integrated loessite paleokarst depositional system early pennsylvanian molas formation paradox basin southwestern colorado u s a abstract mississippian paleokarst served as a dust trap for the oldest known paleozoic loessite in north america the early pennsylvanian molas formation consists of loessite facies sorted angular coarse grained quartz siltstone infiltration facies loess redeposited as cave sediments within paleokarst features of the underlying mississippian leadville limestone colluvium facies loess infiltrated into colluvium surrounding paleokarst towers and fluvial facies siltstone rich fluvial channel and floodplain deposits with paleosols the depositional system evolved from an initial phase of infiltration and colluvium facies that were spatially and temporally related to the paleokarst surface to loessite facies that mantled the paleotopography and to fluvial facies that were intercalated with marine deltaic rocks of the overlying pennsylvanian hermosa formation this sequence is interpreted as a response to the modification of the dust trapping ability of the paleokarst surface loess was initially eroded from the surface transported and redeposited in the subsurface by the karst paleohydrologic system maintaining the dust trapping ability of the paleotopographic surface later the paleotopographic surface was buried when loess accumulation rates exceeded the transport capacity of the karst paleohydrologic system these changes could have occurred because of 1 increased dust input rates in western pangaea 2 rising base levels and or 3 porosity loss due to deposition within paleokarst passageways

tectono stratigraphic evolution of the comondu group from bahia de la paz to loreto baja california sur mexico abstract the late oligocene to mid miocene volcanic and volcaniclastic rocks of the comondu group are well exposed along the main rift escarpment of baja california sur from the bahia de la paz region to bahia concepcion new mapping and stratigraphic analysis of the comondu group from bahia de la paz to loreto reveal facies trends and correlations that form the foundation for a continuous stratigraphic framework for the comondu group along a 300 km long transect on the eastern coast of the baja california peninsula broad but distinct lithostratigraphic trends alluvial fan facies and volcanic and volcaniclastic facies record an overall coarsening upwards package that includes ignimbrite deposits within increasingly proximal alluvial fan deposits both derived from the east geochronology of the unit including 32 isotope ages and 12 previously unpublished 40ar 39ar ages provide the timing of four main increasingly proximal depositional events non marine sandstone defining the base of the comondu group was first deposited between 26 ma and 24 ma emplacement of rhyolitic ignimbrites initiated between 24 ma and 23 ma and marked a westward expansion of volcanic activity affiliated with the sierra madre occidental ignimbrite sequences in southern sinaloa western durango and northern nayarit a change in volcanism occurred at 19 ma to 18 ma with more ignimbrites increased intermediate compositions and the appearance of local vents and proximal volcanic facies a final localized change of volcanism occurred from 14 to 12 ma in the loreto area with an increase of proximal alluvial fan deposits and local volcanoes in the upper comondu group the bulk of the upper comondu group is absent south of the loreto area and has either been removed by erosion as a source for the magdalena fan in the pacific ocean or was focused primarily in the loreto area and northward we use a pre rift tectonic reconstruction of the gulf of california to align broad stratigraphic trends along the peninsula the gulf conjugate margins and within the gulf the cascadia arc of oregon and northern california may be a modern analog for the comondu group with a linear volcanic arc formed above shrinking subducting microplates and a broad backarc region of moderate extension and scattered volcanism

crustal extension and magmatic processes cocorp profiles from death valley and the rio grande rift new crustal scale information on the interaction between normal faulting and magmatic activity is provided by recent cocorp deep seismic reflection profiles in death valley california and by reprocessing of the cocorp data from the socorro area of the rio grande rift new mexico the most striking feature on the seismic sections from these areas is a prominent subhorizontal series of reflectors at mid crustal depth previous studies have suggested that thin tabular magma bodies lie within the mid crustal reflective zones in addition because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the cocorp data these mid crustal horizons are here inferred to be detachments or zones of tectonic decoupling upper crustal cenozoic faults do not appear to penetrate deeper than 15 km in death valley and 13 km in the rio grande rift in death valley these faults are relatively planar with moderate dips 20 to 35 and appear to bound large basement rocks one such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central death valley to the surface location of a 690 000 yr old basaltic cinder cone listric and low to moderate angle normal faults are evident on the reprocessed new mexico data and constitute the structural component of upper crustal extension in particular a listric master fault traceable to depths as great as 13 km is inferred to underlie the albuquerque basin unlike the death valley data no faults are observed to merge with the socorro magma body per se rather subhorizontal to moderately west dipping packages of reflections are imaged between the base of the faulted upper crust 13 km depth and the mid crustal magma body about 20 km depth the middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion events seen in the middle and lower crust are generally subhorizontal and prominent layering is observed a band of reflections attributed to the crust mantle boundary is evident on most seismic sections the upper mantle appears seismically transparent on some of the profiles the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections at least one of which is a layer of magma this association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes

mesozoic orogenic gold deposits in mexico abstract although orogenic gold deposits in mexico were mostly formed during the cenozoic they started forming during the late cretaceous or perhaps even earlier mylonitic zones in the oaxacan metamorphic complex span either permo triassic or triassic jurassic ages but none of these have been formally associated with the existing orogenic gold occurrences given the current scarcity of age data for them any interpretation concerning the age patterns or distribution of orogenic gold deposits in mexico other than those in the sonoran belt is hitherto deemed to be precarious a few known mesozoic examples are part of the quitovac la choya deposits in the caborca orogenic gold belt cogb in sonora and los uvares in the los cabos block southernmost part of the baja california peninsula the cogb constitutes the most economically important and geologically consistent mineralized area in mexico with respect to orogenic gold deposits also the cogb arguably correlates with the mother lode trend in california and nevada whereas the cluster of deposits in the los cabos block may have correspondent similarly mineralized areas in mainland southwestern mexico jalisco block other areas with orogenic gold deposits in mexico which have been traditionally overlooked or scantly mentioned are the northernmost part of the baja california peninsula and the arteaga and oaxacan metamorphic complexes although no geochronological data are available from these besides the overly lack of such data one of the typical problems concerning the characterization and metallogenesis of orogenic deposits in mexico has been their inaccurate or incorrect ascription to other genetic paradigms or types of deposits such as epithermal or porphyry cu au deposits therefore misguiding exploration efforts the formation of orogenic gold deposits in mexico has been commonly associated with the laramide orogeny in association with cataclastic or mylonitic deformation and shear zones their depth of formation under lithostatic pressure has ben estimated at a range between 3 and 10 km alteration assemblages in them are generally dominated by quartz sericite pyrite and may be carbonate rich the hypogene mineralogy of orogenic gold deposits in mexico is normally dominated by sulfides like pyrite galena sphalerite and whereas gold normally occurs as electrum also known as unspecified ag rich gold au rich silver or as free gold the latter may either be a hypogene or supergene mineral in which case it is likely to be due mostly to the leaching off of ag from electrum te rich associations are rare and have complex parageneses that consist typically of silver and or gold tellurides the mineralizing fluid precipitation into quartz veins occurred at temperatures 350 c and are co 2 rich deeply seated crustal brines from metamorphic sources with salinities typically ranging between 2 and 10 wt nacl equiv but attaining up to 33 wt nacl equiv

miocene paleomagnetism and tectonic setting of the baja california peninsula mexico a paleomagnetic pole calculated from 64 virtual geomagnetic poles for miocene volcanic rocks of the baja california peninsula 86 1 n 142 6 e 95 3 9 is similar to a reference pole for the north american craton 87 4 n 129 7 e 95 3 0 consistent with the baja peninsula having been adjacent to the mexican mainland in its pre gulf of california position from at least early miocene time to the pliocene opening of the gulf this result is in agreement with marine geophysical and geologic evidence from the gulf of california and baja peninsula and does not support a long period nondipole geomagnetic field in the region during miocene time or 10 of northward transport of the peninsula since then inferred from smaller data sets included in this work by previous workers this study also indicates that a pre gulf of california northward translation 9 of latitude and clockwise rotation 24 of the baja peninsula with respect to north america determined from paleomagnetic studies of cretaceous rocks within peninsular california occurred prior to miocene time paleomagnetic results from late cretaceous to early tertiary basement rocks in western sinaloa mexico are similar to those results from cretaceous rocks of the baja peninsula supporting the early tertiary northward translation of the peninsula between 45 and 34 ma and suggesting that the tectonic boundary separating peninsular california from north america at that time was east of the present day gulf of california

late cenozoic basin and range structure in western mexico adjacent to the gulf of california the area that surrounds the gulf of california was intensely faulted during the late cenozoic prior to opening of the gulf in a representative 10 000 km 2 area of southern sinaloa the faults consist of a predominant north northwest striking set having normal displacement and a subsidiary east northeast set having largely strike slip displacement two domains are recognized one in which both fault trends are abundant and one in which east northeast faults are minor in the former north northwest faults form a series of mostly southwest tilted half grabens filled with upper tertiary sediments displacement on individual faults is commonly as much as several kilometers east northeast faults probably represent accommodation zones between areas of differential extension in the second domain north northwest faults form an extensive graben system major faults dipping 40 to 70 into the graben and having several kilometers of cumulative displacement are spaced every 5 to 10 km bedding attitudes indicate that fault blocks are rotated as much as 65 dependent upon assumptions about subsurface geometry of the faults total extension may range from 20 to 50 fault geometry and stress orientations calculated from fault slip data indicate that the least principal stress was east northeast k ar ages of three north northwest striking dikes indicate that east northeast extension began as early as 32 ma ages of tilted volcanic rocks however indicate that measurable tilting and therefore most faulting began after about 17 ma faulting having similar timing style and orientation occurred throughout the area surrounding the gulf of california as far south as nayarit on the mexican mainland and as far north as areas in sonora that are unequivocally within the basin and range province this continuity and similarity in characteristics of faulting around the gulf to those of early basin and range faulting in the united states indicate that the area around the gulf is part of the basin and range province this basin and range faulting probably created the widely recognized but debated proto gulf of california the present gulf of california opened by ocean floor spreading and transform faulting in a zone already weakened by basin and range extension the lack of correspondence between areas of extension and likely areas of crustal thickening related either to laramide contraction or magmatism argues against a model of basin and range extension related solely to spreading of overthickened crust

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

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

geodetic and tectonic analyses along an active plate boundary the central gulf of california the gulf of california is traversed by the shear plate boundary between pacific and north american plates and because of several islands in its central part offers the possibility of direct geodetic measurements of plate motion a geodetic network of 150 km aperture and comprising 11 stations was measured in 1982 and 1986 by laser trilateration methods the deformations deduced from the comparison of the two epochs indicate right lateral shear strain covering the entire gulf rather than localized movements in the eastern part of the network between the axial islands and the sonoran coast significant right lateral shear deformation occurs with a relative displacement of about 23 12 cm over 4 years in the northwestern region canal de ballenas a right lateral displacement of about 17 4 cm is observed whereas in the southwestern part of the network canal sal si puedes the deformation remains very weak this suggests that south of the canal de ballenas the plate boundary is locked a tectonic analysis of neogene and quaternary faults in baja california sonora and the central islands of the gulf permitted the reconstruction of the stress pattern evolution of this area these data also indicate the predominance of right lateral motion on a nw se trending zone within a regional framework characterized by an approximately n s compression and an e w extension the geodetic results are discussed in comparison with the neotectonic analysis and the seismic data available in the area the data suggest a broad strain accumulation zone covering the totality of the central gulf of california a nw se relative velocity of about 8 3 cm yr is found between the two sides of the gulf during the 1982 1986 interval

active tectonics of the loreto area baja california sur mexico abstract quaternary faulting and coastal uplift characterize the rifted margin of baja california near loreto the process by which strain between the north american and pacific plates became partitioned across the baja peninsula and the gulf of california is partially recorded in the tectonics of the loreto region the loreto fault which consists of a north south trending northern segment and a northwest southeast southern segment records part of this process as do other faults offshore during the pliocene the southern loreto fault was actively providing accommodation space for rapid sedimentation in the loreto basin basin subsidence which abruptly slowed at the end of the pliocene was replaced by uplift which occurred throughout the quaternary the quaternary history of the northern loreto fault is recorded in fault scarps cutting quaternary deposits fault studies indicate that cumulative normal slip on the northern loreto fault over the quaternary was only about 30 m latest pleistocene deposits are offset by about 5 m uplifted marine terraces which may record either isostatic uplift of the coastal region or faulting events offshore suggest rates of coastal uplift between 0 08 m ka to 0 16 m ka for the last 120 ka these results strongly suggest that since the end of the pliocene strain was transferred from the loreto fault to other faults probably offshore strain between the pacific and north american plates may therefore largely occur in the gulf of california rather than onshore in the loreto region despite the low quaternary strain rates recorded the offshore faults and the loreto fault may both represent significant seismic hazards albeit with long recurrence intervals

backscattering and geophysical features of volcanic ridges offshore santa rosalia baja california sur gulf of california mexico abstract volcanic ridges formed by series of volcanic edifices are identified in the central part of the gulf of california between isla tortuga and la reforma caldera santa rosalia region isla tortuga is part of the 40 km long tortuga volcanic ridge tvr that trends almost perpendicular to the spreading center of the guaymas basin the rosalia volcanic ridge rvr older than tvr is characterized by volcanic structures oriented towards 310 following a fracture zone extension and the peninsular slope it is interpreted that most of the aligned submarine volcanic edifices are developed on continental crust while isla tortuga lies on oceanic like crust of the guaymas basin from a complete bouguer anomaly map it is observed that the alignments of gravity highs trending 310 and 290 support the volcanic and subvolcanic origin of the bathymetric highs volcanic curvilinear structures lava flows and mounds were identified from backscattering images around isla tortuga and over a 400 m high virgenes high where the tvr and the rvr intersect a refraction wide angle seismic profile crossing perpendicular to the virgenes high together with gravity and magnetic data indicate the presence of shallow intrusive bodies presumably of basaltic or andesitic composition it is inferred that most volcanic edifices along the ridges have similar internal structures we suggest that the growth of different segments of the ridges have a volcano tectonic origin the older rvr lies along the extension of a fracture zone and it probably is associated with pliocene ne sw extension

radial dikes laccoliths and gelatin models the northern adel mountains of central montana provide several excellent examples of laccoliths tangentially attached to feeder dikes that are members of radial swarms the radial distribution of the dikes appears to be controlled by stresses imposed by the load of an overlying central volcano model experiments using colored gelatin to simulate magma injected into a tank of uncolored gelatin duplicate the geometry of the radial dikes and field relations between feeder dikes and laccoliths as observed in the field

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

stratigraphy and geochronology of the comondu group near loreto baja california sur mexico abstract upper oligocene to middle miocene volcanic and sedimentary rocks in the loreto region baja california sur are widely exposed and make up the comondu group as redefined here following mcfall the comondu group is part of a volcanic arc and forearc basin that formed along the northwestern margin of mexico regional to detailed scale mapping stratigraphic analysis and geochronology in a 10 20 km wide and 70 km long belt from the gulf escarpment to the coast near loreto reveal three main units in the comondu group and a composite thickness of 1 5 2 km 1 the lower clastic unit 30 19 ma contains 200 300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin 2 the middle breccia and lava flow unit 19 15 ma is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows 3 the upper lava flow and breccia unit 15 12 ma consists almost entirely of up to 600 meters of andesite lava flows a few kilometers west of loreto that are part of a composite volcano north and south of loreto for up to 25 km the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers the three units of the comondu group in the loreto area correlated with the salto pelones and ricason formations of the bahia concepcion area the lower clastic unit of this study is time equivalent to the marine san gregorio el cien and isidro formations on the western and southern baja california peninsula the middle and upper units of the loreto area are the proximal equivalent to the type section of the comondu group near the village of comondu during deposition of the lower part of the comondu group the arc lay to the east on mainland mexico in the sierra madre occidental and then migrated to the position of the gulf of california at about 25 ma at 19 ma the proximal part of the arc migrated abruptly 50 km westward to approximately the position of carmen island and at 15 ma the arc once again stepped west forming eruptive centers along the coast near loreto until 12 ma also at 15 ma the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment

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

neogene structural evolution of the sierra san felipe baja california evidence for proto gulf transtension in the gulf extensional province abstract the sierra san felipe located in the gulf extensional province of northeastern baja california experienced a complex deformation history of integrated normal and strike slip faulting block rotations and extension parallel folding as a result of neogene transtensional plate margin shear between the pacific and north american plates the eastern range front of the sierra san felipe is defined by three left stepping en echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones the las cuevitas santa rosa and huatamote detachments comprise ne to se dipping moderate to low angle normal faults that accommodate between 1 5 and 9 km of broadly e to se directed extension and show strong along strike displacement gradients hanging wall half grabens are infilled with northwest tilted miocene pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement stratigraphic relationships indicate that faulting on the las cuevitas and santa rosa detachment faults initiated synchronously as a kinematically linked fault system before 7 ma 9 8 ma based on the timing of footwall exhumation during the so called proto gulf phase of rifting paleostress calculations suggest a transtensional stress regime with ne to se directed extension and permutating vertical to n s trending subhorizontal contraction fault kinematics and paleostress orientations of the fault array do not vary through time but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments our data indicate that clockwise vertical axis block rotations and constrictional folding were an integral part of the deformation history in the sierra san felipe since rifting began in the late miocene and may have played an important role in facilitating the transfer of deformation between the main gulf escarpment and offshore faults in the gulf of california these observations support the hypothesis that middle miocene to present oblique divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the gulf extensional province

neogene paleostress changes in the basin and range a case study at hoover dam nevada arizona at hoover dam 40 km southeast of las vegas nevada well exposed highly faulted miocene rocks provide an excellent opportunity to study the paleostress history of a very small area within a region where previous geologic studies indicate clockwise rotation of paleostress and a nearness to a major strike slip fault zone boundary within 0 5 km3 of rock the sense of slip was determined on almost 500 separate faults the fault slip data show internal consistency with respect to lithology size of faults and location within the small area with respect to fault slip however the data represent an in homogeneous mixture of primarily strike slip and dip slip motions from this mixture it is possible to resolve two distinct stress fields with directions of extension that differ by 60 each stress field corresponds to a mixture of strike slip and dip slip faults and thus the orientations of 1 and 2 are not tightly constrained if however the data are first separated into strike slip and dip slip faulting modes and subsequently searched for sub populations that correspond to contrasting paleostress orientations the computations yield tightly constrained tensors that illustrate two distinct stress fields with subhorizontal 3 axes that trend n50 e and n75 w the 1 and 2 axes permutate in two vertical planes that strike n40 w and n15 e because 1 and 2 are close in value relative to 3 these relationships suggest that strike slip and dip slip faulting belong to the same tectonic regime qualitative observations of polyphase slip fault fault offsets and fault bedding geometric relationships when evaluated in the context of changes in 3 orientation and permutations of 1 and 2 provide a basis for a two stage late cenozoic structural evolution at hoover dam these stages are 1 strike slip faulting partly pre tilt and dip slip faulting and associated stratal tilting and 2 mostly post tilt complexly interrelated strike slip oblique slip and dip slip faulting the qualitative evaluations indicate that strike slip and dip slip faulting alternated in time during the second stage of deformation and may have done so during the first these mixed mode movements probably represent stress oscillations in time and space rather than discrete stress reorganizations in contrast the two different orientations of 3 either represent a major clockwise rotation of the stress field or a major counterclockwise rotation of the rocks during the faulting history each alternative is consistent with regional geologic relationships and the choice of which is correct cannot be made within the small area that was studied

plate interactions control middle late miocene proto gulf and basin and range extension in the southern basin and range middle late miocene proto gulf 12 6 ma extension around the gulf of california gulf extensional province is commonly interpreted as resulting from partitioning of oblique pacific north american plate motion into strike slip displacement along the margin and east northeast extension perpendicular to the margin within the north american plate we propose that this mechanism also applies to kinematically similar predominantly east northeast extension that occurred at the same time throughout the southern basin and range province from southern arizona and new mexico to the trans mexican volcanic belt new field and 40ar 39ar data in sinaloa and durango confirm that this episode of extension occurred on the mainland side of the gulf and in the basin and range east of the sierra madre occidental which is generally considered the eastern margin of the gulf extensional province published data indicate the middle late miocene episode also occurred across the northern and southern ends of the sierra madre where the gulf extensional province connects with the basin and range 1 from central sonora into southern arizona and new mexico and 2 from nayarit into central mexico north of the trans mexican volcanic belt this episode appears to have avected an area that continues to the eastern edge of the basin and range province in texas and san luis potosi recognition that this episode of extension avected the entire southern basin and range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the gulf extensional province alone published plate reconstructions require 160 to 110 km of east northeast extension between 12 and 6 ma if taken up solely within the gulf extensional province this would have generated 66 to 78 extension which is much greater than observed spread across the entire southern basin and range it requires only 20 total extension which is more consistent with observations of cumulative extension between 12 and 6 ma extension was partitioned into the gulf extensional province because 1 it lies between two stable batholith belts mesozoic peninsular ranges on the west and mid tertiary sierra madre occidental on the east that resisted extension and 2 the gulf was thermally weakened by immediately preceding arc magmatism extension in the main basin and range province in part probably avoided the relatively strong batholithic crust of the sierra madre occidental 2000 elsevier science b v all rights reserved

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

late cenozoic extension of northeastern baja california mexico the pattern and history of normal faulting in a portion of northeastern baja california strongly suggest that the area developed as a result of east west crustal extension during late cenozoic time geometric and kinematic data strongly suggest that this area developed in concert with the opening of the proto gulf of california and the distension of the basin and range province in addition these data coupled with regional tectonic timing relations preclude the application of an oblique rifting model to this area two intervals of extensional tectonics have affected the region since the early miocene at present it is not known if strain was continuous or discontinous with respect to time the first and most intense period initiated between 17 and 9 m y ago was east west directed as determined from tilt patterns orientation of intra terrane transcurrent faults and fault plane kinematic indicators this deformation was dominated by high angle normal faulting and resulted in the disruption of a region wide low relief erosion surface new patterns of sedimentation were established and controlled by north trending grabens that filled with alluvial fan and marine deposits sporadic silicic eruptions occurred in conjunction with the extension thus allowing resolution of the complex history of tilting that accompanied normal faulting tilting and antithetic faulting resulted from the volume compensation required by the downward flattening of normal faults and by fault related monoclinal flexing and reverse drag effects extension during this interval was complete or substantially reduced by 6 m y b p the second interval of east west extension younger than 6 m y b p has been much less intense than the earlier event yet it seems to have utilized the same structures silicic volcanism also waned during this interval giving way to mafic eruptions related to rifting of the present gulf of california

late miocene to recent transtensional tectonics in the sierra san fermin northeastern baja california mexico basins and ranges within part of the gulf of california extensional province mexico have experienced complex distributed deformation including normal and strike slip faulting and block rotations linked to dextral shear at the pacific north america plate boundary in the sierra san fermin and southern sierra san felipe northeastern baja california normal faulting began between 12 5 and 6 ma although most extension occurred between about 6 and 3 ma strongly influencing thickness and distribution of ash flow tuffs and sedimentary deposits extension is generally 10 in 6 ma rocks and somewhat more in 12 5 ma rocks inversion of kinematic data interpreted together with published paleomagnetic data suggests that the axis of least principal stress was oriented between w e and sw ne in late miocene time our data indicate an important change in the amount of dextral shear but not necessarily the least principal stress direction wsw ene at about 3 ma structural constraints limit significant sinistral strike slip faulting conjugate to the dextral plate boundary to the last 3 my progressive changes in the geometry of faulting through time are consistent with regional strain partitioning within the pacific north america plate boundary zone and are predicted by physical and analytical models of oblique divergence as the orientation of the stretching vector changes to lower and lower values

magmatic tectonic interaction during early rio grande rift extension at questa new mexico pre caldera and caldera related igneous rocks near questa new mexico provide a well constrained example of upper crustal extension over a magma chamber extensional strain exceeding 200 occurred over many square kilometers in a region now underlain by a batholith the most intense deformation followed emplacement of intrusions related to formation of questa caldera at 25 7 ma and preceded the cooling of 24 6 ma plutons detailed mapping of the southern part of questa caldera indicates the sequence 1 extension concurrent with plutonism resulting in an overlying carapace of highly tilted blocks bounded by low angle faults 2 renewed intrusion into overlying units with the orientation of pluton tops dikes and molybdenum bearing alteration zones controlled by previously formed low angle faults 3 reactivation or continued motion of low angle normal faults possibly aided by hydrothermal fluids and 4 high angle normal faults that formed after the magmatic system waned early high angle faults remained active again possibly aided by fluids during their rotation to low angles at upper crustal levels the magma chamber is believed to have provided a zone of ductility into which overlying brittle normal structures were rooted at mid levels in the crust regional magmatism thermally softened the crust and allowed it to deform in a ductile manner regional extension appears to have been focused in a thermally and mechanically weakened area during the period of maximum plutonic activity the features at questa provide insights into processes that may have occurred in similar systems in the basin and range province and elsewhere

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

miocene detachment faulting predating epr propagation southern baja california at the southern tip of the baja california peninsula we characterize the onshore structures and kinematics associated with crustal necking leading up to the pliocene breakup and early east pacific rise seafloor spreading from a combination of tectonic field investigations k ar and cosmogenic isotope dating and geomorphology we propose that the los cabos block represents the exhumed footwall of a major detachment fault this north trending detachment fault is marked by a conspicuous low dipping brittle ductile shear zone showing a finite displacement with top to the se ending to the ese this major feature is associated with fluid circulations which led to rejuvenation of the deformed cretaceous magmatic rocks at a maximum of 17 5 ma the detachment footwall displays kilometer scale corrugations controlling the present day drainage pattern this major detachment is synchronous with the development of the san jose del cabo basin where syntectonic sedimentation took place from the middle miocene to probably the early pliocene we propose that this seaward dipping detachment fault accommodates the proximal crustal necking of the baja california passive margin which predates the onset of formation of the east pacific rise spreading axis in the cabo puerto vallarta segment our data illustrate an apparent anticlockwise rotation of the stretching direction in baja california sur from 17 ma to the pliocene

the inverse problem in microtectonics and the separation of tectonic phases abstract the algorithm of inverse problems is applied to three cases in microtectonics a collection of striations measured on pre existing faults may be processed automatically and interpreted in terms of tectonic stress a comparison is made between two different methods formerly proposed by the authors both methods are tested with field data the theoretical results agree with independent field criteria for a separation of tectonic phases

miocene pleistocene tectono sedimentary evolution of bahia concepcion region baja california sur mexico abstract bahia concepcion is one of the largest fault bound bays in the gulf of california it is one of the area s best examples of an extensional basin and an accommodation zone related to the late miocene extension in the gulf of california region extensional tectonics in the gulf region initiated from middle to early late miocene in a simple east west manner the units within the comondu group are tilted between 14 and 45 in opposite directions e w as a direct result of a single main extensional episode this event produced subsidence that created the depocenters for nearshore marine basins the oldest sedimentary units present in the region are assigned to the late miocene early pliocene tirabuzon formation up faulted granodiorite basement occurs on the peninsula concepcion and at punta san antonio as a direct result of the late miocene extensional episode extension on the bahia concepcion zone was responsible for development of a divided half graben structure first flooded in late pliocene time the tectono sedimentary evolution of bahia concepcion is recorded by three stratigraphic stages 1 pre rift strata represented by the comondu group 2 a syn rift unconformity and 3 a post rift strata represented by the late miocene to pliocene flat laying sedimentary units this triad of stratigraphic stages clearly varies from other basins along the margin of the gulf of california upper pleistocene terrace deposits generally conform to the 6 12 m elevation regionally associated with substage 5e their uniformity in elevation inside and outside bahia concepcion indicates that tectonic uplift was locked in step throughout the immediate region at least since that time

structural controls on the continent ocean transition in the northern gulf of california in the gulf of california the pacific north america plate boundary changes character from an oceanic type spreading center and transform fault system to the south to a region of diffuse continental deformation to the north the presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric heat flow gravity or seismic data which indicate significant differences north and south of latitude 30 n we suggest instead that north of 30 n a continent ocean transition begins which we name the wagner transition zone wtz diffuse deformation characterizes the wtz where slip occurs along reactivated north to nnw striking normal faults developed during late miocene or pliocene ene directed extension transtensional deformation varies from ene directed extension along dip slip faults in the west to dextral shear along the coast to dextral oblique slip along inferred north to nnw striking faults submerged in the northern gulf by accounting for rotational and extensional plate motion deformation in northeastern baja california vector constraints require that submerged structures accommodate 30 mm yr of slip in a direction slightly clockwise of the relative plate motion direction the juxtaposition of the discrete spreading center system in the central gulf with the diffuse wtz appears to have been a stable configuration since 4 6 ma perhaps controlling the evolution of spreading center jumps between upper and lower tiburon and delfin basins due to the juxtaposition of kinematically partitioned structural domains different histories of prerift extension and subduction related arc magmatism along the length of the gulf partly related to the migration of the rivera triple junction may explain the location of the continent ocean transition

when still waters ran deep deep water and bottom water the waters that fill the deep parts of ocean basins form when surface waters become dense and sink today this occurs in the northern north atlantic and around antarctica but not in the north pacific there surface waters do not become dense enough to sink more than a few hundred meters in the past however it seems things were different recently okazaki et al offered new insight into the ancient history of the ocean from radiocarbon data and modeling analyses 1 they suggest that deep water formed in the north pacific at the beginning of the transition out of the last ice age

stress and fracture evolution in a cooling pluton an example from the diamond joe stock western arizona u s a abstract an analytical model of fracturing in a crystallizing cylindrical pluton is applied to the natural fracture distribution in the diamond joe stock western arizona u s a the model accounts for three inputs to the stress during cooling thermal contraction magma pressure and regional tectonics fracture type shear or tensile orientation and density were determined at over 300 locations throughout the 8 km diameter stock a distinct pattern of fractures radiates outward from the center of the stock a deflection of these fracture trends toward the regional maximum principal stress direction provides a constraint on the magnitude and orientation of regional stress at the time of crystallization and cooling a calculated model of the fracture pattern suggests that regional maximum and minimum principal stress were 75 and 15 mpa with the orientation controlled by local tectonism the distribution of fractures appears to be insensitive to variations in magma pressure except near the wall of the chamber hence little constraint on magma pressure could be derived from the model thermal gradients during cooling and regional stress appeared to have a greater effect than magma pressure on the overall fracture distribution larger thermal gradients during the early stages of crystallization led to abundant early tensile fractures assuming that the crystalline portions of the pluton behaved elastically during cooling and that all strain was accommodated by fractures the variation of theoretical strain is similar to the distribution of fracture densities within the stock

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

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

tension fractures and extensional tectonics a characteristic pattern of normal faults that we have studied in several regions illustrates successive geometries relating to increasing amounts of extension from 10 50 gulf of suez egypt to 50 100 western gulf of california mexico up to 200 southern basin and range usa tension fractures formed at an early stage of extension have an essential role the resulting structure is described as a multiple tilted packs of cards model in which rotational deformation within the major blocks is accommodated by normal faulting on previously formed tension fractures thus defining smaller blocks of second and third generation

miocene pliocene exhumation along the west salton detachment fault southern california from u th he thermochronometry of apatite and zircon 1 the salton trough is the northernmost segment of the active gulf of california oblique rift system the main rift related structure in the western salton trough is the low angle west salton detachment fault wsdf footwall and hanging wall apatite and zircon u th he ages record a significant contrast in thermal history across the wsdf confirming pliocene normal slip along the fault apatite u th he ages record rapid exhumational cooling from 5 to 2 ma consistent with the timing of accelerated tectonic subsidence recorded in wsdf hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern san andreas fault in the salton trough our results indicate that the wsdf caused at least 2 3 4 km of exhumation and 8 10 km of approximately e directed horizontal extension since 5 ma slip rate of the wsdf at yaqui ridge was likely in the range of 2 3 5 km ma from 7 to 2 ma the wsdf may have been active well before 5 ma middle miocene apatite u th he ages from samples at high elevations have steep age elevation gradients and suggest that exhumation may have initiated by 12 ma about when subduction ceased along a large segment of the baja california margin and consistent with the age of onset of extension in northeastern baja california

cretaceous to cenozoic sequential kinematics in the forearc arc transition effects of changing oblique plate convergence and the san andreas system with implications for the la paz fault southern baja california mexico we studied metasediments and mylonitic arc granitoids from the forearc arc transition of southern baja california mexico thin section analyses and field evidence show that metamorphism of the forearc arc transition is of the high t p active margin type the heat was provided by cretaceous arc intrusions field observations and thin section analyses including the time temperature deformation path demonstrate that the study area was first affected by dextral ductile shearing followed by ductile sinistral possibly transpressive strike slip parallel to the magmatic arc during the cretaceous both intervals are related to changing oblique plate convergence and thus identified as trench linked strike slip effects the geometric relationship between arc dipping foliation stretching lineation and shear sense indicates that the arc may have been pressed onto the rocks of the study area during sinistral shearing the sinistral interval lasted up until regional cooling early cenozoic because the la paz fault is closely associated with the forearc arc transition it must have the same cretaceous to early cenozoic kinematic history the northern segment of the la paz fault is a modern brittle strike slip fault interpreted as a dextral synthetic fault of the san andreas system which opened the gulf of california mar de cortes golfo de california we found no evidence for miocene basin and range extension

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