocity back azimuth radiation pattern and geometrical spreading we show that the a coefficient corresponds to the gradient of logarithmic amplitude and the b coefficient corresponds approximately to the local wave slowness a and b vector fields are interpolated to explore a second compatibility relation through solutions to the helmholtz equation for most wavefields passing through the eastern u s we show that the a coefficients are generally orthogonal to the b coefficients where they are not completely orthogonal there is a strong positive correlation between b and changes in geometrical spreading which can be further linked with areas of strong energy focusing and defocusing we finally obtain isotropic rayleigh wave phase velocity maps for 15 periods between 20 and 150 s by stacking results from 37 earthquakes which show a velocity change along the approximate boundary of the early paleozoic continental margin we also observe two low velocity anomalies one centered over the central appalachians where eocene basaltic volcanism has occurred and the other within the northeastern u s possibly associated with the great meteor hotspot track

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

large scale thin skin tectonics the lithosphere is to a first approximation a layered system generally pressure temperature and composition are strongly dependent on depth and vary relatively little laterally these parameters can affect greatly the response to stress of rocks in the crust and upper mantle and are responsible for the stratification of many characteristics such as seismic response rheology and kinematics in the lithosphere asthemosphere system e g chen and molnar 1983 on a smaller scale lithology which often is also horizontally stratified plays a major role in determining strength e g engelder and geiser 1980 pore pressure can also greatly affect strength hubbert and rubey 1959 suppe 1981

geochemistry petrogenesis and tectonic setting of amphibolites from the southernmost exposure of the appalachian piedmont the uchee belt of the southern appalachian piedmont consists of a sequence of amphibolite amphibolitic gneiss and gneissic calc silicate surrounded by highly strained mylonitic rocks associated with motion along the bartletts ferry goat rock fault system amphibolites from three major localities within the belt the hudson mill rapids davis mill and lindsey creek localities have been analyzed for their major and trace element chemistry the mafic amphibolites are concluded to have been derived from intrinsically basaltic protoliths as determined by the presence of patchy zoning in relict igneous plagioclase normative mineralogy niggli trends and a variety of diagnostic geochemical criteria davis mill amphibolites are characterized by high ree abundances and flat ree pattern la yb sub n 1 3 to 2 3 similar to volcanic rocks from pacific type island arcs geochemical parameters such as high la nb and v ni ratios low ti v and ti zr ratios and discriminant trace element diagrams based on elements generally considered immobile during metamorphism indicate an island arc and back arc basin affinity for the amphibolites hudson mill rapids amphibolites characterized by a wide range of ree concentrations and strongly negative ce anomaly are interpreted as having been derived from parental basalts produced in a back arc basin by melting of a descending slabmore plus mantle wedge the chemical diversity of the amphibolites and the presence of thrusts strike slip faults and highly strained rocks are consistent with the premise that the western uchee belt is a deformed composite part of an extensive arc system less

first evidence for high grade himalayan age synconvergent extension recognised within the western syntaxis nanga parbat pakistan we present evidence for horizons of medium to high grade 400 600 c deformation accompanying normal sense displacement in high strain himalayan gneisses and schists from the southeastern nanga parbat massif the western syntaxis in the pakistan himalaya in their present orientation the broadly n s trending steeply dipping gneisses show microstructural and outcrop scale evidence for dextral and sinistral shear in discrete layers we interpret these as horizons of thrust and normal motion within the footwall of the original main mantle thrust once neogene antiformal folding is removed this is the first report of significant normal motion in the main mantle thrust footwall in the nanga parbat syntaxis and may indicate that synconvergent extension in the himalaya extended to the western syntaxis this episode of extension could correspond to a period of similar normal motion in the central himalaya or represent a separate event earlier in the orogeny

geophysical anomalies along strike of the southern appalachian piedmont bouguer gravity anomalies and aeromagnetic anomalies display a northward increase of values in the charlotte and carolina slate belts from georgia to virginia analysis of these data with constraints imposed by seismic reflection seismic refraction conductivity measurements and surface geology imply that this increase is due to a corresponding increase in the percentage of mafic rocks in the upper crust a regional trend in the bouguer field is determined for an observed eastward decrease in crustal thickness based on seismic refraction measurements a comparison of the calculated regional with surface boundaries shows that in places the crustal thinning occurs more than 50 kilometers west of the inner piedmont charlotte belt transition the comparison also shows that the crustal thinning may take place west of exposed grenville basement the evidence thus indicates that the southern appalachian gravity gradient is not likely related to a crustal or lithospheric suture the evidence further indicates that because the upper crustal and deep crust upper mantle variations are independent in terms of spatial position the upper crust of the charlotte slate belts is decoupled from the lower crust perhaps at subhorizontal boundaries seen on reflection data

the northern suture pakistan margin of a cretaceous island arc the northern suture is a fault separating the cretaceous kohistan island arc terrain northwest himalayas from palaeozoic sediments of the asian plate to the north the kohistan arc includes volcanic and sedimentary rocks andesitic lavas tuffs volcaniclastics slates and limestones metamorphosed to greenschist facies and intruded by the two phase kohistan batholith asian continental margin sediments are mainly of shelf type are variably metamorphosed and intruded by the karakoram batholith the northern suture is a zone of melange from 150 m to 4 km wide and contains blocks of volcanic greenstone limestone red shale conglomerate quartzite and serpentinite in a slate matrix it has a strong planar fabric but in many places bedding is preserved in blocks and matrix and depositional rather than tectonic contacts are seen between the two the melange is inferred to be an olistostrome largely derived from the kohistan arc formed in a small back arc basin between kohistan and asia limestone blocks in the melange are dated as aptian albian post tectonic intrusions yield radiometric ages from 111 to 62 ma the northern suture therefore probably formed in the early late cretaceous during closure of the back arc basin the tethys ocean lay south of kohistan where the main mantle thrust represents the westward continuation of the indus tsangpo suture

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

mazeno pass pluton and jutial pluton pakistan himalaya age and implications for entrapment mechanisms of two granites in the himalaya zircon and monazite u th pb ion microprobe analysis were performed on the mazeno pass pluton and the jutial pluton two leucogranite bodies within the nanga parbat haramosh massif nphm pakistan himalaya zircon rim ages and monazite ages indicate the mazeno pass pluton in southwest nphm intruded at 1 40 0 05 ma the jutial pluton to the north similarly yields concordant zircon and monazite ages suggesting crystallization at 9 45 0 06 ma the jutial pluton was subsequently intruded by leucogranite dikes at 5 3 ma as revealed by monazite ages concordancy of u pb and th pb accessory mineral ages demonstrates the robustness of the technique on young rocks both plutons some of the youngest in the himalaya have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting deformation enhanced ascent together with new field observations these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif

cooling history of the nw himalaya pakistan fission track and 40ar 39ar cooling ages indicate that the late tertiary cooling history of the himalayan ranges of northern pakistan is largely a function of uplift and erosion interpretation of cooling ages which range from under 0 5 ma to over 80 ma suggests that during the late tertiary long term uplift rates at least doubled from under 0 2 mm yr to in some cases well over 0 5 mm yr uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions the association of very rapid uplift and erosion with the nanga parbat haramosh massif can be explained by a locally vigorous collision of india with eurasia near a promontory of indian crust the resultant rapid uplift of the nanga parbat haramosh massif reactivated the main mantle thrust melange zone with a reversed sense of motion discontinuities in the coolingage distribution along the main mantle thrust in the southern swat hazara region may be the result of the thermal effects of overthrusting

precollision tilt of crustal blocks in rifted island arcs structural evidence from the kohistan arc 1 we describe results from field and microstructural investigations in the unusually large chilas plutonic complex which was emplaced within the mesozoic kohistan island arc northern pakistan magmatic and deformational fabrics of the chilas gabbronorite are concordant with magmatic fabrics of associated gabbros diorites and tonalites and show that this reportedly huge intrusion is composed of juxtaposed plutonic bodies with predominantly ssw dipping flow structures over the northeastern part of the complex which turn to sw ne striking subvertical attitudes to the west evidence for multiple intrusions includes concordant and discordant intrusive and or mylonitic contacts between gabbronorites and associated rocks owing to structural and kinematic consistency we attribute transitions from magmatic to more localized mylonitic fabrics to higher finite strain and or strain rate produced from drag of magma against its margin we discuss this structural pattern and conclude that the chilas gabbronorite emplaced incrementally into generally south dipping listric faults that dominated arc extension about 85 ma in addition several sets of hornblende pegmatite indicate that brittle deformation occurred in the presence of residual melt during consolidation of the main body the latest pegmatite set is composed of variably striking vertical extensional veins that show no relationship to the magmatic fabric we argue that the original attitude of these veins along with that of the enclosing chilas complex has been mostly preserved we hypothesize that the northward dip of the southern side of the kohistan is inherited from late cretaceous rotation of crustal blocks in the hanging wall of the listric faults

towards resolving the metamorphic enigma of the indian plate in the nw himalaya of pakistan the pakistan part of the himalaya has major differences in tectonic evolution compared with the main himalayan range to the east of the nanga parbat syntaxis there is no equivalent of the tethyan himalaya sedimentary sequence south of the indus tsangpo suture zone no equivalent of the main central thrust and no miocene metamorphism and leucogranite emplacement the kohistan arc was thrust southward onto the leading edge of continental india all rocks exposed to the south of the arc in the footwall of the main mantle thrust preserve metamorphic histories however these do not all record cenozoic metamorphism basement rocks record paleo proterozoic metamorphism with no cenozoic heating neo proterozoic through cambrian sediments record ordovician ages for peak kyanite and sillimanite grade metamorphism although ar ar data indicate a cenozoic thermal imprint which did not reset the peak metamorphic assemblages the only rocks that clearly record cenozoic metamorphism are upper paleozoic through mesozoic cover sediments thermobarometric data suggest burial of these rocks along a clockwise pressure temperature path to pressure temperature conditions of c 10 11 kbar and c 700 c resolving this enigma is challenging but implies downward heating into the indian plate coupled with later development of unconformity parallel shear zones that detach upper paleozoic cenozoic cover rocks from neoproterozoic to paleozoic basement rocks and also detach those rocks from the paleoproterozoic basement

observations and tectonic setting of historic and instrumentally located earthquakes in the greater new york city philadelphia area a catalog of 383 earthquakes in southeastern new york southwestern connecticut northern new jersey and eastern pennsylvania including metropolitan new york city and philadelphia is compiled from historical and instrumental data from 1677 through 2006 a magnitude felt area relationship is used to calculate the equivalent magnitude mblg prior to the advent of abundant instrumental data in 1974 revised locations are computed for a number of historic earthquakes most hypo centers are concentrated in older terranes bordering the mesozoic newark basin in the reading manhattan and trenton prongs and in similar rocks found at a shallow depth beneath the coastal plain from south of new york city across central new jersey historic shocks of mblg 3 and larger were most numerous in the latter zone the largest known event mblg 5 25 occurred just offshore of new york city in 1884 many earthquakes have occurred beneath the 12 km wide ramapo seismic zone rsz in the eastern part of the reading prong where station coverage was the most ex tensive since 1974 the southeastern boundary of the rsz which is nearly vertical extends from near the surface trace of the mesozoic ramapo fault to depths of 12 15 km because the mesozoic border fault dips about 50 60 southeast earth quakes of the rsz are occurring within middle proterozoic through early paleozoic rocks which faults within the rsz are active is unclear well located activity in the manhattan prong since 1974 extends to similar depths but cuts off abruptly at all depths along a northwest striking boundary extending from peekskill new york to stamford connecticut that boundary which is subparallel to brittle faults farther south is inferred to be a similar fault or fault zone those brittle features may have formed between the newark hartford and new york bight basins to accommodate mesozoic extension the great valley in the northwestern part of the study region is nearly devoid of known earthquakes while few focal mechanism solutions and in situ stress measurements of high quality are available the maximum compressive stress is nearly horizontal and is oriented about n64 e similar to that in adjacent areas the catalog is likely complete for events of mblg 5 since 1737 3 5 since 1840 and 3 0 since 1928 extrapolation of the frequency magnitude relationship indicates that an event of mblg 6 0 is expected about once per 670 yr online material observations and tectonic setting of historic and instrumentally located earthquakes in the greater new york city philadelphia area

a deep crustal electromagnetic sounding in the georgia piedmont electromagnetic soundings in the georgia piedmont using a horizontal loop source indicate an upper layer with conductivity variations from less than 2 10 5 to 2 5 10 4 s m extending to about 15 km underlain by a zone with conductivity of more than 10 3 s m at one site the conductance of any intermediate layer is limited to 2 s to a depth of 15 km the conductivities of the upper layer agree with those expected from the surface geology and the increase in conductivity occurs at a depth which correlates generally with reflections detected by the consortium for continental reflection profiling s seismic profile of the area a possible source of the high conductivity is water saturated sediments trapped when the region was overthrust in the appalachian orogeny

leucogranites of the himalaya karakoram implications for magmatic evolution within collisional belts and the study of collision related leucogranite petrogenesis abstract two types of leucogranite have been identified in a collisional setting in the himalaya type a e g manaslu high himalaya are syn tectonic crustal melts related to hot over cold thrusting and subsequent fluid fluxing type b e g baltoro karakoram is related to lower crustal melting during following crustal thickening and thermal re equilibration followed by crystal fractionation for type b melting is triggered promoted in the lower crust by input of heat and material from the sub continental lithospheric mantle which is contemporaneously melting due to removal of the slab input and downward displacement both these granite types can be explained in terms of recent thermal models for continental collision and so might be expected to be found in other orogenic terranes these models also show that extension and thinning will occur as a result of the crust becoming gravitationally unstable due to thickening some time after collision thinning will lead to upwelling and melting of asthenosphere which could then intrude the base of the crust and promote further melting this could explain the common occurrence of a type granitoids as final plutonic events in orogenic terranes following intrusion of the collision related type a and type b granites existing genetic classification schemes only classify granites in terms of source type or broad environmental affinity e g collisional to further subdivide such broad environmental fields it is considered that processes and a general evolutionary model must be used source type is not suitable due to the number of source components that are often involved and the overlap of these components from one environment to another one or two selected features of a granite are not however sufficiently exclusive to identify different petrogenetic histories we propose that an evaluation of field style mineralogy isotope and elemental geochemistry should be used to subdivide the collisional granite field into type a and type b or manaslu and baltoro types respectively

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

petrogenetic and tectonic significance of young leucogranites from the northwestern himalaya pakistan in the himalaya of northwestern pakistan small discordant bodies of leucogranite intrude metamorphosed basement of the indian crust as the first step in assessing the tectonic significance of these granites we have used the shrimp ion microprobe to obtain u pb dates on zircon zircons of igneous appearance are scarce in the leucogranites and with one exception are associated with xenocrystic components which occur as discrete grains and as cores rimmed by young zircon zircons of igneous appearance are high in u 5000 to 40 000 ppm low in th u 0 01 to 0 05 and frequently colored a distinct blue xenocrystic components give a range of proterozoic ages consistent with ages previously reported for indian crust in sharp contrast zircons of igneous appearance yield clusters of concordant tertiary ages two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 ma swat and about 50 ma naran together with previously determined 40ar 39ar and fission track cooling ages these ages document immediate and rapid metamorphism and denudation following eocene collision three leucogranite dikes from northern localities within the nanga parbat haramosh massif are very young having emplacement ages of about 2 3 ma 5 ma and 7 ma the formation and emplacement of these leucogranites during the rapid late tertiary denudation of the nanga parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

tectonic style of the appalachian allochthonous zone of southern quebec seismic and gravimetric evidence abstract an integrated interpretation of seismic reflection and gravity results yields an image of the structural units nappes overthrust sheets and underthrust slabs of the cambrian ordovician metasedimentary and metavolcanic sequences of the appalachian province of southern quebec at depth and the relative position of the underlying precambrian crystalline basement the tectonics related to crystalline precambrian basement may be correlated with four interpretations and combinations of these a decollement of paleozoic sedimentary and volcanic piles over a rigid crystalline basement is most probable considering the interpretation of gravity and especially seismic data the tectonic style at depth is rather unresolved from surface geological information the elaboration of a two dimensional model is constrained by physical properties of rocks maximum depth extents of individual bodies and seismic reflectors in addition to surface geology finally a minimum of 1500 km of shortening and iapetan closing is suggested

more seismic evidence on the location of grenville basement beneath the appalachians of quebec maine summary high quality multichannel seismic 133 fold and refraction wide angle reflection 1 to 3 km receiver spacing and 30 to 60 km shot spacing data have been collected across the northern appalachians in quebec and maine an integrated interpretation of the seismic data from the southeastern quebec western maine region provides strong evidence that the rocks of the predominantly oceanic dunnage zone are allochthonous having been thrust westwards over precambrian grenville basement during and subsequent to the closing of the iapetus ocean

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

reprocessing and enhanced interpretation of the initial cocorp southern appalachians traverse abstract reprocessing of the 1978 1980 cocorp southern appalachian seismic reflection data has produced improved images of structures related to the emplacement of the blue ridge inner piedmont allochthon the results enhance and extend the interpretation presented previously that the blue ridge and inner piedmont are allochthonous above a shallow and shallow dipping detachment that can be followed from outcrop at the blue ridge valley and ridge transition to at least beneath the carolina terrane the continuity of reflections in the new images supports the interpretation that the southern appalachian detachment is not rooted on the east side of the inner piedmont but rather projects as a low angle detachment or zone of decoupling to beneath the coastal plain an implication of this geometry is that terranes such as the carolina terrane between autochthonous north america and the alleghanian suture beneath the coastal plain are detached thin flakes

geology of the northern part of the nanga parbat massif northern pakistan and its implications for himalayan tectonics the northern outcrop termination of indian continental crust lies at the nanga parbat massif from where this contribution presents new field data the tectonic contact with the structurally overlying kohistan island arc is concordant and ductile is associated with interleaving of nanga parbat and kohistan lithologies and may be correlated with the main mantle thrust found elsewhere in the nw himalayas this ductile shear zone is locally overprinted by cataclastic faults associated with exhumation of the massif but overall the northern outcrop termination of the massif is controlled by erosion through a gently northward plunging antiformal structure this folds both the main mantle thrust and the underlying concordant 9layered unit9 of the nanga parbat basement thus there is no indication that the massif acted as a promontory to the indian continent during collision nor that it is a structure entirely bound by neotectonic faults ductile shear fabrics associated with the 9main mantle thrust9 are cross cut by leucogranite sheets and pegmatites these may represent the stockwork to a significant crustal melt granite body described here in the northern massif this jutial granite shows many geochemical characteristics in common with similar bodies in the high himalayas which are consistent with anatexis of the buried indian continental basement rocks however the granite is enriched in heat producing elements particularly th 22 ppm and shows extremely high 87 sr 86 sr ratios 0 88 the high concentrations of radiogenic sr are also an attribute of a suite of hitherto enigmatic leucogranitic pegmatites that laces the nanga parbat massif suggesting that these may represent a stockwork to a largely buried body of which the jutial granite is a small exposure the enrichment in heat producing elements within the granite may reflect similarly high heat production in the source indian continental crust requiring in turn a fundamental re examination of the thermal evolution of this crust during himalayan metamorphism and exhumation

rejuvenation of rbsr mica ages during shearing on the northwestern margin of the nanga parbat haramosh massif abstract the shear zone bordering the western margin of the nanga parbat haramosh massif nphm separates terranes with distinct but well documented cooling histories this study was undertaken in order to examine isotopic behaviour and if possible to date deformation or cooling within the contact zone itself rb sr muscovite and biotite ages and petrographic descriptions are presented for a range of metamorphic lithologies and granite sheets which have been affected to varying degrees by greenschist facies extensional crenulation this deformation has been kinematically linked to the differential uplift of the nphm ages for metamorphic lithologies and granite sheets are similar and are considered together determined muscovite ages mostly lie in the range 9 22 ma whilst the majority of biotite ages lie in the range 2 11 ma mica ages from the shear zone are significantly more variable than ages determined for unsheared lithologies from outside the shear zone even closely spaced samples yield variable mica ages and in some cases there is a significant difference between muscovite and biotite ages determined for the same sample muscovite ages yield a bimodal distribution with groups of ages in the range 17 21 ma and 8 12 ma the older group is thought to reflect the original purely thermostatic cooling of lithologies through the muscovite closure temperature prior to formation of the nphm structure in contrast the younger group of muscovite ages reflects variable degrees of isotopic resetting during shearing at temperatures below the muscovite closure temperature associated with the relative uplift of the nphm the majority of biotite ages also reflect partial to complete resetting during shearing however biotite samples from structurally competent amphibolite horizons preserve older 16 ma ages indicating that there was locally a strong lithological or fluid control on the degree of resetting the flanks of the nphm cooled through the muscovite closure temperature significantly earlier than the deeper structural levels presently exposed within the core of the nphm greenschist grade deformation was occurring within the shear zone at the same time that deeper levels of indian crust were passing through the muscovite closure temperature suggesting that the shearing reflects the relatively passive extensional movement of rocks sliding off the flank of the actively growing nanga parbat structure the results highlight the isotopic complexities of long lived shear zones present in orogenic belts but show that useful radiometric age information may be obtained if the regional tectonic history is well understood and if the samples for dating come from well constrained structural settings

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

role of large scale tectonic forces in intraplate earthquakes of central and eastern north america central and eastern united states ceus have experienced large intraplate earthquakes yet at present there is no comprehensive model to explain stresses strain and seismicity in this intraplate setting models to explain the intraplate stresses in ceus include glacio isostatic adjustment ridge push effects local stresses along preexisting fracture zones and large scale convection in this paper we present a self consistent model of the dynamics of ceus that explains the stress field responsible for these intraplate earthquakes the earthquakes represent slow ongoing deformation associated with forces arising from a combination of lithosphere topography and structure together with the effects of density driven mantle flow using gps data we calculate strain rates that are likely to arise from tectonic effects and conclude that intraplate strain rates associated with tectonic effects are unlikely to exceed 1 x 10 9 year 1 we test several models of lateral viscosity variations by comparing model stress orientation output with earthquake moment tensors shmax directions from stress inversion and p axes of earthquakes a model that satisfies stress and earthquake constraints and also strain rate magnitude constraints requires high viscosity 1025 pa s craton and old oceanic lithosphere of the western atlantic block and weaker 5 x 1024 pa s accreted appalachian terrane other strength contrasts within the lithosphere are likely present incorporation of these into future models using constraints from seismology along with refined geodetic measurements and improved estimates of crust and upper mantle densities is needed to further refine long term dynamic models and better evaluate the hazards associated with this very slow ongoing permanent deformation within the eastern and central united states

nd isotopic characterization of metamorphic rocks in the coast mountains alaskan and canadian cordillera ancient crust bounded by juvenile terranes nd isotopic data are reported for 52 samples from the crustal region between the alexander wrangellia terrane and the stikine terrane of the alaskan and canadian cordillera this region is composed of the gravina belt a jurassic cretaceous assemblage of volcanic and clastic sedimentary rocks the taku terrane a terrane of probable early permian to late triassic age and four assemblages of metamorphic rocks that occur to the west of and within the coast mountains batholith the gravina belt has end t values that range from 1 1 to 8 3 similar to values of the underlying alexander terrane and consistent with the interpretation that it is a juvenile belt that formed in a back arc or intra arc basin within the alexander terrane mid cretaceous plutons that were emplaced into the gravina belt have end t values of 4 4 to 5 7 and were probably produced by mantle derived melts that incorporated some alexander terrane crust the taku terrane has end 0 values that range from 5 5 to 3 3 with corresponding depleted mantle model tdm ages of 440 to 1430 ma a mid cretaceous pluton intruding the taku terrane has an end t value of 5 1 a value indistinguishable from those determined for cretaceous plutons intruding the gravina belt metamorphic rocks east of and structurally overlying the taku terrane are divided into the tracy arm assemblage end 0 26 to 0 tdm 800 2450 ma the endicott arm assemblage end 0 10 to 1 3 tdm 950 1500 ma the port houghton assemblage end 0 9 4 to 1 1 tdm 550 1500 ma and the ruth assemblage end 0 9 4 to 2 0 tdm 650 1300 ma these isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from precambrian continental crust the metamorphic rocks from these assemblages are lithologically very similar to rocks of the yukon tanana ytt terrane of eastern alaska and yukon territory and have such similar u pb detrital zircon ages and nd isotopic compositions to ytt rocks that they are considered part of that terrane possible tectonic scenarios that can explain the present geometry of the ytt with respect to the alexander wrangellia and stikine terranes include 1 the ytt is the upturned stratigraphic basement of the stikine terrane 2 part of the ytt was structurally emplaced beside the stikine terrane in a transpressive tectonic regime 3 the stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the ytt before the final accretion of the alexander wrangellia terrane