incremental 40ar 39ar thermochronology of mylonitic rocks from the northern snake range nevada a 40ar 39ar thermochronologic study of muscovite biotite and potassium feldspar from footwall rocks of the northern snake range decollement nevada was undertaken in order to constrain the timing and temperatures of mylonitic deformation during extension and the subsequent cooling uplift history of these mylonitic rocks mylonitic deformation has been bracketed between 37 ma and about 24 ma and minimum temperatures during ductile thinning were about 300 c mineral cooling ages along the east flank of the range suggest a minimum tmax gradient of about 55 c ma this steep thermal gradient is the result of a localized heat source and or shearing of colder rocks with older cooling histories over hotter rocks with younger cooling histories although mylonitic deformation may have spanned a 13 m y period 37 ma to 24 ma deformation was not synchronous throughout the range on the west flank of the range age spectra from muscovite and microcline samples indicate that rocks were at temperatures well below 325 c and deformation had ceased by 30 ma while on the east flank of the range lower plate rocks were at temperatures above 325 c and ductile deformation was still ongoing this is also reflected in chrontours of published k ar muscovite ages along with 40ar 39ar ages which monotonically decrease in age from west to east these data provide evidence for a temperature difference across the range within the same structural horizon from about 30 to 24 ma these relations suggest a lateral thermal gradient the east flank of the range cooling more slowly than the west flank alternatively the data suggest either an eastward dip of lower plate units prior to the onset of tertiary mylonitic deformation or that layering dipped eastward due to deformation in an east dipping top to the east shear zone both cases result in differential cooling of lower plate units as they pass through a series of subhorizontal isotherms the 40ar 39ar data along with published finite strain structural and quartz petrofabric data provide constraints on the formation and geometric evolution of metamorphic core complex detachment faults

the summit erosion surfaces of the inner betic cordillera their value as tools for reconstructing the chronology of topographic growth in southern spain erosion surfaces at the top of mountain ranges have suggested to geomorphologists that the earth s crust can undergo rapid pulses of uplift during which rates of denudation fail to balance with rates of crustal uplift thereby preserving much of the initial plateau topography as it rises to greater elevations here we analyze a regionally extensive population of mappable planar surfaces which crown several major tectonically active sierras of the inner south eastern betic cordillera se spain based on a synthesis of long term landscape evolution involving landform reconnaissance dem based morphometric analysis structural geology sediment provenance analysis and field stratigraphy we conclude that the summit surfaces are a mosaic of discrete post orogenic erosion surfaces that each developed in relation to mediterranean marine base levels at a time when the sierras formed a series of low elevation islands they are not therefore fragments of a once regionally continuous paleoplain the summit surfaces were partly the result of shallow angle detachment faulting during the middle miocene in response to lithospheric slab detachment where evidence exists that the resulting low relief topography was reworked by marine abrasion during the tortonian sea level rise the terminal age of the surfaces is latest miocene age bracketing based on stratigraphic correlations among the many depocentres of the region and on published thermochronological data indicates that some erosion surfaces locally formed in less than 3 million years 8 ma 5 7 ma they were subsequently uplifted to their current elevations 1 7 to 2 km by pliocene to quaternary neotectonics in at least two major pulses

thermotectonic history of the chiplakot crystalline belt in the lesser himalaya kumaon india constraints from apatite fission track thermochronology abstract fission track ages and confined track length distribution of apatite samples separated from the chiplakot crystalline belt ccb of the lesser himalayan crystalline lhc zone located to the south of the main central thrust mct munsiari thrust mt in kumaon india have been determined ages from the ccb along the kali and darma valleys fall in two distinct groups in the northern part of the ccb the ages range from 9 8 0 6 to 7 6 0 6 ma with a weighted mean of 9 6 0 1 ma while in the southern part the ages vary from 17 9 0 9 to 12 9 1 1 ma with a weighted mean of 14 1 0 1 ma the bimodal distribution of track lengths indicates that the ages are mixed ages rather than simple cooling ages the apatite fission track aft ages and already published structural data of the ccb suggest a complex erosional denudation history within the upper 3 4 km of the crust of the ccb the ages further indicate that the ccb was thrust into place earlier than the middle miocene i e at the time of development of the mct since then these rocks have remained within the upper 3 km of crust and were affected by only moderate to slow erosion and exhumation these results have important implications for the tectonic evolution of the lhc zone to the south of the mct mt the exhumation of the lhc zone in different parts of the himalaya was not uniform in the kumaon himalaya it was not controlled as in the himachal himalaya by any major tectonic event since it was thrust over the lesser himalayan meta sedimentary lhms zone and underwent moderate to slow erosion and exhumation

tourmaline kar ages compared to other radiometric dating systems in alpine anatectic leucosomes and metamorphic rocks cyclades and southern spain abstract k ar ages of tourmalines from alpine anatectic leucocratic rocks in the migmatite dome on naxos greek cyclades and along ultramafic intrusions in southern spain are older than the corresponding mineral ages they are interpreted as cooling ages closely approaching the age of magmatism suggesting a closure temperature exceeding that of hornblende rb sr whole rock analyses from metagranites and gneisses of the palaeozoic suite of the mulhacen complex of the betic cordilleras southern spain yield errorchrons of between 275 and 191 ma several k ar tourmaline ages of these rock units are higher than the corresponding whole rock ages pointing to the incorporation of excess radiogenic ar of the young tourmalines three of them give ages of 80 85 ma indicating possible examples of incomplete degassing and or possible incorporation of excess radiogenic ar in the mineral during the alpine metamorphism the tourmaline ages are compared with the ages of the coexisting micas these have a range of k ar ages consistently higher than the corresponding young alpine rb sr ages of 15 6 12 5 ma pointing to a complex behavior for radiogenic ar during the alpine metamorphism

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

neogene exhumation and relief evolution in the eastern betics se spain insights from the sierra de gador the betics are a key area to study an orogenic landscape disrupted by late orogenic extension new low temperature thermochronology ltt data ahe and aft coupled with geomorphic constraints in the sierra de gador alpujarride complex are used to reconstruct the cooling history and evolution of relief during the neogene we document three stages i a fast cooling event between 23 and 16 ma associated with the well known extensive tectonic exhumation of the alpujarride unit ii a period of slow cooling between 16 and 7 2 ma related to a planation event and iii a post 7 2 ma surface uplift associated with the inversion of the alboran domain undetected by ltt the planation event followed by this late uplift can explain the occurrence of inherited low relief surfaces overlain by tortonian messinian platform deposits at the top of the range finally we propose that the sierra de gador is a more transient landscape than the nearby sierra nevada this article is protected by copyright all rights reserved

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

normal fault thermal regimes and the interpretation of low temperature thermochronometers abstract exhumation rates inferred from thermochronometers are dependent on spatial and temporal variations in temperature in active extensional mountain belts the temperature field is complicated by tectonic and surface processes including 1 lateral heat flow across large range bounding normal faults due to the juxtaposition of a cool hanging wall and a relatively warmer footwall 2 uplift and erosion of the footwall 3 sedimentation and burial of the hanging wall 4 lateral heat refraction around low thermal conductivity sediments deposited in the hanging wall basin and 5 3d temperature variations due to high relief topography developed on the footwall we explore these mechanisms through a series of 2d conductive thermal models designed to investigate the effect of tectonics and topography on apatite fission track aft and u th he thermochronometer data models were tuned to the geometry and kinematics of the wasatch mountains utah usa the principal parameters in our model are exhumation and burial rates ranging from 0 2 to 5 7 mm per year at the range front and decreasing with distance from the fault surface morphology taken from usgs digital elevation models and basin geometries inferred from seismic and gravity surveys predicted aft and u th he ages were generated using cooling rate dependent annealing and diffusion kinetic models results indicate after 10 million years of exhumation footwall u th he and aft closure temperature isotherms within 10 km of the fault are advected upward 500 and 1000 m respectively from their initial position the upward advection of isotherms and the 2d nature of the thermal regime can result in erroneous exhumation rates calculated from plots of sample elevation versus age using 1d thermal models for simulations with a uniform vertical uplift rate and canyon and ridge topography 1d and 2d exhumation rate differences were 20 70 for u th he and 10 for aft data samples collected perpendicular to fault strike up the range front are sensitive to the exhumation rate and footwall tilt differences between 1d and 2d range front exhumation rates were 10 95 for u th he and 10 40 for aft data furthermore 1d thermal models are incapable of deciphering footwall tilt at simulated exhumation rates of 3 4 and 5 mm per year predicted u th he and aft ages form closely spaced near vertical lines on a plot of elevation versus sample age therefore suggesting a decreased sensitivity of data to topography and surface processes at high exhumation rates despite large errors in previously collected aft data from the wasatch mountains our model suggests a revised fault adjacent exhumation rate of 0 5 0 6 mm per year compared to previous estimates of 0 7 mm per year based on 1d thermal models

tectonic significance of the present relief of the betic cordillera abstract tortonian calcarenites of the betic cordillera were deposited in coastal or very shallow marine environments and represent an ideal marker for estimating vertical movements from the late miocene to the present a map showing the heights at which these tortonian marine rocks are situated has a clear correlation with the present relief indicating that today s relief has been formed since the tortonian there is also a good correlation between present relief and the bouguer anomaly distribution in the betic cordillera as well as with crustal thickness likewise the present relief is directly related to the geodynamic setting of a horizontal n s to nnw sse compression and an almost perpendicular extension along with isostatic readjustment existing in the betic cordillera from the tortonian as a result of these regional stresses faults and folds have produced notable vertical movements the highest rates of uplift of the betic cordillera coincide with large antiforms in particular those of the sierra nevada and the sierra filabres several subsiding sectors also exist for example the granada basin or the guadalquivir basin the foreland guadalquivir basin has a complex history because the uplift in its eastern sector and subsidence in the western sector coexisted during the late tortonian today the whole betic cordillera is characterized by differential regional uplift even in the aforementioned subsiding sectors

transposition of foliations and superposition of lineations during polyphase deformation in the nevado filabride complex tectonic implications detailed structural analysis in a ca 80 km2 area of the western nevado filabride complex betic cordillera reveals a heterogeneous internal structure characterized by multiple cross cutting foliations and lineations that locally transpose earlier ones the large scale geometry of these fabrics conflicts with continuous westward to south westward tectonic transport related to thrusting or crustal extension and mismatches a previously inferred extensional detachment in the area multiple crenulation lineations can be distinguished in the field and correlated with five foliation intersection axes fia1 5 preserved in garnet and plagioclase porphyroblasts of the western sierra nevada these indicate crustal shortening in different directions associated with vertical foliation development and intermitted stages of gravitational collapse producing horizontal foliations the large spread of lineation and fold axes trends in the nevado filabride complex results from the mixed presence of multiple generations of these structures whose distinction is critical for tectonic models the five principal fia trends remarkably match successive vectors of relative africa iberia plate motion since the eocene suggesting that deformation of the nevado filabride took place during this period although peak metamorphism in at least some of its parts was reached as late as the middle miocene

structural and thermal constraints on the initiation angle of detachment faulting in the southern basin and range the chemehuevi mountains case study the cenozoic normal fault system exposed in the chemehuevi mountains of the southern cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching there three stacked brittle low angle normal faults that formed at depths as great as 10 12 km cut gently down section northeastward through deformed proterozoic and mesozoic crystalline basement hanging wall blocks are displaced relatively northeastward the upper crust above the chemehuevi detachment fault was pulled apart along high angle normal faults that rotated to more gentle dips through time in contrast rocks of originally mid crustal affinity in the footwall were only gently rotated and accommodated minor extension application of 40 ar 39 ar and fission track thermochronology to rocks in the footwall of the chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting at the onset of extension between 22 and 24 ma granitic rocks exposed in the southwestern and northeastern portions of the footwall were at 200 c and 350 400 c respectively separated by a distance of some 23 km down the known slip direction this gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre extension isothermal surfaces and constrains the exposed part of the chemehuevi detachment fault to have had a regional dip initially of about 15 to 30 the fault system apparently cut gently down through the upper crust to a minimum depth of 10 12 km the deepest exposed parts of the system today and was domed from midcrustal depths and locally denuded during continued slip together the structural and thermochronologic data confirm the suggestion that faults accommodating large magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles that is 30

continental subduction intracrustal shortening and coeval upper crustal extension p t evolution of subducted south iberian paleomargin metapelites betics se spain abstract the nevado filabride complex represents the basement and permo triassic cover of the south iberian passive margin metamorphosed during miocene continental subduction under the alboran crustal domain it forms a nappe stack with the thick calar alto unit 6 km thick sandwiched between the ragua unit below and the bedar macael unit above the nappe stacking occurred by ductile flow along basal syn metamorphic shear zones structural analysis of the calar alto unit together with thermobarometric data of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of intracrustal shortening in the context of a strongly extended orogen multiequilibrium thermobarometry shows that the calar alto schist underwent a prograde p t path at mantle depths from 1 6 gpa at 450 500 c to 1 0 gpa at 550 570 c during the growth of the schistosity s1 parallel to the compositional layering after detachment from the downgoing slab and underplating the unit underwent nearly isobaric cooling by up to 250 c from peak metamorphic conditions the main crenulation cleavage s2 and the mylonitic foliation sm developed within the crustal nappe stack during decompression and heating sm at the base of the unit grew between 0 8 gpa at 400 c and 0 4 gpa at 500 c while s2 at the top of the unit developed between 0 6 gpa at 300 c and 0 2 gpa at 400 c these shortening structures developed simultaneously to the activity of overlying middle miocene extensional detachments intracrustal shortening and coeval upper crustal extension resulted in a mode of exhumation where the material rise entailed an important horizontal component of displacement at rates of approximately 15 mm year being exhumed along an s shaped p t path after peak metamorphic conditions and its detachment from the subducting slab

neogene coralline algal growth forms and their palaeoenvironments in the almanzora river valley almeria s e spain abstract the almanzora river valley is an elongate e w depression containing mainly neogene sediments during the late tortonian several horizons of crustose coralline algae developed in three distinct sedimentary contexts a on coastal platforms and talus slopes b on fan deltas and c associated with corals in patch reefs growing at the furthermost extremes of coastal fan deltas in the former two cases the predominant growths are as rhodoliths associated with mobile substrates in the coastal systems an increase in depth results in a change in both the morphology and the composition of the rhodoliths with depth there is a significant increase in the diversity of algal species and a concomitant increase in the size of the algal covering compared to the nucleus and thus the morphology of the rhodolith depends less and less on the shape of the nucleus itself the massive crusts and columns which mark the limit of development of the shallow water rhodoliths in the deeper examples are covered by branching and thin leafy laminar growths in the fan deltas the development of the rhodoliths is similar to that of the deeper coastal zones in the patch reefs the specific composition of the algal associations changes somewhat and the growths developed on a stable substrate under low energy conditions are mainly vertical branches arising from thin crusts

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

effect of water on the rheology of experimentally deformed quartzite empirical flow laws have been determined for simpson quartzite samples deformed to mechanical steady state in the quartz stability field using griggs blacic solid medium deformation apparatus experiments were conducted on samples both as received dry and with water added via the dehydration of a talc confining medium wet best fits of the power law type yield a stress exponent of 2 72 0 19 an activation enthalpy of 134 32 kj mol 1 and a preexponential constant of 1 16 1 15 0 58 10 7 mpa 2 72 s 1 for the dry quartzite law and are 2 61 0 15 145 17 kj mol 1 and 5 05 5 00 10 6 mpa 2 61 s 1 for the wet quartzite the enhanced hydrolytic weakening of the wet experiments appears to affect the flow laws mostly in the preexponential constants possibly as a defect concentration term that is higher in the wet than in the dry law over the range of experimental conditions 750 900 c 10 7 to 10 4 s 1 and 1 0 1 25 gpa confining pressure dry specimens are 1 5 2 5 times stronger than wet samples deformed at the same conditions the microstructures produced are analogous to those observed in many natural quartz tectonites recrystallization is well developed in the wet but not the dry specimens at flow stresses below 500 mpa microstructures indicative of recovery are well developed a comparison of our creep activation enthalpies with those determined from diffusion experiments are consistent with deformation of wet simpson quartzite that is limited by oxygen self diffusion inconsistencies between the present study and previous studies are probably the result of differences in sample material sample treatment and sample assemblies chemical environment

timing of tectonic events in the alpujarride complex betic cordillera southern spain ar ar and fission track dating from four low grade sections in the alpujarride complex sheds light on the timing of the early contractional history of these high pressure metamorphic units and their subsequent exhumation during the early miocene extensional collapse of the betic orogen white mica grains from the lowest grade rocks yield partly reset detrital ar ar ages where deformation and recrystallization are more intense however they yield reproducible ages as old as c 48 ma which we interpret as the approximate timing of the main contractional event and associated high pressure metamorphism in these units rocks from slightly higher grade sections give reproducible ar ar ages between 30 and 40 ma but these younger ages probably reflect the interplay of growth and cooling in fine grained micas heated to between 300 c and 400 c during prograde metamorphism in each of the four areas studied the low grade rocks are separated by an extensional detachment or shear zone from medium grade rocks below in two areas these detachments have been folded around major overturned north vergent folds ar ar data from structurally below the detachments and fission track ages from all structural levels indicate the onset of significant cooling in the early miocene which corresponds to the age of the extensional event the folds must therefore be early miocene or younger

lateral interaction between metamorphic core complexes and less extended tilt block domains the alpujarras strike slip transfer fault zone betics se spain abstract the alpujarras area in southeastern spain exhibits one of the scant documented examples of extension related strike slip faults bordering core complexes in the world faults of the alpujarras fault zone define a regional scale complex ene striking transfer zone that marks the boundary among the sierra nevada elongated dome a highly extended constricted core complex and a less extended domain formed by large scale tilt blocks detailed mapping and structural analysis show that the alpujarras fault zone is an integral part of the wsw directed normal fault systems that thinned the betic hinterland during the middle miocene to recent time fault patterns and palaeostress analysis both indicate that dextral movement along the strike slip faults was induced by a local stress field with a sub horizontal e w to ese wnw trending maximum principal stress axis which is synchronous with the regional stress field driving the normal fault systems palaeostress analysis also indicates subsequent variations in the stress field with a sub horizontal nw se to n s trending maximum principal stress axis thus producing both the tectonic inversion of the northern fault of the alpujarras system and shortening of the unloaded extensional detachment footwall a simplified kinematic model for the tectonic evolution of the alpujarras area from the middle miocene to recent emphasizes the kinematic coupling of normal faults and strike slip transfer zones in the extensional process

tectonic denudation and topographic development in the spanish sierra nevada 1 the denudation history of the rapidly uplifting western part of the spanish sierra nevada was assessed using apatite fission track aft ages and 10be analyses of bedrock and fluvial sediments major contrasts in the denudation history are recorded within the 27 km2 rio torrente catchment upland areas are characterized by low relief low slope angles and locally the preservation of shallow marine sediments which have experienced 2 km of denudation since circa 4 ma the minimum denudation rates of 0 4 mm yr 1 derived from aft also contrast with the slow medium term 104 years erosion rates 0 044 0 015 mm yr 1 estimated from 10be measurements at high elevations the local medium long term contrasts in denudation rates within the high sierra nevada indicate that much of the unroofing occurs by tectonic denudation on flat lying detachments in lower elevation parts of the catchment rapid river incision coupled to rock uplift has produced 1 6 km of relief implying that the rivers and adjacent hillslopes close to the edge of the orogen are sensitive to normal fault driven changes in base level however these changes are not transmitted into the low relief slowly eroding upland areas thus the core of the mountain range continues to increase in elevation until the limits of crustal strength are reached and denudation is initiated along planes of structural weakness we propose that this form of tectonic denudation provides an effective limit to relief in young orogens

tertiary structure and thermal history of the harquahala and buckskin mountains west central arizona implications for denudation by a major detachment fault system the harquahala and buckskin mountains lie in the footwall of the whipple buckskin bullard detachment system in the harquahala mountains mesozoic fabric and structure are progressively more intensely overprinted by penetrative tertiary deformation toward the northeastern pan of the range tertiary mylonitic deformation is recognized by the presence of deformed miocene mafic dikes and characteristic textural features lineations in the mylonite trend 040 060 and megascopic kinematic indicators mostly indicate top to the northeast sense of shear the crest of an antiform defined by tertiary mylonitic fabric does not coincide with the topographic crest of the range reorientation of mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones as opposed to a single zone that was then bent the northeastern part of the brown s canyon granite acts as a large low strain lozenge in the southeast limb of the foliation arch mylonitic foliation along the southeast side of the range above this lozenge is strongly oblique to the trend of the detachment fault but the relative orientation is consistent with left oblique normal shear evidence of tertiary plastic deformation is absent southwest of sunset pass minor low angle normal faults in the southwestern harquahala mountains and little harquahala mountains dip northeast the low angle normal faults are cut by nw trending high angle right oblique faults in the central and southwestern harquahala mountains 40ar 39ar age spectra from k feldspar muscovite and hornblende and total gas ages from biotite indicate late cretaceous to early tertiary cooling to argon closure temperatures biotite and k feldspar from the area northeast of sunset pass record rapid early to middle miocene cooling reconstruction of the original geometry of the detachment system based on thermal differences indicated by contrasting cooling histories and the orientation of an early miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30 and 40 thus the harquahala mountains are a lilted block exposing of the order of a 10 km section through the pre tertiary crust heterogeneous proterozoic gneiss sparse paleozoic and mesozoic metasedimentary rocks and an oligocene plutonic complex are extensively overprinted by tertiary mylonitic fabrics in the buckskin mountains hornblende 40ar 39ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early tertiary time except in the vicinity of the oligocene plutonic complex feldspar and biotite 40ar 39ar cooling ages suggest that the footwall of the whipple detachment system experienced a more uniform cooling history in the buckskin mountains than in the harquahala mountains cooling ages between about 13 and 20 ma are recorded throughout the range with no consistent spatial pattern of ages the eagle eye detachment fault southeasternmost extension of the whipple buckskin bullard fault system becomes a transfer fault along the southeastern side of the harquahala mountains this fault separates two distinct extensional terranes on the northwest approximately 50 km of crustal extension has been accommodated on the whipple fault system while on the southeast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent big horn lake pleasant extended terrane as the deformation becomes more distributed maximum slip on any one normal fault decreases to the point where no single fault has enough slip to denude crustal levels at which tertiary plastic deformation has occurred

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

geological relationships and u pb zircon and 40 ar 39ar tourmaline geochronology of gneisses and tourmalinites from the nevado filabride complex western sierra nevada spain tectonic implications abstract the nevado filabride complex the lowest complex in the betic hinterland forms a stack of alpine nappes the tectonic units consist of metasedimentary sequences whose ages are not well constrained gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex new radiometric data from u pb zircon dating on gneisses from the western sierra nevada confirm the presence of late carboniferous intrusive rocks in the betic hinterland these results combined with available data from the literature and a detailed structural analysis suggest that the gneisses represent a single late variscan magmatic event evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations in conjunction with geochemical data u pb zircon 314 7 304 23 ma and 40 ar 39 ar tourmaline 319 85 5 81 317 85 3 67 ma geochronology a pre late carboniferous age for the basal formation of the nevado filabride sequence can be inferred the gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites respectively superposed tectonic units rather than a continuous palaeozoic sequence is supported by the occurrence of palaeozoic rocks at the top of the nevado filabride complex the nappe tectonics as evidenced from the contractional character of the unit boundaries and the superposition of higher grade on lower grade metamorphic rocks is additionally supported by the superposition of older on younger rocks as revealed from radiometric data

timing and kinematics of deformation in the malaguide complex internal zone of the betic cordillera southeast spain the betic cordillera of southern spain forms the westernmost of the mediterranean alpine orogenic belts and is divided traditionally into internal and external zones in the external zone fold and thrust belt unmetamorphosed cover rocks of the iberian margin were deformed from the early miocene onward in contrast internal zone rocks are dominantly metamorphic and are generally assumed to have been deformed and metamorphosed in an alpine event resulting from late cretaceous palaeogene convergence between africa and europe in the sierra espuna at the northern edge of the internal zone in the eastern betics the uniquely preserved permo triassic to miocene stratigraphy of the unmetamorphosed malaguide complex offers a rare opportunity to date the internal zone deformation kinematic data thrust geometries and structural facing of folds suggest that the imbricate thrust stack of malaguide rocks formed as a result of nnw directed motion within present geographical coordinates a fossilized front to this thrust stack preserved beneath a thick sequence of oligocene conglomerates indicates that the timing of the thrust deformation is late eocene in age deformation propagated into the foreland basin in the late oligocene with renewed thrusting subsequently the whole stack has been folded into a n nw vergent regional recumbent fold the espuna fold during the early miocene

fold and fault interactions during the development of an elongated narrow basin the almanzora neogene quaternary corridor se betic cordillera spain 1 elongated corridors grow in different tectonic settings mainly in areas deformed by fold and fault sets parallel to the borders the almanzora corridor however is a good example of an e w elongated asymmetrical narrow basin developed in a changing stress scenario generated by the interaction of fold and fault sets oriented parallel and oblique to the corridor borders the initial crustal thickening process was mainly related to the development of the e w oriented sierra de los filabres large antiform which constitutes a major heterogeneity during the tortonian a nw se convergent tectonic setting oblique to this already emerged antiform gives rise to a large dextral simple shear band that determines the development of a major fold las estancias antiform the asymmetrical location of minor tectonic structures faults and minor folds and the distribution of sedimentation denudation areas that finally configure the almanzora corridor synform furthermore development of nw se oriented normal faults points to ne sw orthogonal extension in a new stress field oblique to the corridor borders this region illustrates that the elongated character of the corridors may persist inherited from the orientation of previous crustal rheological heterogeneities in addition marine transgression may be produced in compressional settings favored by the synform development during initial stages of crustal thickening finally led to the relief uplift

high pressure metamorphism in metabasites from the betic cordilleras s e spain and its evolution during the alpine orogeny the nevado filabride complex is the lowest tectonic unit of the betic zone sensu stricto ss of the betic cordilleras s e spain the upper series of this complex consists of a metamorphosed sequence intruded by basic and ultrabasic igneous rocks high pressure metamorphism in the eclogite and blueschist facies is recorded in the metabasites but this was partially obliterated by further successive metamorphic stages in the almandine amphibolite and greenschist facies

a structural and palaeomagnetic study of a section through the eastern subbetic southern spain a well exposed 2 km section through the eastern subbetic immediately to the south of the crevillente wrench fault zone can be divided into an upper thrust sheet of triassic rocks a middle thrust sheet of cenomanian turonian basinal sediments and a lower thrust sheet of jurassic limestones unconformably overlain by upper cretaceous palaeocene sediments structural relationships suggest that the middle sheet overthrust the lower sheet and both units were subsequently folded and imbricated this sequence was then overthrust by the upper sheet incorporating a footwall duplex of cenomanian turonian sediments these structures can be related to the complex deformation associated with a single overthrusting event the dominant thrust transport direction is to the east strongly oblique to the overall trend of the orogen however a palaeomagnetic study isolates stable components which indicate no cumulative vertical axis rotation of the thrust sheets the section is cut by strike slip faults whose displacements appear to be relatively minor the triassic rocks of the upper thrust sheet form a highly disrupted thrust sheet extending over much of the subbetic rather than being local to the crevillente fault zone this section through the subbetic can be interpreted as the result of a major east directed thrust of basinal sediments of the median subbetic over marginal shelf facies of the internal subbetic strongly oblique to the general ese trend of the orogen the shortened sequence is cut by a later set of strike slip faults which are probably related to movement on the crevillente fault

kinematic and thermal evolution during two stage exhumation of a mediterranean subduction complex 1 we examine the kinematic and thermal evolution of the nevado filabride complex nfc an early to middle miocene subduction complex within the betic cordillera of southern spain thermobarometry from the nfc in the sierra alhamilla reveals an inverted geothermal gradient with peak temperatures and pressures of 560 c and 13 kbar reached in the uppermost tectonic unit the nfc cooled during exhumation defining a linear pt path that remained within the stability field of kyanite until the aluminosilicate breakdown reaction geo and thermochronology suggest the nfc was subducted and exhumed to the surface within 10 m y recording an early stage of fast cooling from peak t through 250 c followed by a late stage of slower cooling as the rocks approached the surface the exhumation is associated with a progressive change in kinematics with early co axial fabrics showing nne ssw trending stretching lineations and late non coaxial fabrics showing ssw trending lineations a new tectonic model is proposed to explain these data in which rocks at the leading edge of the iberian margin were subducted southeastward beneath the thin hot alboran lithosphere to form the nfc the rocks at the top of the slab began to be exhumed once they reached 48 67 km depth they were initially exhumed rapidly along the top of the subducting slab in a subduction channel geometry then upon reaching the middle crust were captured by a low angle detachment fault with a wsw shear sense and final exhumation was accommodated more slowly in a core complex geometry

displacement transfer from borders to interior of a plate a crustal transect of iberia abstract a n s crustal transect of the whole iberian plate is presented the displacement transfer during the alpine deformation from the border ranges cantabrian and betic chains toward the interior ones was analyzed by means of balanced crustal sections inconsistencies in shortening values between upper and middle lower crust in the cantabrian mountains basque cantabrian zone imply a mid crustal detachment transferring orogenic shortening southward toward the spanish central system depending on the shortening value attributed to the central system this mid crustal detachment could involve the entire iberian plate connecting the cantabrian mountains and the betic chain the very small crustal root in the central system can be explained as a result of the tectonic load caused by the upper crustal shortening that was transferred from the border ranges we conclude that a simple shear model rather than a pure shear with lithospheric folding better explains the whole iberia deformation finally the total alpine shortening of the iberian plate is estimated between 267 and 292 km

rheological evolution of a mediterranean subduction complex abstract we use field and microstructural observations coupled to previously published p t time histories to track the rheological evolution of an intracontinental subduction complex exposed in the betic cordillera in the western mediterranean region the body of rock we focus on known as the nevado filabride complex nfc was originally part of the upper crust of the iberian margin it was subducted into hot asthenospheric mantle then exhumed back toward the surface in two stages an early stage of fast exhumation along the top of the subducting slab in a subduction channel and a late stage of slower exhumation resulting from capture by a low angle detachment fault rooted at the brittle ductile transition each stage of deformation in the nfc was punctuated by changes in the dominant deformation mechanism deformation during initial subduction of the complex was accommodated by pressure solution creep in the presence of a fluid phase the grain sizes stress magnitudes and estimated strain rates for this stage are most consistent with a thin film model for pressure solution in which the diffusion length scale is controlled by the grain size during the early stages of exhumation within the subduction channel deformation transitioned from pressure solution to dislocation creep due to increases in temperature which resulted in increases in both water fugacity and grain size each of which favor the dislocation creep mechanism differential stress magnitudes for this stage were 10 mpa and are consistent with simple models of buoyancy driven channel flow with continuing subduction channel exhumation deformation remained within the dislocation creep field because sequestration of free water into hydrous retrogressive minerals suppressed the pressure solution mechanism differential stresses progressively increased to 100 mpa near the mouth of the channel during cooling as the rocks moved into mid crustal levels during the final core complex stage of exhumation deformation was progressively concentrated into a narrow zone of highly localized strain beneath a mid crustal detachment fault localization was promoted by a transition from dislocation creep to dislocation creep accommodated grain boundary sliding at temperatures of 350 380 c grain sizes of 4 m and differential stress magnitudes of 200 mpa peak differential stress magnitudes of 200 mpa recorded just below the brittle ductile transition are consistent with byerlee s law in the upper crust assuming a vertical maximum principal stress and near hydrostatic pore fluid pressures overall the distribution of stress with temperature coupled to independent constraints on strain rate from field observations and geochronology indicate that the naturally calibrated hirth et al 2001 flow law for wet quartzite accurately predicts the rheological behavior of mid crustal rocks deforming by dislocation creep

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

the relationship between track shortening and fission track age reduction in apatite combined influences of inherent instability annealing anisotropy length bias and system calibration abstract the relationship between reduced track density 0 and reduced mean confined track length l l 0 during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition compositional range and origin and with spontaneous tracks giving mean confined track lengths varying from 14 5 m down to 12 3 m in all four apatites induced tracks show a relationship between l l 0 and 0 composed of two regions at higher values the two appear to be equal while at lower values l l 0 is greater than 0 this can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks data from three apatites of uniform composition show near identical relationships in a sample showing a range of apatite compositions the point of divergence from the 1 1 relationship is somewhat higher than in the uniform composition apatites because individual grains anneal at different rates and the presence of highly annealed grains affects the track density more strongly than the mean track length spontaneous tracks show similar relationships to those observed for induced tracks in each sample when the data are renormalised to allow for the natural annealing present in the spontaneous tracks previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data thereby distorting the apparent relationship data for the multi compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks this consistency suggests that a single process is responsible in all three cases implying that a laboratory based description of annealing can be applied to geological conditions with confidence recognition that spontaneous tracks in apatite are commonly annealed suggests that in general the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths since the latter is a manifestation of the thermal history the fission track age should be interpreted in similar terms

the role of extension in the miocene denudation of the nevado filabride complex betic cordillera se spain the internal zone of the betic cordillera se spain consists of a nappe stack of three complexes the deepest of which is the nevado filabride complex the zone is separated from the overlying alpujarride complex by a crustal scale shear zone that has variously been interpreted as a thrust or an extensional detachment a suite of 74 new apatite and zircon fission track results have been obtained from the nevado filabride complex and these have been used to define regional cooling patterns for the complex rapid cooling 105 c 200 c ma 1 is spatially related to the tectonic contact with the overlying alpujarride complex cooling to near surface temperatures occurred first in the east sierra de los filabres during the mid serravallian 12 1 ma and was completed by the early tortonian 9 8 ma in the west sierra nevada there is no correlation between fission track age and sample elevation these results are consistent with tectonic unroofing of this complex a finding that favors extension as the mechanism by which the two complexes were brought into contact extension spans the middle and earliest upper miocene 12 8 ma in the study area and therefore lasted much longer than previously documented a hypothesis is advanced which links oblique convergence between the iberian plate and the betic internal zones resulting in crustal contraction at depth with orogen parallel extension in the middle and upper crust

new insights on the sorbas basin se spain the onshore reference of the messinian salinity crisis the sorbas basin is the land reference of the messinian salinity crisis msc that affected the mediterranean sea in the latest miocene its stratigraphy has been re visited using calcareous nannofossils and planktonic foraminifers which provide a reliable biostratigraphic frame and lead to particularly specify the relationships between the sorbas and zorreras members with yesares evaporites the evaporites overlie a shallowing upward sequence ending with the deposition of the reef unit and terminal carbonate complex tcc on the periphery of the basin the reefal carbonates of the tcc are overlain by clastic deposits that are foreset beds of post msc gilbert type fan deltas developed on the northern edge of the basin these sedimentary structures are separated from reefal carbonates and the reef unit by the messinian erosional surface mes the various facies of the sorbas member have been correlated with the bottomset beds of the gilbert type fan deltas despite some differences in palaeobathymetry in the southeastern periphery of the basin the mes separates the sorbas member from the yesares gypsums in the central part of the basin a hiatus characterizes the contact between these members the zorreras member postdates the msc and entirely belongs to zanclean its white lago mare layers are lagoonal deposits the fauna of which is confirmed to result from mediterranean paratethys high sea level exchange after the post msc marine reflooding of the mediterranean basin this study allows to re assert the two step scenario of the msc clauzon et al 1996 with the following events at 5 971 5 600 ma minor sea level fall resulting in the desiccation of this peripheral basin with secondary fluctuations at 5 600 5 460 ma significant subaerial erosion or lack of sedimentation caused by the almost complete desiccation of the mediterranean sea instantaneous marine reflooding accepted at 5 460 ma followed by continuing sea level rise

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

mountain front migration and drainage captures related to fault segment linkage and growth the polopos transpressive fault zone southeastern betics se spain abstract the polopos e w to ese wnw oriented dextral reverse fault zone is formed by the north alhamilla reverse fault and the north and south gafarillos dextral faults it is a conjugate fault system of the sinistral nne ssw palomares fault zone active from the late most tortonian 7 ma up to the late pleistocene 70 ky in the southeastern betics the helicoidal geometry of the fault zone permits to shift se directed movement along the south cabrera reverse fault to nw directed shortening along the north alhamilla reverse fault via vertical gafarillos fault segments in between since the messinian fault activity migrated southwards forming the south gafarillos fault and displacing the active fault related mountain front from the north to the south of sierra de polopos whilst recent activity of the north alhamilla reverse fault migrated westwards the polopos fault zone determined the differential uplift between the sierra alhamilla and the tabernas sorbas basin promoting the middle pleistocene capture that occurred in the southern margin of the sorbas basin continued tectonic uplift of the sierra alhamilla polopos and cabrera anticlinoria and local subsidence associated to the palomares fault zone in the vera basin promoted the headward erosion of the aguas river drainage that captured the sorbas basin during the late pleistocene

tertiary extension in the old woman mountains area california evidence from apatite fission track analysis apatite fission track ages and confined track length distributions from the old woman mountains area southeastern california record evidence for periods of rapid cooling during the late cretaceous following mesozoic orogenesis and during miocene crustal extension apatites from the clipper marble and turtle mountains give fission track ages between 62 and 69 ma as a result of cooling of these upper crustal rocks after regional late cretaceous plutonism and metamorphism in the piute mountains apatite fission track ages decrease from 70 ma with mean track lengths of 13 5 m to 14 m from west to east intermediate aged samples from the piute mountains have mean track lengths between 11 and 13 m due to their extended residence in the apatite annealing zone during the early tertiary this pattern is the result of 20 30 of westward tilting of the piute mountains block as indicated by the dip of tertiary strata the apatite data indicate that tilting occurred after 18 ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range knowing the amount of tilting and the apparent paleotemperatures across the piute mountains allows us to estimate that the synextensional geothermal gradient was 50 20 c km apatites from the northeastern old woman mountains yield fission track ages of 68 ma with mean track lengths of 13 8 m these samples record cooling after batholith emplacement at 73 ma in the northwestern old woman mountains apatite fission track ages decrease to 20 ma and have mean track lengths 15 m indicating very rapid cooling we interpret this age pattern to be the result of minor eastward tilting of the old woman mountains block after 18 19 ma or flexural unroofing of the northwestern old woman mountains due to movement on the ship mountains fault extension in the old woman mountains area occurred between 19 and 16 ma which is coincident with extension in the colorado river extensional corridor and later than major extension in the mojave extensional belt

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

tectonic and metamorphic evolution of the temsamane units external rif northern morocco implications for the evolution of the rif and the betic rif arc located at an intermediate position in the external rif nappe pile the temsamane units northern morocco are characterized by an abnormally intense metamorphism and a penetrative ductile deformation we present new metamorphic data showing that in spite of their external position in the rif part of the temsamane units underwent medium pressure low temperature mp lt metamorphism at c 7 9 kbar and 330 430 c possibly during the oligocene structural data show that the exhumation of these units during middle to late miocene times was characterized by an intense approximately east west stretching and by top to the west shear senses we tentatively propose two possible origins for the mp lt temsamane units 1 an internal origin related to the subduction and the hp lt event recorded in the internal rif alboran domain or 2 an external origin implying a second subduction system within the external rif parallel to and almost contemporaneous with that of the alboran domain this tectonometamorphic evolution of the temsamane units is set within the context of the external rif evolution at a larger scale we show that the exhumation history of the temsamane units which strongly resembles that documented in the core of the internal betics is compatible with the westward slab retreat occurring during the middle to late miocene in the betic rif region

the western mediterranean region as an upper mantle transition zone between two lithospheric plates abstract the upper mantle structure of the western mediterranean region i e the western mediterranean basins with the surrounding land areas has been investigated by means of surface wave analysis phase velocities of teleseismic rayleigh waves have been obtained from the records of a specially designed long period seismic station network the dispersion data have been regionalized employing a computerized procedure and have then been inverted by means of the so called hedgehog method the upper mantle shows pronounced lateral heterogeneities which are correlated with the tectonic activity of the region some deep anomalous features are revealed such as the soft continent like structure of the lithosphere under the alboran sea the absence of lithospheric roots below the mountain range of the high atlas the continental platform like upper mantle along the northern margin of africa while the upper mantle under the provencal and the algerian basins shows oceanic or back arc basin like structure the balearic promontory and particularly the valencia trough are shown to possess upper mantle structure with characteristics close to those of a continental rift system we therefore suggest that the boundary at the level of the upper mantle between the african and the european plates should rather be viewed as a broad transition zone involving the upper mantle of the entire western mediterranean region across which the plate interaction takes place

the lepontine alps as an evolving metamorphic core complex during a type subduction evidence from heat flow mineral cooling ages and tectonic modeling new surface heat flow data are presented for the valle maggia region within the high grade metamorphic core of the lepontine alps the steady state values range from 44 4 to 55 2 mw m2 and are anomalously low compared with those measured around the periphery of the lepontine these differences are considered to reflect the combined effects of differential uplift and erosion superimposed on a transient low heat flow regime which has existed since the miocene the uplift denudation history of the lepontine is modeled by using various mineral geochronometers two major pulses of accelerated uplift are recognized at circa 22 m y b p and circa 10 m y b p respectively these events directly coincide with times of dominant crustal convergence in the helvetic and jura stages of a subduction from known pmax tmax conditions of oligocene metamorphism estimates of low mantle heat flow and experimentally determined values of crustal heat production best fit thermal models are presented to explain surface heat flow and mineral cooling age data for the metamorphic core a two stage underplating model is shown to be consistent with the excavation history of the lepontine and with geological and geophysical data from more external parts of the central alps this information is integrated to explain the establishment of a crustal scale duplex in the metamorphic core complex contemporaneous with thin skinned tectonics in the foreland

uplift and denudation of the central alaska range a case study in the use of apatite fission track thermochronology to determine absolute uplift parameters apatite fission track thermochronology aftt on granitic samples collected in the central alaska range in conjunction with geologic constraints from basins to the north nenana basin and south cook inlet of the range is used to constrain the timing amount rate and pattern of surface uplift rock uplift and denudation since the late miocene the conversion from a thermal frame of reference apatite fission track data to an absolute frame of reference with respect to mean sea level which requires constraining the paleoland surface elevation the paleomean annual temperature and the paleogeothermal gradient is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone paz apatite ages at denali mount mckinley range from 16 ma near the summit 6 km elevation to 4 ma at 2 km elevation a distinctive break in slope in the apatite age profile at an elevation of 4 5 km also marked by a change in confined track length distributions marks the base of an exhumed apatite paz rock uplift and denudation are greatest at denali decreasing southward away from the mckinley strand of the denali fault system as shown by progressively older apatite ages 7 35 ma from a suite of samples along the kahiltna glacier a correlative decrease in topography occurs southward from the fault the central alaska range lies within an arc defined by the denali fault with the highest peaks including denali concentrated at the arc apex patterns of rock uplift and denudation within the central alaska range mimic topography between early and late miocene and possibly earlier the central alaska range was most likely an area of relative tectonic and thermal stability rock uplift denudation and mean surface uplift of the denali region began by the late miocene 5 6 ma being 8 5 km 5 7 km and 2 8 km respectively at average rates of 1 5 km m y 1 km m y and 0 5 km m y the amount of rock uplift denudation and surface uplift decreases to 3 km 2 km and 1 km at little switzerland some 45 km south of the denali fault we conclude that the topographic and rock uplift patterns of the central alaska range the shape and proximity of the mckinley strand of the denali fault to these patterns the timing of the onset of rock uplift and denudation at 5 6 ma and a significant change in relative plate motion between north america and the pacific plates circa 5 6 ma are all inherently related

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth

patterns and average rates of late neogene recent uplift of the betic cordillera se spain abstract the facies distribution in the sedimentary units infilling a series of neogene basins has been used to reconstruct the relief generation and uplift across the internal zone of the betic cordillera in southern spain uplift amounts and average rates can be estimated using the current elevation of the outcrops of well dated deposits indicative of ancient sea level positions coral reefs and coastal conglomerates record the initial development of emergent betic relief during the langhian continental and marginal marine deposits indicate the existence of a large island centred on the present sierra nevada sierra de los filabres chain by the end of the middle miocene the precursor of the sierra nevada sierra de los filabres chain originally part of this large island remained emerged whilst the surrounding areas were re invaded by the sea during the early tortonian at the end of the tortonian the inland basins granada and guadix basins became continental while the sierras de la contraviesa sierra de gador and sierra alhamilla emerged separating the alboran basin from the alpujarra tabernas and sorbas basins which became narrow passages of the mediterranean sea in contrast the sierra cabrera emerged during the late messinian suggesting a progressive uplift from west to east of the sierras south of the sierra nevada sierra de los filabres chain during the pliocene only the low areas closest to the present day coast remained as marine basins and progressively emerged throughout this stage the highest average uplift rate recorded is 280 m ma for the sierra de gador although the average uplift rates of upper neogene coastal marine rocks since depositon have maximum values of approximately 200 m ma most of the uplift of the betic mountains took place before the early pliocene the recorded uplift of neogene rocks was highest at the margins of western sierra nevada where peaks higher than 3000 m occur the average rates of uplift were lower to the east of this major relief the main sierras and depressions in the present day landscape correspond respectively to the emergent land in which uplift was concentrated and to the marine basins that existed before the final emergence of the region the altitude of the sierras reflects the time at which they became emergent the highest mountains being the first to rise above sea level

evidence for internal zone unroofing from foreland basin sediments betic cordillera se spain tertiary sediments at the boundary of the internal and external zones in the eastern betic cordillera record evidence of compressive deformation in the internal zone and its subsequent exhumation sediments on the north side of the sierra espun associated with the structurally highest unit of the internal zone the malaguide complex are interpreted as a foreland or piggyback basin which formed in response to the deforming internal zone basin evolution has been reconstructed using an integrated analysis of sedimentary facies petrography heavy minerals and structural data the basin fill contains eocene shallow marine carbonate shelf ramp sediments early oligocene fan conglomerates and coralgal reefs transitional to probable slope deposits of the late oligocene and a late oligocene to mid miocene deep water sequence with pelagic faunas the provenance analyses illustrate an unroofing history of the internal zone orogenic edifice locally uplifted mesozoic to eocene carbonates at the thrust front were the source of lower oligocene conglomerates clastic detritus supplied to the late oligocene to aquitanian basinal sediments was mainly derived from the permo triassic and palaeozoic basement of the malaguide complex the presence of small amounts of first cycle metamorphic minerals indicates the start of exhumation of the deeper metamorphic alpujarride complex from the burdigalian onwards a metamorphic terrane dominated the source area of the deep water sediments and by the langhian high pressure minerals from the deepest parts of the internal zone orogenic edifice appear in the sedimentary sequence rapid exhumation recorded in the early miocene sediments may be the result of extensional exhumation of the internal zone metamorphic complexes

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

mars evidence for geologically recent advance of the south polar cap an impact crater near the south pole characterized by pristine appearing secondary crater chains is partly covered with more than a kilometer of polar layered terrain deposits continuous with the main polar deposit the observed relationships strongly suggest that the crater formed near the edge of the cap in amazonian time was buried by polar cap advance and that some retreat occurred these data appear to indicate that the south polar cap has been active in the geologically recent past

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

insights into extensional events in the betic cordilleras southern spain new fission track and u pb shrimp analyses abstract a combination of the u pb shrimp and zircon fission track analyses has been used to constrain the age and evolution of the extensional tectonics in the betic cordilleras southern spain the studied rocks are dolerite dykes intruded in the malaguide complex and in the underlying los reales nappe and their country rocks the dykes are grouped in two nearly perpendicular systems characterized by n s strikes in the los reales nappe and e w strikes in the malaguide complex two periods of extension have been identified the age of the older event ranging from 33 to 25 ma is constrained by the intrusion of the dykes 33 ma and the joint cooling of the los reales malaguide domain from 25 ma onwards this event is associated with the structural emplacement of the malaguide complex over the los reales nappe along a brittle ductile extensional detachment and is tentatively ascribed to the tectonic collapse of the orogenic belt a minimum time span of 10 m y is established between the onset of the extensional collapse and the widespread aquitanian ht lp metamorphism in the present geographic frame the kinematics of this event is characterized by a change from an e w extension at lower levels los reales nappe to a n s extension in upper levels malaguide complex a younger extensional event of miocene age 18 ma has been deduced based on the fission track analysis within the calizas alabeadas member of the malaguide complex

discovery of clinoenstatite in the luobusa ophiolitic mantle peridotite recovered from a drill hole tibet abstract clinoenstatite is an uncommon mineral and has been documented in sulu ultrahigh pressure uhp garnet pyroxenite however clinoenstatite clen was rarely reported in the previous studies of ophiolitic mantle peridotites the studied ophiolitic peridotites are recovered from a drilling hole of the luobusa ophiolitic massif tibet that is unique in occurrences of super reducing uhp minerals in both chromitite and peridotite our petrographic and tem studies reveal abundant clen as lamellae associated with diopside exsolution in orthoenstatite oren from the luobusa mantle peridotite in addition termination of clen lamellae and guinier preston zones nanometer scale precipitates with compositions between enstatite and diopside are also observed in the oren both clen lamellae and host oren have nearly identical mg0 9fe0 1 2si2o6 compositions the clen lamellae in host oren from two core samples have even numbers of 9 a 1 0 0 fringes without twins and are oriented parallel to 1 0 0 of the host oren the crystallographic data of the clen are d100 9 06 a r1 d020 4 34 a r2 d120 4 03 a r3 r1 r2 90 r1 r3 64 and r2 r3 26 whereas the host oren have d100 18 1 a r1 d020 4 48 a r2 d120 4 34 a r3 r1 r2 90 r1 r3 77 and r2 r3 13 no antiphase domain boundaries are observed in tem images it probably indicates that the clinoenstatite is the low p polymorph of clinoenstatite lclen with p21 c symmetry the exsolution of diopside and the presence of guinier preston zones are attributed by cooling of primary high t ca bearing very minor oren the transformation of lclen from oren may have occurred mainly by cooling along the p t boundaries between the oren and lclen stability fields shear stress only enhances the transition the occurrence of clen lamellae in oren and the existence of super reducing uhp minerals in peridotite provide an additional constraint for a very deep mantle origin of the luobusa peridotite

early miocene continental subduction and rapid exhumation in the western mediterranean new lu hf garnet growth ages from high pressure rocks from the structurally lowest terrane nevado filabride complex in the betic cordillera of southern spain indicate early miocene subduction of continental crust requiring significant revision of the timing and processes of orogeny in the western part of the alpine orogenic system mafic eclogite and pelitic schist give lu hf ages in the range 18 14 ma and fission track data show that the terrane was exhumed by 12 8 ma the overlying terrane alpujarride complex shows early tertiary high pressure temperature metamorphism this was followed by a thermal event that accompanied rapid crustal extension at 22 18 ma leading to the formation of the alboran sea backarc basin the evidence therefore suggests two phases of continental subduction separated by a phase of large scale intraorogenic extension the two terranes are now separated by a mid late miocene low angle extensional fault the timing and paleogeographic setting suggest that the nevado filabride terrane originally was close to the rifted south iberian margin and was subducted during westward motion of the extending upper terrane between the converging african and iberian plates

active transfer fault zone linking a segmented extensional system betics southern spain insight into heterogeneous extension driven by edge delamination pliocene and quaternary tectonic structures mainly consisting of segmented northwest southeast normal faults and associated seismicity in the central betics do not agree with the transpressive tectonic nature of the africa eurasia plate boundary in the ibero maghrebian region active extensional deformation here is heterogeneous individual segmented normal faults being linked by relay ramps and transfer faults including oblique slip and both dextral and sinistral strike slip faults normal faults extend the hanging wall of an extensional detachment that is the active segment of a complex system of successive wsw directed extensional detachments which have thinned the betic upper crust since middle miocene two areas which are connected by an active 40 km long dextral strike slip transfer fault zone concentrate present day extension both the seismicity distribution and focal mechanisms agree with the position and regime of the observed faults the activity of the transfer zone during middle miocene to present implies a mode of extension which must have remained substantially the same over the entire period thus the mechanisms driving extension should still be operating both the westward migration of the extensional loci and the high asymmetry of the extensional systems can be related to edge delamination below the south iberian margin coupled with roll back under the alboran sea involving the asymmetric westward inflow of asthenospheric material under the margins

paleomagnetic rotations and the kinematics of the gibraltar arc paleomagnetic investigations of a mesozoic limestone sequence around the gibraltar arc show that there have been large systematic rotations about a vertical axis and imply that there must have been a significant component of westward motion within the betic rif orogenic belt rotations of the late jurassic and late cretaceous limestones in the betic cordillera of southern spain are clockwise with the exception of the unrotated sierra gorda whereas the jurassic sites in the rif mountains of morocco are rotated counterclockwise except in the area around tetuan these data are generally consistent with formation of the arc either by a westward moving alboran microplate or by extensional collapse of a collisional mountain chain the former model however fails to recognize the nonrigid nonplate like character of the alboran domain and lacks a definable driving mechanism a model of a collapsing east west elongated ridge that takes into account the obliquity of the convergence along the active margin may provide mechanisms both for the observed rotations and for the large westward component of motion and therefore can explain the geometry of the gibraltar are

closure of the africa eurasia north america plate motion circuit and tectonics of the gloria fault we examine the closure of the current plate motion circuit between the african north american and eurasian plates to test whether these plates are rigid and whether the gloria fault is an active transform fault we also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries and compare the predicted direction of motion along the african eurasian plate boundary in the mediterranean with the direction of slip observed in earthquakes from marine geophysical data we obtain 13 transform fault azimuths and 40 3 m y average spreading rates 34 of which are determined from comparison of synthetic magnetic anomaly profiles to 140 observed profiles slip vectors from 32 earthquake focal mechanisms further describe plate motion detailed magnetic surveys north of iceland provide 11 rates in a region where prior plate motion models had few data magnetic profiles north of the azores triple junction record a rate of 24 mm yr 4 mm yr slower than used by prior models gloria and sea beam surveys accurately measure the azimuths of seven transform faults our plate motion model fits six of the seven within 2 two transform faults surveyed by gloria side scan sonar lie near famous area transform faults a and b and give azimuths 13 clockwise of them because recent studies show that short offset transforms such as transforms a and b are in many places oblique to the direction of plate motion we exclude azimuths from transforms with less than 35 km offset the best fitting and closure enforced vectors fit the data well except for a small systematic misfit to the slip vectors on right lateral slipping transforms slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths whereas on left lateral slipping transforms slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths we search the long eurasia north america boundary for evidence of an additional plate but find no systematic misfits to the data in particular if a spitsbergen plate exists and moves relative to eurasia its motion is less than 3 mm yr an africa eurasia euler vector determined by adding the eurasia north america and africa north america euler vectors is consistent with the gloria fault trend and with slip vectors from eastern azores gibraltar ridge focal mechanisms a small circle centered at the africa eurasia closure enforced pole fits the trace of the gloria fault the model in which closure was enforced predicts 4 mm yr slip across the azores gibraltar ridge and west northwest convergence near gibraltar 45 more oblique than suggested by a recent model based on compressive axes of focal mechanisms moreover our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between gibraltar and sicily because closure enforced vectors fit the data nearly as well as the best fitting vectors we conclude that the data are consistent with a rigid plate model and with the gloria fault being a transform fault

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

mid cretaceous paleogeography and paleoceanography of the betic seaway betic cordillera spain abstract sedimentary and micropaleontological features of cretaceous formations in the betic seaway have been examined in the external zone of the betic cordillera southern spain planktic and benthic foraminiferal assemblages were studied to establish a biostratigraphic framework and to provide estimates of paleo waterdepth further environmental interpretations are based on the analysis of clay minerals sedimentary petrography and organic geochemistry we suggest the existence of several subbasins with differing subsidence histories created by transform tectonics along the axis of the betic seaway a comparison with mid cretaceous sequences of adjacent dsdp odp holes indicates that the general paleoceanographic conditions within the seaway were similar to the north atlantic our results are integrated in a paleogeographic reconstruction for the western mediterranean during the late albian

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

crustal types and tertiary tectonic evolution of the alboran sea western mediterranean multichannel seismic reflection images across the transition between the east alboran and the algero balearic basins show how crustal thickness decreases from about 5 s two way traveltime twtt 15 km thick in the west east alboran basin to 2 s twtt typical of oceanic crust 6 km thick in the east algero balearic basin we have differentiated three different crustal domains in this transition mainly on the basis of crustal thickness and seismic signature boundaries between the three crustal domains are transitional and lack evidence for major faults tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic suggesting a strong relationship between magmatism and crustal structure stratigraphic correlation of lithoseismic units with sedimentary units of southeastern betic basins indicates that sediments onlap igneous basement approximately at 12 ma in the eastern area and at 8 ma in the western area linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent from west to east thin continental crust modified by arc magmatism magmatic arc crust and oceanic crust middle to late miocene arc and oceanic crust formation in the east alboran and algero balearic basins respectively occurred during westward migration of the gibraltar accretionary wedge and shortening in the betic rif foreland basins arc magmatism and associated backarc oceanic crust formation were related to early to middle miocene subduction and rollback of the flysch trough oceanic basement subduction of this narrow slab beneath the alboran basin was coeval with collision of the alboran domain with the iberian and african passive margins and subsequent subcontinental lithosphere edge delamination along the betic rif margins

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

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

recent and present day stresses in the granada basin betic cordilleras example of a late miocene present day extensional basin in a convergent plate boundary the diffuse convergent boundary between the eurasian and african plates in the western mediterranean is associated with a seismicity zone more than 300 km wide although the two plates are converging nw se the betic and rif cordilleras contain extensional structures that have been active since the miocene the extensional tectonics in the region which occurred simultaneously with the uplift of the cordillera have been analyzed in the southeastern sector of the late miocene to recent granada basin using earthquake focal mechanisms the determination of paleostresses from the study of the orientation and kinematics of microfaults and the study of the major structures both the geological surface data and the focal mechanisms indicate present day regional conditions of ne sw extension with triaxial to prolate stress ellipsoids however the stress field is heterogeneous with local variations in stress over time with different stresses sometimes even acting simultaneously in adjacent areas the most frequent changes consist of pluridirectional or ne sw extension favored by the prolate character of the stress ellipsoids and nw se subhorizontal compression favored by the regional tectonic setting strike slip faults are scarce even though they are the most likely structure to be expected in a region with sw ne extension and nw se compression seismicity is concentrated in the upper crust and may correspond to the activity of low to high angle normal faults similar to the surface faults although they can not be correlated with them the lower cutoff of this seismicity probably coincides with the 300 c isotherm and suggests a low thermal gradient for the region present day regional stresses have 1 vertical at the surface but in depth plunge toward the sw

nature and impact of the neotectonic deformation in the western sierra nevada spain abstract the current high relief of the western sector of the sierra nevada was practically nonexistent before the tortonian as attested by the marine late neogene outcrops that reach a height of 1830 m which would easily surpass 2000 m if not for erosion the significant uplift produced over 3500 m has caused considerable thinning detachments and gravitational slides in the alpujarride units the general displacements of these movements are towards the wnw to ssw on the whole the morphology is fan like adapting itself to the border of the sierra nevada this border contains normal conjugate nw se and n s to nne ssw faults the latter also with sinistral strike slip character these faults have accommodated the uplift of the sierra nevada within an almost n s compressive context and linked e w extension this sector coincides with the western boundary of the nevado filabride complex and in its southern continuation with the western limit of the lower alpujarride units seeming to indicate a fault or faults reaching deep into the basement uplift has taken place unevenly over the last few million years with three noteworthy periods during the deposition of the block formation in the late tortonian towards the early middle pleistocene and in the middle late pleistocene this has resulted in considerable deepening of the fluvial net surrounding sierra nevada at the present time

tectonic inversion of an asymmetric graben insights from a combined field and gravity survey in the sorbas basin the formation of sedimentary basins in the alboran domain is associated with the exhumation of metamorphic core complexes over a ca 15 ma period through a transition from regional late orogenic extension to compression an integrated study coupling field analysis and gravity data acquisition was performed in the sorbas basin in the southeastern betic cordillera detailed field observations revealed for the first time that extensional tectonics occurred before shortening in this basin two extensional events were recorded with nw se to n s and ne sw kinematics respectively the first of which being likely responsible for the basin initiation tectonic inversion of the basin then occurred around 8 ma in an overall ca n s shortening context 2d gravity sections reveal that the basin acted as an active depocenter as the basin floor locally exceeding 2 km depth is characterized by a marked asymmetric architecture based on this integrated study we explore a new evolutionary scenario which can be used as a basis for interpretations of the neogene tectonic history of the southeastern betics

the betic segment of the lithospheric trans alboran shear zone during the late miocene abstract the betic sinistral transcurrent shear zone betic cordilleras crosses the southeastern part of spain the study of the geodynamics of this area is approached from tectonic sedimentary magmatic and geophysical data large strike slip movements tectono sedimentary processes and magmatic events are seen at the boundary between two different lithospheric slabs they are located in a thinned compartment with a high geothermal gradient the large scale movements vertical and horizontal and the intensity of magmatic activity are the expression of the mechanical behaviour of this hot and thin crust all the geological events from the late miocene to recent are integrated in a crustal scale cross section which shows that the betic sinistral transcurrent shear zone widely affects the lithosphere it appears that subduction zones are not the only geodynamic context in which calc alkaline magmatism occurs

thermal modeling of extensional tectonics application to pressure temperature time histories of metamorphic rocks one and two dimensional finite difference models are used to generate theoretical pressure temperature time ptt paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere pure shear and via movement of crustal blocks along rooted large scale low angle normal faults simple shear the temperature time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear and experience a greater amount of posttectonic cooling the depth temperature paths of rocks uplifted from deep crustal levels 30 km depth via pure shear extension are often characterized by a period of nearly isothermal high dp dt uplift during the early stages of extension a modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces ptt paths characterized by a more protracted period of nearly isothermal uplift in contrast simple shear uplift along low angle normal faults produces almost linear depth temperature paths with moderate dp dt and isothermal uplift is not observed for a given initial geotherm the single factor controlling the ptt paths of rocks unroofed below gently dipping normal faults that dip less than 30 is the rate at which unroofing takes place unroofing rate is defined as the rate of uplift relative to the surface and the syntectonic ptt paths are insensitive to fault dip or horizontal displacement rate current geothermometric and geobarometric techniques yield typical uncertainties in real pressure temperature and temperature time data of 1 kbar and 50 c thus it will be difficult to distinguish between these various extensional mechanisms on the basis of ptt data derived from a single sample or from a single structural level although use of data from samples collected from a broad range of structural levels may improve resolution ultimately the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to 0 5 kbar and 25 c

crustal extension in the northern apennines the transition from compression to extension in the alpi apuane core complex an interpretation of northern apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge after a model proposed by platt 1986 followed by the initiation and development of continental rifting during cretaceous to eocene time an accretionary wedge formed as remnant tethyan oceanic crust subducted beneath the rotating corsica sardinia microplate microplate collision during the oligocene was characterized by the rapid imbrication of buoyant continental crust of the italian continental margin the record of which is preserved within the duplex structure geometry of the alpi apuane region the overthickened wedge geometry returned to a more stable configuration by developing extensional features during the miocene both listric normal faults at upper crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid crustal levels are recorded it is proposed that large scale regional extension with associated volcanism beginning in the messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex further rifting perhaps aided by subduction and back arc processes in the southern apennines led to the formation of the tyrrhenian sea as an oceanic basin both the apennines and north american core complexes record evidence of crustal thickening followed by crustal thinning and finally of continental rifting this suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

inversion of apatite fission track data for thermal history information apatite fission track length distributions and ages provide an integrated record of the low temperature 20 150 c thermal history of the host rock over geologic time scales the information content of these data is quantified by applying a stochastic optimization technique simulated annealing to determine the range of thermal histories compatible with synthetic apatite fission track data given a mathematical description of fission track annealing the goodness of fit between the observed and predicted data parameters and a slowly decreasing control parameter are treated as analogous to the free energy and temperature respectively of a thermodynamic system on this basis a boltzmann type probability distribution is defined and used to accept or reject random thermal history perturbations a priori information used to constrain the model space may include the temperature range in which the solutions are sought present day temperature the complexity of the thermal history and the maximum duration of the data sensitive thermal history this approach allows determination of the range of thermal histories compatible with an observed confined track length distribution and associated fission track age and it provides an estimate of the probability distribution of temperatures at a specific time results for synthetic data sets for diverse model thermal histories suggest that the resolution of thermal history is generally poor parts of the thermal history to which the data are most sensitive can be resolved to no better than about 10 c for ideal data bounds on acceptable time temperature space are sensitive to the assumed complexity of the thermal history but are relatively insensitive to typical uncertainties associated with the data application of this approach to real data is fundamentally limited by the uncertainty associated with extrapolating annealing models to geologic conditions

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

king s trough reactivated pseudo fault of a propagating rift summary a compilation of magnetic data in the western north atlantic shows that seafloor spreading magnetic anomalies 26 to 18 are dislocated across a pseudo fault with a direction mirroring that of king s trough it is interpreted that this feature and king s trough were formed by northward propagation of the rift initiated by a hotspot that created the azores biscay rise the rift propagation stopped when king s trough became part of the plate boundary between eurasia and iberia at chron 17 most of the present day structure of king s trough was created by extensional motion across this plate boundary from chron 17 to 9 6c