late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

field test of autogenic control on alluvial stratigraphy ferris formation upper cretaceous paleogene wyoming internally generated autogenic sedimentary processes can obscure signals of tectonic movements climate conditions or sea level fl uctuations in alluvial basins the stratigraphic effects of autogenic dynamics have been considered negligible on time scales of 10 4 10 6 yr however recent physical and numerical models have shown that over basin fi lling time scales self organization in sedimentary systems can produce stratigraphic patterns similar to those resulting from changing basin boundary conditions here we present the fi rst fi eld based test for autogenic control on stratigraphy in an ancient alluvial basin the ferris formation upper cretaceous paleogene hanna basin wyoming is composed of clusters of closely spaced channel deposits separated by intervals dominated by overbank material a pattern that has been proposed as an example of a potentially autogenic stratigraphy in order to evaluate controls on ferris formation stratigraphy spatial patterns of key channel properties were analyzed these variables including sand body dimensions paleofl ow depth maximum clast size paleocurrent direction and sediment provenance were chosen because they can change in response to climate tectonic or sea level forcing and are commonly measurable in ancient alluvial successions no clear statistical trends were detected in the measured variables which suggests that external forces did not likely control stratigraphic organization in the unit this study demonstrates that autogenic dynamics in natural fi eld scale systems can produce organized stratigraphic patterns over much larger spatial and temporal scales than is typically presumed this fi nding emphasizes the need for further understanding of autogenic stratigraphy and greater care interpreting climate tectonic and eustatic changes from alluvial basins

tectonic implications of a late mesozoic fold and thrust belt in northwestern nevada the luning fencemaker fold and thrust belt of northwestern nevada formed east of the mesozoic sierran arc in a back arc setting where noncontinental crust localized marine deposition and subsequent deformation the fold thrust belt was active in middle or late jurassic and early cretaceous time and evolved during major northwest southeast contraction comparable shortening was not developed in the sierran arc and for much of the mesozoic the arc and the back arc regions were at least partially decoupled and separated by the regionally extensive pine nut fault system which underwent substantial left lateral displacement development of the fold thrust belt and associated strike slip fault is attributed to oblique subduction the shortening axis of the fold thrust belt and inferred displacement of the strike slip fault are incompatible with right oblique subduction and call for an interval of left oblique convergence during part of jurassic cretaceous time

connection between igneous activity and extension in the central mojave metamorphic core complex california the development of metamorphic core complexes and associated low angle detachment faults commonly is intimately associated with synextensional igneous activity in most areas studied to date the relation of magmatism to extension is obscured by imprecise dating and by the overprint of later tectonic events we present data from the early miocene central mojave metamorphic core complex cmmcc which indicate that extension was accompanied by igneous activity as reflected by prekinematic synkinematic and postkinematic plutons and coeval volcanic rocks deposited in the associated extensional basins the principal intrusion is an early miocene granite pluton exposed in outcrops across an area greater than 400 km2 dikes adjacent to the pluton are common in the mitchel range at the buttes and at fremont peak the overall orientation of the pluton and associated dikes is west northwest roughly perpendicular to the extension direction results of u pb analyses on zircon from two pluton and two dike samples yield ages of 20 to 23 ma two other dike samples yield inconclusive results synextensional basins formed by detachment faulting during the core complex development rocks in these basins compose the jackhammer and pickhandle formations and filled an elongate nw trending trough more than 50 km long the 4oar 39ar ages for tuff beds are as old as 23 8 0 3 ma near the base of the lower pickhandle formation and as young as 21 3 0 5 ma in the uppermost lower pickhandle hence volcanism and plutonism are coeval the diversity of intrusive relations relative to the timing and development of the mylonitic fabric in the cmmcc precludes any simple cause and effect relationship between magmatism and extensional deformation rather magmatism and extension may have been localized at a releasing bend in a transfer fault system which links extension in the cmmcc with extension in the colorado river area to the east

active faults in the zagros and central iran abstract active tectonic movements in the northwestern zagros include right lateral slip at the rate of about 10 mm a along the main recent fault which inherits the position of the main thrust now inactive and active thrusting and accompanying folding distributed between several zones southwest of the main recent fault in the southeastern zagros the fars province there are several right lateral faults that extend n s obliquely to the overall trend of the zagros fault and fold belt these may be either branches of the main recent fault or faults accommodating relative broadening of the outer zagros in its southeastern segment the main thrust in the southeastern zagros also remains inactive the ipak north tehran and mosha fault zones and several minor structures in the eastern alborz form the e w trending active fault system with combined reverse and left lateral slip on the ipak and mosha zones lateral movements with the late quaternary mean rate exceeding 1 mm a dominate over vertical fault movements together with right lateral faults stretching northeast of zagros the faults of the alborz may accommodate east directed motion of the iranian microplate

age of the peach springs tuff southeastern california and western arizona sanidine separates from pumice of the early miocene peach springs tuff are concordantly dated at 18 5 0 2 ma by two isotopic techniques the peach springs tuff is the only known unit that can be correlated between isolated outcrops of miocene strata from the central mojave desert of southeastern california to the western colorado plateau in arizona across five structural provinces a distance of 350 km thus the age of the peach springs tuff is important to structural and paleogeographic reconstructions of a large region biotite and sanidine separates from bulk samples of the peach springs tuff from zones of welding and vapor phase alteration have not produced consistent ages by the k ar method published ages of mineral separates from 17 localities ranged from 16 2 to 20 5 ma discordant 40ar 39ar incremental release spectra were obtained for one biotite and two of the sanidine separates ages that correspond to the last gas increments are as old as 27 ma the 40ar 39ar incremental release determinations on sanidine separated from blocks of peach springs tuff pumice yield ages of 18 3 0 3 and 18 6 0 4 ma laser fusion measurements yield a mean age of 18 51 0 10 the results suggest that sanidine and biotite k ar ages older than about 18 5 ma are due to inherited ar from pre tertiary contaminants which likely were incorporated into the tuff during deposition sanidine k ar ages younger than 18 ma probably indicate incomplete extraction of radiogenic 40ar whereas laser fusion dates of biotite and hornblende younger than 18 ma likely are due to postdepositional alteration laser fusion ages as high as 19 01 ma on biotite grains from pumice suggest that minerals from pre tertiary country rocks also were incorporated in the magma chamber

volume loss fluid flow and state of strain in extensional mylonites from the central mojave desert california abstract compositional changes in mylonitic rocks from the central mojave metamorphic core complex california indicate that large volume loss 20 70 attended formation of mylonite along the waterman hills detachment fault strong silica depletion and significant mobility of normally immobile elements imply large fluid rock ratios during mylonitization possibly as a result of multiple pass hydrothermal convection connecting near surface and mid crustal alteration regimes dissolution of quartz during mylonitization indicates that the fluid was undersaturated with respect to silica and therefore was probably not magmatic or connate large volume loss in a shear zone should result in apparent flattening strains yet mylonites throughout the complex including the mylonites that record large volume loss exhibit constrictional strains this inconsistency raises questions about the applicability of the shear zone model to cordilleran metamorphic core complexes

tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california several authors recently have proposed the rolling hinge model for the formation of extensional detachment faults in metamorphic core complexes in this model isostatic uplift of the denuded footwall of a major normal fault causes the near surface part of the fault to rotate from a higher to a lower dip for the rolling hinge model to be acceptable the footwalls of detachment faults must have undergone the strain pattern dictated by the rolling hinge in this paper the structural development of the footwall of the central mojave metamorphic core complex is used as a field test tertiary structures in the lower plate of the core complex record ne directed ductile extensional shear followed by cataclastic ne side up shear with a significant component of contraction along the previous extension direction this strain history agrees with the rolling hinge model and particularly with a version of the model in which footwall uplift occurs by a combination of elastic flexure and flexurally driven upper crustal failure similar late stage layer parallel contraction appears to be present in some other tectonically denuded terranes of the basin and range suggesting that the flexural failure rolling hinge model for extensional terranes may be widely applicable

mid tertiary structural evolution of the old woman mountains area implications for crustal extension across southeastern california investigations in the old woman mountains area provide insights into mid tertiary extensional tectonism in southeastern california the old woman mountains area is a moderately extended region that lies between two highly extended terranes the central mojave extensional complex and the colorado river extensional corridor two normal faults and a high angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks these blocks correspond to the four mountain ranges of the area the old woman piute little piute and ship mountains other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys deformation occurred in part before but primarily after deposition of the peach springs tuff an 18 5 ma regional stratigraphic marker transport directions of hanging walls are westward on the west side of the old woman mountains area and east southeastward on the east side we suggest that the old woman mountains area is a less extended portion of a continuous extensional terrane that stretches from the central mojave desert across the colorado river to the transition zone of the colorado plateau lower strain in the old woman mountains area than in most of the colorado river extensional corridor and central mojave extensional complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low angle simple shear zone shifted at about 18 20 ma from a position far to the west to just east of the old woman mountains area

stratigraphic architecture of extensional basins insights from a numerical model of sedimentation in evolving half grabens we present a numerical model for the tectonostratigraphic evolution of half grabens which duplicates many of the stratigraphic relationships observed in extensional terranes of the southwestern united states and elsewhere the model also generates insights and predictions on the impact of fault geometry rates of faulting and rates of erosion and sediment transport on landscape evolution stratal geometry and facies distributions three processes control basin development in the model faulting erosion deposition and isostasy assumptions are thin plate flexure normal fault hanging wall deformation by vertical simple shear linear dependence of sediment flux on slope and perfect sorting of sediment during deposition model limitations reflect oversimplification of the dynamics of sediment erosion and redistribution during periods of fault motion half grabens bounded by listric faults accumulate strongly asymmetric sedimentary units the degree of under or overfilling of the basin is controlled in the short term by basin geometry fault dip and in the longer term by extension rates sediment supply and out of plane sediment transport after fault motion ceases erosion and sedimentation redistribute the loads on the lithosphere resulting in progressively more symmetric sedimentary units which overlap the original basin boundaries model facies architecture is affected by changing subsidence patterns stratigraphic geometries and sediment flux under most conditions syntectonic gravels are trapped near the basin edge on the fault bounded side but prograde far out into the basin on the hinge opposite the fault side when fault movement ceases coarse grained sediment from the fault bounded side may initially be able to reach farther out into the basin but overall retrogression is observed on both sides of the basin as topography is eroded and as posttectonic sedimentation buries former sediment sources changing fault dips and lithospheric rigidity affect facies patterns whereas altering the ratio between rates of erosion and sediment transport affects landform development and depositional geometries

tertiary basin development and tectonic implications whipple detachment system colorado river extensional corridor california and arizona a stratigraphic and structural record of synchronous extension and detachment faulting is preserved in predominantly miocene deposits of the whipple detachment system colorado river extensional corridor in the mohave mountains and aubrey hills of arizona and the eastern whipple mountains of california near parker dam these deposits comprise four unconformity bounded sequences composed of locally derived epiclastic and volcanic rocks and the peach springs tuff the three older sequences represent syntectonic units that were deposited coeval with detachment faulting and the fourth is interpreted to be postextensional the sequences are correlated between four fault bounded regions which are the remnants of four different depositional basins similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units the basins developed in about the same positions relative to each other and to volcanic sources as they occupy at present this is shown by gradational changes of pre tertiary rock types between regions systematic variations in the abundance of magmatic units and correlative volcanic units that occur in two adjacent regions the basins formed in the early miocene from segmentation of the upper crust into blocks bounded by high angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low angle detachment fault extreme rotation of one large crustal block which constitutes the central mohave mountains is recorded by a major unconformity in the lower miocene section of one basin because coeval sections that formed in adjoining basins do not record this rotation the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently these proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the whipple detachment system

regional stratigraphy of the zagros fold thrust belt of iran and its proforeland evolution the latest neoproterozoic through phanerozoic stratigraphy of the zagros fold thrust belt of iran has been revised in the light of recent investigations the revised stratigraphy consists of four groups of rocks each composed of a number of unconformity bounded megasequences representing various tectonosedimentary settings in the lowest group ranging in age from latest precambrian to devonian the uppermost neoproterozoic to middle cambrian rocks constitute a megasequence of evaporites siliciclastic deposits and interlayered carbonates which were deposited in pull apart basins that developed by the najd strike slip fault system this megasequence is overlain by a second one middle to late cambrian in age which consists of shallow marine siliciclastic and carbonate rocks representing deposition in an epicontinental platform the overlying shales siltstones and partly volcanogenic sandstones of ordovician silurian and devonian age are local remnants of stratigraphic units that were extensively eroded during development of several major unconformities the second group consists of two megasequences one permian and the other triassic composed of widespread transgressive basal siliciclastic rocks and overlying evaporitic carbonates of an equatorial epi pangean very shallow platformal sea the third group is composed of four megasequences formed of shallow and deep water carbonates with some siliciclastic and evaporite deposits which accumulated on a neo tethyan continental shelf during earliest jurassic through late turonian time the fourth group comprises siliciclastic and carbonate deposits of a largely underfilled nw to se trending forward and backward migrating late cretaceous to recent proforeland basin which has evolved as an integral part of the zagros orogen this last group consists of three megasequences ix x and xi with distinctive lateral and vertical facies variations which reflect specific tectonic events megasequence ix comprises uppermost turonian to middle maastrichtian prograding and retrograding siliciclastic and carbonate deposits whose accumulations reflect emplacement obduction of ophiolite slivers and subsequent collisional events in the zagros orogen megasequence x consists of uppermost maastrichtian to upper eocene siliciclastic and carbonate rocks which deposited first progradationally in front of the zagros orogenic wedge with reduced contractional tectonic activity and then retrogradationally due to intensified thrust stacking in the interior parts of the orogen megasequence xi consists of oligocene and lower miocene carbonate strata deposited retrogradationally shortly after a period of intensified late eocene thrust faulting in the deformational wedge and an overlying succession of upward coarsening northeasterly derived siliciclastic deposits of lower miocene to recent age which are composed of erosional byproducts of the southwest vergent zagros thrust sheets

additional paleomagnetic results from the sierra nevada further constraints on basin and range extension and northward displacement in the western united states modal analysis of new paleomagnetic results that have a skewed distribution from four plutons in the kings canyon national park area central sierra nevada produces a pole position concordant with the results of frei and others 1984 from the yosemite area these studies suggest a that stable magnetization resides in the more mafic plutons and inclusions in the sierra nevada and b that the amount of internal deformation within the central sierra nevada does not exceed a few degrees at the 95 confidence level the mean pole of both studies limits the relative displacement between the sierra nevada and cratonal north america to 6 8 of clockwise rotation and 6 6 of poleward displacement the clockwise rotation agrees well with the model of frei and others 1984 for basin and range extension which predicts 2 of clockwise rotation for the sierra nevada the poleward displacement suggests right lateral shear of 600 km within the basin and range between 90 and 35 ma

large magnitude miocene extension in the central mojave desert implications for paleozoic to tertiary paleogeography and tectonics this is the published version copyright 1990 american geophysical union all rights reserved

a petrologic comparison of triassic plutonism in the san gabriel and mule mountains southern california triassic magmatism in the southwest u s cordillera forms a semicontinuous magmatic arc extending from northwestern nevada to southeastern california quartz monzodioritic and quartz monzonitic rocks and associated diorites and granites are widespread in southeastern california and we suggest that these rocks represent exposure of a structurally deeper part of the triassic arc where it was emplaced into comparatively thick proterozoic crust elemental and isotopic data suggest that triassic quartz monzodiorites and quartz monzonites in the mule and san gabriel mountains were derived from a relatively undepleted nonradiogenic mafic lithospheric source with virtually no upper crustal interaction very limited data for associated triassic diorites indicate a wide range in composition and a surprisingly radiogenic isotopic signature younger triassic granites record a strong geochemical signature of interaction with continental crust including inherited zircon and high initial sr ratios but comparatively less radiogenic pb isotopic compositions the major and trace element geochemistry of late triassic plutonic rocks in southeastern california is similar in many respects to alkalic components of the triassic arc in the mojave desert however contemporaneous rocks farther north have a calc alkalic signature perhaps reflecting the variation in age and composition of lithosphere across which the triassic arc was constructed

quantifying the hierarchical organization of compensation in submarine fans using surface statistics abstract stratigraphy is often interpreted within hierarchical or scale dependent frameworks that subdivide deposits based on distinct jumps in characteristics such as duration of deposition or scale while the interpretation is logically valid few studies quantitatively demonstrate that the jumps exist rather recent work has quantitatively shown some characteristics of stratigraphy to be fractal or scale invariant compensational stacking the tendency for sediment transport systems to preferentially fill topographic lows is a concept widely used in stratigraphic interpretation here we use the compensation index a metric that quantifies the strength of compensational stacking in sedimentary deposits to describe the architecture of stratigraphy exposed in outcrops of submarine fan strata in the carboniferous ross sandstone representing contrasting architectural styles 1 predominantly lobe elements and 2 predominantly channel elements in both datasets the stratigraphic architecture is classified into hierarchical classes of beds stories and elements results are the following first at both sites we document statistically significant increases in the strength of compensation across larger hierarchical levels supporting the use of hierarchical interpretations of stratigraphy it is therefore plausible for some characteristics of sedimentary systems to be hierarchical and others to be fractal second we document that lobe elements stack more compensationally than channel elements we interpret this pattern to document that compensation increases along a longitudinal transect through this distributive submarine fan

large volume rhyolite ash flow eruptions and submarine caldera collapse in the lower mesozoic sierra nevada california the mineral king roof pendant in the southern sierra nevada california provides a view of a triassic and early jurassic submarine caldera complex which was tilted to vertical and intruded by middle cretaceous granitoids an estimated 90 km3 of rhyolitic and 10 km3 of andesitic volcanic rocks are interstratified with marine sandstones siltstones limestones and conglomerates periods of volcanotectonic activity alternated with periods of marine sedimentation volcanic eruptions at mineral king resulted in 1 deposition of widespread very thick sheets of rhyolite ash flow tuff and 2 construction of small 1 8 km3 andesitic stratocones growth faults fault talus aprons slide blocks slumps and debris flows found in association with these volcanic deposits indicate that the eruptive phases represent periods of great seismic activity interuptive phases were periods of volcanic and tectonic quiescence during which sequences of shallow to deep marine sediments were deposited four different times at mineral king eruption of large volumes of rhyolite ash flow tuff resulted in catastrophic caldera collapse and ponding of single ash flow tuffs to thicknesses greater than 0 5 km only minimum volumes can be estimated for three of these intracaldera tuffs because they are truncated by younger granites volumes are estimated to be at least 25 km3 each placing these tuffs in the lower size range of epicontinental ring structures the fourth caldera the vandever mountain caldera is entirely preserved in the roof pendant the vertically dipping strata of the vandever mountain caldera provide an unusual opportunity to examine a submarine caldera in cross section evidence for caldera subsidence during the eruption of the vandever mountain rhyolite ash flow tuff includes the following 1 the great thickness of the ash flow tuff and the rapid accumulation of the tuff during progressive draw down of a compositionally zoned magma chamber 2 high angle faults which pond the ash flow tuff and offset underlying but not overlying stratigraphy and 3 megabreccia and slide blocks up to 0 5 km long that were derived from the caldera wall and floor and incorporated into the ash flow tuff as it filled the caldera disruption of the caldera floor was intense near the caldera margin but negligible along most of its length owing to pistonlike subsidence of an intact cylinder of crust small volume ash flow eruptions occurred along an incipient leaky ring fracture zone prior to the caldera forming eruption of the vandever mountain ash flow tuff the vandever mountain caldera was not affected by resurgent doming or late stage rhyolite magmatism in the cross sectional view provided by the mineral king roof pendant

nature of the southwestern boundary of the central mojave tertiary province rodman mountains california tertiary volcanic and sedimentary rocks in the central mojave desert are largely confined to the barstow bristol trough an elongate tectonic depression filled with lower miocene strata the southwestern boundary of the trough is sharply defined and separates thick 2 km tertiary sequences from an area to the southwest that is virtually devoid of tertiary rocks segments of this boundary have been interpreted as a buttress unconformity as a right slip sidewall fault that formed the lateral boundary of an early miocene extensional terrane and as a dip slip fault caused by flexure of the north american plate above the subducted mendocino fracture zone the silver bell fault in the rodman mountains central mojave desert is the best exposed segment of the boundary and relations exposed along it can be used to test these hypotheses the tertiary sequence north of the silver bell fault is at least 2 3 km thick and comprises at least 1 9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed tertiary section tuff layers intercalated in the lower part of the clastic deposits are 23 24 m y old indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central mojave metamorphic core complex structural and stratigraphic data indicate that the silver bell fault is primarily a dip slip fault the tertiary section is folded into a west trending upright to overturned syncline located near the southern edge of exposed tertiary rocks the syncline records late cenozoic north south shortening along the northwest striking calico fault which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip southwestward dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension however bedding dip does not decrease upward in the tertiary section paleocurrent directions trend obliquely updip and northeast dipping normal faults are rare stratal tilting postdated the accumulation of the exposed section which therefore was not deposited during rotational normal faulting the new data contradict interpretation of the silver bell fault as a major right slip fault bounding the central mojave extensional terrane although they permit small magnitude extension across the fault the amount of early miocene crustal extension in this area therefore is apparently small in spite of stratal tilting and the lateral limit of large magnitude crustal extension in the central mojave block is farther northwest the data are consistent with the fault having formed by failure during plate flexure above the subducted mendocino fracture zone

correlation of lineaments and groundwater quality in dasht e arjan fars sw of iran the ne oriented dasht e arjan graben is located 65 km west of shiraz and has resulted from the active kare e bas fault segmentations this extensional graben bounded by two fault system east arjan and west arjan to the shahneshin and salamati anticline in these study using landsat 7 etm images with resolution 2 5 m and directional filtering in the four azimuths and semi automatic technique for linear structure in the study area using the obtained data from extracted lineaments the rose diagrams of the main strike lineaments are well confirm with field measurements of faults with n56 4 e direction the structural lineaments of the study area show that the dasht e arjan area is underlain by the limestone sandstone and marl landsat imagery of the area has been analyzed and interpreted in order to determine the lineament and groundwater quality across the area the fracture is structurally controlled and mostly influences both the groundwater and surface water pollution and flow directions in the dasht e arjan using visual interpretation determining the lineaments on the satellite image is very difficult and subjective and it requires an experienced interpreter in this study the lineament analysis is undertaken to examine the orientation of the lineament the relationship between lineaments and tectonic features and groundwater quality lineament density maps show that the lineament density is high around areas areas having high lineament density represent areas with relatively high groundwater pollution field observations agreed with the results from the analysis of the imagery

hydrochemical characteristics and groundwater quality assessment in bushehr province sw iran the chemical quality of groundwater of bushehr southwest of iran was assessed for its suitability for drinking purposes hydro geochemical studies were carried out in this area to identify the geochemical processes and their relation to groundwater quality a total of 19 water samples were collected from the aquifer the samples were then analyzed for different physicochemical properties such as ph total dissolved solids tds total hardness th calcium magnesium carbonate bicarbonate sulfate and chloride concentrations in this study the average tds content was in the range of 4419 10 066 mg l l and other important parameters of water such as th 1200 3500 mg l 1 and chloride 1046 3855 mg l 1 were also higher than the maximum permissible limits specified by who on the basis of concentrations of major elements studies of the study area showed that the total samples collected are unsuitable for drinking linear increase in sodium and chloride of the total dissolved ion indicated a dissolution of halite in the study area salinity of the aquifer is mainly a result of the dalaki river recharge dissolution of evaporated minerals intraformation and also agricultural returned water the dissolution of evaporite minerals such as halite and gypsum has increased the concentration of total dissolved solids and of sulfate in the shahpour river and also groundwater entering the study area has caused salinity in this river

late paleozoic extension in the great basin western united states geologic mapping in the toiyabe range in central nevada has revealed the existence of normal faults of probable mid mississippian to early permian age that strike roughly east west and dip northward additional evidence of uplift and erosion followed by mafic volcanism and subsidence suggests that much of the central and southern toiyabe range was affected by late paleozoic extension similar patterns of late paleozoic uplift and subsidence together with local basaltic volcanism are widespread in the western united states suggesting that the continental margin was dominated by extension or transtension in mississippian to permian time this extension was coeval with convergence between north america and south america across the ouachita and marathon belts and the dynamic interaction of these two margins may by analogy with the cenozoic tectonics of asia have given rise to complex late paleozoic deformation in the ancestral rocky mountains and adjacent areas of the interior western united states

paleogeographic and tectonic implications of the geology of the tiefort mountains northern mojave desert california geologic mapping petrologic and structural analyses and u pb dating in the tiefort mountains northern mojave desert california provide data on paleozoic paleogeography and mesozoic intra arc tectonics metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late precambrian cordilleran passive margin facies recognition of precambrian augen gneiss and metasedimentary rocks with craton margin protoliths suggests that the craton miogeocline boundary is in or near the tiefort mountains the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area the metasedimentary units are intruded by middle jurassic granite and diorite orthogneisses dated by u pb as ca 164 and ca 160 ma deformed rocks record amphibolite facies metamorphism and mylonitization during middle to late jurassic southeast vergent ductile shear pervasively annealed microstructures indicate that recrystallization outlasted deformation at south tiefort mountain the mylonitic fabric is deformed by northwest trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 14 ma at tiefort mountain synkinematic biotite granite gneiss is dated as ca 105 ma and mylonitic fabrics are folded about north trending axes the presence of the cretaceous orthogneiss suggests that two deformational events jurassic and cretaceous in age may have affected part of the study area dikes dated as ca 82 ma that crosscut all fabrics are interpreted to be part of a regional intrusive event the timing and vergence of shear zones at south tiefort mountain are similar to contractional shear zones in adjacent areas forming a belt of contractional deformation from the central mojave desert to the east sierran thrust system this deformation may be related to a period of late middle to late jurassic sinistral oblique subduction

the zagros folded belt fars iran constraints from topography and critical wedge modelling summary the late miocene tectonics of the zagros folded belt fars province has for long been related solely to folding of the cover controlled by a ductile decollement between basement and the sedimentary cover however geological constraints topography analysis and seismotectonic studies reveal that basement thrusting may produce locally significant deformation in the cover to determine how the deep seated deformation in the basement may contribute to the overall topography we first examine the filtered large and short wavelengths of the topography we find that the short wavelength component of the topography 20 25 km including the zagros folds is superimposed on the differential uplift at the regional scale in other words the regional base level of folded marker horizons remains parallel to the regional topography of interest modelling reveals that the salt based wedge model alone is not able to reproduce the large wavelength component of the topography of the zagros folded belt this reveals that when a thick relatively to its overburden layer of salt forms the basal decollement it is generally too weak and cannot support the growth of significant topography we then test an alternative thick skinned crustal wedge model involving the crust of the arabian margin which is decoupled above a viscous lower crust this model satisfactorily reproduces the observed topography and is consistent with present day basement thrusting topography analyses and geological constraints we conclude that basement involved thickening and shortening is mechanically required to produce the shape of the zagros folded belt since at least 10 ma finally the involvement of the basement provides mechanical and kinematic constraints that should be accounted for cross sections balancing and further assessing the evolution of zagros at crustal or lithospheric scales

geophysical and geomorphological approach for locating groundwater potential zones in sukinda chromite mining area the impact of mining causes deterioration of environment and decline of groundwater level in the adjoining mining areas which influences groundwater source for domestic and agriculture purposes this necessitated locating and exploiting of new groundwater source a fast cost effective and economical way of locating and exploration is to study and analyze remote sensing data interpreted remote sensing data were used to select sites for carrying out surface geophysical investigations various geomorphologic units were demarcated and the lineaments were identified by interpretation of false color composite satellite imageries the potential for occurrence of groundwater in the sukinda valley was classified as very good good moderate and poor by interpreting the images sub surface geophysical investigations namely vertical electrical soundings were carried out to delineate and demarcate potential water bearing zones integrated studies of interpretation of geomorphologic lineaments and geophysical data aquifer thickness were used to prepare groundwater potential map the studies reveal that the groundwater potential of shallow aquifers is due to geomorphologic features and the potential of deeper aquifers is determined by lineaments and degree of weathering

cenozoic crustal rotations in the mojave desert from paleomagnetic studies around barstow california paleomagnetic data from the mojave desert around barstow california suggest 55 to 75 of clockwise rotation of this area in earliest miocene time followed by 23 of counterclockwise rotation in the early miocene and no rotation after 18 ma earliest miocene clockwise rotation of the barstow area is suggested by data from nine sites in the oligocene miocene lane mountain quartz latite and jackhammer formation and may be related to oroclinal bending of the southern sierra nevada batholith data from 13 sites in the early miocene pickhandle formation indicate counterclockwise rotation which coincides with early miocene e ne w sw extension and detachment faulting in the mojave desert other parts of the mojave desert experienced variable senses and amounts of rotation at this time which may be related to drag along transfer zones within the upper plate while the lower plate was not rotated these results suggest that miocene extension in the mojave desert is related to similarly oriented extension in the colorado river extensional corridor early miocene to pliocene rocks of the andesite and dacite of the calico mountains barstow formation andesite of murphy s well and black mountain basalt show no evidence for significant rotation of the barstow area after 18 ma most other paleomagnetic data from elsewhere in the mojave are consistent with a lack of significant post 18 ma rotations suggesting that post 10 ma right lateral faulting there has produced relatively little crustal rotation the exception to this generalization is the northeast corner of the mojave desert which underwent substantial post 18 ma clockwise rotation paleomagnetic flattening data also indicate that post oligocene n s translation of crustal blocks in the mojave desert has been insignificant

digital terrain modelling a review of hydrological geomorphological and biological applications the topography of a catchment has a major impact on the hydrological geomorphological and biological processes active in the landscape the spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques many geographic information systems are being developed that store topographic information as the primary data for analysing water resource and biological problems furthermore topography can be used to develop more physically realistic structures for hydrologic and water quality models that directly account for the impact of topography on the hydrology digital elevation models are the primary data used in the analysis of catchment topography we describe elevation data sources digital elevation model structures and the analysis of digital elevation data for hydrological geomorphological and biological applications some hydrologic models that make use of digital representations of topography are also considered

pennsylvanian and early permian paleogeography of east central california implications for the shape of the continental margin and the timing of continental truncation pennsylvanian and early permian paleogeographic features in east central california include a southeast trending carbonate shelf edge and turbidite basin that we infer paralleled a segment of the western margin of the north american continent this segment of the continental margin was oblique to an adjoining segment on the north that trended southwestward across nevada into easternmost california we propose that the southeast trending segment of the margin originated by tectonic truncation of the originally longer southwest trending segment in early or middle pennsylvanian to late early permian time significantly earlier than a previously hypothesized late permian or early triassic continental truncation event we interpret the truncating structure to have been a sinistral transform fault zone along which a continental fragment was removed and carried southeastward into the caborca hermosillo region of northern mexico where it is now represented by exposures of late proterozoic and paleozoic miogeoclinal rocks

three dimensional numerical modeling of eustatic control on continental margin sand distribution abstract eustasy constitutes a key control on continental shelf accommodation along with tectonism the combined effect of these processes influences relative sea level which is thought to regulate the location of facies stratigraphic architecture and distribution of sediment to basin margins here we used a nonlinear diffusion based numerical stratigraphic forward model to investigate the impact of the eustatic curve we ran two models one using a eustatic curve derived from the new jersey margin u s a and the other a base case with no eustatic variation but which were otherwise identical we conclude that changing the eustatic curve resulted in a 0 9 difference in cumulative deep water 200 m sand volumes between models this is because the no eustatic variation model resulted in deep water sand delivery events with magnitudes similar to those observed in the eustatic model which were due to the delta becoming fixed at the shelf edge and the subsequent temporary suspension of avulsive processes in the eustatic model maximum deep water sand accumulation occurred during short term sea level falls but peak sand delivery was not well related to the amplitude of the sea level fall itself further examination of the relationship between rates of relative sea level and deep water sand delivery show a poor correlation r 2

an expanded view of jurassic orogenesis in the western united states cordillera middle jurassic pre nevadan regional metamorphism and thrust faulting within an active arc environment klamath mountains california basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic ultramafic intrusive complexes delineate a middle jurassic arc terrane within the klamath mountain province of northern california exposures of the supracrustal volcaniclastic rocks are restricted to a single fault bounded terrane but the deeper level intrusive complexes were emplaced into most if not all the pre late jurassic terranes of the klamath mountain region the pre late jurassic terranes thus constitute the basement of the middle jurassic arc u pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus k ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of 177 159 ma the active arc and its basement were imbricated by a compressive deformational event the signature of which included thrust faulting isoclinal folding and regional metamorphism several diverse lines of evidence including k ar dating of metamorphic rocks crosscutting relations of dated intrusive complexes to thrust faults and u pb dating of synmetamorphic intrusive complexes establish a distinctly pre nevadan middle jurassic age ongoing at 169 ma and over by at least 161 ma for this compressive deformational episode outboard and structurally beneath the middle jurassic arc and its basement are several terranes that collectively comprise the western jurassic belt these terranes were deformed and regionally metamorphosed during the late jurassic nevadan orogeny which occurred within the time interval of 157 150 ma as upper jurassic plutons with 150 to 142 m y old zircon ages have contact aureoles that overprint the nevadan fabric and the 157 m y old rogue formation was deformed in the nevadan event the middle and late jurassic compressive deformational events were thus distinct and separated by as much as 15 20 m y the relation between middle and late jurassic magmatism and deformation suggests that the klamath mountain province records the evolution of a considerably long lived arc system that evolved above an east dipping subduction zone in addition we suggest that this are system may represent an oceanic continuation of the long recognized early mesozoic arc terrane of the western u s cordillera

hydrogeological factors their association and relationship with seasonal water table fluctuation in the composite hardrock aravalli terrain india seasonal rise of groundwater level manifests aquifer recharge through infiltration spatial variation of aquifer recharge within the same basin or terrain is a function of terrain heterogeneity governed by different hydrogeological factors however assessment of relations between water level fluctuations wlf with various hydrogeological factors is not straightforward in the present study various hydrogeological factors that could influence aquifer recharge in the deformed crystalline aravalli terrain of india have been analysed frequency plots have been used to assess the variations in wlf under different geological parameter classes seasonal wlf associated with these factors have been compared using various statistical parameters parametric and non parametric statistical tests have been used to determine the statistical significance of fluctuation difference the study infers that saturated thickness lineament lineament intersection and drainage beside surface elevation and well depth are the main geological factors influencing aquifer recharge in the aravalli terrain median values under these factors are integrated and compared with the interpolated values of mean wlf at hypothetical well locations they are found to closely resemble each other this infers capability and applicability of the technique in identifying key factors governing wlf and in predicting wlf at unexplored locations

late cretaceous and early miocene andean type plutonic activity in northern makran and central iran two calc alkaline plutonic complexes bazman and natanz intruded through the south eastern active and south western ancient continental margins of central iran have been dated by the rb sr whole rock isochron method at 74 2 and 24 4 5 ma respectively detailed trace element studies together with low 87 sr 86 sr initial ratios 87 sr 86 sr ratios and the gabbros at least were intruded some 10 ma earlier

evidence of uppermost proterozoic to lower cambrian miogeoclinal rocks and the mojave snow lake fault snow lake pendant central sierra nevada california this is the published version copyright 2010 american geophysical union all rights reserved

a big fan of signals exploring autogenic and allogenic process and product in a numerical stratigraphic forward model of submarine fan development distinguishing an allogenic signal from trends and patterns produced by autogenic processes is a critical element in interpreting understanding and predicting strata lobyte3d is a new reduced complexity model of dispersive flow over an evolving topography on fan systems that produces surprisingly complex potentially hierarchical strata despite a simple formulation two submarine fan model scenarios are run one with constant sediment input and one with a sinusoidal variation in sediment input with an oscillation period of 25 ky and a peak to trough 80 volume change both model scenarios show that flows cluster to produce lobes which migrate and can rapidly switch location runs tests that can detect thickening and thinning bed trends and spectral analysis that detects the frequency of any signal present suggest that strata can be ordered even in the absence of any allogenic signal with cycles and trends in bed thickness but no single characteristic frequency in the oscillating supply scenario an allogenic signal is present in places particularly in the axial mid fan but may be difficult to distinguish from the autogenic signal with only limited outcrop data and without knowing a priori how the allogenic signal is likely to be preserved in complex and incomplete strata based on these limited model results we hypothesize that analysis of mid fan vertical sections using simple power spectrum analysis and counting of the significant peaks present across a range of frequencies may allow identification of a signal bump that could be evidence of the presence and nature of allocyclic forcing further lobyte3d modeling work will explore if and how the signal bump is preserved with input signals across a range of frequencies and amplitudes to guide further data collection and interpretation in outcrop and subsurface strata

sedimentation and extensional basin evolution in a miocene metamorphic core complex setting alvord mountain central mojave desert california usa the lower miocene clews formation at alvord mountain mojave desert california comprises an upward coarsening sequence of synextensional continental deposits of variable thickness this sequence pinches out to the west against the alvord mountain pre tertiary basement complex and thickens eastward to c 300 m over a distance of 7 km two stages of sedimentation are recognized in the formation during the initial stage of sedimentation the depocentre was delineated by a lacustrine system that was bounded to the west by small sheetflood dominated alluvial fans and to the north by a southerly flowing fluvial braidplain lacustrine mudstone siltstone and carbonate suggest an initially closed basin conglomerate to the west represents small sheetflood dominated fans igneous clasts and a south east palaeoflow direction suggest a proximal low relief source in the western alvord mountain area pebbly sandstone of the southward prograding braidplain has a metasedimentary provenance in the paradise range to the north the second stage of deposition was dominated by coarse conglomerate and breccia west to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the cronese hills 8 km to the east these strata reflect rapid westward progradation of sheetflood and debris flow dominated alluvial fans that advanced across the axial braidplain and lacustrine system basin development is interpreted to have been controlled by regional ne directed extension on a detachment fault associated with the central mojave metamorphic core complex 30 km to the west the alvord mountain cronese hills region was initially transported as a single hangingwall block on the e dipping detachment initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall subsequent westward progradation of alluvial fans out of the cronese hills is believed to record the propagation of a nw striking sw dipping normal fault antithetic to the low angle e dipping detachment fault

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

delineation of shallow resistivity structure around malvan konkan region maharashtra by neural network inversion using vertical electrical sounding measurements modeling resistivity profiles especially from hard rock areas is of specific relevance for groundwater exploration a method based on bayesian neural network bnn theory using a hybrid monte carlo hmc simulation scheme is applied to model and interpret direct current vertical electrical sounding measurements from 28 locations around the malvan region in the sindhudurg district southwest india the modeling procedure revolves around optimizing the objective function using the hmc based sampling technique which is followed by updating each trajectory by integrating the hamiltonian differential equations via a second order leapfrog discretization scheme the inversion results suggest a high resistivity structure in the north western part of the area which correlates well with the presence of laterites in the south western part a very high conductive zone is observed near the coast indicating an extensive influence of saltwater intrusion our results also show that the effect of intrusion of saline water diminishes from the south western part to the north eastern part of the region two dimensional modeling of four resistivity profiles shows that the groundwater flow is partly controlled by existing lineaments fractures and major joints groundwater occurs at a weathered semi weathered layer of laterite clayey sand and the interface of overburden and crystalline basement the presence of conduits is identified at a depth between 10 and 15 m along the dhamapur kudal and parule oros profiles which seems to be potential zone for groundwater exploration the nw se trending major lineaments and its criss cross sections are indentified from the apparent and true resistivity surface map the pseudo section at different depths in the western part of the area near parule shows extensive influence of saltwater intrusion and its impact reaching up to a depth of 50 m from the surface along the coastal area further the deduced true electrical resistivity section against depth correlates well with available borehole lithology in the area present analyses suggest that hmc based bnn method is robust for modeling resistivity data especially in hard rock terrains these results are useful for interpreting fractures major joints and lineaments and crystalline basement rock and also for constraining the higher dimensional models

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

on the frequency distribution of turbidite thickness the frequency distribution of turbidite thickness records information on flow hydrodynamics initial sediment volumes and source migration and is an important component of petroleum reservoir models however the nature of this thickness distribution is currently uncertain with log normal or negative exponential frequency distributions and power law cumulative frequency distributions having been proposed by different authors a detailed analysis of the miocene marnoso arenacea formation of the italian apennines shows that turbidite bed thickness and sand interval thickness within each bed have a frequency distribution comprising the sum of a series of log normal frequency distributions these strata were deposited predominantly in a basin plain setting and bed amalgamation is relatively rare beds or sand intervals truncated by erosion were excluded from this analysis each log normal frequency distribution characterizes bed or sand interval thickness for a given basal grain size or basal bouma division measurements from the silurian aberystwyth grits in wales the cretaceous great valley sequence in california and the permian karoo basin in south africa show that this conclusion holds for sequences of disparate age and variable location the median thickness of these log normal distributions is positively correlated with basal grain size the power law exponent relating the basal grain size and median thickness is different for turbidites with a basal a or b division and those with only c d and e divisions these two types of turbidite have been termed thin bedded and thick bedded by previous workers a change in the power law exponent is proposed to be related to i a transition from viscous to inertial settling of sediment grains and ii hindered settling at high sediment concentrations the bimodal thickness distribution of thin bedded and thick bedded turbidites noted by previous workers is explained as the result of a change in the power law exponent this analysis supports the view that a and b divisions were deposited from high concentration flow components and that distinct grain size modes undergo different depositional processes summation of log normal frequency distributions for thin and thick bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power law trend thus the occurrence of both log normal and segmented power law frequency distributions can be explained in a holistic fashion power law frequency distributions of turbidite thickness have previously been linked to power law distributions of earthquake magnitude or volumes of submarine slope failure the log normal distribution for a given grain size class observed in this study suggests an alternative view that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters in addition to the settling velocity of the grain sizes present

the signal and the noise forward modeling of allocyclic and autocyclic processes influencing peritidal carbonate stacking patterns recent carbonate sequence stratigraphic models suggest that the degree of order present in successions of carbonate parasequences is determined by temporal variations in eustatic amplitude and by varying levels of autocyclicity ordered parasequence stacking is suggested to occur during greenhouse periods dominated by low amplitude eustatic oscillations conversely disordered vertical successions are said to occur when amplitudes of eustatic oscillations are high or when autocyclic processes dominate and thus drown the periodic external signal these assertions of causal mechanism are tested here by application of a three dimensional stratigraphic forward model the three dimensional model generates parasequences via a ginsburg type mechanism of sediment transport and shoreline progradation parasequence stacking is controlled by a superimposed longer term eustatic oscillation and parasequences generated are analyzed using runs analysis and durbin watson autocorrelation results from the modeling suggest that autocyclic parasequences generated by shoreline progradation form ordered vertical successions when accommodation changes driven by a low amplitude relative sea level oscillation suggesting that partly autocyclic strata may not be inherently any less ordered than entirely allocyclic strata addition of stochastic processes such as mosaic carbonate production and fluctuations in regional sediment transport direction tend to decrease the level of measurable order present in the autocyclic strata increasing the frequency and amplitude of relative sea level oscillations also tends to decrease the measurable order in stacked autocyclic peritidal parasequence thicknesses these results suggest that controls on parasequence stacking patterns are likely to be more complex and varied than suggested by recent sequence stratigraphic models more outcrop and modeling investigation is required with appropriately circumspect interpretations applied

slope fan depositional architecture from high resolution forward stratigraphic models abstract submarine fans in tectonically active continental slope basins are targets of petroleum exploration and production these slope fans commonly comprise compensationally stacked sandy and muddy architectural elements including mass transport deposits weakly confined to distributary channel and lobe deposits and leveed channel deposits the lateral continuity and vertical connectivity of these architectural elements are important uncertainties in reservoir characterization that influence fluid flow behavior during hydrocarbon production here we use a simple forward stratigraphic model to simulate the stratigraphic patterns and illuminate the likely distribution of fine scale sub seismic heterogeneity in a slope fan we used published seismic reflection horizons from the tectonically active columbus basin offshore trinidad to define the top and base of a pleistocene submarine fan we then simulated the stratigraphic evolution of the slope fan with a series of dionisosflow forward stratigraphic models all variables were kept constant during the simulations in order to test the hypothesis that the autogenic evolution of the surface topography alone as a result of erosion and deposition can produce compensational stacking patterns common in submarine fans a reference case model is similar to the thickness trend of published isochron maps of the trinidad slope fan the reference case model also produced patterns of compensational stacking varying the time step impacts the heterogeneity of the model shorter time steps are characterized by less sediment accumulation which results in less sediment diversion during the subsequent time step more gradual migration of channel deposits shorter offset distances of depocenters and shorter length scale heterogeneity compared to longer time steps thus a key characteristic of slope fan deposits is autogenic compensational stacking without any external forcing which governs heterogeneity in these reservoirs furthermore our results suggest that relatively simple diffusion based models can produce realistic compensation patterns and future work will be focused on higher resolution model calibration to seismic reflection data and the influence of input variables on heterogeneity of channel and lobe deposits of slope fans

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

evolution of deep water stratigraphic architecture magallanes basin chile abstract the 4000 m thick and 20 myr deep water sedimentary fill of the upper cretaceous magallanes basin was deposited in three major phases each with contrasting stratigraphic architecture 1 the oldest deep water formation punta barrosa formation comprises tabular to slightly lenticular packages of interbedded sandy turbidites slurry flow deposits and siltstone that are interpreted to record lobe deposition in an unconfined to weakly ponded setting 2 the overlying 2500 m thick and shale dominated cerro toro formation includes a succession of stacked conglomeratic and sandstone channel fill deposits with associated finer grained overbank deposits interpreted to record deposition in a foredeep axial channel levee system 3 the final phase of deep water sedimentation is characterized by sandstone rich successions of highly variable thickness and cross sectional geometry and mudstone rich mass transport deposits mtds that are interpreted to record deposition at the base of slope and lower slope segments of a prograding delta fed slope system the deep water formations are capped by shallow marine and deltaic deposits of the dorotea formation these architectural changes are associated with the combined influences of tectonically driven changes and intrinsic evolution including 1 the variability of amount and type of source material 2 variations in basin shape through time and 3 evolution of the fill as a function of prograding systems filling the deep water accommodation while the expression of these controls in the stratigraphic architecture of other deep water successions might differ in detail the controls themselves are common to all deep water basins information about source material and basin shape is contained within the detrital record and when integrated and analyzed within the context of stratigraphic patterns attains a more robust linkage of processes to products than stratigraphic characterization alone

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

plate tectonic evolution of the southern margin of eurasia in the mesozoic and cenozoic abstract thirteen time interval maps were constructed which depict the triassic to neogene plate tectonic configuration paleogeography and general lithofacies of the southern margin of eurasia the aim of this paper is to provide an outline of the geodynamic evolution and position of the major tectonic elements of the area within a global framework the hercynian orogeny was completed by the collision of gondwana and laurussia whereas the tethys ocean formed the embayment between the eurasian and gondwanian branches of pangea during late triassic early jurassic times several microplates were sutured to the eurasian margin closing the paleotethys ocean a jurassic cretaceous north dipping subduction boundary was developed along this new continental margin south of the pontides transcaucasus and iranian plates the subduction zone trench pulling effect caused rifting creating the back arc basin of the greater caucasus proto south caspian sea which achieved its maximum width during the late cretaceous in the western tethys separation of eurasia from gondwana resulted in the formation of the ligurian penninic pieniny magura ocean alpine tethys as an extension of middle atlantic system and a part of the pangean breakup tectonic system during late jurassic early cretaceous times the outer carpathian rift developed the opening of the western black sea occurred by rifting and drifting of the western central pontides away from the moesian and scythian platforms of eurasia during the early cretaceous cenomanian the latest cretaceous paleogene was the time of the closure of the ligurian pieniny ocean adria alcapa terranes continued their northward movement during eocene early miocene times their oblique collision with the north european plate led to the development of the accretionary wedge of the outer carpathians and its foreland basin the formation of the west carpathian thrusts was completed by the miocene the thrust front was still propagating eastwards in the eastern carpathians during the late cretaceous the lesser caucasus sanandaj sirjan and makran plates were sutured to the iranian afghanistan plates in the caucasus caspian sea area a north dipping subduction zone jumped during paleogene to the scythian turan platform the shatski terrane moved northward closing the greater caucasus basin and opening the eastern black sea the south caspian underwent reorganization during oligocene neogene times the southwestern part of the south caspian basin was reopened while the northwestern part was gradually reduced in size the collision of india and the lut plate with eurasia caused the deformation of central asia and created a system of nw se wrench faults the remnants of jurassic cretaceous back arc systems oceanic and attenuated crust as well as tertiary oceanic and attenuated crust were locked between adjacent continental plates and orogenic systems

groundwater recharge and exploitative potential zone mapping using gis and god techniques two thirds of the total area of taiwan is mountainous terrain which is the main groundwater recharge source of the plains region this study assesses groundwater recharge and exploitative potential zone in the central division of the mountain areas of taiwan the middle reaches of the jhuoshuei river basin basic information from remote sensing and a satellite phantom is collected to set up the basic data maps using elevation formosa ii images the normalized difference vegetation index drainage distribution slope aspect ratio lineament distribution and land cover a geographical information system is used to integrate five contributing factors namely lithology land cover land use drainage slope and lineaments the criteria for the recharge potential assessment are established to demarcate the potential groundwater recharge zone finally the god rating system is adopted to evaluate the potential exploitation zone three main parameters are considered the groundwater occurrence the lithology of the overlying layers and the depth to groundwater the results show that the middle reaches of the chenyuland river have large potential exploitation zones due to its high rainfall recharge capacity regions west of the jhuoshuei river and the downstream regions of the chingshui river are medium potential exploitation zones because of their high infiltration rates and shallow groundwater levels

structures of the zagros fold thrust belt in iran the zagros fold thrust belt in iran forms the external part of the zagros active orogenic wedge it includes a sequence of heterogeneous latest neoproterozoic phanerozoic sedimentary cover strata 7 to 12 km thick and composed of alternating incompetent and competent layers overlying precambrian crystalline basement with a complex pre zagros structural fabric balancing structures of the cover statigraphic units in the zagros fold thrust belt requires in sequence and out of sequence involvement of the precambrian basement in the deformation six detailed balanced and retrodeformable cross sections which are constructed based on geological and geophysical data across various sectors of the belt show fault bend and fault propagation folds interpreted to have formed by slip on their subjacent thrusts out of sequence basement rooted thrusts as the interpretations in the constructed cross sections suggest have breached the cover basement interface and using incompetent cover strata for propagation cut across the cover structures and have created associated new folds superimposed upon the pre existing structures this style of deformation which has resulted in structural complexity of the belt characterizes a 200 to 300 km wide zone of distributed partly synchronous deformation with along strike and across strike variations it also implies that the zagros fold thrust belt as the external part of the orogenic wedge is still in its subcritical condition with internal deformation to achieve a critical taper in creating structural complexity in addition to out of sequence thrusting mechanically weak layers evaporites and mudstones of the cover strata have played a significant role by providing several detachment horizons shortening estimates across the belt are variable based on the constructed cross sections and their restorations minimum shortening estimates range from 16 percent to 30 percent in different sectors of the belt

avulsion cycles and their stratigraphic signature on an experimental backwater controlled delta river deltas grow in large part through repeated cycles of lobe construction and channel avulsion understanding avulsion cycles is important for coastal restoration and ecology land management and flood hazard mitigation emerging theories suggest that river avulsions on lowland deltas are controlled by backwater hydrodynamics however our knowledge of backwater controlled avulsion cycles is limited here we present results from an experimental delta that evolved under persistent backwater hydrodynamics achieved through variable flood discharges shallow bed slopes and subcritical flows the experimental avulsion cycles consisted of an initial phase of avulsion setup an avulsion trigger selection of a new flow path and abandonment of the parent channel avulsions were triggered during the largest floods 78 of avulsions after the channel was filled by a fraction 0 3 0 13 of its characteristic flow depth at the avulsion site which occurred in the upstream part of the backwater zone the new flow path following avulsion was consistently one of the shortest paths to the shoreline and channel abandonment occurred through temporal decline in water flow and sediment delivery to the parent channel experimental synthetic stratigraphy indicates that bed thicknesses were maximum at the avulsion sites consistent with our morphologic measurements of avulsion setup and the idea that there is a record of avulsion locations and thresholds in sedimentary rocks finally we discuss the implications of our findings within the context of sustainable management of deltas their stratigraphic record and predicting avulsions on deltas

dispersed remnants of a northeast pacific fringing arc upper paleozoic terranes of permian mccloud faunal affinity western u s two fragmentary subparallel belts of terranes within the western north american cordillera contain upper paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting permian faunal affinity these two belts are a volcanic arc related successions of permian mccloud faunal affinity mccloud belt and b subduction related accretionary complexes of permian tethy an faunal affinity cache creek belt this paper supports the hypothesis that the fragmentary terranes of the mccloud belt once constituted parts of a northeast pacific fringing arc system and in constrast to some earlier interpretations concludes that the volcanic arc evolved above an eastward dipping subduction zone the absolute distance between this arc and western north america during the late paleozoic cannot be constrained however there is little evidence to suggest closure of a major 10 km ocean basin or protracted periods of westward dipping subduction parts of devonian to permian volcanic island arc sequences of the western u s cordillera are represented in the northern sierra eastern klamath bilk creek grindstone and chilliwack terranes these scattered volcanic arc remnants share several fundamental characteristics 1 the sequences were constructed across continental affinity basement assemblages 2 they underwent similar tectonic evolution during late paleozoic time such as coeval pulses in volcanism and related depositional histories 3 they contain early permian mccloud type fauna of distinctive biogeographic affinity 4 mccloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the cache creek belt which contain fragments of upper triassic blueschist and permian limestone blocks bearing tethyan permian fusulinids and corals based on the presence of distinctive early permian mccloud fauna the island arc remnants discussed in this paper are referred to as the mccloud belt the similarities between mccloud belt terranes suggest that they were once parts of a long lived late paleozoic volcanic arc complex the stratigraphy of these terranes records the tectonic evolution of the volcanic arc volcanism within the paleozoic arc began as early as the early devonian and was succeeded or accompanied by carbonate and epiclastic sedimentation during the middle and late devonian an influx of detritus derived from sedimentary and metasedimentary rocks during the late devonian and mississippian records the uplift and erosion of arc basement sequences an event that was at least locally accompanied by ongoing volcanism a pulse in volcanism during the pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and locally subaerial exposure which persisted into the early early permian elsewhere during the early permian shallow water carbonate deposition was established with only minor influx of volcanogenic debris during the late permian and locally as early as late early permian a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas most of these sequences are overlain by upper triassic carbonate platformal sequences arc volcanism was recorded at least locally until the middle jurassic paleomagnetic data are used to constrain a schematic pre cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages the inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone with a long lived paleogeographic relation to north america the current structural position of these terranes is largely the result of mesozoic and locally younger deformation

storage thresholds for relative sea level signals in the stratigraphic record the tug of relative sea level rsl set by climate and tectonics is widely viewed as the most important boundary condition for the evolution of deltas however the range of amplitudes and periodicities of rsl cycles stored in deltaic stratigraphy remains unknown experimental results presented here suggest that extraction of rsl cycles from the physical stratigraphic record requires their magnitudes and periodicities to be greater than the spatial and temporal scales of the internal autogenic dynamics of deltas these results predict stratigraphic storage of information pertaining to rsl cycles during icehouse earth conditions however these thresholds commonly overlap with the magnitudes and periodicities of rsl cycles for major river deltas during greenhouse earth conditions suggesting stratigraphic signal shredding this theory suggests quantitative limits on the range of paleo rsl information that can be extracted from stratigraphy which could aid the prediction of deltaic response to climate change

north south contraction of the mojave block and strike slip tectonics in southern california the mojave block of southern california has undergone significant late cenozoic north south contraction this previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate tectonic estimates of pacific north american plate motion and for part of the big bend in the san andreas fault in the eastern mojave block contraction is superimposed on early miocene crustal extension in the western mojave block contractional folds and reverse faults have been mistaken for extensional structures the three dimensional complexity of the contractional structures may mean that rigid block tectonic models of the region based primarily on paleomagnetic data are unreliable

frontal and lateral submarine lobe fringes comparing sedimentary facies architecture and flow processes submarine lobe fringe deposits form heterolithic successions that may include a high proportion of hybrid beds the identification of lobe fringe successions aids interpretation of paleogeographic setting and the degree of basin confinement here for the first time the sedimentological and architectural differences between frontal and lateral lobe fringe deposits are investigated extensive outcrop and core data from fan 4 skoorsteenberg formation karoo basin south africa allow the rates and style of facies changes from axis to fringe settings of lobes and lobe complexes in both down dip frontal and across strike lateral directions to be tightly constrained over an 800 km2 study area fan 4 comprises three sand prone divisions that form compensationally stacked lobe complexes separated by thick packages of thin bedded siltstone and sandstone intercalated with muddy siltstone interpreted as the fringes of lobe complexes lobe fringe facies associations comprise i thick bedded structureless or planar laminated sandstones that pinch and swell and are associated with underlying debrites ii argillaceous and mudclast rich hybrid beds and iii current ripple laminated sandstones and siltstones typically frontal fringes contain high proportions of hybrid beds and transition from thick bedded sandstones over length scales of 1 to 2 km in contrast lateral fringe deposits tend to comprise current ripple laminated sandstones that transition to thick bedded sandstones in the lobe axis over several kilometers variability of primary flow processes are interpreted to control the documented differences in facies association preferential deposition of hybrid beds in frontal fringe positions is related to the dominantly downstream momentum of the high density core of the flow in contrast the ripple laminated thin beds in lateral fringe positions are interpreted to be deposited by more dilute low density parts of the flows the development of recognition criteria to distinguish between frontal and lateral lobe fringe successions is critical to improving paleogeographic reconstructions of submarine fans at outcrop and in the subsurface and will help to reduce uncertainty during hydrocarbon field appraisal and development

identifying ordered strata evidence methods and meaning abstract identifying order or pattern in strata on the basis of qualitative interpretation forms the basis for much current sedimentological and sequence stratigraphic analysis order can be usefully defined as some arrangement of facies or unit thickness that has a discernable trend or pattern that is unlikely to occur by chance because it requires some particular systematic process to form coarsening fining thickening or thinning upward trends and arrangement of strata into cycles are examples of order qualitative interpretations of order often demonstrate little more than an implicit assumption of order this paper defines a robust yet simple to apply quantitative method to identify order in strata and to indicate when order cannot be reliably demonstrated the method is based on two calculated metrics the markov metric m derived from analysis of a vertical facies succession and the runs metric r derived from analysis of observed thicknesses of stratal units most importantly both metrics can be compared with equivalent metrics calculated for disordered strata composed of many randomly shuffled versions of the same lithological units probability values can then be calculated from the comparison between observed and randomly shuffled cases and these p values indicate the degree of evidence present for order in the observed strata several test examples using synthetic strata show that the m and r values can define and identify different degrees and types of stratal order and that the metrics are robust for both stationary and non stationary successions with a range of different lengths and numbers of distinct facies analysis of four outcrop examples two siliciclastic and two carbonate demonstrate that ordered facies successions and thickness trends may be less common than typically assumed none of the four examples analyzed show trends in thickness and only the examples from the book cliffs which represent a bedset scale composite of observations show evidence for facies order the examples demonstrate how a quantitative analysis can lead to better understanding of strata either ordered or disordered and can provide better insight into the validity of current stratigraphic interpretations and models absence of order in many of the analyzed 1d vertical successions may also indicate that we need to focus more on longer term trends and analysis of 2d and 3d stratal geometries

geologic structural and thermochronologic constraints on the tectonic evolution of the sierra mazatan core complex sonora mexico new insights into metamorphic core complex formation 1 the sierra mazatan in northwestern mexico is the southernmost metamorphic core complex in the north american cordillera large magnitude tertiary extension at sierra mazatan involved both ductile and brittle slip along a major normal fault that presently dips 10 15 west extension was polyphase and involved an early period of extension from 25 to 23 ma followed by major slip from 21 to 16 ma total slip was 20 km and occurred at rates of 3 4 mm a this extension predated the plate boundary change to transtension at 12 ma and was largely decoupled from relative pacific north american plate motion numerous lines of evidence suggest that the presently low angle normal fault initiated at a steep dip 50 60 and was rotated to lower angles during slip when corrected for this tilting fault corrugations at sierra mazatan had a similar geometry to the segmentation of many active normal faults which is compatible with their origin as primary fault features many aspects of the sierra mazatan are comparable to large active normal faults indicating that this core complex formed owing to prolonged extension on an otherwise typical high angle normal fault therefore core complexes need not represent a fundamentally unique mode of crustal extension

cretaceous dikes within the jurassic independence dike swarm in eastern california field relationships and new u pb geochronology reveal that a significant number of dikes associated with the independence dike swarm both within the type locality and elsewhere are of cretaceous age a new u pb zircon age for a diorite dike in the type locality of the independence dike swarm in california confirms that some mafic dikes in the swarm are coeval with previously dated late jurassic felsic dikes cretaceous dikes in the eastern sierra nevada batholith are located near and may be genetically related to coeval mafic and felsic plutons however the presence of cretaceous dikes throughout the swarm demonstrates that this is not a local phenomenon and caution therefore should be exercised when using independence like dikes as temporal markers we have not identified any consistent petrographic or chemical criteria that distinguish jurassic independence dikes from cretaceous dikes however jurassic dikes in the sierra nevada commonly contain a sinistral shear fabric that is absent in both dated cretaceous dikes and undated dikes that intrude dated cretaceous plutons despite intruding at least 60 m y apart the orientations and petrology of the jurassic and cretaceous dikes are remarkably similar because dikes were intruded over a time period characterized by large changes in the orientation of plate convergence west of the sierran arc it is unlikely that the uniform structural orientation of jurassic and cretaceous dikes resulted from regional stresses imposed by the subducting plate instead the orientation of jurassic and cretaceous dikes and of many sierran cretaceous plutons may have been controlled by preexisting faults or by the orientation of the continental margin

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

faults in the mojave desert california as revealed on enhanced landsat images previously unknown strike slip and normal faults in the central and eastern mojave desert have been revealed on landsat thematic mapper images enhanced by four component processing this method provides color images on which lithologies are discriminated by their contrasting absorption and reflection primarily at infrared wavelengths and particularly with regard to their ferric iron ferrous iron and hydroxyl contents while retaining landform depiction these discriminants represent a new type of geophysical display for geologic mapping in regions of well exposed bedrock faults are revealed on the images by abrupt spectral and textural contrasts that coincide with aligned topographic features the newly discovered faults form part of an extensive regional network of right shear that connects faults in the death valley region with the san andreas fault system they support a heterogeneous strain model for late cenozoic tectonic evolution of the region regional structural relations indicate a westward migration of the locus of strain through time some of the newly identified faults bound blocks that have experienced contrasting rotational histories since early miocene time

regional gravity studies in southeastern california western arizona and southern nevada bouguer gravity anomaly low pass and band pass maps of southeastern california western arizona and southern nevada are analyzed to determine regional crustal structure based on 100 250 km band pass wavelength filtering of the bouguer anomaly map a low amplitude gravity maximum is associated with the highly extended terrane along the colorado river region of california arizona and nevada two crustal models constructed using bouguer gravity anomaly data and constrained by geological mapping aeromagnetic data and seismic refraction data are consistent with the dome shaped midcrustal lens of probable dioritic composition centered under the whipple mountains proposed by mccarthy et al 1989 although other models are not precluded this midcrustal lens extends from the western edge of the colorado river extensional corridor to east of the buckskin and hualapai mountains in arizona dioritic material is exposed in various mountain ranges in the colorado river extensional corridor and may be part of the source of the gravity maximum that is associated with each range

cenozoic evolution of the mojave block of southern california the recorded cenozoic history of the mojave desert region of southern california began in the latest oligocene when intense volcanism and tectonism interrupted a long early tertiary silence volcanism commenced across the region in an east west band ca 24 22 ma northwest of barstow volcanism was accompanied by intense crustal extension and development of a metamorphic core complex outside of this relatively restricted area extension was minor or absent after extension ceased ca 18 ma volcanism shifted to small volume eruptions of basalt post extensional deformation has largely been by strike slip faulting along northwest striking dextral faults and west striking sinistral faults and total dextral slip across the mojave desert region since the early miocene is 45 60 km strike slip deformation has been overprinted locally by intense north south contraction that is the dominant style of deformation in the western mojave block paleomagnetic data indicate that parts of the mojave block were rotated clockwise although the magnitude and timing of this rotation are poorly determined the best evidence for large 45 rotation comes from the area east of barstow where large clockwise declination anomalies and mesozoic and cenozoic dikes with anomalous strikes may reflect early miocene clockwise deflection along the mojave river fault volcanism and tectonism in the mojave block resulted from interactions among the north american pacific and various oceanic plates patterns of volcanism and tectonism do not correlate with growth of slab windows beneath the continent but do correlate with the position of the subducted mendocino fracture zone plate circuit reconstructions suggest that the driving force for extension was divergence between the pacific and north american plates along the transform margin that separated the two this hypothesis accounts for the direction magnitude and rate of extension in the mojave block

a speculative reconstruction of the middle paleozoic continental margin of southwestern north america geologic relations and data from strontium and oxygen isotopic studies are used together to speculate on the paleogeographic positions of various crustal blocks or terranes in southwestern north america with respect to the middle paleozoic continental margin the southwestern margin of the middle paleozoic continent based on these data is quite irregular in its present configuration but most of the irregularities can be accounted for by strike slip displacement of continental fragments during several post middle paleozoic tectonic events we conclude that large scale late paleozoic sinistral displacements and mesozoic and cenozoic dextral displacements have been the major events affecting the configuration of this margin our preferred model includes the following elements 1 500 km of left lateral displacement of a large part of the margin during a late paleozoic truncation 2 slivering of the margin by right lateral faulting during the mesozoic amounting to about 275 km of total displacement and 3 further deformation of the margin during the cenozoic primarily by right lateral faulting oroclinal bending and crustal extension

metamorphic evolution of exhumed middle to lower crustal rocks in the mojave extensional belt southern california usa the waterman metamorphic complex of the central mojave desert was exposed as a consequence of early miocene detachment dominated extension however it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment s tectonic thickening by overthrusting and two periods of extension the metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble calc silicate quartzite mafic granulite pyribolite amphibolite migmatite and biotite schist the latter group of rocks was affected by an initial series of high grade metamorphic events m1 and m2 and a localized lower grade overprint m3 the initial metamorphism m1 can be separated into two stages along its high grade p t path m1a a granulite facies metamorphism at 800 850 c and 7 5 9 kbar and mlb an upper amphibolite facies overprint at 750 800 c and 10 12 kbar m1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of h2o and is associated with intense ductile deformation d1 and minor local partial melting m1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and h2o activity and is accompanied by little or no deformation m2 developed at lower pressures and temperatures 650 750 c 4 5 5 5 kbar and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins effects of m3 are confined to the mitchel detachment zone an anastomosing early miocene detachment fault and are characterized by local ductile brittle deformation d2 of the pre existing high grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions 300 350 c 3 5 kbar the initial overprint m1a represents metamorphism devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and possibly magmatic thickening the increasing pressure transition of m1a to m1b reflects a period of continued compressional tectonism thrusting and influx of h2o in part locally related to crystallization of partial melts the near isothermal decompression between m1b and m2 probably represents a pre 112 ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust following the initial extensional event the metamorphic complex remained at depths of 10 17 km for at least 90 ma until it was uplifted following miocene extension m3 develops locally in response to this second extensional period resulting from the early miocene detachment faulting

oblique opening and noncoaxial emplacement of the jurassic independence dike swarm california abstract several features of the independence dike swarm ids indicate that dikes opened obliquely to their walls in a sinistral sense the ids is largely latest jurassic in age 148 ma and has been traced for more than 600 km along its nw se strike from east central to southern california field and petrographic observations from the ids in the sierra nevada indicate that i some and perhaps all of the dikes initially opened perpendicular to their margins and then were sinistrally sheared and ii sinistral displacement occurred during and shortly after dike injection dike emplacement overlapped in time with the formation of wall rock mylonite zones which accommodated shear varying in sense from sinistral to west side down reverse we suggest that the ids intruded dilatant fractures within a regional sinistral shear system the swarm is located within or along the eastern flank of the cordilleran subduction related magmatic arc the ids and associated wall rock deformation may record partitioning into the magmatic arc of the sinistral component of strongly oblique late jurassic subduction

the full range of turbidite bed thickness patterns in submarine lobes controls and implications a widely misused criterion to interpret lobe deposits in submarine fan systems at outcrop and in core and well logs is a thickening and or coarsening upward profile lobe deposits from the laingsburg depocentre sw karoo basin demonstrate that a full range of bed thickness patterns exists within lobes when lobes are defined by their laterally extensive bounding surfaces that are marked by abrupt facies changes five types of bed stacking patterns are identified thickening upward thinning upward thickening then thinning upward thinning then thickening upward and constant the abrupt bounding surfaces are interpreted to record avulsion of feeder distributive channels the stratigraphic bed thickness pattern preserved in a lobe reflects the internal organization of smaller scale lobe elements rather than lobe wide initiation and progradation as implied by a thickening upward only pattern the full range of bed thickness patterns in lobes can be used to understand the stacking of lobe elements the evolution of sediment deposition through time and space and the relative movement of depocentres

linking onshore offshore sediment dispersal in the golo source to sink system corsica france during the late quaternary abstract spatial and temporal relationships between climate tectonics and sea level have a primary control on sediment transport storage and deposition in onshore and offshore depositional environments although many simplistic models have tried to predict onshore system behavior and sediment partitioning between onshore and offshore depositional environments in response to changes in these boundary conditions the alluvial response to changes in external forcings coupled with autogenic processes can be highly complex and unpredictable the golo source to sink system on the eastern margin of the island of corsica in the mediterranean sea provides an ideal laboratory to study sediment partitioning in both onshore and offshore realms the terrestrial part of the system consists of the golo river which debouches onto a narrow shelf 10 km which in turn passes into a narrow 45 km confined basin known as the corsica trough dominated by submarine fan deposits estimates on timing of late quaternary fluvial aggradation are compared with timing of sediment storage on the shelf and on the basin floor fan the results indicate that onshore deposition and storage of sediment in the fluvial system may occur both during sea level highstand transgression and lowstand volume calculations from the alluvial record show that onshore storage is relatively low 13 relative to the overall sediment budget comparing estimated deposition rates with a sediment prediction model further suggests that deep sea fan sedimentation rates at times may be up to 50 higher or 25 lower than what is being supplied by the river indicating temporary storage and release of sediment on the shelf on average however there is good agreement between predicted and calculated sediment volumes in the golo system the study demonstrates the value of investigating the entire source to sink system in order to obtain a comprehensive understanding of sediment dispersal between onshore and offshore depositional environments over 104 year timescales observations suggest that alternating periods of aggradation and degradation of the golo river are controlled by system thresholds controlled from within the catchment comparison with climate proxy data and timing of major alluviation events elsewhere in the mediterranean region supports the notion that each source to sink system has a unique threshold that must be exceeded to induce regional aggradation and subsequent terrace formation the same is also inferred for timing of sediment dispersal to the deep sea environment in addition preservation of onshore sediment is controlled by local factors such as uplift rate and its impact on the aggradational response it is therefore expected that the onshore record may be diachronous on local regional and global scales making detailed correlation difficult except for the most extreme events finally stream incision rates appear to be one to two orders of magnitude higher than regional hinterland denudation rates indicating that the golo system is characterized by increasing landscape relief

speculative reconstruction of the mojave snow lake fault implications for paleozoic and mesozoic orogenesis in the western united states available evidence suggests that the mojave snow lake fault a major early cretaceous dextral fault with up to 400 km displacement formerly existed in what is now the axial part of the sierra nevada batholith regional relations suggest that the fault formerly continued in western nevada the mojave desert and in rocks now exposed in the salinian block restoration of 400 km of early cretaceous dextral displacement on the mojave snow lake fault leads to speculative but testable reconstructions of mesozoic and paleozoic tectonic features of california and nevada insights derived from these reconstructions include the following 1 two regionally extensive jurassic thrust belts of opposed vergence appear to have intersected in the northern sierra nevada the eastern east vergent thrust belt which includes the fencemaker and liming thrusts in western nevada and several east vergent thrusts in the northern sierra may be a contractional belt along which an early jurassic interarc or marginal basin was closed in middle to late jurassic time the western west vergent thrust belt is part of the classic nevadan orogen of the western klamath mountains and the western sierra metamorphic belt in which collisions and or thrusting of various island arc terranes occurred during middle and late jurassic time 2 the early mesozoic continental arc included paired belts of triassic jurassic blueschist and melange and igneous rocks including late triassic and younger plutons and volcanic rocks this conclusion is not new but the reconstruction provides a slightly different geometry than previously thought 3 sonomia the late paleozoic island arc terrane of the western u s may have formed a relatively narrow belt extending from northwestern nevada to the latitude of the western mojave desert the geologic development of the paleozoic basement complex of sonomia was very different from that of the cordilleran miogeocline in death valley and the mojave desert requiring that a continuation of the sonoma suture belt or golconda thrust must have extended southward through the sierra nevada batholith to the mojave region 4 a heterogeneous belt of permian to triassic or triassic deformation in the mojave and eastern sierra nevada together with the golconda allochthon in nevada may represent the protracted response to the collision of sonomia with the western margin of north america the contractional deformation continued during the evolution of the early mesozoic continental arc 5 several predictions can be made about the nature of concealed basement within the sierra nevada batholith and within western nevada the reconstruction suggests that elements of the antler and sonoma orogenic belts should underlie areas of the batholith and western nevada as a result of northward displacement along the mojave snow lake fault from the western mojave and southern sierra nevada

evolving early mesozoic convergent margin deformation central klamath mountains northern california the stuart fork terrane in the central klamath mountains of northern california is a coherent late triassic blueschist belt which records early plate convergence in western north america complex deformation and polyphase metamorphism of the terrane directly reflect evolving convergent margin processes two major periods of deformation are recorded in the stuart fork terrane d1 an early late triassic phase of synmetamorphic deformation which produced complex folds and possible southwest directed reverse faults and d2 a late stage mid jurassic deformation which formed simple open folds as a result of westward thrusting of the entire terrane over neighboring terranes during regional shortening d2 thrust imbrication of the stuart fork and adjacent terranes also produced a melange zone characterized by disrupted tectonic slivers of these terranes each of these episodes of shortening is related to convergence and at least one d1 accompanied high pressure blueschist facies metamorphism during subduction more importantly however it is possible to distinguish deformation features which formed during initial subduction from those which formed as a result of final thrusting or accretion of the entire assemblage to the continental margin as regional contraction progressed through the arc and forearc region the stuart fork terrane is a lithologically and structurally distinctive lithotectonic assemblage which cannot be directly correlated with other terranes of the western paleozoic and triassic subprovince nonetheless the stuart fork and other similar terranes of the central klamath mountains share a common origin as oceanic materials which were accreted against the continental convergent margin in mid mesozoic time during east directed subduction

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

mylonitization in the lower plate of the buckskin rawhide detachment fault west central arizona implications for the geometric evolution of metamorphic core complexes abstract in metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down dip continuation of a detachment fault or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault detailed microstructural fabric and strain data from mylonites in the buckskin rawhide metamorphic core complex west central arizona constrain the structural development of the lower plate shear zone widespread exposures of 22 21 ma granitoids of the swansea plutonic suite enable us to separate miocene strain coeval with core complex extension from older deformation mylonites across the lower plate consistently record top to the ne directed shear miocene quartz and feldspar deformation recrystallization mechanisms indicate 450 500 c mylonitization temperatures that were relatively uniform across a distance of 35 km in the extension direction quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction strain recorded in the swansea plutonic suite is also relatively uniform in the extension direction which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault altogether these data suggest the mylonitic shear zone initiated with a 4 dip and was unroofed by a more steeply dipping detachment fault system lower plate mylonites in the buckskin rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down dip continuation of a detachment fault

40ar 39ar age spectra for biotite and hornblende from plutonic rocks in the victorville region california 40ar 39ar data from plutonic rocks in the mojave desert near victorville california provide preliminary insight into the complex thermal history of this region and suggest that some plutons here may be significantly older than any plutons in the sierra nevada to the north near victorville multiple deformation and metamorphism of platform miogeoclinal strata is inferred to postdate deposition of permian rocks and predate intrusion of monzonite plutons monzonite that intrudes multiply deformed paleozoic marble and that is unconformably overlain by the mesozoic triassic or jurassic fairview valley formation has hornblende with an age spectrum typical of severe 40ar loss we interpret the age of the highest temperature increment fusion 233 14 m y as a minimum age for this pluton the total gas age for the hornblende is only 169 9 m y biotite from the same rock exhibits a plateau age of 76 59 1 24 m y the fairview valley formation is overlain by volcanic rocks of the sidewinder sequence these rocks were folded and intruded by quartz monzonite that postdates all penetrative deformation in the region biotite hornblende tonalite at quartzite mountain is part of one of these younger quartz monzonite plutons and its biotite yields a plateau age of 73 16 1 11 m y this age is the same at the 95 confidence level as the age of biotite from the older monzonite thus this younger thermal event was at least the partial cause for the argon loss pattern displayed by the monzonite hornblende hornblende gabbro that intrudes paleozoic marble at sidewinder mountain and is in turn contact metamorphosed by later plutonic rocks has hornblende with a disturbed 40ar 39ar age spectrum the 1025 1050 and 1075 c steps contain 44 64 of the total argon and yield a common age of 119 2 3 4 m y which we interpret as the best estimate for the age of the hornblende