continental growth at convergent margins facing large ocean basins a case study from mesozoic convergent margin basins of baja california mexico abstract mesozoic rocks of the baja california peninsula form one of the most areally extensive best exposed longest lived 160 my least tectonized and least metamorphosed convergent margin basin complexes in the world this convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins a progression from highly extensional phase 1 through mildly extensional phase 2 to compressional phase 3 strain regimes this trend is largely due to the progressively decreasing age of lithosphere that is subducted which causes a gradual decrease in slab dip angle and concomitant increase in coupling between lower and upper plates as well as progressive inboard migration of the arc axis this paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin phase 1 consists of late triassic to late jurassic oceanic intra arc to backarc basins that were isolated from continental sediment sources new progressively widening basins were created by arc rifting and sea floor spreading and these were largely filled with progradational backarc arc apron deposits that record the growth of adjacent volcanoes up to and above sea level inboard migration of the backarc spreading center ultimately results in renewed arc rifting producing an influx of silicic pyroclastics to the backarc basin rifting succeeds in conversion of the active backarc basin into a remnant backarc basin which is blanketed by epiclastic sands phase 1 oceanic arc backarc terranes were amalgamated by late jurassic sinistral strike slip faults they form the forearc substrate for phase 2 indicating inboard migration of the arc axis due to decrease in slab dip phase 2 consists of early cretaceous extensional fringing arc basins adjacent to a continent phase 2 forearc basins consist of grabens that stepped downward toward the trench filled with coarse grained slope apron deposits phase 2 intra arc basins show a cycle of 1 arc extension characterized by intermediate to silicic explosive and effusive volcanism culminating in caldera forming silicic ignimbrite eruptions followed by 2 arc rifting characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites this extensional rifting cycle was followed by mid cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent caused by a decrease in slab dip as well as a possible increase in convergence rate phase 2 fringing arc terranes form the substrate for phase 3 which consists of a late cretaceous high standing compressional continental arc that migrated inboard with time strongly coupled subduction resulted in accretion of blueschist metamorphic rocks with development of a broad residual forearc basin behind the growing accretionary wedge and development of extensional forearc trench slope basins atop the gravitationally collapsing accretionary wedge inboard of this ongoing phase 3 strongly coupled subduction together with oblique convergence resulted in development of forearc strike slip basins upon arc basement the modern earth is strongly biased toward long lived arc trench systems which are compressional therefore evolutionary models for convergent margins must be constructed from well preserved ancient examples like baja california this convergent margin is typical of many others where the early to middle stages of convergence phases 1 and 2 create nonsubductable arc ophiolite terranes and their basin fills in the upper plate these become accreted to the continental margin in the late stage of convergence phase 3 resulting in significant continental growth

kinematics of the western caribbean collision of the cocos ridge and upper plate deformation subduction of the cocos plate and collision of the cocos ridge have profound effects on the kinematics of the western caribbean including crustal shortening segmentation of the overriding plate and tectonic escape of the central american fore arc cafa tectonic models of the panama region pr have ranged from a rigid block to a deforming plate boundary zone recent expansion of gps networks in panama costa rica and colombia makes it possible to constrain the kinematics of the pr we present an improved kinematic block model for the western caribbean using this improved gps network to test a suite of tectonic models describing the kinematics of this region the best fit model predicts an euler vector for the counterclockwise rotation of the cafa relative to the caribbean plate at 89 10 w 7 74 n 1 193 ma 1 which is expressed as northwest directed relative block rates of 11 3 1 0 16 5 1 1 mm a 1 from northern costa rica to guatemala this model also predicts high coupling along the nicoya and osa segments of the middle american subduction zone our models demonstrate that the pr acts as a single tectonic block the panama block with a predicted euler vector of 107 65 w 26 50 n 0 133 ma 1 this rotation manifests as northeast migration of the panama block at rates of 6 9 4 0 7 8 4 8 mm a 1 from southern costa rica to eastern panama we interpret this motion as tectonic escape from cocos ridge collision redirected by collision with the north andes block which migrates to the northwest at 12 2 1 2 mm a 1

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

discovery of lower cretaceous synmetamorphic thrust tectonics in french lesser antilles la desirade island guadeloupe implications for caribbean geodynamics located east of guadeloupe the island of la desirade exhibits the oldest rocks of the lesser antilles arc and the eastern caribbean plate an old magmatic basement is composed of 1 late jurassic ophiolitic complex with meta basaltic pillow lavas and interbedded radiolarites 2 acid igneous complex comprising meta quartz diorite and meta rhyolitic lavas flows and 3 meta diabasic microdioritic dyke swarm complex we present and discuss the discovery of synmetamorphic thrust tectonics in this island based on detailed structural analysis we evidenced two main compressive events the first event d1 is characterized by pervasive folding associated with thrust development in the northeastern part of the island a major thrust fault the grand abaque thrust has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit the second deformation event d2 is featured by a main pervasive cleavage s2 observed at the regional scale associated with upright folding f2 and by the development of two sets of conjugated steeply dipping strike slip shear zones dextral 130 n and sinistral 20 n striking respectively microstructural observations indicate that greenschist facies metamorphism is clearly contemporaneous with the development of d1 and d2 superimposed tectonic structures ar ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a d2 event these analyses yield well defined and concordant plateau ages at 106 2 1 7 ma and 107 2 1 8 ma thus shortening and thickening tectonics occurred during lower cretaceous albian in the available framework of caribbean geodynamics the synmetamorphic thrust tectonics we discovered in the french lesser antilles is related to the collision between an aptian albian oceanic plateau and the andean cordilleran east dipping subduction zone our results support subduction polarity reversal during lower cretaceous

u pb geochronological constraints on the triassic jurassic ayu complex southern mexico derivation from the western margin of pangea a abstract we present la icp ms laser ablation inductively coupled plasma mass spectrometry u pb detrital and igneous zircon data of poly deformed metamorphic and igneous rocks of the ayu area southern mexico these rocks were previously inferred to be part of the late paleozoic acatlan complex but new age data indicate that they formed in the mesozoic and should be placed in the newly designated ayu complex the ayu complex comprises polydeformed metasedimentary rocks chazumba lithodeme of a turbidite like protolith that are intercalated with boudinaged ortho amphibolites with transitional arc to morb tholeiitic geochemistry in the south the metasedimentary sequence is affected by a ca 171 ma partial melting which formed the magdalena migmatite migmatization was accompanied by 171 168 ma intrusions of granodioritic dioritic and granitic dikes and sheets as well as pegmatite bodies which are characterized by inherited zircon populations of ca 260 290 320 360 420 480 880 990 and 1080 1250 ma that are also found in the chazumba lithodeme u pb detrital zircon dating of seven metasedimentary samples from the migmatized and unmigmatized chazumba lithodeme yielded youngest detrital zircons and clusters of 192 198 214 250 266 and 291 ma and are interpreted to reflect the late triassic middle jurassic deposition of turbiditic rocks the transitional arc tholeiitic geochemistry of the chazumba amphibolites is consistent with turbidite sedimentation in a back arc environment along a rifted passive margin close to a contemporaneous magmatic arc inferred flattening of the subduction zone led to subduction erosion during the early middle jurassic and underthrusting of the chazumba lithodeme to depths equivalent to amphibolite facies metamorphism steepening of the subducting slab and diachronous rifting within the gulf of mexico contributed to extensional tectonics recorded on the mexican mainland and facilitated the tectonic exhumation of the chazumba lithodeme by normal faulting along the reactivated providencia shear zone during the middle late jurassic more generally the documentation of arc back arc assemblages in the ayu complex requires deposition adjacent to a subducting ocean and thus supports a pangea a reconstruction that was synchronous with the breakup of pangea

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

tectonic wedges and offset laramide structures along the polochic fault of guatemala and chiapas mexico reaffirmation of large neogene displacement fault wedges along the polochic fault of northern guatemala and chiapas mexico record left lateral slip of from 20 to 65 km that accompanied documented neogene slip of 130 km across this north american caribbean plate boundary fault wedges can be correlated to source areas consistent with left lateral displacement along the polochic fault they occur adjacent to restraining bends in three areas where changes in strike of the fault have been mapped these wedges which have been previously interpreted as allochthons emplaced along low angle reverse faults are bounded by high angle faults folds that terminate against the polochic fault are part of a regional fold belt that originated during the campanian through paleocene laramide orogenic event a previous contention that some of these major folds are secondary and related to fault movement is shown to be incorrect the 130 km model of left lateral offset across the polochic fault is reviewed along with evidence for and against a neogene time of major displacement additional evidence for the model is presented 1 late cretaceous granitoid rocks from near motozintla chiapas mexico northern fault block are correlated with granitic rocks of the santa maria batholith in the region of huehuetenango guatemala southern block 2 mineralization at the contacts of the granitoid intrusives of the two areas is correlated 3 points of deflection of laramide structural axes are correlated across the fault 4 a major thrust sheet similar to that of the sierra de santa cruz in eastern guatemala is reassembled by reversing 130 km of left lateral slip which has offset serpentinites north of the fault in western guatemala and those south of the fault in central guatemala

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

the p t t history of blocks in serpentinite matrix melange west central baja california thermochronologic petrologic and geochemical analyses of epidote amphibolite amphilbolite eclogite and blueschist facies blocks in serpentinite matrix melanges from east san benito island and cedros island west central baja california provide constraints on the p t t history of this disrupted terrane results of sup 40 ar sup 39 ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different p t t histories mid jurassic 160 170 ma epidote amphibolite and amphibolite facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction coarse grained blueschist blocks were likely metamorphosed during continued subduction in late early cretaceous time that is 115 100 ma some epidote amphibolite blocks are partially overprinted by blueschist facies mineral assemblages and apparently record both metamorphic events fission track analyses of apatites indicate that the blocks underwent significantly different post metamorphic cooling histories epidote amphilbolite and amphibolite blocks cooled below approximately 100 degrees c form mid jurassic to paleocene time blueschist blocks cooled below approximately 100 degrees c in oligocene miocene time in general mafic blocks have trace element concentrations and ree patterns characteristic of ocean flood basalts this study demonstrates that samples with very different p t t histories can evidently occur over relatively small length scales 1 km in serpentinite matrix melanges 67 refs 16 figs 4 tabs

from static to dynamic provenance analysis sedimentary petrology upgraded abstract the classical approach to sandstone petrology established in the golden years of plate tectonics and based on the axiom that detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes has represented a benchmark for decades the composition of sand and sandstone however simply provides us with a distorted image of the lithological structure of source terranes and gives us little clue whether they are allochthonous or autochthonous orogenic or anorogenic young or old what we may able to see reflected in detrital modes is the nature of source terranes continental arc oceanic and the tectonostratigraphic level reached by erosion in space and time the proposed new approach to the petrology of sand and sandstone 1 starts with a simple classification scheme circulated since the 1960s which is purely descriptive objective and free of ill defined ambiguous terms and 2 focuses on the nature and tectonostratigraphic level of source terranes further steps are essential to upgrade provenance analysis acquiring knowledge from modern settings is needed to properly identify and wherever possible correct for physical and chemical processes introducing environmental and diagenetic bias and thus address nature s complexities with adequate conceptual tools equally important is the integration of multiple techniques ideally including bulk sediment multi mineral and single mineral methods bulk sediment petrography remains the fundamental approach that allows us to capture the most precious source of direct provenance information represented by the mineralogy and texture of rock fragments bulk sediment geochemistry applicable also to silt and clay carried in suspension is a superior method to check for hydraulic sorting chemical weathering and fertility of detrital minerals in different sediment sources detrital geochronology thermochronology and isotope geochemistry reveal the diverse time structures of source rocks and have become necessary complementary techniques in modern provenance analysis inferences on geodynamic processes need independent geological information and come last but if tackled properly they can lead us much farther than the standard label obtained by using triangular diagrams uncritically as if they were infallible oracles

sedimentary record of arc continent collision along mesozoic sw north america siuna belt nicaragua the western margin of the caribbean plate is a typical example where oceanic and continental terranes have amalgamated by subduction collision and strike slip processes the boundaries between these blocks as well as their tectonostratigraphic records are generally covered by younger deposits and dense tropical vegetation which may hamper reconstructing the accretionary evolution of the convergent margin in that context the study of overlap sedimentary assemblages represents an important tool to constrain the accretion timing of terranes in northern central america the geology of the suture zone between the chortis block and the exotic siuna intraoceanic arc indicates that the two terranes were assembled together during a hauterivian arc continent collision approximately 134 131 ma the exotic origin of the intraoceanic arc is based on the nature of metamorphic blocks within and the kinematics of the siuna serpentinite melange the short duration of the collision event is suggested by coeval exhumation of the siuna serpentinite melange and chortis derived coarse sedimentation el amparo formation along the suture zone rapidly followed by onset of pelagic sedimentation rio matis formation although the collision appears to have been short lived and preserved only in the suture zone postcollisional extension affected intra arc to back arc settings of the chortis block and led to the formation of kilometer thick extensional basins we envisage that the convergent margin inboard of sw mexico represented by the fringing guerrero intraoceanic arc and the mixteca continental terrane underwent similar postcollisional extension whereas the western margin of the proto caribbean oceanic realm experienced onset of wsw dipping subduction beneath the accreted siuna intraoceanic arc

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

geodynamic models of terrane accretion testing the fate of island arcs oceanic plateaus and continental fragments in subduction zones 1 crustal growth at convergent margins can occur by the accretion of future allochthonous terranes fats such as island arcs oceanic plateaus submarine ridges and continental fragments using geodynamic numerical experiments we demonstrate how crustal properties of fats impact the amount of fat crust that is accreted or subducted the type of accretionary process and the style of deformation on the overriding plate our results show that 1 accretion of crustal units occurs when there is a weak detachment layer within the fat 2 the depth of detachment controls the amount of crust accreted onto the overriding plate and 3 lithospheric buoyancy does not prevent fat subduction during constant convergence island arcs oceanic plateaus and continental fragments will completely subduct despite having buoyant lithospheric densities if they have rheologically strong crusts weak basal layers representing pre existing weaknesses or detachment layers will either lead to underplating of faulted blocks of fat crust to the overriding plate or collision and suturing of an unbroken fat crust our experiments show that the weak ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion the different types of accretionary processes also affect deformation and uplift patterns in the overriding plate trench migration and jumping and the dip of the plate interface the resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes such as the wrangellia terrane and klamath mountain terranes in the north american cordilleran belt

crustal growth during back arc closure cretaceous exhumation history of cordillera darwin southern patagonia the cordillera darwin metamorphic complex is unique in the andes in exposing kyanite staurolite schist north of the beagle channel in southern patagonia garnet in amphibolite facies pelitic schists from bahia pia has patchy textures whereby some grains consist of clear grossular rich garnet with fine grained s1 inclusion trails truncated by regions of turbid spessartine pyrope rich garnet with biotite muscovite plagioclase and quartz inclusions micron scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress s2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix pseudosection modelling in na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 nckfmashto is used to infer a p t path dominated by decompression from 12 to 9 kbar at t 620 c coupled with garnet mode decreasing from 5 to 1 u th pb in situ dating of s2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72 6 1 1 ma contact aureoles developed adjacent to late granite intrusions include sillimanite bearing migmatites formed at p 6 kbar after 72 ma metamorphism of southern cordillera darwin induced by continental underthrusting beneath the arc related to closure of the rocas verdes back arc basin was terminated by thrusting controlled exhumation with the rocks at p 9 kbar by c 73 ma and 6 kbar by c 70 ma

timing of deformational events in the rio san juan complex implications for the tectonic controls on the exhumation of high p rocks in the northern caribbean subduction accretionary prism abstract an integrated structural petrological and geochronological study was undertaken to constrain the tectonic history and controls on the exhumation of the high p rocks of the rio san juan complex in the northern caribbean subduction accretionary wedge in the main structural units of the complex microtextural analyses were performed to identify the fabrics formed at peak of metamorphism in eclogite facies conditions and during the main retrogressive event toward the low p amphibolite or blueschist greenschist facies conditions u pb shrimp dating on zircon rims 71 3 0 7 ma coupled with 40 ar 39 ar analyses on phengite 70 69 ma in felsic sills placed temporal constraints on the exhumation of the jagua clara serpentinite matrix melange during the blueschist facies stage at the early maastrichtian in the cuaba unit u pb tims zircon ages of 89 7 0 1 ma and 90 1 0 2 ma obtained for the crystallization of tonalitic trondhjemitic melts in the lower guaconejo and upper jobito subunits respectively are similar these ages coupled with a u pb shrimp zircon age of 87 1 8 ma obtained in a garnet amphibolite and a group of older 40 ar 39 ar cooling ages on calcic amphibole constrain the exhumation of the guaconejo subunit from the high p stage to the low p stage at the 90 83 ma time interval further the age data collectively supports a genetic relationship between the distributed extensional ductile shearing the related decompression and the local partial anatexis in the subunit at least from the turonian coniacian boundary to the early campanian a group of younger 40 ar 39 ar ages obtained in the mylonitized amphibolites of the basal jobito detachment zone indicates late ductile deformation and exhumation cooling in the late campanian to maastrichtian 75 70 ma therefore structural and age data established deformation partitioning and reworking of retrograde fabrics during 20 ma in the cuaba unit the different exhumation rates obtained for the jagua clara melange can be explained by uplift in two contrasting tectonic settings a first stage of slow exhumation 1 4 mm yr in the subduction channel largely lower than plate velocities and a second stage of relatively fast exhumation 7 6 mm yr up to the surface therefore the exhumation was temporally discontinuous and the velocity increase at 70 ma probably was triggered in response to the entrance of buoyant material in the subduction zone such as a caribeana continental ribbon or the distal part of yucatan bahamas continental margin in contrast the exhumation path of the guaconejo subunit is composed of a first segment from the baric peak to the low p amphibolite stage at 84 83 ma with an exhumation rate of 7 2 mm yr a second segment around the closure temperature of calcic amphibole at 82 70 ma with a rate of 0 4 mm yr and a third segment to the surface exposure at 60 ma with a rate of 1 8 mm yr these velocity differences can be correlated with the p t path proposed for the exhumation of the subunit with an initial isothermal decompression from the peak in the high p amphibolite to the eclogite facies produced by distributed extensional shearing followed by a relatively slow cooling at low p in the newly acquired structural position and the final tectonics mainly partitioned in the late jobito detachment zone the initial fast exhumation can be related to a major modification in the convergence conditions across the intra oceanic subduction zone as a jump of the basal subduction thrust beyond a previously accreted arc terrane

u pb geochronology of the type nazas formation and superjacent strata northeastern durango mexico implications of a jurassic age for continental arc magmatism in north central mexico u pb ages of ignimbrites and detrital zircon analyses from middle jurassic through lowermost cretaceous strata in northeastern durango mexico indicate the age of local volcanism and the age range respectively of the jurassic continental margin arc in north central mexico where it is termed the nazas arc the type nazas formation along the rio nazas consists of red continental sandstone and siltstone ignimbrites and intermediate flows that provide an important record of the extensional jurassic nazas magmatic arc overlying shallow marine sandstone and shale of late jurassic to earliest cretaceous age represent deposits of younger extensional basins that are part of the border rift system which extended from southern arizona to the gulf of mexico the younger strata constitute volcanic litharenite and lithic arkose derived from the jurassic volcanic rocks and from basement rocks of the coahuila block to the north two ignimbrites in the upper part of the nazas formation have statistically indistinguishable ages of 170 2 and 169 2 ma the ages are consistent with their stratigraphic order and position above volcanic lithic sandstone with a maximum depositional age of ca 180 178 ma calculated from detrital zircons six detrital zircon age populations were defined from four sandstone samples n 377 zircon grains of the nazas formation and overlying la casita and carbonera formations the most prominent age clusters 10 of the entire grain population have ranges ca 1277 890 ma grenville 22 of total ca 486 310 ma early paleozoic 11 and ca 202 159 ma jurassic two populations constituting 54 of total grains the jurassic age cluster indicates that nazas arc magmatism in the region took place during the early and middle jurassic and ended by early late jurassic time the nazas formation correlates with arc related middle jurassic strata in sonora southern arizona and southeastern california that contain interbedded eolian sandstone derived from the jurassic ergs of the colorado plateau absence of interbedded eolianite in the nazas formation thus indicates that the outcrops of north central mexico were likely not translated to their present locations along a regional late jurassic transcurrent fault system as postulated by some workers detrital zircon data from the upper jurassic and lowermost cretaceous strata indicate that jurassic volcanic rocks covered the coahuila block directly north of the study area at the end of middle jurassic time sandstone composition and detrital zircon ages record erosional unroofing of the coahuila block whereby removal of lower and middle jurassic volcanic and volcaniclastic strata exposed grenville pan african and peri gondwanan basement of the block during late jurassic time

contrasting exhumation p t paths followed by high p rocks in the northern caribbean subduction accretionary complex insights from the structural geology microtextures and equilibrium assemblage diagrams abstract the rio san juan metamorphic complex exposes a segment of a high pressure subduction accretionary complex built during convergence between the caribbean island arc and the north america continental margin it is composed of accreted arc and oceanic derived metaigneous rocks serpentinized peridotites and minor metasediments forming a structural pile combined structural geology microtextural relations multi equilibrium calculations and thermodynamical modelling together with published isotopic ages allow reconstructing the metamorphic p t t paths of each nappe unit and their links to the structural evolution in all units of the complex three major stages m1 to m3 in the tectonothermal evolution have been distinguished the m1 stage corresponds to the prograde evolution towards the pressure peak of metamorphism under blueschist or eclogitic facies conditions the m2 stage is related to the main retrogressive event and is characterized by the s2 l2 fabric development in all lithologies and at all scales the m3 stage represents continuous exhumation from ductile to ductile brittle deformation regimes however the shape of the retrograde p t path the age of the exhumation related d2 structures and the tectonic significance of d2 deformation are different in each structural unit in the upper structural levels of the jagua clara serpentinitic matrix melange the counter clockwise p t path of the eclogite blocks is typical of rocks exhumed in the early stages of intra oceanic subduction zones the clockwise p t path obtained for the lower cuaba unit is characterized by a strong isothermal decompression from the garnet epidote amphibolite and eclogite facies pressure peak this p t evolution can be explained by rapid exhumation caused by extensional tectonics in relation to a major modification of convergence conditions across the subduction zone the p t path also explains local syn m2 partial melting processes because it crosses the wet solidus for iat mafic compositions the p t path obtained for the high p guineal schists with exhumation trajectory following the burial trajectory can be related to exhumation during active subduction this exhumation was most likely driven by a combination of underthrusting of tectonic units and erosion processes available geochronological data and t t p t estimates reveal a late campanian to maastrichtian retrograde m2 metamorphism in the lower structural units of the complex during a consistent d2 top to the ne ene tectonic transport a similar tectonic transport has also been recognized in the metasedimentary nappes of the samana complex during eocene to earliest miocene these relations indicate a northeastward progradation of deformation during the successive tectonic incorporation of arc oceanic and continental derived terrains to the developing caribbean subduction accretionary complex

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

microplankton response to environmental conditions in the alboran sea western mediterranean one year sediment trap record abstract the present work analyses the seasonal evolution of planktonic assemblages and particle fluxes through the water column in the eastern alboran sea western mediterranean at 35o55 47 n 01o30 77 w a sediment trap was deployed below the influence of the almeria oran front aof a semi permanent geostrophic front during july 1997 to june 1998 overall the temporal variability of coccolithophore planktonic foraminifer diatom benthic and wind carried biogenic particle fluxes is linked to the seasonal evolution of sea surface hydrological structures maximum planktonic fluxes were found during high productivity periods and wind induced upwelling following a trimodal pattern with maximum fluxes in july 1997 november december 1997 and april may 1998 these periods were characterized by vertical mixing and the full development of anticyclonic gyres in the alboran sea the annual flux of coccolithophores was dominated by the small gephyrocapsa group and emiliania huxleyi whereas turborotalita quinqueloba and globigerina bulloides dominated the foraminiferal fluxes and chaetoceros resting spores rs were predominant in the diatom assemblage benthic specimens were also collected with the sediment trap suggesting a variable influence of bottom water activity wind driven particles phytoliths and fresh water diatoms were collected along the year but their fluxes followed the local wind regime the high sea surface temperature sst during fall due to weaker than usual westerly winds and the pressure anomaly prevailing in the alboran sea during early winter were reflected in the planktonic assemblages by the proliferation of warm lower photic layer inhabitants and or oligotrophic taxa of coccolithophores florisphaera profunda planktonic foraminifers globigerinoides ruber and globorotalia inflata and diatoms leptocylindrus danicus these unusual climatic conditions in the eastern alboran sea must have been caused by the 1997 1998 enso event

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

evidence for precollisional subduction erosion in ancient collisional belts the case of the mid european variscides analysis of the fore arc remnants and suture zone rocks in the european variscides using criteria established for the circum pacific margins geometry sedimentary evolution and style of deformation shows that precollisonal orogenic evolution can also be reconstructed for ancient collisional belts the collision zone of the lower plate thrust belt rhenohercynian zone and the upper plate arc and fore arc remnants saxothuringian zone and saar basin indicate subduction erosion of the upper plate by some 50 km length of upper plate lithosphere prior to collision removal started during early convergence in the famennian ca 365 ma and lasted until the late visean onset of collision ca 330 ma part of this material may have been underplated below the upper plate and has since resurfaced as high pressure rocks contributing to the metamorphic core of the orogen

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

geochemical characteristics of the rio verde complex central hispaniola implications for the paleotectonic reconstruction of the lower cretaceous caribbean island arc abstract new geochronological trace element and sr nd isotope data for metabasalts dolerites and amphibolites from the rio verde complex central hispaniola are integrated with existing geochemical data for mafic volcanic rocks and metamorphic derivatives from the los ranchos amina and maimon formations giving new insights into magma petrogenesis and paleotectonic reconstruction of the lower cretaceous caribbean island arc back arc system u pb and 40ar 39ar age data show that the rio verde complex protoliths were in part coeval with volcanic rocks of the los ranchos formation upper aptian to lower albian the geochemical data establish the existence of gradients in trace element parameters nb yb th yb zr yb zr ba and normalized ti sm y and yb abundances and nd isotope compositions from throughout hispaniola which reflect differences in the degree of mantle wedge depletion and contributions from the subducting slab the rio verde complex mafic rocks and some mafic sills and dykes intruding in the loma caribe peridotite have a transitional iat to n morb geochemistry and a weak subduction related signature and are interpreted to form in a rifted arc or evolving back arc basin setting the los ranchos amina and maimon formations volcanic rocks have arc like characteristics and represent magmatism in the volcanic front trace element and nd isotope modeling reproduce observed data trends from arc to back arc and suggest that the variations in several geochemical parameters observed in a sw direction across the caribbean subduction system can be explained from the progressively lower subduction flux into a progressively less depleted mantle source the low nb contents and high end i values in both arc and back arc mafic rocks imply however the absence of a significant lower cretaceous plume enriched component in order to explain these observations a model of proto caribbean oceanic lithosphere subducting to the sw at least in the 120 110 ma interval is proposed to cause the observed magmatic variations in the lower cretaceous caribbean island arc back arc system in this context arc rifting and initial sea floor spreading to form the rio verde complex protoliths occurred in the back arc setting of this primitive island arc built on the ne edge of the caribbean plate