geochronology geochemistry and tectonic evolution of the western and central cordilleras of colombia abstract autochthonous rocks of the pre cretaceous continental margin of nw south america the tahami terrane are juxtaposed against a series of para autochthonous rock units that assembled during the early cretaceous allochthonous oceanic crust of the caribbean large igneous province collided with and accreted onto the margin during the late cretaceous we present the first regional scale dataset of zircon u pb la icp ms ages for intrusive and metamorphic rocks of the autochthonous tahami terrane early cretaceous igneous para autochthonous rocks and accreted oceanic crust the u pb zircon data are complemented by multiphase 40ar 39ar crystallization and cooling ages the geochronological data are combined with whole rock major oxide trace element and ree data acquired from the same units to constrain the tectonic origin of the rock units and terranes exposed in the western cordillera cauca patia valley and the central cordillera of colombia the tahami terrane includes lower paleozoic orthogneisses 440 ma that may have erupted during the active margin stage of the rheic ocean basement gneisses were intruded by permian continental arc granites during the final assembly of pangea triassic sedimentary rocks were subsequently deposited in rift basins and partially melted during high t metamorphism associated with rifting of western pangea during 240 220 ma continental arc magmatism during 180 145 ma is preserved along the whole length of the central cordillera and was followed by an early cretaceous out board step of the arc axis and the inception of the quebradagrande arc that fringed the continental margin back stepping of the arc axis may have been caused by the collision of buoyant seamounts which were coeval with plateau rocks exposed in the nicoya peninsular of costa rica rapid westward drift of south america closed the quebradagrande basin in the late aptian and caused medium high p t metamorphic rocks of the arquia complex to exhume and obduct onto the continental margin subduction beneath hot spot derived rocks of the caribbean plateau 100 92 ma formed an intra oceanic arc 92 75 ma which collectively comprise the late cretaceous caribbean large igneous province the remnant ocean basin located between south america and the caribbean large igneous province was partly consumed via continental subduction forming the large antioquia batholith the caribbean large igneous province collided and accreted to south america during 75 70 ma along the cauca almaguer fault resulting in the cessation of both arcs and the paleocene onset of subduction beneath the accreted oceanic crust

pliocene uplift of the eastern iberian margin inferences from quantitative modelling of the valencia trough we present the results of quantitative modelling of subsidence in the valencia trough and associated uplift in the catalan coastal ranges and the balearic islands basement subsidence moho depth and gravity anomalies are calculated for different effective elastic thicknesses eet and durations of rifting along two profiles constrained by recently collected multichannel seismic and gravity data the tectonic evolution of the catalan coastal ranges cannot be explained with traditional two layer lithospheric stretching models for the valencia trough but is consistent with a thermomechanical model invoking lithospheric necking combined with a late stage uplift event the modelling of the topography of the balearic islands suggests an interaction of extension in the valencia trough with extension in the south balearic basin our study supports a rifting model with low values for the flexural rigidity an intermediate level of necking and one finite duration stretching event propagating from the northeast towards the southwestern part of the valencia trough the modelling provides constraints on the magnitude of a large scale pliocene uplift of the eastern margin of the iberian peninsula

low temperature thermochronology of the northern cordillera real ecuador tectonic insights from zircon and apatite fission track analysis the post early cretaceous tectonic development of the northern andean segment north of 5 s is loosely constrained by a series of partially and fully reset k ar ages new apatite and zircon fission track data from three traverses across the cordillera real of ecuador reveal distinct periods of accelerated crustal cooling during the cenozoic fission track ages range from 65 to 14 ma zircon and 44 to 8 ma apatite with apatite mean track lengths ranging from 14 6 to 12 1 m these results record rapid cooling rates during 43 30 ma 23 15 ma and 10 0 ma occurring at temperatures below 275 c the events reflect up to 9 km of crustal exhumation in various regions of the cordillera in response to relative changes in the kinematics of the farallon nazca and south american plates the exhumation history of the cordillera real was not continuous but rather evolved during separate phases these phases persisted for 10 myr and are partly coincident with tectonic phases of the central andes although extend for longer time periods possibly as a result of isostatically driven crustal exhumation large spatial variations in the zircon and apatite fission track data within the cordillera real indicate that local fault bounded massifs have experienced different thermal histories since 43 ma major variations are not restricted to the regional scale and exist within individual terranes faulted slivers within terranes demonstrate highly variable exhumation histories suggesting that deformation was not dominated by terrane bounding faults additionally n s variations in exhumation depths suggest that ne sw trending faults have been active since 10 9ma

a deep seismic sounding investigation of lithospheric heterogeneity and anisotropy beneath the iberian peninsula abstract the dimensions of the iberian peninsula the facility of firing large charges in the surrounding waters the well known and relatively uniform geology and prior knowledge of the crustal structure resulted in it being chosen as a study area for the investigation of the seismic structure of the lower lithosphere via refraction wide angle reflection seismic profiling the iberian lithosphere heterogeneity and anisotropy experiment iliha with a star shaped arrangement of six long range dss profiles was carried out in october 1989 the models derived from a first interpretation of the recorded data are presented three crustal profiles cover the same western and central part of the hercynian massif as the mantle profiles the resulting interpretations all include a middle as well as a lower crustal layer above the mantle the velocities of the layers in all three models are similar however the layer depths vary beneath the profiles interpretation of the mantle derived data suggests a layered lower lithosphere one reversed line and an intersecting unreversed line indicate the layering penetrates to at least 90 km depth the homogeneity of these layers contrasts strongly with the heterogeneous hercynian surface geology velocities derived from reflected data from the deep layers suggest the constituent materials are either anisotropic or that the layers suffer a slight regional dip

a hf isotope perspective on continent formation in the south peruvian andes convergent continental margins are the primary host of both growth and loss of continental crust continental growth largely occurs via subduction driven magmatism whereas continental loss largely occurs via subduction erosion and sediment subduction because the latter typically involves partial recycling into magmas both growth and loss of continental crust can be represented in the magmatic record the degree of crustal recycling can be estimated from the initial hf isotope signatures in both magmatic and detrital zircon grains recent insights into the geodynamic evolution of the peruvian margin in combination with a new dataset of hf isotopic data on zircon from the carboniferous to early cretaceous enable us to 1 compare the geodynamic history of the southern peruvian margin with its hf isotopic evolution and 2 quantify the crustal growth between 500 and 135 ma the data exhibit a correlation with trends in isotope composition v time and reflect the dominantly extensional regime that prevailed from the onset of subduction from 530 ma to c 135 ma this study demonstrates that the peruvian margin experienced continental growth with juvenile input to arc magmatism of 30 45 on average and illustrates the use of u pb and hf isotopes in zircon as a tool to trace episodes of crustal growth through time supplementary material hf istopic analyses on zircon a1 and a2 and new u pb zircon ages a3 are available at http www geolsoc org uk sup18661

geochronology and nd isotopic data of grenville age rocks in the colombian andes new constraints for late proterozoic early paleozoic paleocontinental reconstructions of the americas abstract new u pb zircon crystallization ages and 40ar 39ar cooling ages from the colombian andes confirm the existence of rocks metamorphosed during the orinoquian orogenic event ca 1 0 ga of northern south america end t 1 1 ga for these rocks range from 3 9 to 0 91 which is interpreted as a mixture of late archean early proterozoic crust with juvenile material produced during the 1 1 ga orogenic event the colombian grenville age rocks are part of a much longer metamorphic pericratonal belt sporadically exposed along the andes in western central peru southern bolivia and northern argentina in addition nd model tdm ages for the colombian rocks range from 1 9 to 1 45 ga similar to those obtained in the grenville province of the eastern u s and in the mexican basement placing constraints on late proterozoic early paleozoic paleocontinental reconstructions

crustal structure beneath the rif cordillera north morocco from the rifsis wide angle reflection seismic experiment the different geodynamic models proposed since the late 1990s to account for the complex evolution of the gibraltar arc system lack definite constraints on the crustal structure of the rif orogen here we present the first well resolved p wave velocity crustal models of the rif cordillera and its southern continuation toward the atlas made using controlled source seismic data two 300 km long wide angle reflection profiles crossed the rif along ns and ew trends the profiles recorded simultaneously five land explosions of 1tn each using 850 high frequency seismometers the crustal structure revealed from 2 d forward modeling delineates a complex laterally varying crustal structure below the rif domains the most surprising feature seen on both profiles is a 50 km deep crustal root localized beneath the external rif to the east the crust thins rapidly by 20 km across the nekkor fault indicating that the fault is a crustal scale feature on the ns profile the crust thins more gradually to 40 km thickness beneath middle atlas and 42 km beneath the betics these new seismic results are in overall agreement with regional trends of bouguer gravity and are consistent with recent receiver function estimates of crustal thickness the complex crustal structure of the rif orogen in the gibraltar arc is a consequence of the miocene collision between the iberian and african plates both the abrupt change in crustal thickness at the nekkor fault and the unexpectedly deep rif crustal root can be attributed to interaction of the subducting alboran slab with the north african passive margin at late oligocene early miocene times

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

cretaceous alkaline intra plate magmatism in the ecuadorian oriente basin geochemical geochronological and tectonic evidence abstract small volumes of cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the ecuadorian oriente foreland basin they are characterized by a restricted range of compositional variation low lile hfse ratios and sr nd isotope values within the range of oceanic island basalts oib reflection seismic data show that a pre existing nne ssw triassic and jurassic rift controls the location and occurrence of these alkaline eruptive sites radiometric ages 40ar 39ar incremental heating method and the biostratigraphic record of their surrounding sediments indicate a nne ssw systematic age variation for the emplacement of this alkaline volcanism from albian 110 5 2 ma in the northern part of the oriente basin to campanian 82 2 2 0 ma in the west central part the geochemical geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the ssw into the region underlying the pre existing triassic and jurassic rift thin spot we propose that subduction was abandoned subsequent to the accretion of allochthonous terranes onto the ecuadorian and colombian margin in the latest jurassic earliest cretaceous causing the relict slab material corresponding to the eastwards directed leading plate to roll back unmodified asthenospheric mantle migrated into the region previously occupied by the slab this resulted in partial melting and the release of magmatic material to the surface in the northern part of the oriente basin since at least aptian times then magmatism migrated along the ssw trending central wrench corridor of the oriente basin during the upper cretaceous probably as a consequence of the lateral propagation of the transpressive inversion of the triassic jurassic rift eventually the late cretaceous east dipping andean subduction system was renewed farther west and the development of the compressional retro foreland oriente basin system halted the cretaceous alkaline magmatic activity

heat flow heat production and lithospheric thermal regime in the iberian peninsula abstract the first heat flow and heat production maps of the iberian peninsula and its margins using previously acquired data are presented the surface heat flow map includes 553 determinations carried out on water and mining exploration wells oil exploration wells and on the seafloor the surface heat flow varies noticeably from the iberian mainland 65 10 mw m 2 to the atlantic and the mediterranean margins where the heat flow reaches values of about 40 50 mw m 2 and 80 100 mw m 2 respectively the heat production map consists of 664 determinations carried out on rock samples from the variscan iberian massif and the betics the higher values are obtained for granitic rocks 2 5 3 5 w m 3 whereas metasediments and basic rocks reach values of 1 2 5 w m 3 and nearly zero respectively the lithospheric structure deduced for the iberian peninsula by combining heat flow heat production and elevation data indicates that most of the iberian mainland is characterized by a lithospheric thickness of 110 5 km this value is maintained across the west atlantic margin whereas towards the mediterranean margin the lithospheric thickness decreases down to 60 40 km this study also suggests that the heat production in the southern variscan iberian massif must be noticeably higher 3 7 0 5 w m 3 than in the rest of the areas in order to fit the measured heat flow crustal thickness and elevation data

eclogites of the ojen nappe a record of subduction in the alpujarride complex betic cordilleras southern spain eclogites interlayered with amphibolites and migmatites occur in an upper unit ojcn nappe of the alpujarride complex betic cor dilleras estimates of p t conditions reveal a retrograde meta morphic path from an edogitic stage at 15 kbar and c 730 c to amphibolite facies conditions at 5 8 kbar or less and c 650 c the ronda peridotites belonging to the uppermost alpujarride nappe los reales nappe structurally overlie the eclogite bearing unit it is suggested that eclogitic metamorphism and overthrusting of the peridotites reflect dfierent stages of the same convergent episode

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

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

facies partitioning and sequence stratigraphy of cool water mixed carbonate siliciclastic sediments upper miocene guadalquivir domain southern spain during the late miocene the guadalquivir basin and its satellite basin the ronda basin were under atlantic cool water influence the aim of our study is to develop a sequence stratigraphic subdivision of the ronda basin fill and to provide models for the cool water carbonates the upper miocene of the ronda basin can be divided into three depositional sequences sequence 1 is early tortonian sequence 2 late tortonian to earliest messinian and sequence 3 messinian in age the sediments were deposited in a ramp depositional system sequence 1 is dominated by conglomerates and marls in sequence 2 and sequence 3 carbonate deposits dominate in the inner ramp whereas siliciclastics preferentially occur in the middle and outer ramp bryomol carbonate sediments occur in all sequences whereas rhodalgal carbonates are restricted to sequence 3 in bays protected from siliciclastic influx rhodalgal deposits formed under transgressive conditions a bryomol factory occurs in zones of continuous siliciclastic supply this distribution results from facies partitioning during the flooding of the ronda basin which has a rugged and irregular relief embayments were protected from siliciclastic influx and provided regions with less hydraulic energy

an evaporite bearing accretionary complex in the northern front of the betic rif orogen the guadalquivir accretionary complex forms a largely oblique prism at the northern edge of the betic rif orogen where miocene sediments plus allochthonous evaporite bearing units were accreted during the displacement of the alboran domain towards the west traditional interpretations end the tectonic structuring of the betic cordillera at the present topographic front beyond which gravitational and or diapiric processes would predominate however this study shows pervasive tectonic deformation in the outer prism with coherent oblique shortening kinematics which is achieved through an alternation of roughly n s arcuate thrust systems connected by e w transfer fault zones these structures accord well with the geophysical models that propose westward rollback subduction the main stage of tectonic activity occurred in the early middle miocene but deformation lasted until the quaternary with the same kinematics evaporite rocks played a leading role in the deformation as evidenced by the suite of ductile structures in gypsum distributed throughout the area s and l gypsum tectonites scaly clay fabrics and brittle fabrics coexist and consistently indicate westward motion top to 290 with subordinate n s contraction almost perpendicular to the transfer zones this work reveals ductile tectonic fabrics in gypsum as a valuable tool to elucidate the structure and deformational history of complex tectonic melanges involving evaporites above the decollement level of accretionary wedges

lateral variation of the crust in the iberian peninsula new evidence from the betic cordillera abstract new results from a seismic refraction wide angle reflection survey carried out in the betic cordillera in autumn 1989 contribute to a better picture of its deep structure one nw se profile cuts across the iberian massif the external and internal betics the structure of the crust in the iberian massif shows characteristics similar to those found in previous experiments the lower crust is found as a distinct layer 12 km thick with an average velocity of 6 8 km s 1 the moho is found at about 35 km depth this structure extends southeastward until a 3 4 km upwelling of the moho about 30 km north of the present day surface boundary between the external and internal betic units further southeast the moho deepens to 38 km and the lower crust is no longer seismically detected the absence of differentiated lower crust beneath part of the external betics may be related with the mesozoic rifting of the south iberian passive margin instead this absence under the internal betics may be caused by rifting in conjunction with the collisional evolution of the orogen a wnw ese profile lying in the internal betics shows the presence of a prominent reflector at 10 12 km depth this seems to be a widespread feature in the internal betics and may be interpreted as a detachment surface the moho is found at 38 km depth rising strongly in the easternmost betics seismic data suggest a thin crust in the offshore area southwest of malaga probably containing a massive zone of high velocity rocks which is also supported by available geophysical and geological data

tethys atlantic interaction along the iberia africa plate boundary the betic rif orogenic system abstract initial se dipping slow subduction of the ligurian tethys lithosphere beneath africa from late cretaceous to middle oligocene twisting to a later faster e dipping subduction of the subcrustal lithosphere is proposed as an efficient geodynamic mechanism to structure the arcuate betic rif orogenic system this new subduction related geodynamic scenario is supported by a kinematic model constrained by well dated plate reconstructions tectonic sedimentary and metamorphic data sets the slow initial se dipping subduction of the ligurian tethys realm beneath the malaguide upper plate unit is sufficient to subduct alpujarride and nevado filabride rocks to few tens of kilometers of depth in middle eocene times the shift from se to e dipping subduction during latest oligocene early miocene was possibly caused by both the inherited geometry of the highly segmented ligurian tethys domain and by the fast roll back of the subducted lithospheric slab the early miocene rather synchronous multiple crustal and subcrustal processes comprising the collision along the betic front the exhumation of the hp lt metamorphic complexes the opening of the alboran basin its flooring by hp alpujarride rocks and subsequent ht imprint can be explained by the fast nw and w directed roll back of the ligurian tethys subcrustal lithospheric slab the w retreat of the ligurian tethys lithosphere in middle late miocene times could partly explain the initiation of its lateral tear and consequent subcrustal processes from latest miocene onward the betic rif system evolved under both the northerly push of africa resulting in tightening at crustal and subcrustal levels and by the distinct current dynamics of the steep lithospheric slab the sw directed scape of the rif fold belt is one of the most striking evidences linked to the recent evolution of the squeezed betic rif system between africa and iberia

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

petroleum systems of the upper magdalena valley colombia abstract in the upper magdalena valley colombia four petroleum systems were identified two petroleum systems are located in the girardot sub basin and the other two in the neiva sub basin limestones laterally changing to shales of the lower part of the villeta gp deposited during albian and turonian marine flooding events constitutes the main source rocks of the oil families these rocks contain 1 4 toc and type ii kerogen the littoral quartz arenites of the caballos albian and monserrate maastrichtian fms are the main reservoir rocks seal rocks are cretaceous and paleocene shales overburden includes the cretaceous rocks and the tertiary molasse deposited simultaneously with development of two opposite verging thrust systems during cenozoic time these deformation events were responsible for trap creation except for the villarrica area where the source rock reached maturity during the paleocene generation occurred during miocene two oil families are identified each in both sub basins one derived from a clay rich source and the second from a carbonate rich source rock lithofacies of the lower part of villeta gp geochemical source rock to oil correlations are demonstrated for the three of the petroleum systems up dip lateral migration distances are relatively short and faults served as vertical migration pathways a huge amount of oil was probably degraded at surface as a result of miocene deformation and erosion

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

chronology and tectono sedimentary evolution of the upper pliocene to quaternary deposits of the lower guadalquivir foreland basin sw spain this paper presents new litho chrono and magnetostratigraphic data from cores of 23 exploratory boreholes drilled in the abalario and marshlands areas of the lower guadalquivir basin the western sector of the guadalquivir foreland basin sw of spain the lithologic logs of these boreholes identify four main sedimentary formations namely almonte sand and gravel lebrija clay and gravel marismas clay and abalario sand respectively interpreted as proximal alluvial distal alluvial alluvial estuarine and aeolian from radiocarbon and magnetostratigraphic data these formations were dated as upper pliocene to holocene in the marshlands area three main sedimentary sequences are present an upper pliocene to lower pleistocene sequence of the almonte and lebrija lower unit formations a pleistocene sequence of the lebrija upper unit and the lower marismas formations and a latest pleistocene to present day sequence of the upper marismas formation the three sequences began as a rapid alluvial progradation on a previously eroded surface and a subsequent alluvial retrogradation in the third sequence estuarine and marsh sediments accumulated on top of the alluvial sediments the aeolian sands of the abalario topographic high developed coeval to alluvial and estuarine sedimentation after the first alluvial progradation and continuously until the present correlation with the surrounding areas show that the sequences are the result of the forebulge uplift of the northern margin of the basin sierra morena and the adjacent neogene oldest sediments of their northern fringe both form the main source area of the study formations this uplift occurred simultaneous to the flexural subsidence sse tilting of the southern part of the basin where sedimentary aggradation dominated

southern menderes massif an incipient metamorphic core complex in western anatolia turkey in the southern sector of the menderes massif north of selimiye milas augen gneisses interpreted as a deformed peraluminous granite have been dynamothermally metamorphosed and are surrounded by and intrude a regionally metamorphosed palaeozoic envelope the granitic rocks exhibit a moderately dipping mylonitic foliation and nne ssw trending mineral elongation lineation the progressive deformation of the granitic rocks produces a structural sequence typical of an extensional shear zone marked from bottom to top by a very thick extensive zone of mylonites followed in turn by brecciated mylonite and cataclasite the kinematic indicators exhibit a top to the south down dip sense of shear these structures are attributed to exhumation of the granitic rocks of the massif along a major south dipping normal sense shear zone that accommodated crustal extension during late oligocene collapse of the orogen in western turkey thus the southern menderes massif may be interpreted as the exhumed footwall of a major extensional shear zone and possibly as an incipient core complex

the early andean magmatic province eamp 40ar 39ar dating on mesozoic volcanic and plutonic rocks from the coastal cordillera northern chile abstract the early andean magmatic province eamp consists of about 150 000 km 3 of volcanic and plutonic units in the coastal cordillera of northern chile and southern peru and represents a major magmatic mesozoic event in the world for which the precise age of the thick volcanic series was unknown thirty 40 ar 39 ar analyses were carried out on primary mineral phases of volcanic and plutonic rocks from northern chile 18 30 24 s reliable plateau and mini plateau ages were obtained on plagioclase amphibole and biotite from volcanic and plutonic rocks despite widespread strong alteration degree in the arica tocopilla and antofagasta 700 km apart regions the ages obtained on lava flows constrain the volcanic activity between 164 and 150 ma and no n s migration of volcanism is observed the uppermost lava flows of the volcanic sequence at the type locality of the la negra formation extruded at ca 153 150 ma suggesting the end of the volcanic activity of the arc at that time the oldest volcanic activity occurred probably at ca 175 170 ma in the iquique area although no plateau age could be obtained the plutonic bodies of the same regions were dated between ca 160 and 142 ma indicating that they were partly contemporaneous with the volcanic activity at least one volcanic pulse around 160 ma is evidenced over the entire investigated reach of the eamp according to the ages found in arica tocopilla michilla and mantos blancos regions the episodic emplacement of huge amounts of subduction related volcanism is observed throughout the whole andean history and particularly during the jurassic southern peru northern chile and southern argentina these events probably correspond to periodic extensional geodynamic episodes as a consequence of particular subduction conditions such as change of obliquity of the convergence change in the subduction angle slab roll back effect or lower convergence rate that remain to be precisely defined

crustal parameters in the iberian peninsula abstract the structure of the crust in the iberian peninsula has been investigated for the last 15 years by spanish and portuguese groups in close collaboration with other european institutions the first experiments were carried out in portugal mueller et al 1973 with the aim of investigating the crustal structure of the hercynian belt in the southwest corner of the iberian peninsula other experiments have been subsequently realized to study different aspects of the crust in various regions of portugal in spain the main effort has been focused in alpine areas with the first experiments in the alboran sea and the betic cordilleras working group for deep seismic sounding in spain 1974 1975 1977 working group for deep seismic sounding in the alboran sea 1974 1975 1978 follow up experiments until 1981 completed the work in the betic cordillera extensive experiments were carried out in the pyrenees in 1978 further surveys covered the balearic islands in 1976 the valencia trough in 1976 and 1983 and the celtiberian chain or iberic system in 1981 the hercynian belt has only been studied in detail in the northwest corner of spain in 1982 with smaller studies in the central iberian massif in 1976 and 1986 mostaanpour 1984 has compiled some crustal parameters crustal thickness average crustal velocity and pn velocity for western europe meanwhile more complete data are available for the iberian peninsula the results presented here were derived from a large number of seismic refraction experiments which have been carried out mostly along or close to coastal areas of the iberian peninsula offshore explosions of various sizes were used as the energy source in most cases in addition to some quarry blasts unfortunately this leaves most of the inner part of the iberian peninsula unsurveyed our purpose is to summarize some of the crustal parameters obtained so far and to detail the appropriate literature for the interested reader

flexure of the continental lithosphere with multilayered rheology summary in this paper a model of flexure of the continental lithosphere is derived taking into account crustal and mantle rheologies bending of the continental lithosphere is modelled with a double yield stress envelope three layers brittle elastic and ductile for the crust and three analogous layers for the mantle portion the deformation of the layers is controlled by the rheological properties of quartz rich crustal rocks and olivine rich mantle rocks the influence of various factors such as the depth of moho strain rates thermal structure of the lithosphere boundary conditions and topographic load is examined results show that the mechanical strength of the continental lithosphere in the horizontal and vertical directions is primarily controlled by the present thermal structure of the plate boundary forces and moments and the applied topographic load this explains why mountainous regions may be more locally compensated than adjacent regions we also thus are able to explain why many continental plates have apparent effective rigidities much smaller than those predicted on the basis of their geological ages the model is then applied to the tien shan tarim area central asia and original topography and gravity data are used to constrain parameters of the model we found that the model satisfactorily matches the data and is also able to predict the thermal state of the plate and the location of the deep seismicity

superimposed folding and oblique structures in the palaeomargin derived units of the central betics sw spain the subbetic units consist of a deformed wedge of sedimentary rocks deriving from the south iberian palaeomargin in the westernmost segment of the alpine mediterranean orogenic belt in the central betics shortening produced folding thrusting and nappe stacking the structural trend is ne sw to ene wsw and the internal structure of the thrust sheets which shows no preferred vergence is strongly controlled by the rheology of the decollement level a viscous layer of triassic evaporites and of the overlying carbonate rock sequence the relationships of piggyback basin miocene sediments with the main structures show that the latter developed during the aquitanian burdigalian transition superimposed folding is described for the first time in the study area the origin of the interference cannot be determined at present but may be related to initially oblique structures refolded during progressive deformation a revision of previously published palaeomagnetic data in the light of the data presented in this paper in conjunction with a model of the structural evolution of the subbetic units of the western betics permits us to establish milestones of the tectonic evolution of the external zones in the northern branch of the gibraltar arc

geochemistry and tectonic setting of pre collision cretaceous and paleogene volcanic rocks of ecuador pre collision cretaceous lower tertiary volcanic rocks of western ecuador from the coastal area pinon and san lorenzo formations and western cordillera macuchi and celica formations consist of three rock suites the first suite includes tholeiites of ocean floor affinities that have distinct light ree depleted patterns and low la nb la hf and th hf ratios the island arc tholeiites of the second suite have lower contents of ti zr hf and nb the third suite comprises calc alkalic rocks mainly continental margin andesites and dacites the continental volcanic arc rocks crop out in an area east of the tholeiitic rocks the boundary between the two areas is the major suture separating continental south america from accreted terrenes to the west a model is suggested for the geodynamic evolution of what is now western ecuador during the late cretaceous it includes two east dipping subduction zones of turonian to santonian ages and the accretion of the oceanic island arc to the continent

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

crust and upper mantle structure of the central iberian meseta spain summary quarry blasts recorded along three lines on the central iberian meseta are used in an attempt to interpret the crustal structure the results of the interpretation of the data together with published surface wave and earthquake data suggest a layered structure of the crust having the following features the basement in some areas covered by up to 4 km of sediments has a p velocity of 6 1 km s 1 a low velocity layer between 7 and 11 km depth seems to exist on the basis of both p and s interpretation of seismic data a thick middle crust of 12 km has a p velocity of 6 4 km s 1 and overlies a lower crust with a mean p velocity of 6 9 km s 1 and a possible slight negative gradient the mean vp vs ratio for the crust is about 1 75 the moho is reached at about 31 km depth and consists of a transition zone at least 1 5 km thick the p velocity of the upper mantle is close to 8 1 km s 1 and the s velocity about 4 5 km s 1 which gives a vp vs ratio of 1 8 for the uppermost mantle a tentative petrological interpretation of the velocities and composition of the layers is given

the early cretaceous san juan plutonic suite ecuador a magma chamber in an oceanic plateau sections through an oceanic plateau are preserved in tectonic slices in the western cordillera of ecuador south america the san juan section is a sequence of mafic ultramafic cumulates to establish that these plutonic rocks formed in an oceanic plateau setting we have developed criteria that discriminate intrusions of oceanic plateaus from those of other tectonic settings the mineralogy and crystallization sequence of the cumulates are similar to those of intra plate magmas clinopyroxene predominates throughout and orthopyroxene is only a minor component rocks of intermediate composition are absent and hornblende is restricted to the uppermost massive gabbros within the sequence the ultramafic cumulates are very depleted in light rare earth elements lree whereas the gabbros have flat or slightly enriched lree patterns the composition of the basaltic liquid in equilibrium with the peridotite calculated using olivine compositions and ree contents of clinopyroxene contains between 16 and 8

high temperature 350 c thermochronology and mechanisms of pb loss in apatite abstract natural processes driven by heat flow can be understood using quantitative reconstruction of the thermal history of accessory and common minerals that were formed or modified in these processes thermochronology assumes that isotopes are lost from minerals by thermally activated volume diffusion and forms the basis of many studies of the thermal evolution of the crust however some studies challenge this assumption and suggest that the mechanisms controlling isotope transport in minerals over geological time scales are dominated by aqueous fluid flow within mineral pathways here we test these contrasting hypotheses by inverse modelling apatite uranium lead u pb dates to produce theoretical t t solutions assuming pb was lost by volume diffusion these solutions are compared with independent geological constraints and intra grain apatite u pb dates which demonstrate that volume diffusion governed the displacement of pb this confirmation combined with an inverse modeling procedure that permits reheating and cooling paths to be distinguished between 375 and 570 c provides geologists with a unique tool for investigating the high temperature thermal evolution of accessory minerals using the u pb method

evolution of the late miocene mediterranean atlantic gateways and their impact on regional and global environmental change marine gateways play a critical role in the exchange of water heat salt and nutrients between oceans and seas as a result changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate as well as having a profound impact on local environmental conditions mediterranean atlantic marine corridors that pre date the modern gibraltar strait closed during the late miocene and are now exposed on land in northern morocco and southern spain the restriction and closure of these miocene connections resulted in extreme salinity fluctuations in the mediterranean leading to the precipitation of thick evaporites this event is known as the messinian salinity crisis msc the evolution and closure of the mediterranean atlantic gateways are a critical control on the msc but at present the location geometry and age of these gateways are still highly controversial as is the impact of changing mediterranean outflow on northern hemisphere circulation here we present a comprehensive overview of the evolution of the late miocene gateways and the nature of mediterranean atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins we also consider the possible impact of evolving exchange on both the mediterranean and global climate and highlight the main enduring challenges for reconstructing past mediterranean atlantic exchange

zircon u pb ages of super units in the coastal batholith peru implications for magmatic and tectonic processes zircon u pb ages on 50 plutonic samples from super units in the lima arequipa and toquepala segments of the peruvian coastal batholith range from 188 to 37 m y revealing previously unrecognized jurassic elements the jurassic plutons represent a distinct time defined continental arc separated from the cretaceous arc by a quiescent period of 50 m y only 4 of the 50 samples have discordant zircons and these are all from the arequipa and toquepala segments this regionally limited discordance interpreted to be due to inherited radiogenic pb from the precam brian basement suggests that there are fundamental differences in the nature of the crust beneath the lima and the arequipa toquepala segments on a grand scale magmatism in the coastal batholith generally evolved through time from mafic to siliceous melts but zircon u pb ages show that the occurrence of small bodies of old siliceous and young mafic rocks was also important close age association between all rock types raises the possibility that mantle derived mafic magmas provided the heat that produced siliceous magmas in either the underplated wedge of the subduction complex or the lower crust throughout the period of batholith emplacement in general age results confirm that super units have short time spans for the santa rosa paccho and linga super units however mapping generalizations were made in the absence of thorough isotopic age and trace element data remapping of these super units with supporting geochemical and geochronological studies will be required before the emplacement history of the batholith is fully understood dated plutons in the paccho super unit yield ages between 64 0 and 39 0 m y they are thus considerably younger than has been previously recognized and cannot belong to one super unit their young ages also verify the eastward migration of the locus of magmatism through time a migration rate of 1 3 mm yr computed for the rocks along latitude 11 08 1 s on the basis of the 105 m y old patap super unit and a 61 m y old pluton in the paccho super unit suggests that the geometry of the andean subduction zone in peru was very stable during batholith emplacement

evidence for an active sinistral shear zone in the westeren alboran region a compilation of gravity seismicity neotectonics geology tomography and topography data from the alboran region reveals distinctive differences between the eastern and western part of the alboran region and betic cordillera calculated profiles of integrated crustal strength reveal that lateral differences in mechanical behaviour are caused by marked inherited differences in crustal make up the two domains with different tectonic behaviour are separated by a n025 trending lineament in the western alboran and central betic cordillera this lineament can be interpreted as an upper mantle lower crustal sinistral shear zone accommodating a large part of the convergence between africa and iberia

u pb re os and 40ar 39ar geochronology of the nambija au skarn and pangui porphyry cu deposits ecuador implications for the jurassic metallogenic belt of the northern andes new u pb re os and 40ar 39ar dates are presented for magmatic and hydrothermal mineral phases in skarn and porphyry related ores from the nambija and pangui districts of the subandean zone southeastern ecuador nambija has been one of the main gold producing centers of ecuador since the 1980s due to exceptionally high grade ores average 15 g t but frequently up to 300 g t au pangui is a recently discovered porphyry cu mo district the geology of the subandean zone in southeastern ecuador is dominated by the i type subduction related jurassic zamora batholith which intrudes triassic volcanosedimentary rocks the zamora batholith is in turn cut by porphyritic stocks which are commonly associated with skarn formation and or porphyry style mineralization high precision u pb and re os ages for porphyritic stocks u pb zircon associated prograde skarn u pb hydrothermal titanite and retrograde stage skarn re os molybdenite from veins postdating gold deposition of the nambija district are all indistinguishable from each other within error 145 ma and indicate a late jurassic age for the gold mineralization previously gold mineralization at nambija was considered to be early tertiary based on k ar ages obtained on various hydrothermal minerals the new jurassic age for the nambija district is slightly younger than the 40ar 39ar and re os ages for magmatic hydrothermal minerals from the pangui district which range between 157 and 152 ma mineralization at nambija and pangui is associated with porphyritic stocks that represent the last known episodes of a long lived jurassic arc magmatism 190 to 145 ma a jurassic age for mineralization at nambija and pangui suggests that the northern andean jurassic metallogenic belt which starts in colombia at 3 n extends down to 5 s in ecuador it also adds a new mineralization style au skarn to the metal endowment of this belt

modeling the evolution of the guadalquivir foreland basin southern spain 1 previous quantitative studies dealing with the origin of foreland basins have focused primarily either on the rheological basis of the lithosphere mechanical response or on the relationship between orogenic loading and sediment geometry to link the evolution of the guadalquivir foreland basin south iberia with the thermomechanical stratification of the iberian lithosphere we combine quantitative approaches to deep and shallow processes thrust loading lithospheric flexure thickness changes of the crust and the lithospheric mantle and surface mass transport a planform flexural analysis of the present day load distribution shows that basement subsidence is related not only to upper crustal thrusting but also to a deep seated additional load on the basis of the observed gravity and geoid fields we propose this additional load to be related to a lithospheric mantle thickening larger than the coeval crustal thickening further modeling of the evolution of a basin cross section reveals that the architecture of the sedimentary basin is additionally related to the lithosphere rheological response the quantitative study of the evolution of basement faulting and the forebulge uplift of sierra morena leads us to conclude that viscous stress relaxation and or plastic yielding within the lithosphere are key processes to explain the flexural evolution of the basin

geological history of the cretaceous ophiolitic complexes of northwestern south america colombian andes abstract the western cordillera of colombia was formed by intense alpine type nappe forming folding and thrusting the cretaceous 80 120 ma b p tholeiitic material of the western cordilleran nappes has been obducted onto the paleozoic and precambrian polymetamorphic micaschists and gneiss of the central cordillera near yarumal the antioquia batholith 60 80 ma b p intrudes both obducted cretaceous oceanic material and the polymetamorphic basement rock of the central cordillera therefore nappe emplacement and obduction onto the central cordillera occurred during late senonian to early paleocene the nappes travelled from northwest to southeast so that the highest unit the rio calima nappe therefore has the most northwestern source whereas the lowest units originated from a more southeastward direction sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment during cretaceous times the opening of this marginal sea from now on called the colombia marginal basin probably originated by detachment of a block from the south american continent related to the farallon south america plate convergence in the popayan area southern colombia the central cordilleran basement exhibits glaucophane schist facies metamorphism this high pressure low temperature metamorphism is of early cretaceous 125 ma b p age and is related to an undated metaophiolitic complex the ophiolitic material originating from the western cordilleran is thrust over both the blueschist belt and the metaophiolitic complex these data suggest that the occidente colombiano suffered at least two phases of ophiolitic obduction during mesozoic time

early cretaceous granitoids and their implications for the collapse of the dabie orogen eastern china shrimp zircon u pb dating and geochemistry abstract two stages of early cretaceous post orogenic granitoids are recognized in the dabie orogen eastern china which recorded processes of extensional collapse of the orogen the early stage granitoids 132 ma are foliated hornblende quartz monzonites and porphyritic monzogranites they are of high k calc alkaline series and metaluminous to weakly peraluminous with high k 2 o and low mgo contents mg values 32 0 46 0 they contain high sr low y and heavy rare earth elements hree and have high sr y and la yb n ratios without clear negative eu sr and ti anomalies the early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite rich plagioclase and thus indicate the existence of an over thickened 50 km crustal root beneath the orogen at 132 ma the later stage granitoids 128 ma are undeformed fine grained monzogranites fine grained k feldspar granites and coarse grained k feldspar granite porphyry they belong to a peraluminous and high k calc alkaline to shoshonite series and display a flat hree pattern and have strong negative eu sr and ti anomalies with low sr y and la yb n ratios the late stage granitoids are equilibrated with residues rich in anorthite rich plagioclase hornblende ilmenite titanite and poor in garnet indicating that the crust of the dabie orogen became thinner

high mg extrusive rocks from the romeral zone ophiolites in the southewestern colombian andes abstract in the southwestern colombian andes high mg extrusive rocks occur within the mesozoic late cretaceous ophiolitic fragments from the romeral fault zone a major suture between the continental domain of the central cordillera and the oceanic domain of the western cordillera the high mg rocks include picrites and picritic basalts and are associated in places with peridotites showing transitional to extrusive textures plagioclase free picrites characterized by quench textures have been found in two localities and occur as pillow lava rio boloblanco or as clasts of monogenic and polygenic volcanic breccias el encenillo they show similar olivine phyric micro spinifex textures with groundmass consisting of hopper and plate olivine or ca poor pyroxene at el encenillo skeletal clinopyroxene and glass hyaloporphyritic texture occurs in the rio boloblanco picrites at pillow rims chemically these picrites are characterized by mgo contents of 20 and are distinguished by the content of tio 2 and incompatible elements in a high ti type rio boloblanco and a very high ti type el encenillo the latter particularly remarkable for the high contents of ti zr and nb which give evidence of an alkaline tendency phase chemistry is consistently different between the two picrite types but comparably high al contents up to 14 of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions olivine phyric picritic rocks characterized by microlitic non quenched groundmasses occur as clasts of volcanic breccias at el encenillo picritic basalts and are present with comparatively high frequency within major los azules and minor upper chapungo valley ophiolitic fragments from the romeral zone either as lavas and volcanigenic breccias these rocks are mostly richer in olivine than the quench textured picrites contain modal plagioclase and relatively low al pyroxenes as regards chemistry a high ti and very low al group los azules upper chapungo valley and a very high ti group el encenillo can also be distinguished within these picrites a comparison with the ultramafic volcanic rocks from gorgona island in the nearby pacific domain off colombia among which the unique phanerozoic komatiites were discovered indicates close similarities between the rio boloblanco picrites and some gorgona island komatiites olivine phyric komatiites with micro spinifex groundmass regarding their petrochemical features it is suggested that they record a unique event related to the same tectonic environment and consequently that they belong to the same structural unit developed in the late cretaceous and extending from continental colombia to the pacific domain this is in contrast with previous subdivisions of the ophiolitic complexes from southwestern colombia into distinct age groups progressively younger from the central cordillera toward the pacific ocean

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

mantle lithosphere bodies in the alboran crustal domain ronda peridotites betic rif orogenic belt abstract geological and geophysical data are used to demonstrate the existence of intracrustal high density high p wave velocity bodies in the western betics these bodies appear to correspond to buried peridotites similar to those that outcrop in the ronda area a gravity study shows how the gravity field is mainly the result of a combined effect of crustal thinning and the presence of ultramafic bodies the size of the buried high density body as interpreted from gravity and seismic results shows maximum dimensions of about 40 km in length nnw sse about 8 km in thickness and a lateral extension ene wsw of about 70 km the thinning of the crust from 32 35 km to 20 22 km takes place in a narrow area less than 35 km wide our results are compatible with an interpretation in terms of an unrooted peridotite slab dismembering of an initial slab of ultramafic rocks is a possible consequence of the extensional regime that originated the alboran basin

cross frontal cold jets near iceland in water satellite infrared and geosat altimeter data this paper reports detailed in water observations and satellite infrared images which are approximately coincident with a single geosat altimeter track across the iceland faeroes frontal zone the are thermistor chain covered the upper 300 m of the ocean along the track and the first two of a long sequence of noaa satellite infrared images were obtained all within 24 hours of the geosat overpass the data are interpreted as showing cold cross frontal jets related to the formation of cold eddies south of the main frontal boundary implications for the use of altimetry for ocean monitoring are considered

heat flow in the alboran sea western mediterranean abstract the results of the first regional heat flow survey carried out in the alboran basin are presented the survey consists of 98 heat flow measurements obtained using a violin type probe 697 nautic miles of gravity profiles 1446 nautic miles of bathymetric survey and 22 gravity cores a remarkable difference in heat flow patterns exists between the western wab and eastern eab parts of the alboran basin the average heat flow in the wab is 69 6 mw m 2 with a generally increasing trend towards the centre and to the east in contrast the heat flow pattern in the eab shows an average value of 124 8 mw m 2 and it is maintained rather constant for the overall area superimposed on this general pattern there are some local thermal anomalies associated with hydrothermal activity which have been detected in the central wab up to 123 mw m 2 in the south alboran basin sab up to 153 mw m 2 and in the djibouti bank db up to 254 mw m 2 after corrections for thermal refraction sedimentation and cooling of volcanic bodies the resulting heat flow distribution in the wab is smoother but still shows the increasing trend towards the centre and to the east in the eab the application of these corrections did not lead to any noticeable changes a 1 d approach that combines heat flow data crustal structure and elevation shows a dramatic decrease in lithospheric thickness from the wab 50 90 km to the eab 38 40 km likewise the resulting crustal thickness is around 14 16 km in the central part of the wab increasing towards the borders of the basin whereas in the eab the crustal thickness varies between 12 5 and 14 5 km in its western part and between 10 and 11 5 km in its eastern part

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

sediment supply from the betic rif orogen to basins through neogene abstract we present a quantification of total and partial divided by time slices sedimentary volumes in the neogene basins of the betic rif orogen these basins include the alboran sea the intramontane basins the guadalquivir and rharb foreland basins and the atlantic margin of the gibraltar arc the total volume of neogene sediments deposited in these basins is 209 000 km3 and is equally distributed between the internal alboran basin and intramontane basins and the external basins foreland basins and atlantic margin the largest volumes are recorded by the alboran basin 89 600 km3 and the atlantic margin 81 600 km3 the guadalquivir and rharb basins amount 14 000 km3 and 14 550 km3 respectively whereas the intramontane basins record 9235 km3 calculated mean sediment accumulation rates for the early middle miocene show an outstanding asymmetry between the alboran basin 0 24 mm yr and the foreland basins 0 06 0 07 mm yr and the atlantic margin 0 03 mm yr during the late miocene sedimentation rates range between 0 17 and 0 18 mm yr recorded in the alboran basin and 0 04 mm yr in the intramontane basins in the pliocene quaternary the highest sedimentation rates are recorded in the atlantic margin reaching 0 22 mm yr sedimentary contribution shows similar values for the inner and outer basins with a generalized increase from late miocene to present from 3500 to 6500 km3 my interestingly the alboran basin records the maximum sedimentary contribution during the late miocene 5500 km3 my whereas the atlantic margin does during the pliocene quaternary 6600 km3 my the spatial and time variability of the sediment supply from the betic rif orogen to basins is closely related to the morphotectonic evolution of the region the high sedimentation rates obtained in the alboran basin during the early middle miocene are related to active extensional tectonics which produced narrow and deep basins in its western domain the highest sedimentary contribution in this basin as well as in the foreland and intramontane basins is recorded during the late miocene due to the uplift of wide areas of the betics and rif chains the analysis of the sedimentary supply also evidences strong relationships with the post tortonian crustal thickening and coeval topographic amplification that occurred in the central betics and rif with the concomitant evolution of the drainage network showing the fluvial capture of some internal basins by rivers draining to the atlantic ocean the ancestral guadalquivir

mid crustal detachment in the betic system of southeast spain abstract the tertiary orogenic belts of eastern spain can be interpreted using the principles of thin skinned thrusting with a fundamental detachment at mid crustal level in the eastern betics the compression involves an oblique restacking of a stretched continental margin much of the shortening is transmitted on permian and triassic detachments which emerge as forward and back thrust duplexes an interpretation of the guadarrama range of central spain is presented in which significant shortening is shown on a mid crustal detachment propagating from the southeast it is suggested that this represents the external limit of a compressive system linked to the betics the pyrenees and eastern cantabrians are a separate tertiary compressive system again resulting from restacking of an extensional basin during the collision of iberia and france previous authors have proposed mid crustal detachment in the pyrenees observation of the geological map of the iberian ranges suggests a broadly similar structural origin may apply in which case the iberian ranges may represent the most external zone of a major pyrenean system again linked by a mid crustal detachment thus it is suggested that the whole of eastern spain is made up of two major compressive systems linked at mid crustal level

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

puncoviscana folded belt in northwestern argentina testimony of late proterozoic rodinia fragmentation and pre gondwana collisional episodes stratigraphic correlations and tectonic analysis suggest that the puncoviscana fold belt of northwestern argentina was an intracontinental basin with bimodal igneous suites that formed in connection with the breakup of the rodinia supercontinent at 800 ma several lines of evidences point to an initial lithosphere rupture possibly induced by a rising mantle plume the earliest synrift igneous products are represented by ultra potassic dykes and alkaline lava flows of high lree hree and low zr nb y nb ratios the dyke emplacements and the initiation of rifting were probably synchronous they pass laterally and upwards middle part of the puncoviscana succession into basalts of alkaline transitional character oib like source the distinctive chemical feature of these lavas are very similar to the source of oceanic island basalts thus they are thought to represent a magmatism associated with the rift and rift drift transition stage during this stage of rifting probably true oceanic crust was formed the upper part of the puncoviscana sequence late precambrian lower cambrian in age comprises a thick and monotonous sequence of pillow lavas massive basaltic flows and minor volcanic breccias and hyaloclastites these lavas exhibit morb trace element characteristics with high feot and tio2 low k2o and p2o5 flat light ree spectra little or no depletion in nb and ta this volcanism consists of the major and latest effusive episode from the puncoviscana basin which was slightly modified by subduction processes the geodynamical model proposed for the generation of these volcanic rocks could have been developed in two stages in the first stage the volcanic event is compatible with a progressive opening of a continental rift leading to formation of a mature oceanic basin in contrast the second stage shows the effects of a completed wilson cycle including a primitive volcanic arc which continued until the accreted cuyania arequipa belen antofalla caba terrane against the proto gondwana western borderland of the amazonian shield 535 ma

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

high resolution seismic imaging of an active fault in the eastern guadalquivir basin betic cordillera southern spain abstract we calculated the high resolution seismic velocity poisson s ratio crack density and saturation ratio structures in and around the source areas of the torreperogil seismic series october 2012 april 2013 this seismic series characterized by a large number of low magnitude below mw 3 7 or md 3 9 and very shallow microearthquakes took place in the guadalquivir basin a large flexural foreland basin with a linear ene wsw trending bounded to the north by the iberian massif and to the south by the betic cordillera and filled from a middle miocene to plio quaternary sedimentary sequence in the upper layers of the crust strong low velocity anomalies are extensively distributed under the central zone which together with high poisson s ratio and crack density values may correspond to rocks which are less likely to fracture perhaps due to the accumulation of tectonic and seismic stress 93 of the earthquakes occurred at depths of up to 8 km which could indicate that the base of the seismogenic zone lies at this depth the seismic series was concentrated in layers of strong structural heterogeneities in the boundary area between low and high anomalies which were likely to generate earthquakes due to differential strain accumulation beneath the region the high velocity areas are also considered to be strong yet brittle parts of the fault zone which may generate earthquakes at depths of between 5 km and 9 km by contrast low velocity areas are less prone to fracture allowing seismic slippage to take place from 2 to 4 km depth the best estimate of the depth of the main shock m blg 3 9 is 7 6 km which could tend to nucleate at the base of the seismogenic zone at the fault end on the boundary between a low velocity zone to the east and a high velocity zone to the west indicating the fault plane which separates both areas laterally assuming that this seismic contrast is one of the main torreperogil faults it could imply that stress has accumulated in an existing fault zone with lateral heterogeneity in velocity

the geodynamic evolution of the internal zone of the betic cordilleras south east spain a model based on structural analysis and geothermobarometry the internal zone of the betic cordilleras consists of several superimposed major thrust sheets with different p t t evolutions on the basis of an integrated field microscopic and laboratory study the tectono metamorphic history of the mulhacen complex and almanzora unit has been reconstructed in detail the mulhacen complex has been af ected by at least five phases of penetrative deformation which have been labelled dx d dx d and dx d and d are related to continent continent collision which is indicated by high pressure low temperature wilt and subsequent intermediate pit metamorphic conditions d is related to crustal thinning and heterogeneous extension during this event the almanzora unit was juxtaposed against the mulhacen complex this phase was succeeded by the establishment of low pressure high temperature lpiht conditions and at least two phases of folding and overthrusting the almanzora unit shows a comparable tectono metamorphic evolution pt d however the pit conditions prior to d indicate a higher crustal position with respect to the mulhacen complex during the collisional event key w alpine orogeny betic cordilleras spain continent continent collision crustal thickening crustal thinning deformation structures geothermobarometry p t t path

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

lateral inhomogeneities of the hercynian crust in central spain abstract an azimuthal layout of deep seismic profiles has been partially completed in the central iberian massif data from two new profiles and a reinterpretation of previous profiles make possible two dimensional models for the hercynian crust in central spain from which some lateral crustal inhomogeneities can be established these inhomogeneities due to alpine intraplate tectonics coincide with the spanish central system and consist of a 5 km crustal thickening with respect to the surrounding areas

modification of an oceanic plateau aruba dutch caribbean implications for the generation of continental crust the generation of the continental crust may be connected to mantle plume activity however the nature of this link and the processes involved are not well constrained an obstacle to understanding relationships between plume related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships particularly in ancient greenstone belts studies of younger analogous regions may help to clarify these relationships on the island of aruba in the southern caribbean a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed the mafic lavas show geochemical and isotopic affinities with other basaltic picritic and komatiitic rocks that crop out elsewhere in the caribbean these are well documented as belonging to an 88 91 ma plume related oceanic plateau which is allochthonous with respect to the americas and is thought to have been formed in the pacific region the 85 to 82 ma tonalitic rocks share some geochemical characteristics high sr and ba low nb and y with archaean tonalite trondhjemite granodiorite ttg suites field relationships suggest that deformation of the plateau sequence possibly related to collision with a subduction zone was synchronous with intrusion of the aruba batholith new incremental heating 40ar 39ar dates combined with existing palaeontological evidence show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence sr nd isotopic data for both rock suites are uniform 87sr 86sri 0 7035 endi 7 whereas pb isotopes are more variable plateau sequence 206pb 204pb 18 6 19 1 207pb 204pb 15 54 15 60 208pb 204pb 38 3 38 75 aruba batholith 206pb 204pb 18 4 18 9 207pb 204pb 15 51 15 56 208pb 204pb 38 0 38 5 this suggests that there has been a minor sedimentary input into the source region of the batholith however the limited time interval between basaltic and tonalitic magmatism makes a normal subduction related origin for the tonalites improbable instead models involving derivation of tonalite from partial melting of the plateau sequence or alternatively genesis in an unusual subduction zone environment are investigated

a deep seismic reflection survey across the betic chain southern spain first results abstract two land seismic reflection profiles across the betic chain have imaged the deep structure of the crust belonging to two different crustal domains to the north one profile samples the crust of the variscan iberian massif that underlies the sedimentary cover of both the guadalquivir foreland basin and the south iberian crustal domain the upper crust is non reflective along the profile but the lower crust is reflective in the northern half of the profile between 7 and 12 s two way travel time in this segment of the profile the mono is also well imaged the quality of the data diminishes significantly as the profile enters the neogene guadix baza basin a second profile crossing the alpine metamorphic complexes of the betics reveals a reflective lower crust and a conspicuous reflection in the upper crust that is continuous for about 10 km at 11 s a distinct reflection has been interpreted as the reflection moho deeper reflections are also seen in the central part of the profile in a segment about 10 km long comparison with available refraction and wide angle reflection data shows some differences in both the crustal configuration and in the depth to the moho

flexural behavior of the continental lithosphere in italy constraints imposed by gravity and deflection data a procedure is developed that systematically matches observations of plate flexure and gravity anomalies at simple convergent boundaries to the bestfitting two dimensional flexural geometry obtained by a thin elastic sheet of specified elastic strength the procedure allows analytical solution for three independent variables one of which is the initial preflexure elevation of the reference surface and two of which are related to the vertical forces and bending moments that act on the plate application to four profiles constructed across the apennine convergent system in italy shows that the observed depth of the basal pliocene surface which marks the base of the foredeep basin and the observed bouguer gravity anomalies can be fit by a two dimensional elastic plate model with an effective elastic thickness of about 20 km on three profiles and about 10 km on the fourth profile the initial elevation prior to flexure is determined to have been about 1000 m below sealevel on all profiles large bending moments and or vertical shear forces are required to act at the effective plate ends in order to maintain the observed deflection moments between 0 5 1017 and 3 1017 n and shear forces between 3 1012 and 3 1012 n m gravity modelling indicates that on the two northernmost profiles these terminal forces can not be supplied by a static mass at crustal depths and suggests that on all four profiles the terminal forces are supplied by anomalously dense material present at depths of 75 150 km beneath and west of the west coast of italy this dense material probably corresponds to westward subducted lithosphere of the adriatic plate extending from the northern apennines to the southern apennines and calabria this study suggests that in subduction systems like the apennines that are characterized by backarc extension forces acting on subducted lithosphere at depth can be expressed at the surface in the geometry of the foredeep basin that forms adjacent to the subduction zone

late miocene globorotaliid event stratigraphy and biogeography in the ne atlantic and mediterranean abstract the quantitative study of keeled and unkeeled globorotallids in several late miocene sites of the ne atlantic the north betic and south rifian gateways and the western mediterranean enabled us to recognize a sequence of planktic foraminiferal pf events useful for a high resolution correlation of the mediterranean with the global ocean for the time interval immediately prior to the salinity crisis pf event 1 6 7 ma end of subchron 7n1 is defined by the sudden disappearance or reduction ofgloborotalia menardii group i sinistral in most of the ne atlantic and mediterranean this is probably due to a southward migration of these forms to more subtropical latitudes pf event 2 6 56 ma subchron 6r2 is defined by the abundant occurrence ofgloborotalia menardii group ii dextral between pf events 1 and 2 the temperate waters of the ne atlantic ocean and mediterranean sea were almost completely lacking in keeled globorotaliids the most prominent change in the assemblages the replacement of thegloborotalia menardii group ii by thegloborotalia miotumida group pf event 3 6 4 ma subchron 6n may be correlated with the tortonian messinian boundary and is related to a widespread second southward migration of northern temperate faunas replacing the subtropical ones as the north atlantic latitudinal thermal gradients increased this event also coincides with the beginning of the mediterranean crisis recorded by isotope fluctuations in surface waters and immediately followed by the development of widespread anoxic conditions in the deep and intermediate waters during the deposition of the tripoli formation pf event 4 first abundant occurrence of dextral specimens in theneogloboquadrina acostaensis group approximately between 5 6 and 5 5 ma subchron 5r1 and pf event 5 reduction or disappearance of thegloborotalia miotumida group approximately between 5 6 and 5 5 ma subchron 5r1 pre date the lower evaporites and pf event 6 first abundant occurrence ofgloborotalia margaritae s s subchron 5n1 is contemporary to their deposition the coiling direction of unkeeled globorotaliids highly variable in the first part of the late tortonian became almost 100 dextral pf event a a short time after pf event 2 this group maintained this pattern until the late messinian with the exception of a short interval with a peak of sinistral forms just at the tortonian messinian boundary on comparing these events with the carbon isotope records of different sites of the atlantic ocean we recognized similar trends between the events maximum 13c values in planktic and benthic foraminifera coincide with pf event 1 they decrease between pf events 1 and 2 and increase again immediately after pf event 2 pf event 3 coincides with the beginning of the global late miocene carbon shift

multiple plume events in the genesis of the peri caribbean cretaceous oceanic plateau province the oceanic crust fragments exposed in central america in north western south america and in the caribbean islands have been considered to represent accreted remnants of the caribbean colombian oceanic plateau ccop on the basis of trace element and nd sr and pb isotopic compositions we infer that cumulate rocks basalts and diabases from coastal ecuador have a different source than the basalts from the dominican republic the latter suite includes the 86 ma basalts of the duarte complex which are light rare earth element ree enriched and display relative to normal mid ocean ridge basalts nmorb moderate enrichments in large ion lithophile elements together with high nb ta pb and low th contents moreover they exhibit a rather restricted range of nd and pb isotopic ratios consistent with their derivation from an ocean island type mantle source the composition of which includes the himu high 238u 204pb component characteristic of the galapagos hotspot in contrast the 123 ma ecuadorian oceanic rocks have flat ree patterns and relative to nmorb are depleted in zr hf th and u moreover they show a wide range of nd and pb isotopic ratios intermediate between those of ocean island basalts and nmorb it is unlikely on geochemical grounds that the plume source of the ecuadorian fragments was similar to that of the galapagos in addition because of the nne motion of the farallon plate during the early cretaceous the ecuadorian oceanic plateau fragments could not have been derived from the galapagos hotspot but were likely formed at a ridge centered or near ridge hotspot somewhere in the se pacific

caltepec fault zone an early permian dextral transpressional boundary between the proterozoic oaxacan and paleozoic acatlan complexes southern mexico and regional tectonic implications 1 the tectonic boundary between the grenville age oaxacan and paleozoic acatlan crystalline complexes in southern mexico named the caltepec fault zone cfz is characterized for the first time as a dextral transpressional nnw trending and ene dipping ductile fault zone of early permian age the complexes are welded by a syntectonic magmatic epidote bearing granite along the entire length of the cfz from east to west the 2 6 km wide cfz consists of disrupted and retrograded banded gneisses of the oaxacan complex quartz feldspar mylonite and the syntectonic magmatic epidote bearing cozahuico granite czg with huge xenoliths up to several kilometers long and up to 600 m wide of the proterozoic gneisses thrust westward over metasedimentary tectonites of the acatlan complex the czg shows magmatic fabrics that represent a transition to solid state deformation characterized by subvertical foliation subhorizontal nne and sse dipping mineral stretching lineation and dextral kinematics the megaxenoliths underwent partial melting developing banded migmatites with layers of epidote bearing granitic neosome the parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation migmatization of xenolithic gneisses and granite emplacement along the cfz were coeval the neosome yielded a u pb zircon concordant age of 275 6 1 ma probably dating the peak of the tectonothermal event we interpret the cfz as a major terrane boundary accommodating transpressional interaction between the acatlan and oaxaquia blocks which were amalgamated in an oblique convergent setting by early permian time as the leading edge of gondwana impinged onto the southern margin of laurentia along the marathon ouachita suture to form pangea

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

oxygen and hydrogen isotope geochemistry of s and i type granitoids the cape granite suite south africa abstract the late precambrian cape granite suite of the southwestern cape south africa can be subdivided into six major batholiths saldanha vredenburg darling malmesbury paarl wellington stellenbosch kuils river and peninsula individual plutons have been classified as a i and s type granites on the basis of petrography and chemical composition quartz and feldspar from the cape granites have 18o values which range from 8 6 to 13 4 and 5 0 to 13 6 respectively the 18o value of quartz is a very effective discriminator between s and i type granites and provides the best means to estimate the 18o value of the original magma the s type granites crystallized from magmas having 18o values between 9 5 and 11 4 mean 10 6 the a and i type granites crystallized from magmas having 18o values between 6 6 and 9 9 mean 7 8 there is almost no overlap of 18o values between the s and the i type granites with the boundary at 9 5 of the 31 quartz feldspar pairs analysed the vast majority have values of qtz fsp between 0 and 4 however only eight samples have values of qtz fsp between 0 7 and 1 55 which would be consistent with equilibrium under closed system conditions the remaining samples were affected by minor fluid rock interaction biotite d values range from 113 to 47 and show no correlation with estimated magma 18o value biotite water content or whole rock loss on ignition the variation in biotite d is probably caused by a combination of varying degree of magma degassing and minor low temperature alteration

the influence of the evolution of the pyrenees on adjacent basins abstract important thicknesses of syn and post orogenic sediments are observed in the southern part of the aquitaine basin and in the north of the ebro basin their deposition cannot be explained only as a result of the extensional history of the early cretaceous thus the influence of the compression associated with the pyrenees formation upon the subsidence of these basins is studied europe and iberia are considered to behave as two semi infinite elastic plates the evolution through time of the sedimentary thicknesses is explained by the deflections of these two plates induced not only by the topographic load of the pyrenees but also by additional forces 10 12 n m that may have their origin in the formation of a cold lithospheric root during the shortening a series of calculations is presented for an approximately north south profile across the aquitaine basin the pyrenees and the ebro basin a good fit for the shape of the basins and the gravity anomalies was obtained with flexural rigidities of 1 5 10 23 nm for the northern plate and the outer part of the southern plate and 1 4 10 21 nm for the inner part of the southern plate

along strike variations in the thermal and tectonic response of the continental ecuadorian andes to the collision with heterogeneous oceanic crust oblique to strike geological segmentation in the andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates as well as the upperplate expressions of differing along strike subducted slab age strength and composition we present new white mica and biotite 40 ar 39 ar and zircon and apatite fission track data from several traverses across the cordillera real of ecuador in the northern andes that reveal distinct along strike differences in the timing of accelerated crustal cooling during the cenozoic the data record elevated cooling rates from temperatures of v380 c during v65 55 and v43 30 ma from all sampled regions of the cordillera real and at v15 ma and since v9 ma in the northern cordillera real each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust elevated cooling rates of up to v30 20 c myr were initiated during the palaeocene and eocene early oligocene along the entire contemporaneous margin of ecuador and were driven by the accretion of the oceanic pallatanga terrane and pin on macuchi block respectively onto northwestern south america both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the caribbean plateau and accreted onto the margin during the approximately northeastward migration of the plateau into its current position within ecuador the development of higher topography and elevated cooling rates of up to 50 c myr at v15 ma and since v9 ma are restricted to the region north of 1 30ps and is situated above the postulated subducted flatslab section of the aseismic carnegie ridge plate convergence rate calculations suggest the carnegie ridge collided with the ecuador trench at v15 ma which caused the pre existing coastal provinces to displace to the northeast subsequently driving extension and marine ingressions in southern ecuador and compression and uplift in northern ecuador fl 2001 elsevier science b v all rights reserved

seismic refraction constraints on the geometry of the ronda peridotitic massif betic cordillera spain abstract large ultramafic masses known as the ronda peridotites outcrop in the westernmost part of the internal zone of the betic cordillera in southern spain the bouguer anomaly gravity map shows a pronounced positive anomaly up to 110 mgal although shifted eastwards with respect to the outcropping peridotites three seismic refraction profiles one of them partly reversed crossing the peridotites and an area displaying a gravity high were surveyed in 1977 and 1981 the length of the profiles ranges from 50 to 70 km the normal crustal structure of the internal betics was derived from another 150 km long almeria malaga profile along the south coast of spain where an average crustal thickness of 25 km has been found a p wave interpretation of the seismic data indicates pronounced lateral variations in the upper crust based on first arrivals and one later seismic phase the delay time of pg arrivals corroborates the large offshore cenozoic sediment thickness 1 3 km close to the coast ray tracing modelling of high apparent velocity first arrivals and secondary seismic phases interpreted as reflections shows the presence of a high velocity 7 0 7 2 km s body beneath the gravity high its top is well defined whereas the bottom remains loosely constrained it is suggested that the peridotites outcropping further north are slices of this ultramafic body the moho has been located at 15 20 km in the malaga estepona area the precise depth depending on the alternatives chosen when modelling the seismic data

outcrop and core integrative ichnofabric analysis of miocene sediments from lepe huelva sw spain improving depositional and paleoenvironmental interpretations abstract ichnofabric analysis was conducted in miocene sediments from lepe huelva sw spain based on integrative outcrop and core research to improve interpretations of depositional and paleoenvironmental conditions with special attention to sequence stratigraphy seven intervals were differentiated in outcrops based on stratigraphic and ichnological features consisting of two ichnofabrics ophiomorpha thalassinoides spongeliomorpha ichnofabric characterizes intervals 1 2 6 7 and 8 while palaeophycus planolites phycosiphon ichnofabric characterizes intervals 3 4 and 5 fourteen ichnofabrics were differentiated in the core mainly in view of lithological features including ferruginous material grain size mottled background ichnotaxa and bioturbation index a comparison between outcrop and core ichnofabrics through the upper 13 5 m corresponding to the uppermost tortonian lowermost messinian interval revealed certain similarities as well as some differences a continuous and relatively slow siliciclastic deposition with punctual variations in the sedimentation rate can be interpreted that associated with favorable paleoenvironmental parameters such as aerobic conditions and nutrient availability evidence that a well developed and diverse macroinvertebrate trace maker community existed at that time softgrounds are dominant but occasionally loosegrounds and even firmgrounds could develop the ichnofabric distribution shows long range patterns in outcrop and core and short range patterns exclusively in core long range patterns reflect the last phases of a transgressive system tract with a maximum flooding zone at the end and then a highstand normal regression high frequency short range repetitive patterns in ichnofabrics from core mainly between ichnofabrics 6 8 to 9 from lower to upper part of the pattern can be linked to local flooding surfaces subdividing the maximum flooding zone into parasequences our results reveals the usefulness of the integrative ichnofabric analysis including outcrop and core materials in sedimentary basin analysis assessing paleoenvironmental conditions and improving sequence stratigraphy characterization

the high grade las cruces copper deposit spain a product of secondary enrichment in an evolving basin the las cruces deposit iberian pyrite belt includes a large high grade cementation zone capped by unusual rocks that contain carbonates galena iron sulphides and quartz between the late cretaceous and tortonian the volcanogenic massive sulphides were exhumed and affected by subaerial oxidation that formed paired cementation and gossan zones onset of alpine extension produced accelerated growth of the cementation zone along extensional faults leading to formation of the high grade copper ore at ca 11 ma later replacement of the overlying gossan by sulphide and carbonate rich rocks beneath sealing marl sediments is thought to have involved microbial processes occurring between the messinian ca 7 2 ma and today isotope data show that the cementation zone formed by the mixing of descending acidic waters derived from oxidation of the massive sulphides with upwelling geothermal waters flowing at temperatures above 100 c the c o and sr isotope values of the mineralization 87sr 86sr 0 7101 0 7104 and of the local groundwater 0 7102 0 7104 reflect equilibration with basement rocks and indicate that influence on the ore forming process by marl equilibrated water 0 7091 0 7093 or miocene seawater 0 7086 0 7092 was negligible the high sulphur isotope values of the sulphides in the biogenic zone most 19 to 24 are well above those of the primary sulphides 34s ca 6 8 to 10 3 and likely reflect formation of the biogenic sulphides by reduction of aqueous sulphate in the groundwaters sulphur isotope values of the cementation zone 34s ca 2 4 to 21 7 are also consistent with some contribution of sulphur from the biogenic reduction of aqueous sulphate