jurassic sedimentary melange and associated facies baja california mexico the middle jurassic coloradito formation of cedros island baja california is a fine grained flysch sequence that includes some coarse debris flow filled channels sedimentary melange olistostrome is widespread and is composed of sheared black argillite with megablocks of triassic cherts shallow water origin paleozoic limestone and continental sandstone derived from the north this unit represents the earliest record of arc continent interaction in the baja area the eugenia formation is locally present in erosional contact above the coloradito formation the unit is a northerly derived channel fill deposit in which the lower one third is dominated by metasedimentary detritus and first cycle calc alkaline volcanic detritus dominates above this the long axes of clasts in sheared argillite of the coloradito have orientations parallel to clast imbrication in associated conglomerate channels suggesting shearing due to downslope sliding contrasting deformation styles sheared versus open folded of fine grained coloradito units may be due to abundance of smectite in the latter allowing higher water retention and a more plastic style of deformation jurassic sedimentary sequences on isla cedros record a complex history of explosive island arc volcanism gran canon formation arc continent collision coloradito formation and renewal of magmatic arc volcanism eugenia formation

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

sediment fluxes along the northeastern european margin inferring hydrological changes between 20 and 8 kyr abstract in an attempt to delineate past hydrographic conditions on the northeastern atlantic ocean margin along a latitudinal transect 37 63 n and link the eastern north atlantic ocean with the norwegian sea we have investigated planktonic and benthic stable oxygen isotopes as well as abundances and fluxes of planktonic and benthic foraminifera calcium carbonate organic carbon and detrital grains in six sediment cores studied by the enam project in our joint approach we focused on the major climatic events of the last 20 kyr that is the last glacial maximum lgm the heinrich event 1 h1 the younger dryas yd and the holocene climatic optimum hco simmax temperature estimates suggest glacial ssts off portugal similar to holocene values contradicting the proposed polar front position of climap project members 1976 besides increased planktonic and benthic foraminiferal fluxes in the norwegian sea and the faeroe islands cores point to nordic seas partially ice free with seasonally productive polynas multiple ird spikes observed at the faeroe islands and norwegian sea sites coeval with h1 suggest multiple pulses of the fenno scandian ice sheet at the same time the presence of ird on the portuguese margin cores confirms the southward influence of icebergs the increase in foraminiferal abundances and fluxes that parallel ird spikes at all sites suggests that times of increased productivity either succeed or occur simultaneously with the iceberg surges in the open north atlantic productivity conditions similar to those observed during h1 are also registered during the yd even though planktonic foraminiferal assemblages suggest less cold conditions

rift flank uplift at the gulf of california no requirement for asthenospheric upwelling continental rifts are commonly flanked by zones of high elevation but the cause of uplift remains controversial proposed uplift mechanisms include active and induced asthenospheric upwelling and isostatically driven lithospheric flexure discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension here we focus on the well preserved neogene gulf of california rift the western rift margin is characterized by a prominent east facing kilometer scale escarpment which bounds a west tilted topographically asymmetric rift flank we exploit west draining canyons incised into the rift flank to constrain the timing of uplift to between ca 5 6 and 3 2 ma using 40ar 39ar dating of lavas which show cut and fill relationships to the canyons rift flank uplift closely followed the onset of slip on the principal fault of the loreto rift segment at ca 8 6 ma the age of which we obtain from apatite u th he and fission track thermochronologic analysis of rift escarpment exhumation uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca 6 and 3 ma but post dates a proposed asthenospheric upwelling event by 8 10 ma the timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

evolution of the guerrero composite terrane along the mexican margin from extensional fringing arc to contractional continental arc the western margin of mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins accretion of exotic island arcs by the consumption of entire ocean basins versus accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone we present geologic and detrital zircon evidence that the zihuatanejo terrane of the guerrero composite terrane originated from the latter mechanism the evolution of the zihuatanejo terrane can be explained by extensional and compressional processes operating entirely within the upper plate of a long lived subduction zone that dipped east under the mexican margin this process controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic material during extension followed by accretion and thickening of the crust during contraction prior to this study all mesozoic rocks in the western part of the guerrero composite terrane were considered to be part of a single arc however we divide it into four distinctive tectonostratigraphic assemblages 1 a triassic early jurassic accretionary complex arteaga complex 2 a jurassic to earliest cretaceous extensional volcanic arc assemblage 3 an early cretaceous extensional arc assemblage and 4 a santonian maastrichtian compressional arc assemblage 1 the arteaga subduction complex forms the basement to the zihuatanejo terrane and includes grenville pan african and permian detrital zircon suites that match the potosi fan of the mexican mainland 2 the jurassic to earliest cretaceous extensional volcanic arc assemblage shows a callovian tithonian ca 163 145 ma peak in magmatism extensional unroofing began in this time frame and continued into through the next 3 the early cretaceous extensional arc assemblage has two magmatic peaks one in the barremian aptian ca 129 123 ma and the other in the albian ca 109 ma in some localities rapid subsidence produced thick mainly shallow marine volcano sedimentary sections while at other localities extensional unroofing of all older assemblages resulted in recycling of zircon from all older units 1 2 3 4 for the santonian maastrichtian compressional arc assemblage our new detrital zircon dates show for the first time that arc volcanic rocks of this age are present in the coastal zihuatanejo terrane the contractional arc developed atop assemblages 1 3 which were shortened between turonian and santonian time ca 93 and 84 ma taken together the western zihuatanejo terrane records a more protracted history of arc magmatism than has yet been dated in other terranes of western mexico but it closely matches the history of baja california to the northwest

model for late mesozoic early tertiary tectonics of coastal california and western mexico and speculations on the origin of the san andreas fault cretaceous and early tertiary arc and forearc rocks found along the coast in southern california and baja california have been shown by paleomagnetic measurements to have originated many hundreds or even several thousands of kilometers south of their current locations northward transport also is found in cretaceous batholithic rocks near the edge of the continent in washington and british columbia the consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of north america have been translated along the coast by strike slip faulting strike slip faults are shown to be an expected response to north oblique subduction of the farallon and or kula plates faulting and subduction probably were concurrent activities i e the arc and forearc grew and were displaced simultaneously because displacement was dominantly parallel to subduction related lithic belts arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts subduction related strike slip faulting is shown to be favored by high angles of oblique convergence shallow dip of the subducting slab relatively easy slip on potential faults within the arc or forearc and the existence of a place for the moving sliver to go it is suggested that these conditions are not difficult to satisfy and that subduction related strike slip faulting may thus be a common feature of convergent orogens the location of the san andreas fault inboard from the continental margin may be attributable to localization of the pacific north american transform by a system of precollision subduction related faults

a method for evaluating basement exhumation histories from closure age distributions of detrital minerals we have developed a two dimensional thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement using this model we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata at relatively high mean denudation rates of 0 5 km m y 1 sustained over millions of years magmatic heating events have minimal influence upon the age distributions of detrital minerals such as k feldspar that are moderately retentive of radiogenic ar at lower rates however the effects of batholith emplacement may be substantial we have applied the approach to detrital k feldspars from forearc strata derived from the deeply denuded peninsular ranges batholith prb agreement of the denudation history deduced from the detrital k feldspar data with thermochronologic constraints from exposed prb basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known c 1999 american geophysical union

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

a statistical scaling model for fracture network geometry with validation on a multiscale mapping of a joint network hornelen basin norway 1 fracture patterns are characterized by a complex geometry which involves a large length distribution and nonhomogeneous density distributions here we address the issue of the modeling of this complex geometry over scales through a first order model providing the characterization of the number of fractures of a given length and at a given scale we propose that the simplest model is a power law in both space and fracture length with three main parameters the exponent a of a power law length distribution the fractal dimension d which fixes the scale dependence of the number of fractures and the fracture density we verify the applicability of this model on seven fracture patterns mapped from the metric scale up to almost the kilometric scale in the hornelen basin the model efficiently describes fracture network properties at all scales with a single set of parameters for the hornelen fracture networks we found a simple relationship between basic exponents a d 1 implying that the fracture network is self similar through this analysis we present different methodological developments for deriving the basic exponents a and d and for verifying the consistency of the model overall the main methodological development is about the normalization of the measurements made from different scales of observation finally we discuss both the limitations and the uses of such a model for analyzing the hydraulic and mechanical properties of fracture networks

volcanic vent self similar clustering and crustal thickness in the northern main ethiopian rift 1 the spatial distribution of volcanic vents in the northern ethiopian rift has been analysed in terms of their self similar clustering the fractal distribution of vents has been computed for the monogenetic vents cones domes and maar occurring along the rift valley and in the rift margin the self similar clustering of vents has been computed through the analysis of vent centres coordinates by means of the two point correlation method the vents of the northern main ethiopian rift display a self similar clustering characterised by a correlation coefficient of 1 41 0 02 with a size range defined by a lower cut off of 1 1 km and an upper cut off of 26 5 km this analysis indicates a probable crustal thickness of 27 km as indicated by the value of the computed upper cut off this value compares well with other estimates for the crustal and lithosphere thickness in the ethiopian rift

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

mid cretaceous crustal extension recorded in deep marine half graben fill cedros island mexico marine mid cretaceous fore arc strata on cedros island vargas and pinos formations record the onset of extensional brittle deformation of the upper crust concomitant with initial uplift of blueschist grade rock from great depth the pre extensional vargas formation is chiefly fine grained and it contains facies interpreted as basin plain sand rich fan and slope deposits that onlapped a jurassic unconformity toward the north the syn extensional pinos formation is coarser grained than is the vargas and records the initiation and filling of a northeast southwest trending deep marine half graben structure that formed by reactivation of a jurassic fault zone located along the northern margin of the basin the axis of the half graben acted as a submarine canyon that funneled gravelly sediment gravity flows toward the south west whereas the shoulder of the half graben was draped by sandy turbidites and mud a second episode of faulting on a fault parallel to and synthetic with the older half graben fault produced a stress transfer zone that steepened the intervening block producing a catastrophic slope failure in the basin several points of evidence suggest that the canyon bounding fault zone mapped at the northern margin of the basin was syndepositionally active basin floor scarps are indicated by the presence of large angular blocks derived from the jurassic strata these blocks occur within cretaceous conglomerate and olistostromes and as clast supported breccia asymmetric basin subsidence is recorded in monotonic lithofacies trends that occur laterally away from the inferred fault zone these trends include a decrease in average grain size and bed thickness and an increase in interbedded mudstone and in lateral continuity of individual beds abundant slope failure features including slide blocks as much as 100 m wide by 40 m thick also point to frequently recurring intrabasinal seismicity the facies distribution and basin structures exposed on cedros island are analogous to aspects of the southern viking graben and the ebro turbidite complex modern tectonic and physical analogues for the half graben are found at the kuril margin and in the aleutian are where oblique convergence has produced fault bounded submarine canyons of the same lateral and vertical scale as inferred for the cretaceous basin on cedros island the temporal coincidence between brittle deformation in the upper plate with peak blueschist metamorphism in the lower plate of the convergent margin suggests that initial blueschist uplift began soon after deepest burial

scaling laws of the size distribution of monogenetic volcanoes within the michoacan guanajuato volcanic field mexico abstract the michoacan guanajuato volcanic field displays about 1040 monogenetic volcanoes mainly composed of basaltic cinder cones this monogenetic volcanic field is the consequence of a dextral transtensive tectonic regime within the transmexican volcanic belt tmvb the largest intra continental volcanic arc around the world related to the subduction of the rivera and cocos plates underneath the north american plate we performed a statistical analysis for the size distribution of the basal diameter wco for cinder cones dataset used here was compiled by hasenaka and carmichael 1985 monogenetic volcanoes obey a power law very similar to the gutenberg richter law for earthquakes with respect to their size distribution log 10 n wco log 10 wco with 5 01 and 2 98 therefore the monogenetic volcanoes exhibit a wco size distribution empirical power law suggesting a self organized criticality phenomenon

volcano spacing and plate rigidity in plane stresses which accompany the flexural deformation of the lithosphere under the load adjacent volcanoes may govern the spacing of volcanoes in hotspot provinces specifically compressive stresses in the vicinity of a volcano prevent new upwelling in this area forcing a new volcano to develop at a minimum distance that is equal to the distance in which the radial stresses change from compressional to tensile the inflection point if a volcano is modeled as a point load on a thin elastic plate then the distance to the inflection point is proportional to the thickness of the plate to the power of 3 4 compilation of volcano spacing in seven volcanic groups in east africa and seven volcanic groups of oceanic hotspots shows significant correlation with the elastic thickness of the plate and matches the calculated distance to the inflection point in contrast volcano spacing in island arcs and over subduction zones is fairly uniform and is much larger than predicted by the distance to the inflection point reflecting differences in the geometry of the source and the upwelling areas

spatial analysis of an intra plate basaltic volcanic field in a compressional tectonic setting south eastern australia abstract the newer volcanics province nvp is a pliocene to recent intra plate basaltic volcanic field bvf that has formed in a compressive tectonic setting v

the holocene occurrence of cold water corals in the ne atlantic implications for coral carbonate mound evolution abstract u series dating of constructional cold water corals is a powerful tool to reconstruct the evolution of corals on carbonate mounds here we have investigated the time framework of corals such as lophelia pertusa and madrepora oculata on five different mound settings of the eastern north atlantic on rockall bank and in porcupine seabight sampled at variable depth and location 610 880 m water depth we have found that the past 11 ka reflect a period generally favourable for coral development we further determined local mound growth rates and identified mound surface erosion framework collapse during times of active coral framework construction local vertical mound growth rates vary between less than 5 cm ka 1 and up to 220 cm ka 1 we interpret rates exceeding 15 cm ka 1 as representative of densely populated coral reefs during times of reduced or absent coral development mound evolution rates are by far smaller 0 to 1 the time resolution achieved here furthermore provides first evidence for reduced coral ecosystem activity at 1 8 2 0 ka 4 2 4 8 ka and between 6 and 8 2 ka within the holocene that may be related to climate driven changes of the coral growth environments during glacial periods coral growth in those areas seems apparently extremely reduced or is even absent on mounds

self similar clustering of cinder cones and crust thickness in the michoacan guanajuato and sierra de chichinautzin volcanic fields trans mexican volcanic belt abstract the spatial clustering of basaltic vents in monogenetic volcanic fields has been used as a proxy for crustal thickness in extensional and back arc tectonic settings the basaltic vents have a fractal clustered distribution self similar clustering described by a power law the power law is defined over a range the size range of the distribution of values in this case the vents separation delimited by a lower and an upper cut offs here we apply the fractal clustering analysis to the two largest monogenetic volcanic fields of the trans mexican volcanic belt tmvb a continental arc built on different crustal terranes the michoacan guanajuato volcanic field mgvf located in the central western tmvb includes over 1000 vents of late pliocene to quaternary age built on attenuated crust of mesozoic to tertiary age the sierra de chichinautzin volcanic field scvf in the central eastern tmvb is composed of 220 late pleistocene to holocene vents laying above thicker crust of precambrian to tertiary age monogenetic vents in both volcanic fields show self similar clustering with fractal exponent d 1 67 in the range 1 3 38 km mgvf and d 1 56 in the range 1 5 32 km scvf the upper cut off uco for the power law distribution of the mgvf well fits the crustal thickness below the volcanic field as derived from independent geophysical data the uco value of scvf indicates a crust thickness of about 32 km this value is in agreement with new geophysical data that indicate magma underplating the crust beneath the volcanic field area

spatial relationship between earthquakes and volcanic vents in the central northern main ethiopian rift abstract during the breakup of continents extension is commonly accommodated by connected networks of fluid filled fractures dykes and by faults despite the importance of these two extension mechanisms their spatial relationship in three dimensions is poorly understood primarily because it is difficult to quantify the subsurface distribution of faulting and intrusion in order to address this problem we conduct a quantitative analysis of the spatial distribution and clustering of earthquakes and volcanic vents in the main ethiopian rift in east africa in order to understand how extension by faulting and intrusion is distributed throughout the volcanic rift we use fractal analysis of earthquake epicentres in order to infer the 2d characteristics of the subsurface fault network and directly test our model results against the 3d distribution of earthquake hypocentres our results show that fractal analysis of these features is a reliable means to characterise the 3d properties of the fault network in addition the strong similarity between the properties of the fault network derived from earthquakes and properties of the magma filled fracture network derived from fractal analysis of volcanic vents strongly suggests that these are genetically linked we then explore their spatial link using computation of earthquake and vent density which shows that the zone of seismicity is generally around 20 30 km wide while the zone of vents is narrower and centred within the zone of seismicity this spatial relationship suggests that the faults which form rift axial grabens are induced above a narrower and central zone of diking we also demonstrate significant along rift variation in degree of magmatism and faulting with regions of increased degree of diking inferred from a higher cone density characterised by reduced degree of faulting

palynology and chronostratigraphy of the upper cretaceous in the subsurface of the barinas area western venezuela palynological and well log analyses of cretaceous strata from southeastern venezuela allow the recognition of third order eustatic cycles and are the basis for a chronostratigraphic framework with a datum approximately every 0 8 m y this definition permits the correlation of events in the local geologic evolution with regionally important sedimentological and tectonic events an early cretaceous marine transgression reached areas north of the andes before 113 ma however according to our framework the transgression extended into the barinas area shortly after 98 ma an interval of relatively low sea level from 95 5 ma to 91 5 ma is represented by oil bearing sandstones and dolomitic limestones a second marine transgression reached the area at approximately 91 5 ma and resulted in the deposition of organic rich clastics that are correlatable with source rocks from areas to the north a major hiatus santonian campanian encompasses approximately 11 m y and probably is related to the emplacement of oceanic rocks in western colombia a younger major hiatus maastrichtian eocene encompasses approximately 26 m y and probably is related to the emplacement of the lara nappes in northern venezuela two other minor intra turonian and intra maastrichtian hiatuses are recognized and encompass approximately 1 5 m y each

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

spatial distribution and structural analysis of vents in the lunar crater volcanic field nevada usa volcanoes within monogenetic volcanic fields often are arranged in alignments and clusters which are related to effects of magma source geometry in the upper mantle principal stress orientations and crustal structures on their magma feeding systems we use cluster analysis with dendrogram vent morphometric analysis and field structural data to explore the relationships between volcanoes and tectonic features in the plio pleistocene part of the lunar crater volcanic field lcvf pancake range nevada usa which includes 96 monogenetic volcanic edifices totaling 119 vents structural analysis identified three main sets of faults with dip slip kinematics mostly normal with a few examples of thrust faults striking n s e w and ne sw the ne sw set comprises dip slip faults with a dominant normal component of movement which are consistent with the modern state of stress based upon the world stress map database spatial distribution pattern analysis suggests a clustered distribution of vents in the lcvf and gis based spatial density analysis shows that these clusters trend mostly ne sw morphometric study of the monogenetic cones which provides information on feeder dike orientation where dikes are not directly exposed suggests dominant nne ssw to ne sw orientations of near surface inferred dikes an amount of 27 out of 31 inferred feeder dikes within the lcvf is parallel to the present orientation of the greatest principal horizontal stress hmax as suggested by world stress map data derived from hydrofracturing and earthquake focal mechanisms in some cases dike strike is parallel with that of pre existing quaternary dip slip faults we suggest that the spatial distribution of vents is related to domains of different scales of partial melting and compositional heterogeneity in the upper mantle source which is substantiated by geochemical data the relationship of feeder dikes with respect to shallow tectonic structures although somewhat ambiguous at lcvf is consistent with behavior that is intermediate between volcanic fields with high and low long term magma fluxes

fissural volcanism polygenetic volcanic fields and crustal thickness in the payen volcanic complex on the central andes foreland mendoza argentina shield volcanoes caldera bearing stratovolcanoes and monogenetic cones compose the large fissural payen volcanic complex located in the andes foreland between latitude 35 s and 38 s the late pliocene pleistocene and recent volcanic activity along e w trending eruptive fissures produced basaltic lavas showing a within plate geochemical signature the spatial distribution of fractures and monogenetic vents is characterized by self similar clustering with well defined power law distributions vents have average spacing of 1 27 km and fractal exponent d 1 33 defined in the range 0 7 49 3 km the fractal exponent of fractures is 1 62 in the range 1 5 48 1 km the upper cutoffs of fractures and vent fractal distributions about 48 49 km scale to the crustal thickness in the area as derived from geophysical data this analysis determines fractured media crust thickness associated with basaltic retroarc eruptions we propose that the payen volcanic complex was and is still active under an e w crustal shortening regime

proliferation and demise of deep sea corals in the mediterranean during the younger dryas abstract uranium series and radiocarbon ages are reported for deep sea corals madrepora oculata desmophyllum dianthus lophelia pertusa and caryophyllia smithii from the mediterranean sea u series dating indicates that deep sea corals have persisted in the mediterranean for over 480 000 years especially during cool interstadial periods the most prolific period of growth however appears to have occurred within the younger dryas yd period from 12 900 to 11 700 years bp followed by a short 330 years phase of post yd coral growth from 11 230 to 10 900 years bp this indicates that deep sea corals were prolific in the mediterranean not only during the return to the more glacial like conditions of the yd but also following the rapid deglaciation and transition to warmer conditions that followed the end of the yd surprisingly there is a paucity last glacial maximum lgm coral ages implying they were largely absent during this period when cold water conditions were more prevalent radiocarbon ages show that the intermediate depth waters of the mediterranean generally had 14 c compositions similar to surface waters indicating that these waters were extremely well ventilated the only exception is a narrow period in the yd 12 500 100 years bp when several samples of lophelia pertusa from the ionian sea had 14 c values falling significantly below the marine curve using a refined approach isolation ages isol of 300 years to 500 years are estimated for these intermediate 800 1000 m depth waters relative to surface marine waters indicating a reduction or absence of deep water formation in the ionian and adjacent adriatic seas during the yd contrary to previous findings we find no evidence for widespread intrusion of low 14 c atlantic waters into the mediterranean prolific growth of deep sea corals in the mediterranean ended abruptly at 10 900 years bp with many of the coral bearing mounds on the continental slopes being draped in a thin veneer of mud their demise is attributed to a number of factors including the direct loss of habitat due to high sedimentation that accompanied glacial meltwater pulses together with rising temperatures that would have finally pervaded the deeper water of the mediterranean following the onset of holocene warming

cinder cone clustering in the transmexican volcanic belt implications for structural and petrologic models cinder cone distributions have most often been characterized using univariate statistics here a new technique to volcano distribution studies cluster analysis is applied to cinder cone distribution in the central transmexican volcanic belt tmvb a total of 1016 cinder cones are identified over an area of approximately 60 000 km2 application of cluster analysis reveals structure in cinder cone distribution using a search radius parameter of 16 km 75 of the cinder cones within the central tmvb are found within eight clusters of 45 159 cones each these clusters are each 2000 to 5000 km2 in area only 22 cones are found within clusters of three or fewer cinder cones indicating that clustering is a pervasive phenomenon some petrologic variation is evident among clusters low mg alkaline cinder cones are found within a single cluster 360 400 km from the trench application of alignment analysis techniques including the two point azimuth method the hough transform and two dimensional fourier analysis demonstrates that cinder cone alignments have common orientations on a regional scale within three clusters all located in the southernmost part of the michoacan guanajuato volcanic field in the western portion of the study area these alignments consist of tens of cinder cones are 20 50 km long and are all oriented with azimuths of 020 040 parallel to the direction of plate convergence high mg lavas which last fractionated at pressures in excess of 8 kbar are only found associated with these alignments indicating that magma transport is significantly enhanced in these areas although local alignments of three to six cinder cones occur within mapped fault zones that transect the area regional cinder cone alignment patterns and the distribution of clusters themselves do not appear to be affected by the presence of these fault zones the cluster analysis the alignment analysis and some petrologic data support the hypothesis that the tmvb is segmented near 101 w

biochronology paleoenvironments and stratigraphic sequences of the late albian middle eocene fore arc vizcaino basin western baja california mexico the vizcaino fore arc basin accumulated approximately 4 km 13 000 ft of upper albian middle eocene siliciclastic marine sedimentary rocks derived from the peninsular ranges in baja california data from eight exploratory wells document the micropaleontological content and lithological characteristics of these rocks the strata studied represent mostly neritic upper bathyal marine environments and overlie a basement composed of cretaceous granitic rocks or aptian albian volcaniclastic sedimentary rocks correlative with the alisitos formation we recognize four major depositional sequences within the basin that are related to the regional geology the basal albian turonian sequence 1 represents the initiation of fore arc basin sedimentation contains continental conglomerates that change to bathyal shales and correlates with the lower part of the valle group of the vizcaino peninsula sequence 2 is coniacian paleocene includes basal conglomeratic sandstones grading into maastrichtian bathyal shales and usually overlies a coniacian santonian unconformity sequence 2 is represented at the surface by the rosario group in northwestern baja california and the upper part of the valle group in the vizcaino peninsula sequence 3 is paleocene middle eocene represents continuity of fore arc sedimentation in neritic upper bathyal conditions is capped by a major unconformity and correlates with the sepultura and bateque formations to the north and south of the basin respectively the uppermost miocene pliocene sequence 4 is composed of marine sandstone siltstone unconformably overlying sequence 3 and is correlative with the tortugas formation that represents sedimentation after the end of subduction of the farallon plate beneath the north america plate

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

rapid and accurate u th dating of ancient carbonates using inductively coupled plasma quadrupole mass spectrometry here the potential for rapid and accurate u th dating technique of marine aragonite skeletons deep sea corals lophelia pertusa and secondary calcite deposits speleothems and stalagmites has been explored using inductively coupled plasma quadrupole mass spectrometry icp qms the analytical procedure includes a largely simplified chemical separation technique for uranium u and thorium th using uteva resin the developed technique permits simultaneous quantification of uranium 238u and thorium 232th concentrations and their respective isotopic composition required for u series disequilibrium dating up to 50 u th dates per day can be achieved through icp qms with 234u and 230th reproducibility 2 of 3 4 and 1 respectively the high sensitivity 3 0 105 cps ppb together with low background 0 5 cps on each mass between 228 and 236 amu allowed u th dating of ancient deep water corals 15 260 kyr and stalagmites 30 85 kyr at precision levels of less than 2 consequently the combination of simplified chemistry using uteva with state of the art icp qms isotopic measurements that do not require a u th separation step now provides an extremely rapid and low cost u series dating technology the level of precision is most convenient for numerous geochronological applications such as the determination of climatic influences on ecosystem development and carbonate precipitation as a first example application we present icp qms u th dates of north atlantic deep water coral fragments retrieved in the southeastern porcupine seabight md01 2463g mound therese indicating a purely interglacial growth of deep water corals on so called carbonate mounds over several climate cycles

carbonate budget of a cold water coral carbonate mound propeller mound porcupine seabight high resolution studies from the propeller mound a cold water coral carbonate mound in the ne atlantic show that this mound consists of 50 carbonate justifying the name carbonate mound through the last 300 000 years approximately one third of the carbonate has been contributed by cold water corals namely lophelia pertusa and madrepora oculata this coral bound contribution to the carbonate budget of propeller mound is probably accompanied by an unknown portion of sediments buffered from suspension by the corals however extended hiatuses in propeller mound sequences only allow the calculation of a net carbonate accumulation thus net carbonate accumulation for the last 175 kyr accounts for only 0 3 g cm2 kyr which is even less than for the off mound sediments these data imply that propeller mound faces burial by hemipelagic sediments as has happened to numerous buried carbonate mounds found slightly to the north of the investigated area

evolution of a middle jurassic back arc basin cedros island baja california evidence from a marine volcaniclastic apron a rifted arc ophiolite assemblage and over lying volcaniclastic rocks all of late middle jurassic age represent a fragment of the frontal arc margin of a back arc basin now exposed on cedros island in baja california mexico ophiolite generation was immediately followed by progradation of a deep marine volcaniclastic apron gran cannon formation contemporaneous with the growth of island arc volcanoes this back arc apron shows relatively simple uniform sedimentation patterns that may be recognizable in other back arc basins isolated from terrigenous sediment influx the progradational back arc apron sequence includes from base to top 1 the tuff lithofacies thin bedded well sorted laterally continuous tuffs that thicken into basement paleo lows these were deposited from dilute sediment gravity flows generated by eruptions from a deeply submerged nascent arc 2 the lapilli tuff tuff breccia lithofacies medium to very thick bedded lapilli tuffs and tuff breccias these were deposited from debris flows on proximal parts of the apron and high density turbidites were deposited on more distal parts this lithofacies records increasing generation and resedimentation of scoriaceous and hyaloclastic debris as the summits of the arc volcanoes grew nearer to sea level 3 the primary volcanic lithofacies monolithologic dacite pyroclastic flows were produced during the eruption of differentiated magmas which climaxed the growth of the island arc basalt lava flows were fed from fissures that extended down the apron within the back arc basin pyroclastic rocks of the back arc apron were blanketed with silty basinal turbidites epiclastic facies that record abrupt cessation of volcanism and erosion of the arc within about 10 m y of ophiolite generation reflecting the temporal episodicity of back arc basins it is predicted from this study that the frontal arc side of back arc spreading centers is preferentially preserved in the geologic record relative to the center of the basin it is suggested that the hot frontal arc side of the basin is prone to intra plate volcanism but is unaffected by faulting subsequent to basin initiation several factors lead to slumps occurring more frequently on the arc flank of back arc basins than in any other tectonic setting except perhaps intra arc marine basins

palaeolatitude drift history of displaced terranes in southern and baja california the delineation of microplate tectonic terranes1 which make up western north america the timing of their accretions to western north america and the identification of their original geological settings are all topics of debate2 3 the most recent addition to this family of terranes is the santa lucia orocopia allochthon4 5 sloa fig 1 which is made up of salinia6 7 and several other smaller terranes8 and is located in central and southern california this composite terrane amalgamated in the cretaceous and accreted to southern california by the early palaeogene4 5 we describe here new palaeomagnetic results from localities outboard from the sloa in southern california usa and baja california mexico these results are used to define a newly identified allochthonous terrane in southern and baja california peninsular ranges terrane9 prt which is outboard from the sloa estimate the prt s palaeolatitude drift history for the past 160 myr and infer its original geological setting prt and sloa palaeolatitude drift histories are then combined to produce a new perspective on the tectonic development of southern and baja california over the past 160 myr

the effect of volcanic constructs on rift fault patterns volcanoes in rift situations often show distinctive hour glass fault patterns with increased fault throw as the volcano is approached we present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass for faults to be reorientated there must be a ductile layer below the volcano this can be hot crust at mid ocean ridges continental rifts or weak sedimentary strata increased volcano size or mass and lower brittle ductile ratios lead to increased fault curvature whereas an increased regional extension rate decreases the effect volcanoes on one side of a rift may capture it forming the axis of a new rift with curved or en echelon zones on either side by concentrating extension magma is more easily erupted so volcanoes may erupt more magma of less evolved composition a positive feedback between increased extension and magma eruption rate will lead to rift narrowing which can favor the formation of oceanic crust

spatial distribution of cones and satellite detected lineaments in the pali aike volcanic field southernmost patagonia insights into the tectonic setting of a neogene rift system abstract the relationships between the distribution and morphometric features of eruptive structures scoria and spatter cones maar tuff rings and the fracture network were investigated in the pliocene quaternary pali aike volcanic field southernmost patagonia argentina chile the alkali basaltic basanitic magmas which erupted in this area have nearly primary magma compositions and often bear mantle xenoliths hence magma ascent from deep seated reservoirs was probably very fast with no significant stagnation at crustal levels field surveys and satellite image analysis led to the identification of up to 467 eruptive structures and four main nw se ne sw e w and n s fracture systems the spatial distribution of eruptive cones and fractures was investigated through the computation of power law exponents df for self similar clustering the self similarity of cones and fractures was defined between lower and upper cut offs which were in turn related to the thickness of the fractured mechanical layer the fractal character of cones and fracture distribution clustering in the pali aike volcanic field area was thus correlated with crustal thickness the self similarity of fractures was used to establish the relative chronology of the detected fracture systems the self similar clustering exponent is highest in the e w and nw se fracture systems df 1 78 and 1 77 respectively and lowest in the n s system df 1 65 the self similar clustering of eruptive structures is well defined df 1 45 the intense volcano tectonic activity in the pali aike area marks a major pliocene quaternary phase in the development of the magellan neogene rift system

volcano tectonic setting of the intraplate pliocene holocene newer volcanic province southeast australia role of crustal fracture zones 1 the pliocene holocene newer volcanic province in victoria southeast australia is a wide volcanic field site of the most recent volcanic activity of australia the intraplate signature and the localization on the northern side of the southern ocean have been used in the past to ascribe the volcanic activity either to the presence of a mantle hot spot or to thermal anomalies inherited from the seafloor spreading that separated australia from antarctica during the gondwana breakup the spacing and distribution of groups of eruption points suggests the presence of a 32 km deep mantle thermal anomaly alignments of eruption points indicate a strong tectonic control on magma emplacement mainly along nw se mesozoic cenozoic structures and along n s palaeozoic and e w late cretaceous structures this volcano tectonic setting relates to the interference of the left lateral kinematics of the major n s trending faults associated with the tasman fracture zone and the extensional structures of the otway basin this interpretation is also coherent with stress in situ data and focal mechanisms of earthquakes shmax oriented n150 this interpretation enhances the role of the tasman fracture zone a major sinistral transform on the reactivation of transtensional structures and on triggering the magmatism of the newer volcanic province

fluids mobilization in arabia terra mars depth of pressurized reservoir from mounds self similar clustering abstract arabia terra is a region of mars where signs of past water occurrence are recorded in several landforms broad and local scale geomorphological compositional and hydrological analyses point towards pervasive fluid circulation through time in this work we focus on mound fields located in the interior of three casters larger than 40 km firsoff kotido and unnamed crater 20 km to the east and showing strong morphological and textural resemblance to terrestrial mud volcanoes and spring related features we infer that these landforms likely testify the presence of a pressurized fluid reservoir at depth and past fluid upwelling we have performed morphometric analyses to characterize the mound morphologies and consequently retrieve an accurate automated mapping of the mounds within the craters for spatial distribution and fractal clustering analysis the outcome of the fractal clustering yields information about the possible extent of the percolating fracture network at depth below the craters we have been able to constrain the depth of the pressurized fluid reservoir between 2 5 and 3 2 km of depth and hence we propose that mounds and mounds alignments are most likely associated to the presence of fissure ridges and fluid outflow their process of formation is genetically linked to the formation of large intra crater bulges previously interpreted as large scale spring deposits the overburden removal caused by the impact crater formation is the inferred triggering mechanism for fluid pressurization and upwelling that through time led to the formation of the intra crater bulges and after compaction and sealing to the widespread mound fields in their surroundings

hydraulic connection and fluid overpressure in upper crustal rocks evidence from the geometry and spatial distribution of veins at botrona quarry southern tuscany italy abstract veins are the geologic record of fluids that filled fractures at depth in the crust in southern tuscany italy well exposed oligocene early miocene sandstones hosting vein systems provide insight into the role of pore fluid and the stress state at the time of vein formation the stress ratio 2 3 1 3 and driving stress ratio r p f 3 1 3 were determined by analysing the distribution length and aperture of fractures and veins and the magnitude of fluid overpressure the derived fluid overpressure for the whole vein system ranges from 30 mpa to 64 mpa with an average of 43 mpa these values indicate that veins formed under supra hydrostatic pressure conditions despite their spatial contiguity two different vein arrays show very different stress and driving pressure ratios one vein system is characterised by 0 62 and r 0 60 the other by 0 54 and r 0 78 the described vein systems are an example of a close spatial association of two non hydraulically connected vein systems representing fluids focused through the upper crust

paleogeography and sedimentology of upper cretaceous turbidites san diego california upper cretaceous campanian and maestrichtian marine strata of the rosario group in the san diego area include the point loma formation and overlying cabrillo formation these units contain six facies associations 1 shelf and lagoonal sandstone 2 slope and basin plain mudstone 3 outer fan lobe sandstone 4 middle fan channel fill sandstone 5 middle and inner fan interchannel and channel margin thin bedded turbidites and mudstone and 6 inner fan channel fill conglomerate and sandstone the facies associations define a deep sea fan deposited by westward flowing sediment gravity flows that transported sediments derived chiefly from batholithic and prebatholithic metamorphic rocks of the peninsular ranges the sedimentary basin initially deepened abruptly partly aided by eustatic sea level rise the fan then prograded westward into the basin with a retrogradational phase recorded in the uppermost part of the sequence which is erosionally truncated by transgressive lower eocene conglomerate the fan was deposited along the eastern edge of a forearc basin similar to that of the great valley sequence in northern california the western part of the fan which probably contained mostly outer fan lobe and associated basin plain deposits appears to have been truncated by late cenozoic strike slip faulting associated with the san andreas fault system the fan and remnants of the western part of the basin and associated subduction complex may be present on the northwest in the channel islands region or still farther north

crustal structure of the main ethiopian rift from gravity data 3 dimensional modeling abstract a three dimensional interpretation of the newly compiled bouguer anomaly map of the main ethiopian rift is presented a high resolution 3 d model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben the bouguer anomalies along the axial portion of the rift floor as deduced from the results of the regional and residual separation are mainly caused by the deep seated structures the inferred zone of intrusion which is the main subject of the present study coincides with the maximum gravity anomaly of the rift floor the intrusion is displaced at several sectors along the east west direction and the two major displacements coincide with the locations of the major rift offsets on the surface because of the asthenospheric uplift the crust under the main ethiopian rift is slightly thinned the zone of crustal thinning 31 km coincides with the location of the intrusion beneath the rift floor and the maximum of which is attained in the northern and central sectors of the graben the trend of the crustal thinning zone which is from south to north is the same as the one obtained in the afar depression the southeastern and western plateaus on the other hand show by far the largest crustal thickness in the region 38 51 km in contrast to the afar depression where the crust is partly oceanized the thickness and density of the crust suggest that the main ethiopian rift is underlain by a purely continental crust the deep and relatively large nature of the intrusion leads to the conclusion that a large scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben

textural variation in juvenile pyroclasts from an emergent surtseyan type volcanic eruption the capelas tuff cone sao miguel azores abstract here i present textural data i e vesicularity vesicle size distributions vsd plagioclase crystallinity crystal size distributions csd combined with fractal analyses of particle outlines from a natural succession of alternating fall and surge deposits in the emergent capelas tuff cone azores the textural variation in the capelas succession is surprisingly small considering the wide variety of fragmentation processes vent activity and emplacement mechanisms that are characteristic of emergent eruptions the plagioclase crystal content varies between 24 and 33 vol csd analyses of plagioclase show near linear trends with a slight increase in time for the smallest crystal sizes with surge deposits having more groundmass plagioclase when compared with fall deposits this is consistent with crystallization induced by degassing and decompression at lower eruption rates the vesicularities of the capelas pyroclasts are more variable 18 to 59 vol with vsds displaying kinked trends characteristic of coalescence this is especially evident in the fall deposits and consistent with being formed in continuous uprush jetting with an overall shallow fragmentation level within the conduit bubble coalescence can also be identified in the surge deposits although to a much lesser extent the amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization i e plagioclase in the capelas deposits a relatively broad range of fractal dimensions with average dbox 1 744 and 0 032 for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at capelas in addition to this the fact that the fractal dimensions for both the fall and surge end members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges these results are consistent with emergent eruptions building tuff cones to be a relatively low energy phreatomagmatic landform e g at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes

new geologic mapping and shrimp u pb zircon data in the peninsular ranges batholith baja california mexico evidence for a suture geologic and shrimp u pb zircon studies in the sierra san pedro martir area of northern baja california mexico suggest that the western and eastern parts of the peninsular ranges batholith originated as separate arcs they are now juxtaposed along a well exposed ductile thrust main martir thrust marked by an age discontinuity of at least 10 to 15 m y and sharp changes in rock type metamorphic grade and deformation history the east dipping thrust and adjacent highly deformed rocks were stitched by 108 97 ma plutons that were generated in a region of crustal thickening formed during juxtaposition of the two arcs lack of chemical isotopic and geologic evidence for continentally derived rocks in the western arc may preclude an origin by rifting of the continental margin or development of the arc on a prism of continentally derived sediments adjacent to the continental margin instead the western arc may have originated as an island arc above a subduction zone outboard of north america if so a second concurrent subduction zone along the north american margin must have driven convergence and suturing of the two arcs and the main martir thrust may mark a nonterminal suture within a wider convergence related deformation zone that formed ca 115 108 ma

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

flank cones at mount etna volcano do they have a power law distribution mount etna is currently characterised by intense effusive and explosive activity of its summit vents whereas 319 non active holocene flank cones are spread across its flanks at altitudes of between 2990 and 475 m in volcanic areas the relationships between fracture occurrence and cone growth location are well established with this in mind the spatial distribution of the mount etna flank cones was analysed in order to make some inferences about the fracture systems that feed the cones the positions of the flank cones were acquired by the use of a digital elevation model of the volcano with a geometric resolution of 10 10 m spatial distribution of the cones was analysed through counting box and sand box methods checking for fractal or multifractal behaviours the four data sets analysed consist of the whole number of parasitic cones 319 and cones located on the ne 50 south 143 and west 61 rifts respectively the cones have a non trivial power law distribution the sand box method gave the best results with a fractal exponent df for all cones of 1 41 0 02 over the length range 0 2 10 km the same analysis was performed on the other data sets south rift 1 42 0 02 west rift 1 39 0 02 and ne rift 1 43 0 02 the cones do not have a multifractal distribution as suggested also by the strong similarity between fractal exponents of the different data sets data suggest a strong control over flank cone distribution by fracture length and density these two characteristics are in turn expressions of highly connected fractures activated as magma feeders by the volcano s present stress field we interpret the rifts as inherited structures that represent preferential sites of fracture connectivity

new age constraints on the timing of volcanism and tectonism in the northern main ethiopian rift southern afar transition zone ethiopia abstract forty new k ar and 40 ar 39 ar isotopic ages from the northern main ethiopian rift mer southern afar transition zone provide insights into the volcano tectonic evolution of this portion of the east african rift system the earliest evidence of volcanic activity in this region is manifest as 24 23 ma pre rift flood basalts transition zone flood basalt activity renewed at approximately 10 ma and preceded the initiation of modern rift margin development bimodal basalt rhyolite volcanism in the southern afar rift floor began at approximately 7 ma and continued into recent times in contrast post subsidence volcanic activity in the northern mer is dominated by mio pliocene silicic products from centers now covered by quaternary volcanic and sedimentary lithologies unlike other parts of the mer mio pliocene silicic volcanism in the mer afar transition zone is closely associated with fissural basaltic products the presence of pliocene age ignimbrites on the plateaus bounding the northern mer whose sources are found in the present rift indicates that subsidence of this region was gradual and that it attained its present physiography with steep escarpments only in the plio pleistocene large 7 5 ma silicic centers along the southern afar and northeastern mer margins apparently formed along an e w oriented regional structural feature parallel to the already established southern escarpment of the afar the addis ababa rift embayment and the growth of 4 5 3 ma silicic centers in the addis ababa area are attributed to the formation of a major cross rift structure and its intersection with the same regional e w structural trend this study illustrates the episodic nature of rift development and volcanic activity in the mer afar transition zone and the link between this activity and regional structural and tectonic features

brittle ice shell thickness of enceladus from fracture distribution analysis abstract we determine the depth of fracture penetration in multiple regions of enceladus by performing self similar clustering and length distribution analysis of fractures the statistical characterization of fault population attribute such as length and clustering provide a productive avenue for exploring deformation rate stress transmission mode rheology of the medium and mechanical stratification of the ice satellite through this analysis we estimate the depth of the mechanical discontinuity of enceladus ice shell that is the depth to which fractures penetrate the brittle ice layer above the ductile one in this work we find that for the south polar terrain spt the brittle ice shell interested by fracture penetration is about 30 km and corresponds to the total depth of the ice shell because the spt has a very high thermal gradient and hence fractures likely reach the ocean ice interface in the other regions analyzed the depth of fracture penetration increases from 31 to 70 km from the south pole to northern regions up to 75

chronological constraints on the thermal and tilting history of the sierra san pedro martir pluton baja california mexico from u pb 40ar 39ar and fission track geochronology the tectonothermal history of the four major phases of the sierra san pedro martir pluton and surrounding metamorphic rocks of the mesozoic peninsular ranges batholith of baja california is presented on the basis of u pb 40 ar 39 ar step heating and fission track dating in combination with al in hornblende geobarometry a previous model proposed up to 90 of east side up tilting of the pluton exposing 20 km of crustal section to account for its crescent shape asymmetrical zoning internal structure the eastward younging of k ar dates across the intrusion and eastward increase in the metamorphic grade of the country rocks from greenschist to amphibolite facies the u pb data suggest that the different phases of the pluton were emplaced sequentially from west to east between 97 0 4 1 ma and 93 8 1 1 ma all except one of the 105 40 ar 39 ar age spectra have well defined plateaus and are interpreted as cooling ages samples from the pluton give hornblende and biotite 40 ar 39 ar plateau dates and apatite fission track dates that young from west to east thus hornblende dates decrease from 95 to 91 ma biotite dates decrease from 94 to 88 ma and apatite dates decrease from 72 to 57 ma muscovite biotite and plagioclase from the same rock sample collected at the easternmost phase of the pluton yield concordant 40 ar 39 ar dates of 88 ma the exposed part of the pluton underwent rapid cooling approximately equal to 40 c ma down to 250 c in the first 10 m y after intrusion modeling of track length distribution in apatite is consistent with monotonic slow cooling from ca 80 ma to the present the data do not support a history that includes major tilting of the pluton eastward younging of 40 ar 39 ar and fission track dates may be explained by approximately equal to 15 of east side up tilting of the pluton at or after 88 ma about a north south horizontal axis furthermore the fission track data suggest that part or all of this tilting may have taken place at or after 57 ma and therefore may be a consequence of regional scale crustal extension associated with the opening of the gulf of california in neogene time such tilting is in agreement with the al in hornblende geobarometry for the hornblende biotite intrusive phase that yields pressures of 5 2 0 6 kbar an approximately equal to 15 northeast side up tilt of the crustal block containing this pluton would explain the apparent paleomagnetic inclination discrepancies with cratonic north america and militates against large scale northerly transport of baja california

evolution of subpolar north atlantic surface circulation since the early holocene inferred from planktic foraminifera faunal and stable isotope records abstract past changes in the surface flow regime of two main eastern north atlantic warm water pathways toward the nordic seas were reconstructed based on faunal analyses in combination with carbon and oxygen stable isotope measurements in planktic foraminifera the investigated sites in the surroundings of the faroe islands are located in the transitional area where surface waters of subpolar and subtropical origin mix before entering the arctic mediterranean in these areas large amplitude millennial variability in the characteristics of the upper water column appears modulated by changes in the intensity of the subpolar gyre circulation from 7 8 to 6 ka bp faunal records indicate a deep mixed layer which in conjunction with lighter 18 o values suggest that the inflowing atlantic waters were dominated by a relatively cooler and fresher water mass reflecting a strengthening of the subpolar gyre under conditions of enhanced positive nao like forcing and reduced meltwater input a shift in the hydrographic conditions occurred during the mid holocene centered at 5 ka bp at this time increasing upper water column stratification and the incipient differentiation of the stable isotopic signal of the iceland faroe and faroe shetland surface water masses suggest increasing influx of warmer more saline surface waters from the subtropical gyre as subpolar gyre circulation weakened the mid holocene decline in subpolar gyre strength is presumably related to a shift toward a low state of the nao like forcing associated with decreased solar irradiance later in the holocene from 4 ka bp to present the increased frequency and reduced amplitude of the surface hydrographic changes reflect corresponding fluctuations in subpolar gyre circulation these high frequency oscillations in subpolar gyre strength suggest increased surface circulation sensitivity to moderate freshwater fluxes to the labrador irminger sea basin highlighting the importance of the salinity balance in modulating subpolar gyre dynamics particularly under conditions of low nao atmospheric forcing

the depth dependency of planktonic benthic foraminiferal ratios constraints and applications abstract study of the ratio between planktonic and benthic foraminifera in a great number of areas shows that variation of this ratio with depth can be described using organic matter flux equations essentially the share of benthic foraminifera in the total association is inversely proportional to depth as is the amount of organic matter resulting from primary productivity reaching the sea bottom variation in the p b ratio from area to area appears to be strongly dependent on the amount of inbenthic living foraminifera if the ratios are corrected for this the regressions between the proportion of planktonic foraminifera p and depth appear to be near identical in the gulf of mexico the gulf of california the west coast of the usa and the adriatic sea this regression can be described by depth e 3 58718 0 03534 p a study of the behaviour of this function in nine cores from the adriatic upper quaternary reveals that the palaeodepth reconstructions are not influenced by fluctuations in productivity and temperature but that they are significantly influenced by redeposition of fine grained deposits

self similar clustering distribution of structural features on ascraeus mons mars implications for magma chamber depth in platz t massironi m byrne p k hiesinger h eds 2015 volcanism and tectonism across the inner solar system geological society london special publications 401 203 218

correction of inclination shallowing and its tectonic implications the cretaceous perforada formation baja california abstract a paleomagnetic and magnetic fabric study of the aptian to albian perforada formation the oldest sedimentary unit of the cretaceous valle group was conducted to determine whether the perforada inclination had been shallowed by burial compaction and to correct the inclination for any shallowing the perforada formation is located on the vizcaino terrane of the vizcaino peninsula of west central baja california an accurate paleolatitude will indicate whether the vizcaino terrane is far travelled eighty eight specimens from 43 independently oriented hand samples and cores were collected from 11 sites a small 10 m fold was sampled in order to constrain the age of the magnetization with a fold test thermal and alternating field demagnetization was able to isolate characteristic magnetizations for most samples maximum unblocking temperatures near 580 c indicate that magnetite is the magnetic carrier the characteristic magnetization passes the fold test at the 99 confidence level anisotropy of anhysteretic remanence aar and anisotropy of magnetic susceptibility ams measurements show that five sites had well developed bedding parallel foliations while four sites had poorly developed aar fabrics but foliated ams fabrics the remaining two sites had poorly developed foliations for both ams and aar the mean inclination i 35 6 of the five sites with well developed aar foliations is 13 7 shallower than the mean inclination i 49 3 of the two sites with poorly developed aar and ams fabrics when the inclinations of the sites with well developed aar foliations are corrected using an individual particle anisotropy of 1 5 determined from magnetic extract epoxy samples dried in 30 50 mt dc magnetic fields the corrected mean inclination i 50 1 is indistinguishable from the mean inclination of the sites with poorly developed aar fabrics a corrected formation mean direction for the perforada formation d 311 8 i 47 0 95 9 3 indicates a 33 5 11 1 counterclockwise rotation and 5 6 7 7 of post cretaceous northward motion after 3 of neogene fault motion has been removed these results indicate that the vizcaino terrane has moved little with respect to the peninsular ranges terrane and cratonic north america since the cretaceous

crustal thinning in relationship to the evolution of the afro arabian rift system a review of seismic refraction data abstract crustal and upper mantle structure of major units of the afro arabian rift system has been progressively investigated by seismic refraction surveys in the jordan dead sea rift the red sea the afar depression and in the east african rift of kenya the two extreme structures are oceanic crust flooring the axial trough of the southern red sea rift and 30 35 km thick continental crust underneath the jordan dead sea rift and its flanks underlain by a mantle with pn velocities of 8 0 8 2 km s the remainder of the units have a crustal structure of transitional to uncertain nature in the southeastern red sea the change from 15 18 km thick transitional crust to the 40 km thick continental crust of the arabian shield is probably very abrupt and is the major lateral change in structure in this region in the northern afar depression the crust is only 15 20 km thick in the southern afar depression anomalous mantle with 7 2 7 3 km s velocities occurs at depths greater than 25 km also here an abrupt change to a 40 km thick shield crust with a velocity of 8 0 km s is evident from seismic and gravity data crustal thickness of the east african rift system amounts to 30 36 km a recent trial experiment in the eastern rift of kenya indicates a 35 36 km thick crust underlain by a low velocity upper mantle with 7 5 km s at 23 24 km depth a thin high velocity lens of 7 1 km s velocity may be present within the 22 km thick lower crustal layer averaging 6 6 km s velocity investigations of local earthquake body waves in the western rift provide information that the crust here is of continental type with a thickness of about 32 km overlying mantle material of 8 0 km s p velocity and 4 6 km s s velocity in conclusion a model of preferential rifting of continents is applied to the afro arabian rift system where rifting seems to progress into areas with thick continental crust as seen for example in the levant and the east african rift systems instead of using paths in thin crust

carbonate mound development in contrasting settings on the irish margin cold water coral carbonate mounds formed by framework building cold water corals are found in several mound provinces on the irish margin differences in cold water coral mound development rates and sediment composition between mounds at the southwest rockall trough margin and the galway mound in the porcupine seabight are investigated variations in sediment composition in the two mound provinces are related to the local environmental conditions and sediment sources mound accumulation rates are possibly higher at the galway mound probably due to a higher influx of hemipelagic fine grained non carbonate sediments in both cold water coral mound areas mound growth has been continuous for the last ca 11 000 years before this period several hiatuses and unconformities exist in the mound record the most recent unconformity can be correlated across multiple mounds and mound provinces at the irish margin on the basis of apparent age on the southwest rockall trough margin these hiatuses unconformities are associated with post depositional aragonite dissolution in and lithification of certain intervals while at galway mound no lithification occurs this study revealed that the influx and types of material transported to cold water coral mounds may have a direct impact on the carbonate mound accumulation rate and on post depositional processes significantly the logachev mounds on the sw rockall trough margin accumulate slower but because they contain lithified layers are less susceptible to erosion this net effect may account for their larger size compared to the belgica mounds

patterns of faulting in the ethiopian rift valley abstract some typical and some unusual fault patterns are described from the ethiopian rift valley they include arcuate faults convex in plan towards the downthrown side lines of obliquely tilted faulted blocks up horsts horizontally stratified rhomb horsts athwart tilted strata faults whose polarity switches in conjunction with development of small horsts intersecting fault sets and re entrant and semi circular faults explanations remain wanting for many of these phenomena which show that crustal extension can be complex at the meso scale

mud volcanoes as potential indicators of regional stress and pressurized layer depth abstract although the use of magmatic volcanoes as indicators of tectonic stress is established the application of the same principles to mud volcanoes has been overlooked here we analyse the onshore azerbaijan mud volcano province in the foreland of the greater caucasus and we present the first use of the statistical regional scale distribution of active mud volcanoes to obtain information about 1 contemporary regional tectonic stress and 2 the average depth of pressurized source layers self similar clustering suggests that mud is mobilized from a ca 5 km deep mud prone layer and mud volcano strain indicators allow the estimation of a robust present day direction of regional maximum horizontal stress s h striking ca 30 e these results are in excellent agreement with geophysical information about upper crustal structure in the region and stress data and deformation vectors we conclude that besides other fold and thrust belts mud volcano features could be used as a proxy for active stress orientation and source layer determination in inaccessible settings such as underwater or even the surface of planets where mud volcanoes are considered to be indicators of mobilized fluids

cold water coral carbonate mounds as unique palaeo archives the plio pleistocene challenger mound record ne atlantic through the interplay of a stabilising cold water coral framework and a dynamic sedimentary environment cold water coral carbonate mounds create distinctive centres of bio geological accumulation in often complex continental margin settings the iodp expedition 307 drilling of the challenger mound eastern porcupine seabight ne atlantic not only retrieved the first complete developmental history of a coral carbonate mound it also exposed a unique early pleistocene sedimentary sequence of exceptional resolution along the mid latitudinal ne atlantic margin in this study a comprehensive assessment of the challenger mound as an archive of quaternary palaeo environmental change and long term coral carbonate mound development is presented new and existing environmental proxy records including clay mineralogy planktonic foraminifer and calcareous nannofossil biostratigraphy and assemblage counts planktonic foraminifer oxygen isotopes and siliciclastic particle size are thereby discussed within a refined chronostratigraphic and climatic context overall the development of the challenger mound shows a strong affinity to the plio pleistocene evolution of the northern hemisphere climate system albeit not being completely in phase with it the two major oceanographic and climatic transitions of the plio pleistocene the late pliocene early pleistocene intensification of continental ice sheet development and the mid pleistocene transition to the more extremely variable and more extensively glaciated late quaternary mark two major thresholds in challenger mound development its late pliocene 2 74 ma origin and its middle late pleistocene to recent decline distinct surface water perturbations i e water mass polar front migrations productivity changes melt water pulses are identified throughout the sequence which can be linked to the intensity and extent of ice development on the nearby british irish isles since the earliest pleistocene glaciation induced shifts in surface water primary productivity are thereby proposed to fundamentally control cold water coral growth which in turn influences on mound sediment accumulation and hence coral carbonate mound development throughout the pleistocene as local factors such as proximal ice sheet dynamics and on mound changes in cold water coral density significantly affected the development of the challenger mound they can potentially explain the nature of its palaeo record and its offsets with the periodicities of global climate variability on the other hand owing to this unique setting a regionally exceptional high resolution palaeo record of early pleistocene ca 2 6 to 2 1 ma environmental change including early british irish ice sheet development broadly in phase with the 41 ka paced global climate system is preserved in the lower challenger mound all in all the challenger mound record highlights the wider relevance of coral carbonate mound archives and their potential to capture unique records from dynamic continental margin environments

40ar 39ar geochronology reveals rapid change from plume assisted to stress dependent volcanism in the newer volcanic province se australia here we present 40ar 39ar ages of volcanic features in the cenozoic intraplate newer volcanic province in southeast australia the 5 ma volcanic products in the newer volcanic province can be subdivided into tholeiitic valley filling newer plains basalts and alkaline scoria cones lava shields and maars of the newer cones series plateau ages range from 3 76 0 01 to 4 32 0 03 ma 2 all sources of uncertainties included for the newer plains series with production rates of volcanism decreasing post 4 ma we suggest that magmatism is related to the complex interplay of magma upwelling due to edge driven convection and the cosgrove track mantle plume located in the northeast of the province at 6 5 5 ma plateau ages range from 1290 20 to 41 1 2 2 ka 2 for the newer cones series with a diffuse age progression in the onset of volcanism for these features from east to west analyses of the distribution and geomorphology of these volcanic features indicates a strong control of basement faults on volcanism reflected in alignment of volcanic features along paleozoic north south oriented basement faults in the east and cretaceous northwest southeast oriented extensional features in the west this age progression can be explained by a westerly migration of stress derived from the left lateral strike slip tasman fracture zone this suggests that the general mechanism of volcanism changed from upwelling due to plume assisted edge driven convection prior to 4 ma to stress dependent upwelling at around 1 3 ma

fractal analysis of fracture patterns using the standard box counting technique valid and invalid methodologies abstract on the basis of results of simple box counting analysis it has been suggested in several recent publications that natural fracture patterns are fractal fractal patterns are characterized by self similarity of structure on a range of scales and provide straight line distributions on box counting plots new analysis of a fracture pattern that provided the most convincing straight line box counting curve previously published shows that the curve is non linear and therefore that the fracture pattern is not fractal non linear box counting curves are also characteristic of other natural fracture patterns analysed but spurious linear curves can be obtained if the area analysed extends beyond the mapped area

reconstruction of the formation history of the darwin mounds n rockall trough how the dynamics of a sandy contourite affected cold water coral growth abstract cold water coral mounds formed through a feed back process of cold water coral growth and sediment baffling have been studied all along the ne atlantic continental margin however major questions remain concerning their formation history especially their initiation and early development in relation to the surrounding sediment dynamics for the first time two small mounds located in a sandy contourite have been cored from the top to mound base here the formation history of the darwin mounds located in the northern rockall trough was investigated and reconstructed from two piston cores using a multidisciplinary approach this consisted of ct scanning for quantifying coral density changes with depth grain size analysis to obtain the hydrodynamic trends and radiocarbon and u series dating to place the results into a wider paleoceanographic context the results show that the darwin mounds formed during the early holocene 10 ka bp through sediment baffling mainly by lophelia pertusa the initiation of both mounds shows a similar pattern of increased current velocities resulting in coarser sediment deposition and a relatively high coral density with a peak of 23 vol the mound growth was rapid between 10 9 7 ka bp up to 277 cm ka 1 in one of the mounds with further vibrant growth periods around 8 8 ka bp 6 5 ka bp and 3 4 ka bp the demise of the mounds ca 3 ka bp was likely caused by an intensification in bottom current velocities causing a hostile environment for coral growth in the contourite setting in a wider context the development of the darwin mounds appears to have responded to the relative strength and position of the subpolar gyre which affected food supply to the corals sedimentation rates current speeds and other water mass properties in the area

tectonic setting of blueschist and island arc terranes of west central baja california mexico diverse oceanic rocks exposed in west central baja california mexico are assigned to three shallowly dipping tectonic units the lower plate is a subduction complex consisting of regionally metamorphosed blueschists and divided into structurally and petrologically distinct subterranes the upper plate consists of triassic lower cretaceous are rocks and overlapping mid cretaceous turbidites a fault bounded serpentinite matrix melange between lower and upper plate rocks contains blueschist eclogite amphibolite and variably serpentinized mafic and ultramafic blocks lower plate blueschists were metamorphosed in the late early cretaceous and uplifted along shallowly dipping normal faults during conditions of steady state subduction exotic blocks of blueschist eclogite and amphibolite are derived from a cryptic terrane or terranes record varied p t t histories and were uplifted via tectonic emplacement or diapiric rise of serpentinite along the normal faults