quartz recrystallization regimes c axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone ribeira shear zone se brazil abstract the crystal plastic behavior of quartz mylonites from the ribeira shear zone se brazil a major strike slip structure that was active during a prograde metamorphic phase related to the neoproterozoic brasiliano pan african orogeny was investigated using a multi method approach geothermobarometry results indicate deformational conditions ranging from 300 to 630 c and 500 700 mpa a strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone bulging recrystallization blg dominates within the chlorite zone between 300 and 410 c subgrain rotation recrystallization sgr operates within the biotite zone from 410 to 520 c and grain boundary migration recrystallization gbm dominates in the garnet zone above 520 c the development of quartz c axis textures is mainly governed by temperature and dynamic recrystallization mechanisms textures from blg zone mylonites are characterized by maxima around z sgr zone mylonites display single girdles or asymmetric type i crossed girdles and gbm zone mylonites comprise maxima around y and intermediate between x and z the scarcity or absence of water bearing fluid inclusions in quartz mylonites from the sgr and gbm zones which are dominated by carbonic inclusions suggests water deficient conditions whereas blg zone mylonites are dominated by water bearing inclusions this evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature no effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected and little influence on c axis texture development was observed most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history recording a decompression in the range of 300 500 mpa while microstructural reequilibration effects related to the prograde metamorphism are largely preserved fluid inclusion microstructures and densities from two sgr zone samples preserved evidence for a near isothermal compression within the interior of the ribeira shear zone during the prograde metamorphism

chemical weathering rates and atmospheric soil co2 consumption of igneous and metamorphic rocks under tropical climate in southeastern brazil abstract chemical weathering rates and atmosphere soil co 2 consumption of igneous and metamorphic rocks under tropical climate in southeastern brazil were evaluated using the chemical composition of surface waters and fresh rocks and soil horizon c in the upper sorocaba river basin surface water samples were collected between june 2009 and june 2010 and analyses were performed to assess ph electrical conductivity ec temperature and total dissolved solids tds including na k ca 2 mg 2 cl so 4 2 po 4 3 no 3 and sio 2 fresh rocks and c horizon samples were also collected taking into account their geological context abundance and spatial density to analyze major elements and mineralogy the concentration of tds and dissolved cations anions and silica increased during the dry period in relation to the wet period and the same behavior was observed for ph ec and temperature after corrections of anthropogenic contributions ca 21 t km 2 yr and atmospheric inputs ca 19 t km 2 yr the annual flux due to chemical weathering involving the igneous and metamorphic rocks was ca 29 t km 2 yr the co 2 atmospheric soil consumption in the upper sorocaba river basin was ca 0 2 10 6 mol km 2 yr and when extrapolated to the entire mantiqueira orogenic belt accounted an estimated consumption of 0 07 10 12 mol yr representing 0 6 of the total co 2 consumption flux derived from global average silicate weathering the chemical weathering rates of igneous and metamorphic rocks in the upper sorocaba river basin were estimated at 15 m my respectively the main weathering process in this watershed was the monossialitization with partial hydrolyses of bedrock minerals except quartz which was not weathered and remained in the soil profile the annual specific flux derived from igneous and metamorphic rocks at upper sorocaba river basin could be compared with watersheds in tropical climates however this value is higher than in other north american european asian and african granitoid watersheds and lower than in montane watersheds

tectonic implications of paleomagnetic and geochronologic data from the yukon koyukuk province alaska the paleomagnetic and geochronologic record of alaska is complicated by overprints that make it difficult to relate the terranes of northern and southern alaska to each other and to north america to better understand these relationships and the overprinting paleomagnetic and k ar dating samples were analyzed from the three major geologic units of the yukon koyukuk province ykp of central alaska these units consist of a basal jurassic early cretaceous volcanic island arc assemblage albian cenomanian clastic sedimentary rocks and bimodal eocene volcanic rocks k ar age determinations of the early tertiary volcanic rocks form two distinct groups the older group 65 49 ma is mostly felsic and probably represents the maximum inboard penetration of low angle subduction related volcanism these older rocks were deformed during the regional deformation of the ykp the younger group 44 43 ma is post deformational and composed dominantly of basalt these results bracket a time 49 44 ma when deformation ceased in the province this time interval is coincident with a major transition from convergence to strike slip and extensional tectonic style throughout alaska the geologic units of the ykp display two characteristic paleomagnetic signatures primary directions based on positive reversal conglomerate and or bedding tilt stability tests were obtained from the albian cenomanian sedimentary rocks and eocene volcanic rocks the mid cretaceous sedimentary rocks show about 15 of poleward motion between 90 56 ma a time of high rates of convergence between alaska and the pacific and eurasian plates age equivalent rocks from neighboring regions st matthew island and the alaskan north slope also indicate similar amounts of poleward northward motion which suggests that the whole of western and northern alaska may have moved northward about 10 relative to north america as a more or less coherent block paleomagnetic results from the eocene volcanic rocks indicate that the ykp was in place by 56 ma and formed part of the accretionary nucleus that served as a backstop for the accreted terranes of southern alaska secondary remagnetized directions are recorded in the basal island arc assemblage and in albian cenomanian sediments near younger igneous occurrences the clustering of secondary directions in the geographic reference frame indicates that they were acquired after deformation ceased 49 44 ma which is consistent with the coincidence of the mean of these directions and the 54 44 ma north american reference direction

diachronic collision slab break off and long term high thermal flux in the brasiliano pan african orogeny implications for the geodynamic evolution of the mantiqueira province abstract the mantiqueira province in se brazil and uruguay is a complex brasiliano pan african orogenic province that has recently been the topic of conflicting geodynamic models focusing the possibility of multiple collision events during the ediacaran ordovician assembly of the western gondwana supercontinent a thorough compilation of recently obtained high resolution u pb geochronology and geochemistry in the mantiqueira province provided the following results a considering the entire orogenic system the pre syn and post orogenic periods range from 670 595 ma 620 550 ma and 560 490 ma respectively and are coeval with the intrusion of 3 distinct magmatic sequences within each geographic segment of this large orogenic province evolving from i calc alkaline arc related magmatism to ii syn orogenic anatectic magmatism coeval with the metamorphic climax to iii calc alkaline alkaline including shoshonitic and a type affinity magmatism that reveal significant lower crust and mantle contribution b progressively younger ages are present from south to north irrespective of the orogenic period considered implying that the different geographic sectors were from south to north diachronically amalgamated into the mantiqueira province results provide important evidence for the existence of a single diachronic collision event at 630 590 ma followed by the thermal climax at 615 560 ma slab break off asthenospheric upwelling and mantle underplating that sustained long term high heat flux conditions until 540 490 ma when post tectonic calc alkaline alkaline granitoids probably admixed with small amounts of slab melts formed at depth intruded the middle low crust magmatic and metamorphic sequences during the final stages of thermal and orogenic collapse of the mantiqueira province belts

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

tectonics of the southern portion of the ribeira belt apiai domain abstract the name ribeira belt was originally used in a reference to a broad belt of deformed crust which strikes subparallel to the southeastern brazilian coastline it was originally believed to be neoproterozoic brasiliano panafrican but present investigations indicate that this age seems to be mainly related to the extensive arc related plutonism and tectono metamorphism that affected its most representative formations whereas the time of deposition of the supracrustals would be significantly older in this study the main tectono stratigraphic features of the apiai domain of the ribeira belt are described the lithostratigraphy is reviewed as well the paleogeographic structural geochemical and radiometric data and their implications for the tectonic evolution models for the region the marine sediments of the apiai domain were laid down on the borders of the parana and luis alves cratons and on an intervening mesoproterozoic 1 8 1 1 ga ocean crust they were intensively deformed by ocean closure and collision of the cratonic fragments and they constitute the majority of the local supracrustals designated as the acungui supergroup symmetrical continental passive margin deposits are represented to the northwest by a more proximal facies itaiacoca group probably related to the parana paleocontinent and by the capiru formation to the southeast between them lies the central domain formed by the votuverava group characterized by the remains of gradually deepening carbonatic platform sediments of the lajeado subgroup which grade eastward into distal turbiditic sequences of the ribeira subgroup where ocean floor and immature island arc magmatic rocks are present during the neoproterozoic the apiai domain behaved as a single continental mass into which basic rocks were intruded between 1 1 and 0 85 ga between 0 7 and 0 6 ga the apiai domain evolved into a cordilleran type magmatic arc related to the evolution of a subduction zone possibly situated to the southeast of the luis alves cratonic fragment this subduction caused metamorphism and migmatization of the supra and infra crustal rocks of the region and produced the extensive calc alkaline granitoid plutonism between 0 6 and 0 45 ga the majority of the southeastern continental blocks amalgamated during closure of the adamastor ocean followed by development of a system of regional dextral transcurrent shear zones related to oblique collision and or escape tectonics pull apart basins opened up and were filled with molasse sequences the region was intruded by post tectonic granitoids and then underwent regional cooling

accretion of southern alaska abstract paleomagnetic data from southern alaska indicate that the wrangellia and peninsular terranes collided with central alaska probably by 65 ma ago and certainly no later than 55 ma ago the accretion of these terranes to the mainland was followed by the arrival of the ghost rocks volcanic assemblage at the southern margin of kodiak island poleward movement of these terranes can be explained by rapid motion of the kula oceanic plate mainly from 85 to 43 ma ago according to recent reconstructions derived from the hot spot reference frame after accretion much of southwestern alaska underwent a counterclockwise rotation of about 50 as indicated by paleomagnetic poles from volcanic rocks of late cretaceous and early tertiary age compression between north america and asia during opening of the north atlantic 68 44 ma ago may account for the rotation

formation of interior basins associated with curved faults in alaska the positions of certain nonmarine sedimentary basins in central and southern alaska are spatially related to the geometry of adjacent curved faults these interior basins occur along the northern convex side of these fault systems which change trend from northwest in canada and eastern alaska to east west in central alaska and to northeast in western alaska the basins probably initiated in early to middle tertiary time and deposition within all basins continued through the quaternary broadly synchronous with tertiary strike slip displacements along the tintina denali and border ranges fault systems we examine the spatial and temporal relationship between basins and faults by using a mechanical model of curved strike slip faults based on boundary elements we relate slip along the curved faults to tertiary pacific north american plate motions by using principal stress directions inferred from the plate motion directions locations of the predicted mean stress reductions around the fault bends caused by fault slip correlate well with positions of the interior basins our results suggest that basin subsidence and location were controlled primarily by the geometry of adjacent curved fault systems and the state of stress in alaska during the tertiary mechanical interaction between curved alaskan fault systems increases the tendency for basin subsidence stresses inferred from early tertiary plate motion vectors are not oriented correctly to drive right lateral slip along kaltag farewell and parallel faults in western alaska furthermore these stress states would have produced left lateral displacements for which there is no evidence along these western faults in their present orientations we find that a combination of slightly rotated fault geometry and low friction along the faults appears to facilitate right lateral slip along the faults in western alaska our results show that deformation well inside the continent can be explained in terms of the pacific north american plate motions thus tertiary plate motions have resulted in a broad zone of fault slip uplift and subsidence in the nonrigid north american plate producing several large basins

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

fault valve action and vein development during strike slip faulting an example from the ribeira shear zone southeastern brazil abstract fluid inclusion microthermometry and structural data are presented for quartz vein systems of a major dextral transcurrent shear zone of neoproterozoic cambrian age in the ribeira river valley area southeastern brazil geometric and microstructural constraints indicate that foliation parallel and extensional veins were formed during dextral strike slip faulting both vein systems are formed essentially by quartz and lesser contents of sulfides and carbonates and were crystallized in the presence of co2 ch4 and h2o co2 ch4 nacl immiscible fluids following unmixing from a homogeneous parental fluid contrasting fluid entrapment conditions indicate that the two vein systems were formed in different structural levels foliation parallel veins were precipitated beneath the seismogenic zone under pressure fluctuating from moderately sublithostatic to moderately subhydrostatic values 319 397 c and 47 215 mpa which is compatible with predicted fluid pressure cycle curves derived from fault valve action growth of extensional veins occurred in shallower structural levels under pressure fluctuating from near hydrostatic to moderately subhydrostatic values 207 218 c and 18 74 mpa which indicate that precipitation occurred within the near surface hydrostatically pressured seismogenic zone fluid immiscibility and precipitation of quartz in foliation parallel veins resulted from fluid pressure drop immediately after earthquake rupture fluid immiscibility following a local pressure drop during extensional veining occurred in pre seismic stages in response to the development of fracture porosity in the dilatant zone late stages of fluid circulation within the fault zone are represented dominantly by low to high salinity 0 2 to 44 wt equivalent nacl h2o nacl cacl2 fluid inclusions trapped in healed fractures mainly in foliation parallel veins which also exhibit subordinate h2o nacl cacl2 co2 ch4 and h2o co2 ch4 nacl fluid inclusions trapped under subsolvus conditions in single healed microcracks recurrent circulation of aqueous carbonic fluids and aqueous fluids of highly contrasting salinities during veining and post veining stages suggests that fluids of different reservoirs were pumped to the ruptured fault zone during faulting episodes a fluid evolution trending toward ch4 depletion for co2 ch4 bearing fluids and salinity depletion and dilution approximation of the system h2o nacl for aqueous saline fluids occurred concomitantly with decrease in temperature and pressure related to fluid entrapment in progressively shallower structural levels reflecting the shear zone exhumation history

late palaeozoic palaeomagnetic data for wrangellia resolution of the polarity ambiguity wrangellia and the alaska peninsula terrane are two of the many allocthonous terranes making up southern alaska today palaeomagnetic data from these areas clearly indicate low palaeolatitudes in the early mesozoic because of local and regional rotations the mean declination of the palaeomagnetic vectors is uncertain this in addition to incomplete temporal coverage has led to an ambiguity in the polarity of the vector and hence an ambiguity in whether the palaeolatitudes determined are in the northern or southern hemisphere the data considered here have resolved the ambiguity for wrangellia and by implication for the alaska pennisula terrane and also allow the construction of a highly speculative apparent polar wander curve

the cenozoic denali fault system and the cretaceous accretionary development of southern alaska the juxtaposition of disparate geologic terranes in southern alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the denali fault system in cenozoic time recent geologic investigations in the healy quadrangle strongly suggest that cenozoic horizontal displacements of such magnitude along the denali fault system do not exist in the healy quadrangle isograds and metamorphic facies boundaries of an early late cretaceous metamorphic belt trend across the cenozoic mckinley strand of the denali system without significant horizontal offsets the present geologic makeup of most of southern alaska is primarily the result of the talkeetna superterrane consisting of the previously assembled peninsular terrane and wrangellia colliding with and subsequently being thrust upon the yukon tanana and nixon fork terranes of the ancient north american continent in about middle cretaceous time the leading edge of the talkeetna superterrane faces a wide complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with jurassic and cretaceous flysch the flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks the postcollisional denali fault system developed in cenozoic time across the already accreted continental margin in eastern alaska along an older cretaceous suture

strain accumulation across the denali fault in the delta river canyon alaska deformation along the denali fault in the delta river canyon was determined from geodetic surveys in 1941 1942 1970 1975 and 1979 the data were best for the 1975 1979 interval in that period the average strain accumulation was essentially pure right lateral shear at a rate of 0 6 0 1 rad a a is years engineering shear across a vertical plane striking n87 e the plane of maximum shear is rotated about 30 counterclockwise from the local strike of the denali fault but closely coincides with the strike of a major linear segment of the fault that begins 50 km farther west the deformation between 1941 1942 and 1970 is consistent with a similar rate of strain accumulation if one removes the coseismic strain step contributed by the 1964 alaska earthquake the 1970 1975 deformation is poorly defined owing to uncertainties in the 1970 survey but the strain accumulation during that period is certainly much less than during the 1975 1979 interval the 1975 1979 strain accumulation is interpreted by means of a dislocation model which suggests that the denali fault in the vicinity of the delta river canyon behaves as a leaky transform fault i e a source of spreading as well as lateral slip the block south of the denali fault appears to be moving westward parallel to the strike of the denali fault west of the 147th meridian at the rate of about 20 mm a relative to the north american plate because the linear segment of the denali fault east of the 147th meridian along which the geodetic network is located strikes n63 w accommodation of the westward motion of the southern block requires some spreading within the fault zone as well as right lateral slip on the fault

glacially influenced sedimentation in the later proterozoic of the paraguay belt mato grosso brazil abstract late proterozoic probably early vendian 610 590 ma glaciogenic and turbiditic sediments and metasediments are present along the transitional zone between the amazonian craton to the northwest and the paraguay belt to the southeast in the cuiaba region mato grosso brazil a few exposures of diamictites are associated with conglomerates and graded pebbly sandstones on the cratonic fringe and pass southeastwards into fine grained sediments deposited in the deeper part of the probably marine basin in the paraguay belt these rocks have been affected by the brasiliano orogeny possibly around 570 ma they show increasing deformation and metamorphism from the craton towards the fold belt the sedimentation scheme proposed for the western border of the paraguay belt and the eastern edge of the amazonian craton involves three probably marine main depositional systems to the west on the craton the platform domain can be subdivided into a western inner shelf and an eastern outer shelf the deposits on the inner shelf subdomain show an alternation of dominant massive diamictites sandstones and fine grained sediments with few dropstones this diamictite association is tentatively interpreted as glaciomarine in origin in the outer shelf subdomain the massive diamictites are progressively replaced southeastwards by an association of massive diamictites stratified diamictites and fine grained sediments this diamictite association is interpreted as a whole as resedimented glacial debris deposited by subaqueous debris flows the slope domain located at the transition between craton and fold belt is characterized by strong reworking of the glaciomarine sediments by gravity flows responsible for a type of submarine fan deposition the coarse clastic facies which result from a reworking by rapid downslope transport of cohesive debris flows and gravity flows with high viscosity fill feeder channels in the upper fan eastward inside the belt a progressive sorting in the deeper portions of the fan is shown by the transition from diamictites massive gravels sandstone intercalations with occasional inverse and or normal grading debris flow turbidity current to normal graded finer sediments turbidity current sandstone and siltstone intercalations represent interchannel deposits formed by turbidity currents east of the slope well bedded fine grained sediments were generated by low density turbidity currents direct glacial influence is reduced to the presence of a few isolated clasts or dropstones two distinct upper proterozoic glaciogenic units are known in brazil the older and more widespread one dated at about 950 ma is located on the sao francisco craton and in the adjacent brasiliano fold belts the younger one 600 ma is restricted to the paraguay belt and the adjacent border of the amazonian craton in southern western gondwana late proterozoic glaciation is of mountain type and developed at latitudes of 60 on highs probably generated by pan african brasiliano rifting at the same time in its northern part a widespread ice sheet covered the west african craton

does the moon possess a molten core probing the deep lunar interior using results from llr and lunar prospector 1 it is the main purpose of this study to examine the deeper structure of the moon in the light of four numbers these are lunar mass m mean moment of inertia i second degree tidal love number k2 and the quality factor q accounting for tidal dissipation within the solid body of the moon the former two have been measured by lunar prospector to high precision and more than 30 years of lunar laser ranging llr data have led to an estimate of the second degree tidal love number and quality factor the inverse problem dealt with here of obtaining information on the lunar density and s wave velocity profile from the four numbers follows our earlier investigations by employing an inverse monte carlo sampling method we present a novel way of analyzing the outcome using the bayes factor the advantage lies in the fact that rather than just looking at a subset of sampled models we investigate all the information sampled in different runs i e take into account all samples in order to estimate their relative plausibility the most likely outcome of our study based on the data their uncertainties and prior information is a central core with a most probable s wave velocity close to 0 km s density of 7 2 g cm3 and radius of about 350 km this is interpreted as implying the presence of a molten or partially molten fe core in line with evidence presented earlier using llr regarding the dissipation within the moon

structural development of an accretionary prism by thrust and strike slip faulting shumagin region aleutian trench a combination of seabeam swath bathymetric data closely spaced single channel seismic reflection profiles and multichannel seismic reflection profiles from the shumagin islands region indicates that the large scale deformation of the accretionary prism in this segment of the aleutian trench occurs progressively through several stages 1 folding along axes perpendicular to the plate convergence direction in the region 2 thrust faulting in the direction of plate convergence and 3 oblique strike slip faulting along conjugate west northwest trending right lateral and north northeast trending left lateral faults strike slip faults occur in distinct domains with right lateral oblique faults present west of a major transverse fault at long 161 w and left lateral oblique faults prevalent east of 161 w the structural development of the modern aleutian trench fore arc in this region is probably affected by pre eocene inherited structures that are still active in the present day stress regime a major domain boundary fault located at long 161 w is probably part of a strike slip fault system that also bounds sanak basin farther landward on the shumagin shelf this fault zone is nearly coincident with the mapped position of the western edge of the shumagin seismic gap these transverse structures in the shumagin fore arc probably influence the spatial distribution of stress buildup related to subduction and hence the locations of rupture zones of great interplate earthquakes along this segment of the aleutian trench

the relative contribution of accretion shear and extension to cenozoic tectonic rotation in the pacific northwest large cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the pacific northwest and many tectonic models have been proposed to explain them including 1 rotation of accreted oceanic microplates during docking 2 dextral shear between north america and northward moving oceanic plates to the west and 3 microplate rotation in front of an expanding basin and range province stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations coast range structures basin and range extension and paleomagnetic data from middle miocene 15 ma coast range rocks indicate that dextral shear is responsible for at least 40 of the post 15 ma rotation of the coast range and that basin and range extension is responsible for the remainder reconstructions based on extrapolation of this ratio back to 37 and 50 ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has been a significant contributor to rotation during most of tertiary time changes in the dextral shear rotation rate over the past 50 m y correlate directly with changes in the velocity of the farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost basin and range region and suggest 17 extension since 15 ma 39 since 37 ma and 72 since 50 ma near latitude 42 n

structural evolution of the early tertiary cantwell basin south central alaska the cantwell basin is a paleogene intermontane basin located in south central alaska the cantwell formation which fills the basin is as much as 4000 m thick and consists of a lower sedimentary sequence of fluvial and alluvial fan origin and an upper sequence of calc alkaline volcanic rocks the east trending cantwell basin developed as a southward thickening asymmetrical graben between the hines creek and denali faults substantial dip slip displacement and local dextral strike slip displacement on the hines creek fault during basin subsidence was followed by dip slip displacement in late cenozoic time strike slip movement along the denali fault to the south exerted primary control on basin subsidence and subsequent structural inversion during the paleocene kula plate motion was nearly perpendicular to the axis of the cantwell basin however kula plate motion was oblique to southeastern alaska and produced regional dextral slip on the denali fault system the cantwell basin appears to have developed as a pull apart basin in which northwest striking syndepositional normal faults linked the denali fault with the western part of the hines creek fault in early eocene time an increased rate of plate convergence resulted in substantial displacement on the denali fault concurrent counterclockwise rotation of southern alaska produced a regional constraining bend in the denali fault and resulted in strong compressional deformation of the cantwell basin

the southern kahiltna terrane implications for the tectonic evolution of southwestern alaska regionally extensive and highly deformed jurassic to cretaceous basinal turbidite deposits occur along most of the inboard boundary of the talkeetna superterrane in southern alaska and western canada the southern kahiltna terrane of southwestern alaska consists largely of such basinal deposits regional and detailed studies in and adjacent to this terrane suggest several new conclusions regarding the origin of the basin and its role in the tectonic evolution of southwestern alaska stratigraphic sedimentologic compositional and structural evidence suggests that the upper jurassic to lower cretaceous strata of the southern kahiltna terrane were derived from and deposited on rocks of the adjacent talkeetna superterrane deposition of these clastic rocks apparently postdated arc magmatism in the talkeetna superterrane suggesting deposition in a basin developed on the suture between the talkeetna superterrane and north america if true this observation implies that collision of the talkeetna superterrane with north america began prior to latest jurassic time probably at a more southerly location the superterrane has since been translated northward probably by coast parallel strike slip displacement to its present location where it was structurally juxtaposed with middle to upper creteceous rocks of the kuskokwim group a magmatic overprint indicates that the talkeetna superterrane was in approximately its present location by latest cretaceous to paleocene time

late vendian closure of the adamastor ocean timing of tectonic inversion and syn orogenic sedimentation in the gariep basin abstract new pb pb age data obtained on stromatolitic dolomite from the oceanic marmora terrane in the pan african gariep belt support previous ar ar age data on early amphibole that has been related to sea floor metamorphism and indicate formation of oceanic crust in the neoproterozoic adamastor ocean gariep basin around 600 ma a maximum age for oceanic crust in the gariep basin is given by 741 6 ma which is the best available minimum constraint on continental rifting in the gariep belt tectonic inversion to basin closure overlaps in time with the end of a younger glacial period that is recorded both in the oceanic and continental realms and is correlated with the global c 580 ma marinoan glaciation the rock record up to that time in the marmora terrane is completely different to that in the para autochthonous continental port nolloth zone in the east of the belt but the post glacial sediment successions bear many lithological and geochemical similarities these younger successions are therefore interpreted as reflecting syn orogenic deposits in a foredeep the age of which is constrained between c 580 and 550 ma comparison of three chemostratigraphic profiles through the post glacial carbonate succession shows that enrichment in carbonate 13c is in the first instance a function of proximity to the basin margin and thus of the extent of ocean water stratification

paleomagnetism of eocene volcanic rocks talkeetna mountains alaska previous paleomagnetic studies of eocene rocks deposited in the talkeetna mountains on the northern edge of the combined peninsular wrangellia and alexander terranes the southern alaska superterrane sas yield a paleolatitude of 76 n while late cretaceous rocks on the southern edge of the sas record a 32 n paleolatitude at face value these drastically different paleolatitudes imply that wrangellia moved 44 to the north in only 20 m y requiring a northward component of velocity of about 24 cm yr alternatively the discrepancy might be explained by postulating an unrecognized tectonic boundary separating the two localities allowing different emplacement ages we have tested the unrecognized tectonic boundary hypothesis allowing post eocene emplacement of the sas by sampling eocene volcanic rocks deposited near the southern edge of the sas in the talkeetna mountains our data set comprises measurements from 97 oriented cores collected from 26 lava flows and two tuffs distributed over a distance of 50 km a primary remanence is indicated by positive fold tests and penecontemporaneous intrusion tests age control is provided by nine new k ar whole rock age determinations ranging from 38 8 to 53 6 ma the mean paleomagnetic pole calculated for this study is not significantly different from the pole derived from the eocene flows to the north and gives a paleolatitude of 78 thus ruling out the hypothesis of an unrecognized suture accommodating major post eocene displacement this result has prompted a reevaluation of the data which suggest rapid movement including the precision of the age control the adequacy of averaging of secular variation and the possibility of sedimentary inclination error the bulk of the evidence suggests northward displacement from lower latitudes however the error limits are such that the dramatic velocities previously postulated are not required

geology and geochronology of the betara region in south southeastern brazil evidence for possible statherian 1 80 1 75 ga and calymmian 1 50 1 45 ga extension events abstract the results of geological mapping chemical analysis and radiometric dating of metabasic rocks of betara formation and mapping and dating of those present in the betara basement nucleus together with mylonitic granodiorite and syenogranite are reported here u pb analysis of bulk zircon fractions from the metabasic rocks of the basement nucleus yielded a statherian age of 1790 22 ma while the metabasic rocks from the upper part of the betara formation yielded a calymmian age between 1500 and 1450 ma this age is a minimum for the deposition of the betara formation the older metabasic rocks are associated with post tectonic possibly anorogenic syenogranite while the younger ones are gabbro or very porphyritic ankaramite whose ree patterns are consistent with crystallization from an n morb parent magma the observations and data point to the probable events associated with extensional processes of the end of paleoproterozoic and early mesoproterozoic similar registers of statherian 1 80 1 75 ga and calymmian 1 50 1 45 ga extensional events are recorded in other parts of the south american and african continents the neoproterozoic witnessed the formation and junction of the tectonic slices which formed the apiai domain during the assemblage of western gondwana

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

palaeomagnetism of lower cretaceous tuffs from yukon kuskokwim delta region western alaska during the past decade the prescient arguments1 3 for the allochthoneity of large portions of southern alaska have been corroborated by detailed geological and palaeomagnetic studies in south central alaska4 9 the alaska peninsula10 kodiak island11 12 and the prince william sound area13 fig 1 these investigations have demonstrated sizeable northward displacements for rocks of late palaeozoic mesozoic and early tertiary age in those regions with northward motion at times culminating in collision of the allochthonous terranes against the backstop of nuclear alaska14 15 a fundamental question is which parts of alaska underwent significantly less latitudinal translation relative to the stable north american continent thereby serving as the accretionary nucleus into which the displaced microplates 16 were eventually incorporated17 18 here we present new palaeomagnetic results from tuffs and associated volcaniclastic rocks of early cretaceous age from the yukon kuskokwin delta region in western alaska these rocks were probably overprinted during the cretaceous long normal polarity interval although a remagnetization event as recent as palaeocene cannot be ruled out this overprint direction is not appreciably discordant from the expected late cretaceous direction for cratonal north america the implied absence of appreciable northward displacement for this region is consistent with the general late mesozoic early tertiary tectonic pattern for alaska based on more definitive studies little to no poleward displacement for central alaska though substantially more northward drift for the southern alaska terranes comprising alaska peninsula kodiak island prince william sound area and matunuska valley since late cretaceous to palaeocene time

paleomagnetism and geology of eocene volcanic rocks of southwest washington implications for mechanisms of tectonic rotation paleomagnetic and geologic investigations in eocene volcanic rocks of the southwest washington coast range demonstrate a close relationship between tectonic rotations and the local structural geology the allochthonous middle eocene submarine basalt basement of the crescent formation consists of several fault bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation 20 to perhaps as much as 65 when compared to the eocene reference pole for north america structural analysis shows the differential rotations postdate middle eocene folding of the crescent formation against the continental margin and predate the unconformably overlying upper eocene goble volcanics which are rotated about 23 and do not show the same domains of rotation as the underlying crescent formation post goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults major north northwest trending faults with several kilometers of dextral displacement form the boundaries of cross faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west northwest trending sinistral r riedel shears thirty five to 100 of the observed post late eocene rotations could have occurred by this shear rotation mechanism other paleomagnetic study areas in the coast range and western cascades have a similar fault geometry and may also have undergone significant shear rotations long term northward oblique subduction of the farallon plate beneath the coast range throughout most of the tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large rigid plates

p t fluid deformation regime of the ediacaran serra do cavalo magro orogenic gold deposit ribeira belt brazil abstract the serra do cavalo magro orogenic gold deposit is located in the southern ribeira belt brazil gold quartz veins are hosted in calymmian 1500 1450 ma metasedimentary and metabasic rocks and ediacaran 610 600 ma granitic rocks geochronological data available in previous literature and structural petrological relationships with host rocks and shear zones indicate that the gold mineralization was formed within 580 540 ma during the late stage evolution of the brasiliano pan african orogeny related with gondwana supercontinent assembly petrological modeling indicates peak metamorphic conditions from 560 c and 7 kbar chlorite biotite phyllite to 625 c and 6 8 kbar garnet biotite phyllite recorded in immediate host rocks gold mineralization occurs in quartz shear and extensional veins structurally controlled by ne trending second and third order sinistral transcurrent shear zones the nnw trending subvertical and subhorizontal extensional veins are oriented 55 85 in relation to the fault planes indicating the shear zones were severely misoriented to frictional reactivation deformation within the shear zones was accommodated by bulging recrystallization of quartz aggregates while feldspar aggregates from the granitic protolith underwent cataclastic flow or were replaced by sericite epidote producing phyllonites gold precipitated during stages of vein deformation within microfractures in shear and extensional veins with quartz previously subjected to bulging recrystallization metamorphic ore bearing fluids are recorded in distinct assemblages of low to moderate salinities 1 18 wt nacleq h2o nacl cacl2 and h2o co2 n2 nacl cacl2 fluid inclusions and co2 n2 inclusions microthermometric data indicate entrapment conditions of 250 350 c and 0 3 2 3 kbar recording hydrostatic to supralithostatic fluid pressures in post peak metamorphic stages sharp fluid pressure fluctuations 0 4 1 6 kbar resulted from earthquake rupture events and the fault valve behavior induced circulation of fluids from distinct sources with limited partial fluid mixing favoring the gold transportation and precipitation

late cenozoic uplift of denali and its relation to relative plate motion and fault morphology apatite fission track analysis of samples that cover a 4 kilometer vertical section from the western flank of denali mount mckinley north america9s highest mountain suggests that the mountain massif was formed by rapid uplift 1 kilometer per million years beginning 6 million years ago ma uplift was a result of the morphology of the denali fault and a change in motion of the pacific plate with respect to north america at 5 ma which created opposing tangential vectors of relative movement along the fault and forced the intervening crustal blocks upward

fluid regimes fault valve behavior and formation of gold quartz veins the morro do ouro mine ribeira belt brazil abstract fluid inclusion studies petrographic analyses and structural analyses for neoproterozoic auriferous veins from the morro do ouro mine ribeira belt southeastern brazil are presented the deposit is classified as an orogenic gold deposit the vein type au mineralization at the mine is restricted to structurally controlled domains in a low grade calymmian ca 1500 1450 ma metasedimentary sequence that also hosts syngenetic au mineralization two main auriferous quartz vein systems are present i nw trending subvertical extensional veins and ii ne trending subvertical veins parallel to the axial surface of tight folds the mineralized veins are adjacent to a major dextral transcurrent fault zone and their structural relationships indicate that this fault is severely misoriented for frictional reactivation the nw trending veins present a fluid inclusion assemblage dominated by co 2 n 2 ch 4 inclusions h 2 o co 2 n 2 ch 4 nacl cacl 2 inclusions and h 2 o nacl cacl 2 inclusions both with highly variable salinities 0 5 to 52 wt nacl equivalent while the ne trending veins are dominated by h 2 o co 2 n 2 ch 4 nacl cacl 2 and h 2 o nacl cacl 2 inclusions also of highly variable salinities 1 to 45 wt nacl equivalent the entrapment of fluids occurred at temperatures between 225 and 240 c in the nw trending veins and at approximately 208 c in the ne trending veins involving processes of fluid immiscibility and mixing between fluids of contrasting compositions the nw trending extensional veins were formed predominantly under pressure fluctuating between near lithostatic and strongly supralithostatic conditions 100 250 mpa during pre seismic failure stages the ne trending veins precipitated predominantly under subhydrostatic to lithostatic pressure conditions 10 150 mpa following the discharge of fluids from different reservoirs along the ruptured zone after earthquake rupture stages the hydrothermal fluids were probably enriched with gold through interactions with the host rocks and its precipitation is attributed to changes in physicochemical properties due to fluid immiscibility and mixing between fluids of highly contrasting salinities as a consequence of cyclic fluctuations in the values of fluid pressure and tectonic stresses accompanying episodes of seismogenic fault activity

constraining the age of the iporanga formation with shrimp u pb zircon implications for possible ediacaran glaciation in the ribeira belt se brazil abstract the ribeira belt in se brazil is a neoproterozoic to early palaeozoic orogen whose architecture and history is not yet fully understood the depositional age of many of the sedimentary sequences in the ribeira belt remains unconstrained and with debate concerning their depositional environment and tectonic setting in this paper we present shrimp zircon u pb age constraints for one such problematic unit in the ribeira belt the iporanga formation and discuss the significance of this age with regards to the timing of neoproterozoic glacial events in southeast brazil using a felsic volcanic unit immediately under the iporanga formation and granite cobbles from breccias in its basal parts a reconnaissance shrimp u pb zircon maximum depositional age of 580 ma is assigned for the base of this unit this age is marginally younger than the 625 605 ma ages for intrusions into the lajeado and ribeira subgroups with which the iporanga formation is in tectonic contact this indicates that the lajeado and ribeira subgroups are not stratigraphically equivalent to the iporanga formation as thought previously by some workers the maximum depositional age of 580 ma also places a maximum time constraint on the tectonic juxtaposition of the iporanga formation with other supracrustal units and on the greenschist facies metamorphism and isoclinal folding that affected it the potential glacial origin for the iporanga formation if correct would place it in the late ediacaran provisionally equivalent to the gaskiers glaciation

early tertiary paleomagnetic evidence and the displacement of southern alaska paleomagnetic data from early paleocene lavas a few kilometers north of lake clark just inboard of the peninsular terrane indicate modest post early tertiary latitudinal displacement and about 55 sup 0 of counterclockwise rotation of this region with respect to north america a nearby middle eocene sequence of lava flows however gives no indication of displacement or rotation south of lake clark the paleomagnetism of five paleogene 30 to 40 ma volcanic sequences that are a part of the peninsular terrane proper shows no northward translation and no rotation a major part of the northward movement documented by other studies in the gulf of alaska region may be accounted for by cumulatively significant intraterrane displacements 32 references 3 figures 1 table

vendian cambrian of western gondwana introduction lnstituto de geulogia y paleontologik facultad de ciencias igua 4225 11400 montevideo uruguay e mail gauche chasque apc org department of geological sciences university of cape town rondebosch 7701 south africa e mail hejzdgeology uct ac za neg labise departamento de geologia ufpe c p 7852 recife pe 50670 000 brazil e mail valderez ufie br cig conicet unlp calle 1 n o 644 1900 la plata argentina e mail poire cig museo unlp edu ar

denali fault system of southern alaska an interior strike slip structure responding to dextral and sinistral shear coupling the denali fault system dfs extends for 1200 km from southeast to south central alaska the dfs has been generally regarded as a right lateral strike slip fault along which post late mesozoic offsets of up to 400 km have been suggested the offset history of the dfs is relatively unconstrained particularly at its western end for this study we calculated relative motion vectors at discrete points along the length of the dfs based on the well understood kinematic interaction between the north american pacific and kula plates and the following assumptions 1 the arcuate geometry of the dfs has existed essentially unchanged since the late cretaceous 2 the yukon tanana terrane and other terranes north of the dfs were fixed in situ prior to the accretion of the southern alaskan terranes and 3 tangential and normal relative motion vector components calculated for points along the dfs using the plate model of kelley 1993 describe the plate kinematics of the dfs since the late cretaceous the consequent kinematic model for the dfs predicts that left lateral stresses have acted upon the western end of the dfs for much of its history and conflicting senses of shear exist between the eastern and western ends of the system the offset history of the western end of the denali fault system should be significantly different than the history of the central and eastern sections consequently individual crustal blocks in southeast and southwest alaska may have undergone respectively clockwise and counterclockwise rotations the sense of rotation predicted by our model is in agreement with rotations determined by paleomagnetic studies and provides an alternative model to the alaskan orocline hypotheses

character distribution and tectonic significance of accretionary terranes in the central alaska range the central part of the alaska range near mount mckinley is composed of nine separate tectonostratigraphic terranes that were accreted in southern alaska during late mesozoic time these terranes now form long linear fault bounded belts that are subparallel to the denali fault on the north but oblique to the fault on the south the postaccretion right lateral offset along the denali fault system is about 200 km from north to south the major terranes are 1 yukon tanana terrane metasedimentary and metavolcanic rocks mostly undated but including rocks of known late paleozoic age polymetamorphosed with terminal events in late mesozoic 2 pingston terrane isoclinally folded upper triassic deep water silty limestone quartzite and carbonaceous slate folded with upper paleozoic phyllite chert tuff and minor limestone 3 mckinley terrane upper paleozoic flysch chert and minor limestone intruded by large gabbro sills and dikes and overlain by thick piles of triassic porphyritic pillow lava the top of the section is thick sequence of upper mesozoic conglomerate flysch chert and phyllite 4 dillinger terrane very thick sequence of strongly folded lower paleozoic micaceous sandstone turbidites graptolitic shale and deep water limestone locally overlain unconformably by jurassic fossiliferous sandstone or triassic pillow basalt 5 windy terrane heterogeneous assemblage of serpentinite basalt tuff and chert ophiolite with paleozoic and mesozoic flysch and blocks of mid paleozoic fossiliferous limestone 6 mystic terrane predominantly upper paleozoic flysch and conglomerate but also includes lower paleozoic graptolitic shale pillow basalt and shallow water limestone and upper paleozoic fossiliferous limestone sandstone chert and undated pillow basalt 7 chulitna terrane upper devonian ophiolite overlain by upper paleozoic chert volcanic conglomerate limestone and flysch capped by lower triassic limestone and upper triassic red beds basalt and interbedded basalt and limestone later mesozoic rocks are sandstone chert and argillite 8 westfork terrane jurassic chert sandstone conglomerate and triassic and jurassic crystal tuff 9 broad pass terrane upper paleozoic chert tuff and argillite with blocks of devonian and older limestone locally associated with serpentinite these diverse terranes of mixed oceanic and continental affinities are now juxtaposed to form a complex sequence of folded and faulted rootless nappes major suture zones between them are occupied by intensely deformed upper mesozoic flysch all of the terranes differ markedly from wrangellia the southermost coherent accreted terrane in alaska paleomagnetic measurements from triassic rocks prove that wrangellia originated near the equator probably in the southern heimsphere paleolatitudes for terranes in the central alaska range are not yet determined but other lines of evidence biogeographic and lithologic suggest large scale northern transport for some terranes

limits to northward drift of the paleocene cantwell formation central alaska volcanic rocks of the paleocene cantwell formation in central alaska apparently originated at a paleolatitude of 83 n 95 9 7 as indicated by paleomagnetic results when compared with the paleocene pole for the north american craton the 95 confidence limits of the results suggest that terranes north of the denali fault have moved no more than 550 km northward relative to the north american craton since paleocene time

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

on the mechanism of tectonic transport in zones of oblique subduction abstract many cases of oblique subduction are known throughout the world today and many more must have existed in the past in at least one modern case sumatra the over riding plate has responded to oblique subduction by forming a strike slip fault that traverses the magmatic arc and displaces its leading edge in the direction of the transverse component of oblique convergence conditions favoring this sort of tectonic response are shown to be 1 1 a high angle of oblique convergence 2 2 a low angle of subduction and 3 3 relative thermal softening of the magmatic arc this mechanism may have been responsible in part for northward transport of western cordilleran magmatic arc and subduction zone rocks in the late cretaceous and earliest tertiary it also may help to explain evidence of southward transport and left lateral shear in northern mexico and the caribbean region in pre jurassic time