evolution of a provenance as revealed by petrographic analyses of cretaceous formations in the queen charlotte islands british columbia canada two contrasting marine sedimentary facies the haida formation of sandy and argillaceous sediments and the conformably overlying honna formation of gravelly sediments were formed within different types of tectonic basins during mid to late cretaceous time the sediments of both formations were derived from the east sandstones from the two formations show characteristics of mature magmatic arc provenance and are classified as lithic and feldspathic arenites however the honna sandstones are more feldspathic and less quartzose than the haida sandstones the honna sandstones have many volcanic rock fragments vrf but the haida sandstones do not feldspars of the vrf rich honna sandstones however do not seem to have been derived from the breakdown of volcanic rocks the observed petrographic differences between the two formations can be ascribed to a process in which the volcanic cover and the basement rock denudation took place simultaneously in the source area during deposition of the honna formation

integrated interpretation of 3d seismic data to enhance the detection of the gold bearing reef mponeng gold mine witwatersrand basin south africa we present an integrated approach to the seismic interpretation of one of the world s deepest gold ore body carbon leader reef using three dimensional seismic data ultrasonic velocity measurements at elevated stresses and modified instantaneous attribute analysis seismic wave velocities of the drill core samples quartzite shale and conglomeratic reef from the mine are sensitive to uniaxial stress changes i e they slowly increase with increasing pressure until they reach maximum value at 25 mpa for all the samples seismic velocities are constant above 25 mpa indicating a possible closure of microcracks at stress corresponding to 1 0 km 1 5 km a reflection coefficient of 0 02 computed between hanging wall and footwall quartzites of the carbon leader reef ore body suggests that it may be difficult to obtain a strong seismic reflection at their interface our modified seismic attribute algorithm on the other hand shows that the detection of the lateral continuity of the carbon leader reef reflector can significantly be improved by sharpening the seismic traces three dimensional seismic data reveal that faults with throws greater than 25 m that offset the carbon leader reef can clearly be seen faults with throws less than 25 m but greater than 2 m throw were identified through horizon based attribute analysis while most dykes and sills with thickness less than 25 m were invisible the detection of the lateral continuity of the carbon leader reef reflector and its depth position is greatly improved by integrating the modified instantaneous attributes with controls from borehole observations the three dimensional visualization and effective interpretation of the carbon leader reef horizon shows a host of structurally complex ore body blocks that may impact future shaft positioning and reduce its associated risks

the nature and distribution of upper cenozoic ignimbrite centres in the central andes abstract a wide variety of upper cenozoic ignimbrite centres have been identified and studied on landsat imagery of the central andes some have been investigated in the field lower miocene ignimbrite centres are distributed along an arc which in north chile diverges markedly from the line of the present trench and parallels an arc of lower miocene plutons in the eastern cordillera of bolivia much of the ignimbrite erupted since the late miocene has been erupted around the faulted margins of the altiplano basin of western bolivia particularly where the western and eastern cordilleras converge typical examples of ignimbrite centres are described some centres take the form of ignimbrite shield volcanoes other ignimbrite sheets are more widely dispersed the degree of dispersal probably relates to the height of the eruption column partial or total caldera formation sometimes took place and a few of these calderas have resurgent centres late stage lava extrusions are common and some of these are located on caldera ring fractures

area selection for diamonds using magnetotellurics examples from southern africa abstract southern africa particularly the kaapvaal craton is one of the world s best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it the southern african magnetotelluric experiment samtex was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity variations laterally and with depth initially it was planned to acquire magnetotelluric data on profiles spatially coincident with the kaapvaal seismic experiment however with the addition of seven more partners to the original four through the course of the experiment samtex was enlarged from two to four phases of acquisition and extended to cover much of botswana and namibia the complete samtex dataset now comprises mt data from over 730 distinct locations in an area of over one million square kilometres making samtex the largest regional scale mt experiment conducted to date preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km constructed through approximate one dimensional methods map resistive regions spatially correlated with the kaapvaal zimbabwe and angola cratons and more conductive regions spatially associated with the neighbouring mobile belts and the rehoboth terrane known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions comparisons between the resistivity image maps and seismic velocities from models constructed through surface wave and body wave tomography show spatial correlations between high velocity regions that are resistive and low velocity regions that are conductive in particular the electrical resistivity of the sub continental lithospheric mantle of the kaapvaal craton is determined by its bulk parameters so is controlled by a bulk matrix property namely temperature and to a lesser degree by iron content and composition and is not controlled by contributions from interconnected conducting minor phases such as graphite sulphides iron oxides hydrous minerals etc this makes quantitative correlations between velocity and resistivity valid and a robust regression between the two gives an approximate relationship of v s m s 0 045 log resistivity ohm m 4 5

descriptive nomenclature and classification of pyroclastic deposits and fragments the iugs subcommission on the systematics of igneous rocks herein presents its recommendations on the nomenclature and classification of pyroclastic and mixed pyroclastic epiclastic deposits using descriptive mainly granulometric criteria the recommendations are the result of an international inquiry by means of questionnaires during the last four years

late paleozoic volcanism in the central part of the southern permian basin ne germany w poland facies distribution and volcano topographic hiati based on facies analysis of more than 5 500 m cores of 45 deep wells three large sub provinces have been defined for the lower rotliegend volcanic rocks in the central southern permian basin spb in northeastern germany and western poland additional data came from unpublished descriptions of more than 200 wells the three sub provinces are a the mecklenburg vorpommern sub province mvsp dominated by silica rich lava domes and subvolcanic intrusions b the eastern brandenburg sub province ebsp dominated by a mg andesite shield volcano complex which extends into western poland and c the flechtingen altmark sub province fasp with prominent ignimbrite sheets punctuated by lava domes and flows whereas in ne germany thickness of up to 2 300 m have been found in places in western poland ignimbrites and other pyroclastic deposits as well as andesitic and silica rich lava complexes accumulated successions of a few hundreds of meters a hiatus of up to 30 ma occurs between the lower rotliegend volcanic and sedimentary rocks asselian sakmarian and upper rotliegend ii sediments upper wordian capitanian upper rotliegend i deposits are known from a few wells and outcrops only previous studies postulated solely intrabasinal tectonics to account for this major unconformity however under semiarid to arid conditions as assumed for the rotliegend of the spb both sio2 rich lava complexes and silica poor shield volcanoes can be expected as being extremely resistant to weathering and erosion most probably these bodies drowned in a regolith formed by physical weathering rarely removed by torrential rain thus the silica rich lava complexes and the shield volcanoes in the central european basin system cebs can be viewed as long living morphological highs with intervolcanic depressions in between in these intervolcanic depressions syn to postvolcanic successions of conglomeratic to sandy alluvial fan sediments and lake to mud flat deposits accumulated during the upper rotliegend i they show numerous pedogenic horizons representing times of non deposition during the upper rotliegend ii the remaining volcano topography was filled up with alluvial eolian and playa deposits in some places in western poland covering was not complete until the zechstein latest permian at the same time soil formation and or erosion in the upper part of sio2 rich complexes and shield volcanoes remained subordinate consequently the volcano topographic hiatus on top of the volcanic complexes comprises the maximum period of time whereas in the intervolcanic depressions this time splits into periods of deposition and numerous minor intraformational hiati intrabasinal tectonic activity cannot be ruled out as one major control of the rotliegend depositional evolution in the subsiding spb however the presence of weathering resistant volcanic edifices led to the formation of long lasting depositional gaps in many regions of the central spb

depositional architecture of a maar pitted playa sedimentation in the hopi buttes volcanic field northeastern arizona u s a abstract primary and reworked tephra in the mio pliocene bidahochi formation of northeastern arizona was produced by and partly preserved in maar volcanoes and related hydrovolcanic structures the temporal and geometric relationships among lithofacies of the bidahochi formation illustrate the dynamic interplay between volcanism and sedimentation in an essentially closed sedimentary system maar volcanism in the low relief basinal environments of the bidahochi appears to have resulted in little net aggradation both because the total amount of magma erupted was relatively modest and because the large maar volcano craters produced areas of lowered base level within the basin that formed sediment sinks for much of the newly erupted tephra the volcanically eroded maar craters commonly contain deposits of lakes multiple stream channels and eolian dunes they formed an integral part of the basin s overall depositional environment and affected sedimentation in the playa lake stream systems beyond the vents both during and following the major eruptive period

the geological history of maio cape verde islands the oldest igneous rocks on maio are pillow lavas of mid ocean ridge pillow basalts character which have been tilted and uplifted about 4 km from the ocean floor to outcrop as a partial ring dipping steeply away from a central plutonic complex made up of pyroxenites essexites syenites and carbonatites the ocean floor volcanic rocks are overlain conformably by a stratigraphically continuous pelagic carbonate succession which demonstrates a shallowing depositional environment from the upper jurassic to upper cretaceous times when tuffaceous beds indicate renewed volcanism the tuffs are associated with rudites demonstrating the emergence of the island and amongst the clasts are plutonics indicating upper cretaceous magmatism and the unroofing of the volcano to a substantial depth deformation under compressive stress resulted in the folding and local repetition by thrusting of this sedimentary cover which together with the plutonic core had been intensively injected by major sills the mesozoic succession has been planed off and overlain with marked unconformity by a largely neogene sequence of volcanic and terrestrial sedimentary rocks there is a hiatus throughout the palaeogene and constructional activity appears to recommence with ankaramitic hyaloclastite and lava deltas and subaerial ankaramitic flows these are overlain by fluvial sediments and tuffs stratigraphically above these is an extensive plateau of silica undersaturated lavas olivine melilitites and nephelinites which rest on a planed and locally lateritized surface at topographically higher levels in the eastern part of the island there are thick ankaramitic lavas and pyroclasts which evidently flowed eastward through valleys cut down into the mesozoic strata and appear to be of pliocene age the subsequent history of the island appears to be non volcanic

the dali ash island of rhodes greece a problem in interpreting submarine volcanigenic sediments in the southern part of rhodes greece rhyolitic subaqueous pyroclastic deposits are interbedded with tertiary deep water marine sediments the lowermost and best exposed of these deposits the dali ash is described here the deposit has been previously described as a deep water welded subaqueous ignimbrite this paper shows that there is no evidence of welding and texture previously reported were misidentified the dali ash consists of a lower massive unit 5 m thick overlain by a sequence of ash turbidites 2 5 m thick the lower unit was deposited by a high concentration turbidity current and the ash turbidites by dilute turbidity currents foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water particulate systems a palaeomagnetic study also suggests they were deposited cold

sedimentary basin in fill in the northernmost natal valley hiatus development and agulhas current palaeo oceanography sedimentation over large areas of a marginal plateau the northernmost natal valley nnv is influenced by the tropical sub tropical agulhas current through mobilization of detritus and concentration of sedimentation into localized depo centres current action provides a mechanism for hiatus development both in on shelf and off shelf areas post gondwana break up sedimentary basin in fill is interrupted by several regional hiatuses these correlate with coastal regressions global sea level low stands proposed phases of invigorated oceanic circulation and with global phases of slow deep sea sedimentation an assessment of these mechanisms indicates detritus supply related to sea level changes cannot explain correlation of on shelf and off shelf hiatuses current action may have been a contributory factor in development of a cenomanian turonian hiatus and facies change and a cretaceous tertiary boundary hiatus an early oligocene hiatus clearly marks a change in the agulhas current regime current moulded features asymmetric sediment distributions and re working attest to enhanced current action establishment of a cold bottom layer in the ocean at this time stimulated indian ocean surface near surface circulation new circulation patterns were established after a miocene pliocene boundary hiatus since then the main flow paths have been fairly stable although local depo centre shifts late in the pliocene recent sequence suggest eastward movement of a counter current and cyclonic eddy off maputo

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

rates and factors of stratovolcano degradation in a continental climate a complex morphometric analysis for nineteen neogene quaternary crater remnants in the carpathians abstract the systematic age progression of the neogene quaternary volcanic chain in the carpathians manifested in progressively degraded stratovolcanoes from south to north offers a great opportunity to calculate erosion rates and the factors that control degradation in a moderate continental climate using available and newly determined k ar ages for the cessation of volcanism of nineteen stratocones a great number of morphometric variables including crater diameter and perimeter intracraterial valley length valley order and density dip of internal and external crater rim cone diameter relative height cone diameter height above and distance from base level etc has been examined by regression and factor analyses on the one hand the erosion rates which are calculated from the regression equations of a given characteristic versus age seem to answer the question of to what extent the stratovolcanoes are degraded numerical values of crater enlargement 109 m ma internal valley growth 1 3 km ma average cone lowering 31 5 m ma etc are given moreover using multiple regression equations as age equations the age of any volcano in the same climate can be estimated from the measured characteristics on the other hand there are morphometric characteristics e g dip of crater rim crater circularity intracraterial valley density which are not functions of time three major factors i e the time factor about 40 the size depth factor about 24 and the erosional distance factor about 10 account for about 75 of the stratovolcano characteristics variance and two additional factors eruption style magma viscosity and the central manifestation of volcanism may be responsible for a further 13

magma and sediment i emplacement mechanism of late carboniferous tholeiite sills in northern britain president s anniversary address 1981 like other great tholeiite sheets of the world the midland valley sill and whin sill were emplaced in thick accumulations of sediments which at the time of intrusion were not much older nor had been orogenically deformed however the carboniferous sediments of northern britain had been deposited under the influence of continuous differential subsidence so that they were already disposed in syn sedimentary basins and swells when intrusion began by drawing palinspastic contours of the sills relative to a late westphalian marine band approximating to the surface at the time of emplacement it is established that the dolerites mainly follow bedding planes dipping at angles of as much as 5 down to the bottoms of the basins isopachs of the sills indicate a further relationship for the intrusions are thickest at the bottoms of the basins these facts are not explained by any intrusion mechanism previously postulated though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step and stair transgression of sediment bedding it is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0 5 and 1 0 km below the contemporary surface thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up dip under pressure of head the sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow

magma and sediment ii problems of interpreting palaeovolcanics buried in the stratigraphic column president9s anniversary address 1982 the preservation of volcanoes never more than partial involves a complex interaction of processes and these operate over periods of time which are often undervalued from a review of the relative rates of these processes it seems that short lived structures such as surtseyan ash rings may be preserved albeit in modified form around their whole circumference for larger composite volcanoes however growth and remoulding by activity is balanced by penecontemporaneous and later erosion in such a way that the central and most of the proximal zones are likely to be removed before they can be buried either by transported volcaniclastic or regional epiclastic sediments even the preservation of distal zones may be asymmetrical a one sided hinge line subsidence sedimentation model is therefore suggested to explain the elongate lens shaped aspects of the many extrusive piles known to lie approximately concordantly within stratigraphic sequences formed in both continental and littoral environments

the jurassic succession in west central argentina stratal patterns sequences and paleogeographic evolution abstract jurassic rocks in west central argentina are predominantly marine and marginal marine siliciclastics associated with prominent but volumetrically subordinate carbonates and evaporites facies developments were ruled by paleogeographic persistence within the southern infratropical latitude belt within 40 50 s and by siliciclastic derivation from the patagonian hinterland to the southwest minor volcanic and volcaniclastic supply arrived from the west out of a magmatic belt related to circum pacific convergence timing of the main marine flooding events and general correspondence with the high rank stratal packaging recorded in western north america suggest that global eustasy was also a factor in controlling the local stratigraphic record early jurassic sedimentation occurred within a series of semi isolated depocentres linked to fault bounded triassic troughs the sinemurian toarcian deposits record depocentre expansion and coalescence these trends were coeval with progressively more widespread marine invasions from the northwest and west leading to an elongate marine seaway which connected central patagonia with the pacific domain during the aalenian bajocian the region was subject to a more subdued tectonism and the foreland side of the basin became fringed by an extensive clastic embankment bathonian and early callovian were times when coarse clastics prograded into the basin while the marine embayment shrank as a result of stepwise forced regressions during late callovian to oxfordian globally rising sea level the depocentre witnessed the appearance of cosmopolitan invertebrates and a stratal pattern of basin widening and depositional underbalance that promoted cratonward onlap and inception of widespread carbonate deposition ooidal coralline carbonate development was terminated after a relatively sudden messinian style event that desiccated a large tract of the andean basin and favored massive precipitation of anhydrite in the course of the kimmeridgian the evaporite basin was largely flooded by siliciclastics and turned into a broad and featureless mudflat salina complex linked to a widespread erg and to an ephemeral drainage system by the tithonian at a period of peak oceanic stand marine connection was reinstated and recorded as the most widespread jurassic transgression across the neuquen aconcagua embayment shelfal deposition consisted of molluscs and ooid dominated carbonate terraces that grew in pulses tuned to eustatic fluctuations like in other prolific petroleum provinces around the world the tithonian basinal strata involve widespread euxinic deposits featuring unusually high organic content

etruria formation westphalian c palaeoenvironments and volcanicity on the southern margins of the pennine basin south staffordshire england the westphalian c etruria formation of tansey green south staffordshire comprises three facies associations deposited during a period of extensional faulting subsequent to upper delta plain deposition during westphalian a and b times the lower etruria formation was deposited upon a poorly drained alluvial plain fed by low sinuousity east and northeast flowing streams which were periodically inactive whilst channel geometry remained unchanged continued sedimentation resulted in relative lowering of the water table and development of a second facies association characterized by well drained alluvial plain conditions upon which conifers thrived these alluvial associations include occasional well developed palaeosol horizons indicative of variable drainage conditions continued extensional faulting culminated in alkaline basaltic and phreatomagmatic volcanism and deposition of the third facies association which comprises volcaniclastic and possible epiclastic rocks rapid deposition of the third association resulted in the excellent preservation of a westphalian c conifer flora although extensional tectonism provided the dominant control on sedimentation this is believed to be only of localized importance during a period of variscan compression and associated uplift which characterizes much of the southern margin of the pennine basin

new insights from legacy seismic data reprocessing of legacy 2d seismic data for imaging of iron oxide mineralization near sishen mine south africa abstract two overlapping legacy seismic profiles 130 km long end to end were shot in the 1990s over the kuruman hills on the western margin of the kaapvaal craton in southern africa the 6 s profiles were aimed at investigating the crustal structure of the western kaapvaal craton as well as to locate potential continuation of the witwatersrand gold bearing horizons beneath the cover rocks the latter of which was unsuccessful in this study the legacy seismic data are reprocessed and used to image the iron oxide mainly haematite mineralization found in the kuruman formation of the griqualand west supergroup which outcrops along the two seismic profiles the seismic profiles are located close to the sishen open pit iron mine where one of the world s largest iron ore concentrations 986 mt is mined the reprocessed and merged seismic data are combined with magnetic magnetotelluric borehole and outcrop data to constrain the interpretation and all indicate the mineralization host rocks to have 500 m thickness and 950 m depth the seismic data further reveal seismic reflections associated with multiple iron ore horizons which are affected by a first order scale syncline and numerous near vertically dipping 65 80 normal and reverse faults of various orientations and throws thus providing insight into the structurally controlled iron ore mineralization in the area seismic tomography and magnetotellurics characterize the sediments to have a velocity ranging between 5000 and 6000 m s and a resistivity of 10 m the seismic imaging of the syncline and associated structural disruptions is important for future mining purposes and plans in the area as these structures might have preserved iron oxide mineralization from erosion the reprocessed data thus provide information that could be incorporated in potential future underground mine planning in the area improving the resource evaluation of the iron oxide deposit legacy seismic data are thus shown to hold intrinsic quality and possible untapped potential that can be realized via data reprocessing

magma mixing and petrogenesis of the 13 november 1985 eruptive products at nevado del ruiz colombia abstract magma erupted on november 13 1985 at nevado del ruiz colombia was compositionally diverse with bulk compositions ranging from andesite 59 sio2 and 6 mgo to dacite 65 sio2 and 3 mgo the textural heterogeneity of juvenile products is emphasized by clasts of black scoria gray scoria black scoria with white pumiceous inclusions banded scoria banded pumices and white pumices numerous phenocrysts are out of equilibrium in the andesitic component including magnesian olivine sodic plagioclase and hypersthene plagioclase compositions are bimodal such features and textural heterogeneities are interpreted to result from transfers of phenocrysts by a mechanical mixing process mainly from the felsic component to the mafic one three mineralogical associations have been observed dacitic association andesine an 35 50 hypersthene en 68 70 edenite augite phlogopite magnetite and ilmenite andesitic association labradorite an 55 65 bronzite en 75 78 pargasite augite phlogopite magnetite and ilmenite mafic association magnesian olivine fo 79 87 and bronzite en 78 83 basic magma was not erupted but is thought to be responsible for triggering the eruption it could have been injected into a shallow dacitic reservoir ph2o about 1 kbar destabilized it and mechanical mixing could have started in the magma chamber or through a conduit just before eruption the basic magma evolved completely to andesite as suggested by the experiments of kouchi and sunagawa in 1983 while some original dacitic magma was preserved as complete homogenization was not attained the resulting products are texturally heterogeneous such a genesis explains the mineralogical disequilibria

reflection seismic imaging and physical properties of base metal and associated iron deposits in the bathurst mining camp new brunswick canada the bathurst mining camp northern new brunswick canada is a major base metal producing region that includes the brunswick no 12 and smaller past producing brunswick no 6 deposit sustaining mining activity in the camp requires exploration of orebodies near and adjacent to existing mines in this paper we recovered re processed modeled and interpreted a 2d high resolution reflection seismic profile in the vicinity of the brunswick no 6 open pit with the aim of providing key information on the geological structures associated with mineral deposits at depth the seismic data quality is good to excellent with numerous strong reflections in raw shot gathers resulting from a careful survey design that included test shots and a priori considerations for the geological environment physical properties of lithological units and mineralized zones including gamma gamma conductivity magnetic susceptibility density and sonic data were studied from borehole geophysical measurements conducted near the mine this helped to correlate seismic data and shows that the brunswick horizon a key ore prospecting horizon and associated lithological contacts are strongly reflective the results demonstrate that reflection seismic imaging has been particularly effective for imaging steeply dipping structures of the brunswick no 6 deposit many of which intersect the surface and thus allow for correlation with surface geology massive sulfides and iron formation of the brunswick horizon are identifiable locally within a distinctive reflective package that can be used as a broad guide in the region for the exploration of deep base metal deposits in several locations a change from highly reflective to semi transparent seismic character is explained by the presence of faults that juxtapose highly reflective lithological units with seismically transparent lithologies

intra arc sedimentation in a low lying marginal arc eocene clarno formation central oregon abstract the clarno formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows and cut locally by irregular hypabyssal stocks dikes and sills lateral lithofacies variations are pronounced and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation a range of sedimentary environments is represented in the volcaniclastic sequences including near vent flow and breccia accumulations bouldery high gradient braided streams and relatively low gradient sandy tuff braidplains associated with paludal deposits we infer that the coarse grained volcaniclastic rocks of the clarno formation accumulated largely in volcanic flank and apron settings the formation s stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands and only a few scattered remnants of large central vent volcanoes are known the absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop we infer that the arc was composed of small to medium size volcanoes arranged haphazardly across a broad area the sedimentary succession most probably accumulated in a series of intra arc depressions formed by diffuse block rotation driven by oblique subduction during the eocene