tectonic decapitation of a pliocene mega delta on isla del carmen in the gulf of california mexico and a river ran through it abstractthe tiombo conglomerate on isla del carmen in baja california sur represents a mega delta deposited mainly during the middle pliocene early piacenzian age a cross section with the characteristically arched profile of a fan delta is exposed in sea cliffs 60 m high that extend for 2 km along the east side of the fourth largest island in mexico s gulf of california a tidewater aspect to the delta is proven by the inclusion of fossil rich lenses dominated by the pecten patinopecten bakeri which also occurs abundantly together with many other middle pliocene fossils in the arroyo blanco limestone juxtaposed against the south flank of the delta construction inland the conglomerate fills a broad cross island channel approximately 3 km2 in area that ends with cliff exposures of a lesser thickness overlooking the carmen passage between isla del carmen and the peninsular mainland of baja california based on exposures that fail to reveal the full basal contact the combined fluvial and deltaic parts

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

neogene stratigraphy foraminifera diatoms and depositional history of maria madre island mexico evidence of early neogene marine conditions in the southern gulf of california abstract foraminifera and diatoms have been analyzed from an upper miocene through pleistocene sequence of marine sediments exposed on maria madre island largest of the tres marias islands off the pacific coast of mexico the neogene stratigraphic sequence exposed on maria madre island includes a mid miocene non marine and or shallow marine sandstone unconformably overlain by a lower upper miocene to uppermost miocene upper to middle bathyal laminated and massive diatomite mudstone and siltstone unit this unit is unconformably overlain by lower pliocene middle to lower bathyal sandstones and siltstones which in turn are unconformably overlain by upper pliocene through pleistocene upper bathyal to upper middle bathyal foraminiferal limestones and siltstones these beds are unconformably capped by pleistocene terrace deposits basement rocks on the island include cretaceous granite and granodiorite and tertiary andesites and rhyolites the upper miocene diatomaceous unit contains a low diversity foraminiferal fauna dominated by species of bolivina indicating low oxygen conditions in the proto gulf maria madre basin the diatomaceous unit grades into a mudstone that contains a latest miocene upper to middle bathyal biofacies characterized by baggina californica and uvigerina hootsi along with displaced neritic taxa an angular unconformity separates the upper miocene middle bathyal sediments from overlying lower pliocene siltstones and mudstones that contain a middle to lower bathyal biofacies and abundant planktonic species including neogloboquadrina acostaensis and pulleniatina primalis indicating an early pliocene age significantly this pliocene unit contains common occurrences of benthic species restricted to miocene sediments in california including bulimina uvigerinaformis pliocene to pleistocene foraminiferal limestones and siltstones characterize submarine bank accumulations formed during uplift of the tres marias island area and include abundant planktonic foraminifera such as pulleniatina obliquiloculata and neogloboquadrina duterteri common benthic foraminifera in this unit are indicative of upper bathyal water depths the neogene depositional history recorded on maria madre island involves an early late miocene subsidence event marking formation of the tres marias basin with relatively undiluted diatomaceous sediment deposited in a low oxygen setting subsidence and deepening of the basin continued into the early pliocene along with rapid deposition of terrigenous clastics uplift of the basinal sequence began in late pliocene time accompanied by deposition of upper pliocene pleistocene foraminiferal limestones on a rising submarine bank continued episodic uplift of the neogene deposits brought the island above sea level by late pleistocene time

the morphodynamics of beach ridge formation magilligan northern ireland abstract magilligan foreland is a holocene beach ridge plain comprising three principal sets of sand ridges sets i and ii are related to the mid holocene progradation of the plain while set iii represents recent deposition at the distal margin at present two types of beach ridge are being formed in both cases as part of the general recovery sequence following storm erosion of the dune cliffs mode i beach ridge development is associated with shore normal sediment transport under dissipative wave conditions these ridges form around the high water mark of spring tides through the migration of coalescence of swash bars within 30 to 55 days of a storm mode ii formation is more complex sediment initially stored in nearshore bars is moved alongshore causing bar elongation the bar moves into the intermediate and even reflective wave domains eventually narrowing of the bar leads to severance of a downdrift portion usually at or near the major drift pulse to the north west of the foreland normally this sediment starved remnant welds progressively onto the beach face in the direction of the dominant process gradient east to west but occasionally locally generated counteractive seas move sediment onshore employing the bar as a traverse conduit mode ii beach ridges emerge onto the upper beach around 90 150 days after a storm the beach ridges have distinctive structures depending on their mode of formation and both constitute important sources for dune building sand

the neogene genus streptochilus bronnimann and resig 1971 from the gulf of california abstract four species of the planktonic foraminiferal genus streptochilus from key neogene marine localities are documented in relation to the evolution of the gulf of california s globigerus s latus s macdougallae sp nov and s inglei sp nov planktonic foraminiferal bioevents and strontium isotopes in the bouse tirabuzon carmen and ojo de buey lithostratigraphic units constrain the local distribution range between 6 and 5 3 ma for the last three species whereas s globigerus appears locally at 5 5 ma and disappears between 3 79 and 3 46 ma in the imperial and trinidad formations the last occurrence of streptochilus latus and the first and last occurrences of s globigerus in the ancient gulf of california are correlated with bioevents calibrated in the equatorial pacific therefore they can be used as reliable local biostratigraphic markers the presence of streptochilus in the ancient gulf of california seems to correlate with upwelling in a pattern similar to that observed in the modern oceans

late miocene to pliocene stratigraphic evolution of northeast carmen island gulf of california implications for oblique rifting tectonics abstract this paper describes a thick section of upper miocene to middle pliocene marine sedimentary rocks exposed at punta perico northeastern carmen island in the southern gulf of california these strata overlie miocene volcanic rocks along a low angle unconformity and have a total thickness of 1100 m the section contains the following units 1 lower conglomerate 750 m thick representing submarine debris flows and grain flows 2 conglomerate and sandstone 90 160 m deposited by gravelly grain flows and sandy turbidity currents 3 marlstone and mudstone 120 m with benthic forams that record bathyal water depths 400 500 m and an age of 3 5 3 1 ma 4 dacite breccia and boulder beds 20 m emplaced by submarine avalanches and debris flows and 5 stratified conglomerate and bioclastic limestone 80 m deposited by traction currents in a shallow shelf environment conjugate syn sedimentary faults deformation bands and growth structures are recognized within a 250 m wide belt adjacent to a large n striking w dipping normal fault that bounds the eastern margin of the pliocene section indicating that the eastern normal fault was active during deposition of at least the upper 300 m of the dipping section considering the thickness of the two units below the marlstone 900 m and assuming a range of possible sediment accumulation rates 0 2 1 0 m ka we postulate that deposition of the lower conglomerate was initiated in late miocene or early pliocene time 8 4 ma possibly coincident with initial opening of the gulf of california deposition of the marlstone unit above a probable unconformity represents a major change in basinal tectonics at 3 5 3 1 ma which is about the time when seafloor spreading and transform faulting were initiated in the gulf of california these tentative correlations suggest that the baja california peninsula was structurally and mechanically linked to the deep gulf during late miocene to middle pliocene time

the opening of the gulf of california near loreto baja california mexico from basin and range extension to transtensional tectonics abstract detailed structural field analyses of faults and fractures have led to the reconstruction of the tectonic evolution of the loreto region baja california mexico during the opening of the gulf of california three main tectonic events have been recognized i formation of large scale radial structures along the future main gulf escarpment here interpreted as associated with emplacement of magmatic bodies at shallow levels during the beginning of extension processes in early middle miocene ii ne sw extension basin and range event active since 15 ma producing nw se normal dip slip faults in the oligo miocene volcaniclastic basement and indicating high extension rates iii transtensional tectonics recorded especially in the plio quaternary sediments and associated with the activation of nw se dextral strike slip faults and n s normal faults the tectonic structures related to the basin and range extension are sealed by the plio quaternary sediments of the loreto basin and by the volcanics of the mencenares volcanic complex a huge volcanic field interfingering with the sedimentary succession of the basin

description and kinematics of the se loreto basin fault array baja california sur mexico a positive field test of oblique rift models abstract we present the results of mapping and kinematic analysis on an 18 km2 array of normal and oblique slip faults in the se loreto basin near loreto baja california sur mexico the fault array developed on the western margin of the gulf of california in late pliocene time during the early part of the modern stage of oblique rifting of the gulf the se loreto fault array is an antithetic transfer zone between two large e dipping normal faults the fault array is coherent and formed in one episode the dominant n to nne striking faults primarily dip w and display nearly pure normal slip more ne striking faults show normal sinistral slip and nw striking faults have dextral normal to dextral slip n to nw striking faults have steeper dips and a greater component of dextral slip with more northwesterly strike bulk extension based on strain analysis of faults with striae is oriented nearly e w with little plunge and consistent throughout the fault array the kinematic analysis of the se loreto fault array provides a well constrained test of oblique rift models our results support previous modeling of homogeneous extension in oblique rifted margins when both instantaneous and finite strain are considered positive field tests of analytical and experimental models such as this one indicate that such models may be robust predictors of the obliquity of rifting in ancient mountain belts where brittle fault arrays are preserved but plate motions are unknown

low temperature thermochronology of northern baja california mexico decoupled slip exhumation gradients and delayed onset of oblique rifting across the gulf of california 1 the northern gulf extensional province displays key structural relationships that characterize the magnitude direction and timing of neogene rift related transtension during the opening of the gulf of california apatite fission track and u th he thermochronology from the sierra san felipe document moderate cooling 4 c myr 7 c myr during the early paleogene associated with progressive unroofing caused by erosional downwearing of the ancestral peninsular ranges beginning at 45 35 ma a period of tectonic quiescence with low cooling rates 1 c myr marks the development of a regional oligocene miocene peneplain rift related exhumation began at 9 7 ma and attains 2 5 km in the hinges of two antiformal megamullions decoupling between exhumation and finite displacement in certain fault segments is explained by vertical deflections associated with extension perpendicular folding of the fault surfaces the main faults of the detachment system were active contemporaneously thus forming a mechanically linked array of large displacement normal faults in the hanging wall of the main gulf escarpment the late miocene onset of transtension in the sierra san felipe suggests that widespread deformation may only have localized in the gulf extensional province between 9 and 7 ma some 3 5 ma after a major plate reorganization associated with cessation of subduction in the trench to the west between 12 ma and 9 7 ma plate boundary shearing was likely distributed between the continental borderland west of baja california and the southern basin and range province in mexico

dispersal of hydrothermal vent larvae at east pacific rise 9 10 n segment author posting american geophysical union 2018 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 123 11 2018 7877 7895 doi 10 1029 2018jc014290

evolution of the margin of the gulf of california near loreto baja california peninsula mexico the gulf of california is a prime example of a young oblique divergent plate boundary this type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings a complexity in the gulf of california is that the modern oblique rifting was preceded by a stage of orthogonal rifting we have used extensive mapping as well as stratigraphic geomorphic and structural analysis to determine the history of deformation near loreto on the southern baja california peninsula during formation of the gulf of california our data support the suggestion that middle to late miocene protogulf stage 12 to ca 6 ma orthogonal rifting was overprinted by transtensional structures during pliocene to quaternary time though we cannot closely define the time of change we further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults the loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned down tothe east extensional monoclines and normal faults these structures lie along or as far as 4 km in front east of a steep 1000 1600 m high main gulf escarpment that defines the western topographic margin of the rift zone the loreto segment is further defined by a rise in elevation of the main gulf escarpment of up to 1 km and an 500 800 m increase in structural relief of prerift strata in the escarpment from the segment boundaries to the center of the segment structural analysis of secondary faults of the loreto segment shows that fault populations that are known to be pliocene to quaternary in age are mixed normal and dextral normal faults with a bulk extension direction of west northwest eastsoutheast 2808 1008 in contrast fault populations that cut only prerift oligocene middle miocene volcanic and sedimentary rocks are mainly normal faults with northeast southwest to east northeast west southwest bulk extension directions that average 2458 658 these faults are interpreted to be late miocene in age and formed during protogulf orthogonal rifting thus the pliocene faults record a major change of extension direction of 358 from latest miocene to pliocene time compatible with the onset of oblique rifting in the gulf of california oblique divergent overprinting is mainly expressed as the loreto fault and loreto basin in the northern part of the segment the loreto fault and basin evolved with major fault reorganizations and partial basin inversion many observations suggest that the pliocene loreto fault was linked to two nascent transform faults in the gulf and that carmen island rotated 358 408 clockwise within the complex fault system

synrift stratigraphic geometry in a transfer zone coarse grained delta complex miocene pohang basin se korea the gojusan fan delta is a coarse grained delta complex on the western margin of the miocene pohang basin se korea the deposits consist of five facies associations fa alluvial fan conglomerates fa i shallow marine mouthbar sandstones fa ii fluvial and mouth bar conglomerates fa iii gilberttype foreset conglomerates fa iv and hemipelagic mudstones fa v different facies associations characterize the northern central and southern parts of the delta complex to the north fa iv is laterally juxtaposed with fa i or the basement with scarp derived breccias along the contact centrally and to the south fa i is laterally juxtaposed with fa ii with an abrupt facies change and local inliers of basement rocks along the contact early alluvial fan and mouth bar deposits are overlain by the topset fa iii and truncated by the foreset fa iv of a gilbert type delta in the central part of the fan delta complex whereas fa ii passes transitionally upward into fa iii in the south with the latter extending basinward into a gently inclined shoal water delta front gilbert type and shoal water geometries are therefore developed in the same delta complex the composite delta geometry is interpreted as reflecting 1 its development near an extensional transfer zone where the hangingwall relief was variable over short distances along strike and 2 the operation of intrabasinal faults this interpretation contrasts with previous studies that viewed the delta complex as having formed along a pull apart basin margin

what do the cajon pass stress measurements say about stress on the san andreas fault comment on in situ stress measurements to 3 5 km depth in the cajon pass scientific research borehole implications for the mechanics of crustal faulting by mark d zoback and john h healy

mixed carbonate siliciclastic sedimentation on a tectonically active margin example from the pliocene of baja california sur mexico bioclast rich coarse grained deposits in the pliocene loreto basin provide a record of mixed carbonate and siliciclastic sedimentation at the steep hanging wall margin of this small fault controlled basin sedimentary facies consist of sand to gravel sized carbonate debris mixed with volcaniclastic sand and gravel in a proximal to distal facies tract that includes matrix rich and matrix poor shelly conglomerate impure calcirudite and calcarenite mixed composition turbidites and bioturbated calcarenitic sandstone carbonate material was produced by mollusks and other benthic organisms on a narrow high energy shelf and mixed with volcaniclastic sand and gravel in cross shelf channels these mixtures were transported down a steep subaqueous slope by debris flows grain flows and turbidity currents forming foresets and bottomsets of marine gilbert type deltas this style of mixed carbonate siliciclastic sedimentation has not been documented in detail elsewhere but should be locally abundant in the stratigraphic record of fault bounded basins particularly those with cool or nutrient rich waters that support relatively few binding and framework building faunas recognition of similar facies in other settings can provide useful insights into ancient conditions of carbonate production oceanography climate and tectonics

rocky shores and development of the pliocene pleistocene arroyo blanco basin on isla carmen in the gulf of california mexico arroyo blanco basin on isla carmen preserves a 157 m thick nearly complete record of pliocene pleistocene history in the gulf of california examples of rocky shore geomorphology occur on all margins of this trapezoidal shaped 3 3 km2 basin a shoreline is developed in low relief on miocene andesite from the comondu group at the rear of the basin parallel to the long axis of the island two end walls trace normal faults that stayed active during the life of the basin and maintained steep rocky shores the basin is 64 filled by calcarudite and calcarenite derived from crushed rhodolith debris other facies include shell beds and stringers of andesite conglomerate that define a 4 6 ramp the ramp expanded onshore through pliocene time based on a succession of overlapping range zones for 22 macrofossils typical of lower through upper pliocene strata in the gulf of california the unconformity exposed 1 km inland at the rear of the basin is between miocene volcanics and pleistocene cap rock at an elevat

evolution of a coquina barrier in shark bay australia by gpr imaging architecture of a holocene reservoir analog abstract the holocene hypersaline carbonate system in shark bay australia is represented by microbial deposits stromatolites and thrombolites and a supratidal coquina beach ridge system which overlies the pleistocene bibra formation and is prograding seaward since 4500 years ago over supratidal intertidal microbial deposits as a consequence of late holocene sea level fall and the high volume of bivalve shells available the extent internal architecture and ridge constructional types of coquina deposits in this world heritage area have not been previously investigated in detail by cores tied with geophysical images here we document external and internal architecture of the holocene coquina system in southeast hamelin pool showing three depositional units which comprise the ridge system construction 1 tabular layers complex 2 convex up ridges and 3 washover deposits bivalve skeletons represent almost the total amount of bioclasts with symmetrical semi circular formats and valve sizes 5 9 mm based on ground penetrating radar gpr images the number of prograding layers present in each transect line varies from 252 to 433 which when compared with the measured 14 c ages shows lateral growth of the system at average rates from 10 years per layer on proximal older sequences to 30 years per layer in the younger and thicker packages which display more gentle inclination and sub horizontal arrangement divergent orientation of tabular layers relates to changes in energy of currents and waves and involves time gaps and erosion after earliest crest construction the system experienced a decrease in sea level 3600 3000 14 c years bp that left behind a sequence of parallel tabular layers with dip angles 10 to

translation of terranes lessons from central baja california mexico baja california is one of the best modern examples of a continental block or terrane that is translating hundreds of kilometers along a highly obliquely divergent plate the central domain of baja california is particularly appropriate as a modern analogue for terrane translation because it has a relatively simple late neogene history and is a buoyant continental block that is likely to be preserved in any future accretion to the north american continent in contrast the deep dense oceanic crust of the gulf of california where the main transform faults of the plate boundary lie has little preservation potential it is surprising that there is little strike slip faulting in the central domain of baja california even though the plate boundary has a rift angle angle between rift trend and azimuth of plate motion alpha of only 20 recent field data confirm modeling studies of oblique rifting and show that there is a predictable change in fault type fault orientation and extension direction with changing rift angle that is from secondary structures alone one can predict the approximate obliquity of rifting and the orientation of the plate motion with rift angles of 0 20 strike slip faults at low angles to the rift trend will dominate the secondary structures with rift angles from 20 to 35 there are strike slip faults subparallel to the rift trend and normal faults 20 40 clockwise from the rift trend in dextral shear above 35 normal faults are dominant and there are few or no strike slip faults as secondary structures

tectonics of the pliocene loreto basin baja california sur mexico and evolution of the gulf of california the pliocene loreto basin is a transtensional basin exposed just north of loreto baja california sur that consists of nonmarine to marine sedimentary rocks and interbedded tuffs on the basis of stratigraphic study and 40ar 39ar dating of tuffs the southern loreto basin began to form at 3 4 ma as an alluvial basin with moderate rates of subsidence 0 4 mm yr between 2 46 and 2 36 ma extremely rapid subsidence 5 10 mm yr of the basin was accompanied by deposition of vertically stacked gilbert type fan deltas during this period the southernmost basin began to be cut by an array of dextral normal faults the basin returned to a moderate rate of subsidence from 2 36 to 2 0 ma when volcanism increased within the northern part of the basin the pacific north america boundary was first fully located within the gulf of california beginning at 3 5 ma at which time the zone of transform related deformation widened to initiate formation of the loreto basin the major change in the loreto basin at 2 46 ma may be coeval with the beginning of faulting in southern california on the elsinore and san jacinto faults these widespread events may indicate a minor change in the plate boundary at 2 5 ma

a reappraisal of seafloor spreading lineations in the gulf of california implications for the transfer of baja california to the pacific plate and estimates of pacific north america motion an analysis of seafloor spreading lineations in the southern gulf of california demonstrates that divergence of the baja peninsula from north america since 3 6 ma has been significantly slower than pacific north america pa na motion estimated solely from closures of global plate circuits this discrepancy remains unchanged or increases with modifications to the plate circuits and kinematic data used to predict pa na motion suggesting that it arises from the probably incorrect assumption that baja california has been rigidly coupled to the pacific plate since 3 6 ma a 15 increase in seafloor spreading rates in the southern gulf after 1 ma could have been caused by a transfer of motion along faults west of baja california to the modern day plate boundary or alternatively could indicate a recent speedup in pa na motion both interpretations imply that pa na motion is now 4 2 mm yr 1 2 faster than predicted by the 3 16 myr average nuvel 1a model

fault tectonics of the baja california peninsula and the opening of the sea of cortez mexico abstract a detailed field analysis of neogene and quaternary faults in baja california has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula the deformation which is related to the opening of the gulf affects the whole peninsula but decreases from east to west most observed faults normal and or dextral strike nnw sse to wnw ese their mechanisms include both strike slip and dip slip as well as intermediate motions compressional events have occurred since late neogene times but were probably of minor quantitative importance because reverse faults are rare and small the principal fault pattern includes dextral nnw sse riedel shears and n s tension faults induced by dextral strike slip along two main nw se fault zones bordering the peninsula the gulf of california to the east which is the most important and the tosco abreojos fault to the west the resulting pattern of deformation shows that the eastern part has been a complex transform extensional zone since late miocene early pliocene times

relative role of accommodation zones in controlling stratal architectural variability and facies distribution insights from the fushan depression south china sea in sedimentary basins a better understanding of the controlling effect of accommodation transfer zones on stratal architecture and facies distribution can improve the success rate of locating hydrocarbon reservoirs the fushan depression is a half graben rift sub basin located in the southeast of the beibuwan basin south china sea in this study comparative analysis of seismic reflection palaeogeomorphology fault activity and depositional facies distribution indicates that three different types of inner basin slopes i e multi level step fault slope in the western area slope flexure zone in the accommodation zone area and gentle slope in the eastern area were developed along the southern slope of the fushan depression together with a large scale accommodation zone located at the intersection of the western and eastern fault systems further analysis shows that the accommodation zone played an important role in controlling not only stratal architectural variability in the southern slope but also depositional facies distribution in the accommodation zone area during the high stand stage the deposition of depositional systems was mainly controlled by sediment supply and the nw trending transfer faults major drainage systems entered the fushan depression through the accommodation zone along the direction of sediment supply and sedimentary flow paths were parallel to the accommodation zone axis with sedimentary flows constrained to the adjacent areas of the nw trending transfer faults by contrast during the low stand stage the transfer faults had little control over depositional facies distribution and the sublacustrine fan sediments diverted away from the accommodation zone and flew down along the northeast oblique to the accommodation zone axis the flow division in the lowstand stage might be greatly influenced by flow type and topography in addition the accommodation zone area demonstrated unique hydrocarbon accumulation models different from the western and eastern areas suggesting that the future exploration should be conducted at different levels in the fushan depression

development of asymmetric basins along continental transform faults abstract many basins associated with wrench or transform faults are asymmetrical both longitudinally and laterally the sense of basin asymmetry changes along the strike of transform faults geophysical data from the dead sea rift and other transforms indicate that wrench induced asymmetric basins are bounded on only one side by a transform fault this implies that during the evolution of such basins maximum compression was parallel to the transform the direction of extension can change during the geologic evolution of a basin a possible explanation of this observation takes the strength of the fault into into consideration in the case of a weak fault in a strong crust the horizontal principal stresses would probably rotate to orientations parallel and perpendicular to the fault thus minimizing the shear stress on the main fault basins formed by regional extension such as in the east african rift system sometimes show similar features recent studies have shown that these basins often undergo oblique extension this suggests that the asymmetry in these basins may result from simultaneous strike slip motion and fault normal extension similar to the way asymmetry is produced in large transform basins periods of oblique extension can alternate with periods of orthogonal extension the asymmetry formed during periods of oblique extension is often preserved in the stratigraphic record

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

transform normal extension and asymmetric basins an alternative to pull apart models continental transforms are commonly associated with relatively small scale pull apart basins however much larger scale basins which are not consistent with the pull apart model commonly exist adjacent to transforms structures associated with several basins along the dead sea rift and el pilar fault illustrate this these basins are remarkably asymmetric bounded by essentially linear transform segments on one side and subparallel normal faults on the other suggesting simultaneous strike slip motion and transform normal extension this observation which is incompatible with classical faulting theory can be explained along divergent plate boundaries if the transform fault is much weaker than the adjacent crust analysis of the orientation of the horizontal principal stress direction near a weak transform fault embedded in an otherwise strong crust shows that convergent and divergent plate motion results in a stress field characterized by nearly fault normal compression and extension respectively structural styles along the san andreas dead sea and el pilar transform systems show that predictable changes between these two states have occurred through time along strike of the transforms

carboniferous permian palynostratigraphy of west australian marine rift basins resolving tectonic and eustatic controls during gondwanan glaciations abstract the longest interval of cold climate in earth s phanerozoic history spanned some 55 million years from the namurian late carboniferous to the kungurian early permian when glaciation affected much of high latitude gondwana uncertainty surrounds the timing of glaciation s and the frequency of interglacial episodes ice volumes and the location of ice centres this is because the glacial record is preserved predominantly within marine strata deposited in intracontinental rift basins marginal to onshore ice centres on uplifted basement highs the direct stratigraphic record of glaciation is meagre on adjoining basement highs and extremely difficult to date many basins contain broadly similar threefold lithostratigraphic successions of lowermost glaciomarine diamictite facies middle shales and uppermost carbonate and sandstone rich commonly coal bearing deltaic strata in the absence of absolute age control and infrequent presence of macrofossils such deglaciation sequences are widely assumed to be correlative from basin to basin recording gondwanan deglaciation s and a world wide glacioeustatic sea level rise a newly compiled biostratigraphic database for west australia using first appearance datums of spore pollen types allows correlation of late palaeozoic siliciclastic successions between seven basins bonaparte canning carnarvon collie gunbarrel northern and southern perth basins these extend some 3800 km along australia s western continental margin adjacent to the west australian shield all but the gunbarrel basin contain tripartite glacially influenced successions diamictite shale sandstone biostratigraphic data reveal sharp differences in ages and stratigraphic distributions and marked variations in the thickness of coeval units diamictite shale sandstone successions accumulated diachronously from basin to basin recording a dominant tectonic control on subsidence and relative sea level middle shale units are not exact correlatives between basins and thus are unlikely to simply represent widespread climatically controlled glacioeustatic events instead they are argued to record high relative sea levels during times of maximum synrift subsidence this tectonostratigraphic model can be applied to other intracontinental rift basin successions across gondwana