tracking ancient magmatism and cenozoic topographic growth within the northern andes forearc constraints from detrital u pb zircon ages a comparison of detrital zircon ages from the cenozoic forearc basins in sw ecuador nw peru and those derived from the amotapes massif offers a singular opportunity to shed some light on the evolution of the nw andean margin of south america an extensive set of pre andean u pb zircon ages older than late cretaceous was obtained from 18 detrital samples coming from the talara zorritos progreso and gulf of guayaquil tumbes forearc basins as well as from two metasedimentary rocks of the amotapes massif u pb age analysis on detrital zircon grains allows a general source to sink scenario to be determined for this section of the andes these analyses give consistent insights into the early topographic growth during the modern andean orogeny the occurrence of proto andean magmatic periods in the western margin of gondwana and the evolution of the amotapes massif within the geodynamical setting of northwestern south america zircon ages at ca 320 ma and neoproterozoic clusters for the amotapes metasediments show significant similarities along the strike of the metamorphic belt thus defining a common metasedimentary origin and can be compared with previously dated rocks from the eastern cordillera of peru these strong similarities demonstrate the presence of a wide polyphased metamorphic belt that includes the western parts of the eastern cordillera the northern section of the occidental cordillera of peru and the amotapes massif detrital sediments of the forearc are marked by a major gap in early cretaceous jurassic ages and the minor presence of ordovician famatinian orogeny and carboniferous permian 340 285 ma ages the age spectrum is not compatible with sediment input from the eastern andes and suggests that the external areas of the andean chain e g the western cordillera were already uplifted prior to or immediately after forearc initiation during the paleocene thus representing a pervasive drainage divide throughout all of the cenozoic the amotapes massif or other terrains sharing the same age affinities e g the olmos terrane occidental cordillera of peru are likely to represent the best potential sources for pre andean zircon grains in the forearc age clusters in both source and sink at ca 600 ma 570 560 ma and 520 ma most likely define magmatic periods and may represent first order constraints for the difficult correlation between proto andean magmatic and orogenic periods to the north and south of the huancabamba deflection it is unlikely that the neoproterozoic zircon grains came from the pan african braziliano belts thus suggesting the onset of significant magmatic activity in this section of the andes at around 570 560 ma and probably ca 600 ma the 520 ma cluster is compatible with the most prominent magmatic period in the southern andes related to the end of the pampean orogeny thus suggesting the existence of a regional proto andean magmatic arc during early cambrian times

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

stratigraphic responses to a major tectonic event in a foreland basin the ecuadorian oriente basin from eocene to oligocene times the eocene to oligocene sediments of the ecuadorian oriente basin record two kinds of second order stratigraphic response to the tectonic evolution lower eocene shows evidences of local scale syntectonic deposits this tectonic activity can be related to right lateral convergent movements inverting pre cretaceous extensional structures upper eocene and oligocene sediments are integrated as the expression of an isostatic rebound characterizing a basin scale syntectonic deposition this response is evidenced by a reciprocal architecture of the depositional sequences identified in the sedimentary formations these data have allowed us to propose a new geodynamic model for the paleogene evolution of the oriente basin

three dimensional characteristics of stratospheric mountain waves during t rex abstract measurements from the national science foundation national center for atmospheric research nsf ncar gulfstream v g v obtained during the recent terrain induced rotor experiment t rex indicate marked differences in the character of the wave response between repeated flight tracks across the sierra nevada which were separated by a distance of approximately 50 km observations from several of the g v research flights indicate that the vertical velocities in the primary wave exhibited variations up to a factor of 2 between the southern and northern portions of the racetrack flight segments in the lower stratosphere with the largest amplitude waves most often occurring over the southern flight leg which has a terrain maximum that is 800 m lower than the northern leg multiple racetracks at 11 7 and 13 1 km altitudes indicate that these differences were repeatable which is suggestive that the deviations were likely due to vertically propagating mountain waves that varied systematically in am

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

simple bouguer gravity anomaly field and the inferred crustal structure of continental ecuador the simple bouguer gravity anomaly field of continental ecuador corresponds closely to the physiographic provinces of the country the sierra which includes the andes and their foothills is characterized by a pronounced low with values to 292 mgal which reflects the deep andean root bouguer anomalies over the oriente become less negative away from the sierra chiefly in response to progressive thinning of continental crust eastward the costa between the sierra and the pacific shore in the north has the most positive on land bouguer anomalies 162 mgal so far known in the western hemisphere this part of the costa is underlain by an ancient oceanic plate now welded to the northwestern corner of the otherwise continental south american plate

an autochthonous geological model for the eastern andes of ecuador abstract we describe a traverse across the cordillera real and sub andean zone of ecuador poorly known areas with very little detailed mapping and very little age control the spine of the cordillera comprises deeply eroded triassic and jurassic plutons the roots of a major arc emplaced into probable palaeozoic pelites and metamorphosed volcanic rocks the w flank comprises a jurassic submarine basaltic andesitic volcanic sequence which grades up into mixed jurassic cretaceous volcanic and sedimentary rocks of the inter andean valley the sub andean zone on the e flank of the cordillera comprises a newly recognized cretaceous basin of cleaved mudrocks quartz arenites and limestones east of the syndepositional cosanga fault the cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon bearing oriente basin the principal penetrative deformation of the cordillera real was probably latest cretaceous palaeocene it telescoped the magmatic belts but shortening was largely partitioned into the pelites between plutons the plutons suffered inhomogenous deformation some portions completely escaped tectonism the pelites conserve two foliations the earliest comprises slaty cleavage formed under low or sub greenschist conditions the later is a strong schistosity defined by new mica growth it largely transposed and obliterated the first both foliations may have developed during a single progressive deformation we find inappropriate recent terrane models for the cordillera real and sub andean zone of ecuador instead we find remarkable similarities from one side of the cordillera to the other including a common structural history in place of sutures we find mostly intrusive contacts between major plutons and pelites triassic to cretaceous events occurred on the autochthonous western edge of the archaean guyana shield the latest cretaceous paleocene deformation is interpreted as the progressive collision of an oceanic terrane s with the south american continent young fault movements have subsequently juxtaposed different structural levels through the cordillera real orogen

the mera lahar deposit in the upper amazon basin transformation of a late pleistocene collapse at huisla volcano central ecuador abstract the sub andean zone east of the cordillera real ecuador and out to the amazon basin s western margin has been the depository of voluminous lahars related to volcanic activity in the andean highlands these lahars arrived to the sub andean zone via gravitational transport through narrow river canyons and emplaced volumes surpassing several cubic kilometers over robust inundation zones this paper discusses the origin flow route depositional zone terrace formation and geomorphic significance of the most important lahar deposit yet mapped in this region that of the mera lahar which likely formed from a late pleistocene collapse of huisla volcano followed by impounding by temporary dams behind drainage blocking debris avalanche deposits dad then subsequent rupture of the blockage the actual deposit of the mera lahar has a thickness between 30 and 70 m is mainly comprised of dad breccia is matrix supported c 70 of reddish gray color and is well consolidated based on geochemical and petrographic similarities huisla volcano s dad is the best candidate for the lahar s source huisla volcano is located some 90 km up valley of the bulk of mera lahar s mapped deposit clasts of mera lahar rocks and huisla s dad breccias have 57 61 wt sio2 corresponding to andesites of the calc alkaline series with mean values of 1 1 5 wt for k2o the mapped mera lahar deposit has an actual volume estimated in 1 2 km3 compared to its original estimated volume of 5 4 km3 cross sectional widths of 1 5 to 4 5 km span and extend laterally beyond the actual pastaza river channel where the lahar s deposition produced high standing isolated surfaces that are notable local geomorphic features of the upper sub andean zone the flow modeling program laharflow employing the modern landscape as the topographic base adequately simulates the flow route and inundation zones of the mera lahar

high resolution uk 37 temperature reconstructions in the south china sea over the past 220 kyr past sea surface temperatures sst in the northern and southern areas of the south china sea have been reconstructed for the past 220 kyr using the uk37 alkenone index the sst profiles follow the glacial interglacial pattern exhibiting differences between last glacial maximum and holocene that are 1 3 c larger than those observed at the same latitudes in the atlantic and pacific oceans in termination i both planktonic foraminiferal 18o and sst exhibit well defined bolling allerod and younger dryas events with temperature differences between both periods of 0 8 and 0 4 c in north and south respectively ssts record a constant north south difference of 1 c in the interglacials and nearly 2 5 c in the glacial stages these differences define two distinct climatic and water circulation patterns that correspond with glacial interglacial sea level oscillations which opened and closed water exchange with the tropical indo pacific ocean through the present sunda shelf

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

distinguishing between in situ and accretionary growth of continents along active margins abstract active continental margins represent both sites of crustal loss by tectonic erosion and gain by juvenile magmatism and oceanic arc plateau accretion estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust or juvenile magmatic addition during extension distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks we present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest south america during 190 113 ma semi continuous slab rollback resulted in crustal thinning juvenile magmatism and ultimately the removal of continental slivers negating previous hypotheses that the margin evolved by the accretion of an island arc however basic crustal growth estimates along with comparisons between rocks exposed in northern peru and the northern andes imply that uneven losses of continental crust occurred along northwest south america for mesozoic exposures and that the accretion of the caribbean large igneous province clip onto nw south america probably played a crucial role in maintaining and preserving mesozoic continental crust north of the huancabamba deflection

geology mineralization and geochronological constraints of the mirador cu au porphyry district southeast ecuador the mirador porphyry cu au district is located in the southernmost sector of the northern andean cordillera in zamora chinchipe province southeastern ecuador the district contains two significant porphyry cu au ag mo deposits mirador and mirador norte and an interconnected series of narrow mineralized structures known collectively as chancho the principal mineralization in the porphyries is disseminated to blebby chalcopyrite developed primarily in potassic alteration with overlying chalcocite supergene enrichment zones prior to radiometric dating presented in this study these deposits were considered late jurassic based on close similarity and therefore assumed age equivalence with the well dated panantza san carlos and sutzu porphyry copper deposits located 40 km to the north new u pb zircon ages confirm a middle jurassic age for granodiorite of the zamora batholith at this location 163 8 1 9 ma and late jurassic ages for subvolcanic intrusions 156 2 1 0 and 153 1 1 3 ma re os molybdenite ages average 156 1 0 ma and indicate coeval mineralization between mirador and mirador norte mineralization and related sub volcanic igneous activity are closely associated at mirador and represent the same temporal event recorded at panantza san carlos and sutzu as well as coeval porphyry skarn and epithermal au mineralization extending at least 80 km south defining a north south cu au metallogenic belt spanning over 120 km

transected folds from the western part of the bala lineament wales abstract fold cleavage relationships are described in the cadair idris area of the lower palaeozoic welsh basin which includes the western part of the bala lineament many fold hinges are transected by the broadly contemporaneous caledonian lower devonian cleavage transection angles are amongst the highest yet described from the welsh basin there is a close relationship between fold strike and sense and amount of transection n s folds are clockwise transected ne folds have an axial planar cleavage whilst ene folds are anticlockwise transected similar relationships occur throughout the central welsh basin a model of progressive deformation is described whereby weak regional transpression is modified by local factors viscosity contrasts are recognized as one of the most important controls on the timing of fold development and the amount of transection

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

allochthonous terranes in the andes of ecuador and northwestern peru in ecuador and northwestern peru the andes and adjacent country particularly on the pacific side are composed of at least five distinctive geologic terranes the terranes are distinguished from one another and from cratonic south america to the east by dissimilar basements cover rocks intrusive rocks and bouguer gravity anomaly fields the pinon terrane occupying most of coastal ecuador has a basaltic basement characterized by the largest known on land positive bouguer anomalies in the western hemisphere the tahuin terrane occupies most of northwestern peru and the southwestern corner of ecuador the terrane has an especially complex basement and is the site of generally positive bouguer anomalies the small biron terrane has an unusual basement composed in part of cordierite gneiss and amphibolite that give consistent late cretaceous k ar mineral ages the wedge shaped chaucha terrane lies in part on the western andean slope between the oceanic pinon terrane on the north and the continental bir

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

magmatic dominated fluid evolution in the jurassic nambija gold skarn deposits southeastern ecuador the jurassic approximately 145 ma nambija oxidized gold skarns are hosted by the triassic volcanosedimentary piuntza unit in the sub andean zone of southeastern ecuador the skarns consist dominantly of granditic garnet ad20 98 with subordinate pyroxene di46 92hd17 42jo0 19 and epidote and are spatially associated with porphyritic quartz diorite to granodiorite intrusions endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone exoskarn has an outer k and na enriched zone in the volcanosedimentary unit gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide poor vugs and milky quartz veins and veinlets in association with hematite fluid inclusion data for the main part of the prograde stage indicate the coexistence of high temperature 500 c to 600 c high salinity up to 65 wt eq nacl and moderate to low salinity aqueous carbonic fluids interpreted to have been trapped at pressures around 100 120 mpa corresponding to about 4 km depth lower temperature 510 300 c and moderate to low salinity 23 2 wt eq nacl aqueous fluids are recorded in garnet and epidote of the end of the prograde stage the microthermometric data th from 513 c to 318 c and salinity from 1 0 to 23 wt eq nacl and 18o values between 6 2 and 11 5 for gold bearing milky quartz from the retrograde stage suggest that the ore forming fluid was dominantly magmatic pressures during the early retrograde stage were in the range of 50 100 mpa in line with the evidence for co2 effervescence and probable local boiling the dominance of magmatic low saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system which could have delayed the ingression of external fluids until relatively low temperatures were reached the resulting low water to rock ratios explain the weak retrograde alteration and the compositional variability of chlorite essentially controlled by host rock compositions gold was precipitated at this stage as a result of cooling and ph increase related to co2 effervescence which both result in destabilization of gold bearing chloride complexes significant ingression of external fluids took place after gold deposition only as recorded by 18o values of 0 4 to 6 2 for fluids depositing quartz below 350 c in sulfide rich barren veins low temperature 300 c meteoric fluids 18owater between 10 0 and 2 0 are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities about 100 c and 25 wt eq nacl the latter are similar to low temperature fluids 202 74 5 c with 18o values of 0 5 to 3 1 and salinities in the range of 21 1 to 17 3 wt eq cacl2 trapped in calcite of late veins and interpreted as basinal brines nambija represents a deep equivalent of the oxidized gold skarn class the presence of co2 in the fluids being partly a consequence of the relatively deep setting at about 4 km depth as in other au bearing skarn deposits not only the prograde stage but also the gold precipitating retrograde stage is dominated by fluids of magmatic origin

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

seismological evidence on the geometry of the orogenic system in central northern ecuador south america analysis of the spatial distribution of seismicity beneath central ecuador from a temporary network gives new insights on two main points of the ecuadorian tectonics major structures in the ecuadorian andes are east dipping late jurassic to paleogene sutures reactivated by present day compression the oceanic plate is plunging continuously down to a depth of about 200 km with a dip of 25 35 we also show that the coastal plain acts as a buttress transmitting stresses to the andes beneath which deformation is concentrated

geodynamic evolution of the northern and central andes during early to middle mesozoic times a tethyan model the early to middle mesozoic sedimentary and magmatic history of the northern and central andes i e the nne trending colombian ecuadorian segment and the nw trending peruvian segment exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings the late triassic to late liassic period began with a widespread marine transgression from the latest triassic the colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in peru during this time no significant magmatic activity is recorded except in the emerging colombian area this period is interpreted as the result of rifting of a tethyan oceanic arm which separated the colombian and palaeo mexican margins from latest liassic an important calc alkaline magmatic arc developed along the emergent colombian segment further south the north peruvian shelf probably emerged but marine sedimentation continued in southern peru during middle and early late jurassic times the colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation along the peruvian segment a turbiditic trough emergent areas and continental basins were created and the scarcity of calc alkaline magmatism suggests that only very local subduction took place this period is regarded as one of the southeastward subduction beneath the colombian segment and of tethyan oceanic crust originating in the newly formed colombian oceanic arm this pattern would have induced a chiefly left lateral transform motion along the peruvian segment by kimmeridgian tithonian times the palaeogeographic framework had drastically changed along the colombian segment magmatic activity ceased and continental accretions occurred along dextral strike slip sutures in peru tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation in the coastal area arc related volcanism indicated that subduction took place beneath this segment this geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the tethyan ridges and replacement by pacific spreading centres inducing a roughly northeastward convergence direction

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

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

geomorphic evidence of active deformation and uplift in a modern continental wedge top foredeep transition example of the eastern ecuadorian andes abstract the eastern ecuadorian andes appear as a fold and thrust belt adjacent to a continental foredeep represented by one of the world s largest tropical alluvial megafans the pastaza megafan debouching into the amazonian lowland the apex of the pliocene pleistocene megafan situated in the present day wedge top subandean zone has been cut by an erosion surface the western part of which has been uplifted of 500 m along the frontal thrust forming a poorly dissected plateau the mera plateau this erosion surface erased most of the previous fluvial landscape but preserved a large thrust related anticlinal hinge deforming less erodible underlying strata the mirador fold and smaller sized anticlines this surface has been then incised by two antecedent major rivers the pastaza and the napo and few tributaries the plateau edge is marked by a series of large scale gently sloping landslides clustered along a 70 km long concave eastward line associated with the frontal thrust fault the newly formed immature rivers issued from the landslides or sourced within east dipping remnants of the erosion surface downstream of the landslide line constitute the greatest part of the streams feeding the ecuadorian amazonian basin at 70 to 100 km from the landslide line the drainage abruptly changes from highly immature to mature with a well defined hinge line representing the outer limit of landslide and tectonic control the diversions of the pastaza river indicate ongoing fold growth since at least the late pleistocene in the eastern cordillera and the early holocene in the mera plateau the preserved terraces of the pastaza valley are all degradational and are ascribed to periods of tectonic seismic activity alternating with periods of tectonic quiescence or decreased seismic activity rather than to climatic events 14c dating of the plateau erosion surface and of the upper pastaza terraces indicates that the minimum average incision rate since 18 000 years bp varies locally in the upper pastaza valley from 0 5 to 0 67 cm year 1 increasing from 18 000 years bp to now a comparison of these incision rates with fold and thrust fault uplift rates indicates that incision in the upper pastaza valley was a result of rapid uplift up to 1 cm year 1 along the mirador fold and thrust which caused a restoration of the local equilibrium profile of the upper reach combined with smaller local fault uplift along the westernmost thrust faults the uplift of the whole mera plateau with respect to the upper amazonian basin gives a minimum average uplift rate of 2 8 cm year 1 since 18 000 years bp the overall uplift of the mera plateau and the eastern cordillera is likely to have been caused by a regional scale low angle thrust ramp emerging as the frontal thrust fault

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

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

the metamorphic basement of ecuador the metamorphic rocks of ecuador once thought to constitute a uniform and ancient basement belong to several distinct units of widely differing ages compositions and metamorphic histories the largest area of metamorphic rocks constitutes an unbroken belt on the eastern andean slope from the colombian to the peruvian border the bulk of the rocks in this belt are of cretaceous age and were metamorphosed in the greenschist facies under a barrovian facies series rocks in the extreme north and those south of lat 3 15 s are different and may be of paleozoic age a smaller terrain on the western andean slope in southwestern ecuador is yet more varied it includes four metamorphic terrains the polymetamorphic piedras group of precambrian age the low pressure tahuin group of paleozoic age the high pressure raspas formation of cretaceous age and a low pressure terrain of uncertain age north of the la palma fault several isolated outcrops of metamorphic rocks in ecuador are of uncertain significance

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

geochronological control for the main tectonic magmatic events of ecuador abstract a chronological outline of the principal tectonic magmatic events of ecuador is presented based upon approximately 100 k ar and rb sr age determinations eight magmatic episodes are recognized in southwestern ecuador evidence of plutonic and metamorphic activity in the late precambrian and in the middle to late triassic is found large granitic batholiths of middle to late jurassic age associated with widespread acidic volcanism are traceable along the eastern flank of the andes into colombia early cretaceous volcanism and plutonism is recognized only in southern ecuador although apparently equivalent units continue southwards into the coastal cordillera of peru a great orogenic belt characterized by metamorphism plutonism and volcanism was established in the late cretaceous and continued through the paleogene it is traceable into peru and colombia the macuchi fm of the western cordillera along with the pin on fm and associated sediments of the coastal zone represent the remnants of an island arc and the adjacent oceanic floor respectively that collided with the south american plate during the eocene feininger 1981 in the neogene quartz dioritic intrusions were emplaced in the western cordillera as well as in the cordillera de muelleturo although apparently at different times in the late oligocene and miocene acidic volcanism is noted in the southern ecuadorian andes but not in the north in the quaternary andesitic volcanism and large stratovolcanoes characterize the northern andes while widespread rhyolitic ashflow activity is noted in the southern andes in ecuador the post triassic magmatic episodes show a westerly migration similar to that found in colombia but opposite to that noted in peru

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

geochemical evolution of the quaternary chachimbiro volcanic complex frontal volcanic arc of ecuador abstract the chachimbiro volcanic complex cvc is a composite volcano located in the frontal part of the ecuadorian quaternary arc four periods of activity can be differentiated in chachimbiro 400 ka long volcanic history named ch1 405 7 20 0 298 6 32 9 ka ch2 121 75 23 2 36 08 2 8 ka ch3 36080 280 22730 120 ybp and ch4 5 760 30 ybp respectively the magmatic suite ranges in composition from andesites to rhyodacites and displays systematic increases of sio2 sr y la yb incompatible elements e g lree mree lile and depletion of hree with time these geochemical features coupled with radiogenic isotope data and textural observations e g several resorption and overgrowth cycles in plagioclase phenocrysts suggest an open system evolution for cvc including typical crustal magmatic processes such as recharge assimilation and fractional crystallisation to better constrain the magmatic crustal processes of the cvc we use a multi model approach including three open system models rfc recharge fractional crystallisation afc assimilation fractional crystallisation and ec rafc energy constrained recharge assimilation fractional crystallisation different simulations have been developed to best fit cvc s geochemistry including trace elements sr y ratios and sr isotopes which suggest predominant fractional crystallisation and variable assimilation as the main processes explaining the bulk of the geochemical data of cvc this is consistent with an overall thermal decline of the magmatic system through time the genetic model here presented involves the input of a magmatic recharge component matured in the low mid crust through dominant fractionation of amphibole garnet into intermediate crustal reservoir s where magma undergoes extensive fc

partitioning of oblique convergence in the northern andes subduction zone migration history and the present day boundary of the north andean sliver in ecuador along the ecuadorian margin oblique subduction induces deformation of the overriding continental plate for the last 15 ma both exhumation and tectonic history of ecuador suggest that the northeastward motion of the north andean sliver nas was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends here we present geologic data earthquake epicenters focal mechanisms gps results and a revised active fault map consistent with this new kinematic model all data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system connecting fault segments from the gulf of guayaquil to the eastern andean cordillera although secondary faults are recognized within the cordillera they accommodate a negligible fraction of relative motion compared to the main fault system the eastern limit is then concentrated rather than distributed as first proposed marking a sharp boundary between the nas the inca sliver and the subandean domain overthrusting the south american craton the nas limit follows a northeast striking right lateral transpressional strike slip system from the gulf of guayaquil isla puna to the andean cordillera and with the north south striking transpressive faults along the eastern andes eastward migration of the restraining belt since the pliocene abandonment of the sutures and reactivation of north south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the nas favoring its extrusion as a continental sliver accommodating the oblique convergence of the nazca oceanic plate toward south america

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

late glacial and holocene avulsions of the rio pastaza megafan ecuador peru frequency and controlling factors the geomorphological study by means of remote sensing imagery of the rio pastaza megafan ecuador and northern peru reveals the traces of numerous avulsions one hundred and eight avulsion sites have been defined the location of these sites the available radiocarbon ages as well as historical maps of the seventeenth century enable us to propose an evolution history of the migration and avulsions of the rio pastaza since the last glacial maximum the first avulsions of the rio pastaza occurred after the lgm in a zone close to and roughly parallel to the sudandean front where the developed avulsion gave a distributive pattern to the ancient stream of the rio pastaza in an area located between the modern rio morona and pastaza where they caused the rio pastaza to develop a fan like distributary pattern this is interpreted as a response to thrust related forelimb tilt progressively shifting eastward the rio pastaza and the apex of the megafan this sequence of events ended with the great diversion of the rio pastaza towards the modern rios corrientes and tigre avulsions occurred in the tigre corrientes area between 9200 and 8 500 years cal bp afterwards the rio pastaza was diverted to its present day north south course this last significant avulsion occurred before ad 1691 in the area located between the modern rio morona and pastaza avulsion frequency probably overestimated ranges between 100 and 200 years in the rios tigre and corrientes area avulsion frequency probably underestimated ranges from 300 to 400 years regional tectonics is likely to have triggered most of the avulsions in the morona pastaza area but its influence is restricted to this area the factors controlling the avulsions in the tigre corrientes area are less clear because the frequently described hydrologic driven avulsion as observed in areas characterized by contrasted hydrologic cycles are inconsistent with the characteristics of the hydrologic cycles of the rio pastaza