tectonic implications of early miocene extensional unroofing of the sierra mazatan metamorphic core complex sonora mexico sierra mazatan in northwest mexico is the southernmost metamorphic core complex in the north american cordillera new geologic structural and 4 0 ar 3 9 ar thermochronologic data demonstrate that the core complex detachment fault accommodated 15 35 km of slip at a rate of 3 3 7 7 mm yr between 20 5 and 16 ma these data also suggest that the lower plate was tilted eastward 20 50 since 20 ma indicating that the detachment fault initially dipped 30 60 rapid slip on the fault occurred concurrently with subduction in sonora and thus was not related to the ca 12 ma plate boundary change at this latitude or the late miocene opening of the gulf of california rapid extension at sierra mazatan was synchronous with core complexes along the length of the cordillera suggesting a distinct 20 15 ma core complex event the fundamental causes of which remain a mystery

retrograde fluids in granulites stable isotope evidence of fluid migration widespread retrograde alteration assemblages document the migration of mixed h sub 2 o co sub 2 fluids into granulite facies rocks in the adirondack mountains fluid migration is manifest by 1 veins and patchy intergrowths of chlorite plus minus sericite plus minus calcite 2 small veins of calcite many only identifiable by cathodoluminescence and 3 high density co sub 2 rich or mixed h sub 2 o co sub 2 fluid inclusions the distinct and varied textural occurrences of the alteration minerals indicate that fluid rock ratios were low and variable on a local scale stable isotope ratios of c o and s have been determined in retrograde minerals from samples of the marcy anorthosite massif and adjacent granitic gneisses charnockites retrograde calcite in the anorthosite has a relatively small range in both delta sup 18 o sub smow and delta sup 13 c sub pdb 8 6 to 14 9 and minus 4 1 to 0 4 respectively probably indicating that the hydrothermal fluids that precipitated the calcite had exchanged with a variety of crustal lithologies including marbles and orthogneisses and that calcite was precipitated over a relatively narrow temperature interval values of delta sup 34 s sub cdt that range from 2 8 to 8 3 within the anorthosite can also be interpreted to reflect exchange between orthogneisses and metasediments the recognition of retrograde fluid migrationmore is particularly significant in granulite facies terranes because the controversy surrounding the origin of granulites arises in part from differing interpretations of fluid inclusion data specifically the timing of entrapment of high density co sub 2 rich inclusions results indicate that retrograde fluid migration which in some samples may leave only cryptic petrographic evidence is a process capable of producing high density co sub 2 rich fluid inclusions less

late pliocene to early quaternary extensional detachment in the la pazâel cabo area baja california sur mexico implications on the opening of the gulf of california and the mechanics of oblique rifting terra nova 22 64 69 2010 abstract we demonstrate that pliocene to early quaternary sedimentary formations in baja california sur mexico were deposited syn tectonically over a major detachment associated with the exhumation of mesozoic crust the detachment dips to the ene and is associated with e w stretching this large extensional structure strikes almost parallel to the general trend of the gulf of california and extension is oblique to the east pacific seafloor spreading direction crustal scale stretching in this area was still active after the beginning of seafloor spreading c 3 6 ma ago the detachment is capped by late pleistocene holocene alluvial sediments the deposition of which seems to be partly syn tectonic and controlled by minor stretching subparallel to the present day north american pacific kinematic vector we discuss the implications of our observations on strain partitioning during opening of the california gulf as well as on the structure of the gulf of california margin

pulling apart the mid to late cenozoic magmatic record of the gulf of california is there a comondu arc the composition of the lithosphere can be fundamentally altered by long lived subduction processes such that subduction modified lithosphere can survive for 100 s myrs incorrect petrotectonic interpretations result when spatial temporal compositional trends of and source contributions to magmatism are not properly considered western mexico has had protracted cenozoic magmatism developed mostly in board of active oceanic plate subduction beneath western north america a broad range of igneous compositions from basalt to high silica rhyolite were erupted with intermediate to silicic compositions in particular showing calc alkaline and other typical subduction related geochemical signatures a major oligocene rhyolitic ignimbrite flare up 300 000 km3 switched to a bimodal volcanic phase in the early miocene 100 000 km3 associated with distributed extension and opening of numerous grabens extension became more focussed 18 ma resulting in localised volcanic activity along the future site of the gulf of california this localised volcanism known as the comondu arc was dominantly effusive and andesite dacite in composition past tectonic interpretations of comondu age volcanism may have been incorrect as these regional temporal compositional changes are alternatively interpreted as a result of increased mixing of mantle derived basaltic and crust derived rhyolitic magmas in an active rift environment rather than fluid flux melting of the mantle wedge above the subducting guadalupe plate

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

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

slip re orientation in oblique rifts oblique extension is expected to result in a combination of dip slip and strike slip displacement along faults with strike orthogonal and oblique to the extension direction respectively this general concept is in disagreement with observations from natural oblique rifts where faults show dip slip kinematics indicating pure extension irrespective of the fault strike with respect to the regional extension direction consequently along oblique structures slip is re oriented and oblique to the applied extension direction besides at fault scale slip is re oriented along strike such that it is dip slip at the fault center and becomes highly oblique slip toward the fault tips here we use analogue experiments to show that this discrepancy can be resolved when a preexisting weak zone wz is present in the crust at the onset of oblique extension the wz is implemented within the lower crust and strikes oblique to the extension direction our experimental results show that an inherited wz within the ductile crust favors the re orientation of slip such that oblique extension results in pure dip slip displacement on faults that strike oblique with respect to the extension direction furthermore we show that slip is re oriented along strike of major faults such that the fault center shows dip slip kinematics whereas its tips display strike slip kinematics these findings call into question the use of paleostress reconstructions to constrain plate kinematics in oblique extensional tectonic settings

crustal thickness of the peninsular ranges and gulf extensional province in the californias we estimate crustal thickness along an east west transect of the baja california peninsula and gulf of california mexico and investigate its relationship to surface elevation and crustal extension we derive moho depth estimates from p to s converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at 318n latitude depth to the moho is 33 63 km near the pacific coast of baja california and increases gradually toward the east reaching a maximum depth of 40 64 km beneath the western part of the peninsular ranges batholith the crust then thins rapidly under the topographically high eastern peninsular ranges and across the main gulf escarpment crustal thickness is 15 18 62 km within and on the margins of the gulf of california the moho shallowing beneath the eastern peninsular ranges represents an average apparent westward dip of 258 this range of moho depths within the peninsula ranges as well as the sharp east west gradient in depth in the eastern part of the range is in agreement with earlier observations from north of the international border the moho depth variations do not correlate with topography of the eastern batholith these findings suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that a local airy crustal root does not support the highest elevations we suggest that moho shallowing under the eastern peninsular ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the gulf extensional province that commenced in the late cenozoic support of the eastern peninsular ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and or upper mantle

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

waning miocene subduction and arc volcanism in baja california the san luis gonzaga volcanic field abstract subduction of the guadalupe magdalena microplate beneath baja california ended in the middle miocene and the last volcanic events in the frontal arc extinguished along the present day eastern margin of the baja california peninsula the san luis gonzaga area in the north central gulf coast contains one of the younger arc related volcanic centers in northern baja california the volcanic succession contains three sequences the basal sequence group 1 is composed of stratified pyroclastic deposits up to 500 m thick and subordinate lava flows the near vent facies crop out in tilted fault blocks along the present shoreline whereas the distal facies are exposed across 12 km toward the west and includes epiclastic deposits and at least three ash flow tuffs this sequence is internally concordant and overlies smooth paleosurface developed on granitic basement and pinches out across the gulf escarpment the potrero andesite group 2 is a series of dacite to basaltic andesite lava flows from a shield volcano located 15 km west of today s coastline similar source vents also occurs further south of the san luis gonzaga area a sequence of dacite domes group 3 intrudes the near vent facies of group 1 and contains subordinate volcanic breccia and minor lava flows that overlie group 1 sequence cross cutting relationships and the abundance of volcanic breccia associated with the domes suggest that these domes were emplaced as semi rigid intrusions spines with low explosive activity the san luis gonzaga volcanic suite ranges in composition from basaltic andesite to dacite with predominant plagioclase and pyroxene and variable amounts of hornblende trace element patterns indicate calc alkaline to mildly alkaline magmas with high ba and low nb contents incompatible element ratios and mineralogical characteristics suggest different magma batches and or different amount of crustal assimilation for the three sequences that produced contrasting eruptive styles a pink dacitic tuff that lies in the upper portion of group 1 yielded an 40 ar 39 ar age of 17 2 0 3 ma the most precise of three ages from the andesitic sheet flows of group 2 is 15 4 0 5 ma the dacite domes of group 3 represent the end of miocene arc activity but they are strongly altered and no reliable ages were obtained these data fall within the range of isotopic ages reported for arc lavas 21 to 16 ma from the discontinuous series of isolated volcanic fields in northern baja california this contrasts with the continuous and more voluminous arc volcanism in southern baja california that started earlier 25 ma and ended later 11 ma these space time patterns of arc volcanism reflect the geometry of the subducted farallon slab and the history of its breakup as the rivera triple junction migrated to the south

the 40ar 39ar thermochronology of core complexes and other basement rocks in sonora mexico implications for cenozoic tectonic evolution of northwestern mexico 1 northwestern mexico remains an important region for understanding the tectonic evolution of the north american cordillera however many fundamental aspects of the geologic history of the region remain poorly known we present new 40ar 39ar thermochronology of exposed basement rock from throughout sonora mexico including from metamorphic core complexes these results provide new constraints on the late cretaceous to cenozoic tectonic evolution of northwestern mexico following laramide ca 80 50 ma emplacement most sonoran plutons experienced rapid post magmatic cooling to 400 200 c which we attribute to shallow level emplacement following this rapid cooling plutons cooled slowly 300 c until the late oligocene rapid footwall cooling documents that extension at sonoran core complexes began synchronously at ca 25 ma despite the fact that these regions are separated by 200 km extension at most sonoran core complexes continued until ca 15 ma although extension at the magdalena core complex ceased earlier ca 21 20 ma slip at each core complex was 10 30 km and occurred at average rates of 1 9 mm yr this major period of extension primarily occurred during subduction indicating that relative plate motions between the pacific and north american plates cannot be invoked as a driving force for this phase of extension

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

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

active continental rifting in southern baja california mexico implications for plate motion partitioning and the transition to seafloor spreading in the gulf of california three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern gulf of california focal mechanism solutions for earthquakes of the april 1969 and june 1995 series indicate nearly pure normal displacement across nnw striking planes this seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the gulf extensional province the dominant normal faults control distribution of neogene basins and have been active since middle to late miocene time epicenters of the august 1969 earthquake series lie south of the baja california peninsula and focal mechanism solutions indicate strike slip displacement this seismic activity is likely related to active wnw striking faults in the continental borderland which isolate baja california as a microplate within the pacific north american plate boundary therefore modern pacific north america plate motion is partitioned into at least three distinct fault systems oceanic spreading centers and transforms in the gulf of california normal faults along the western margin of the gulf of california and faults along the pacific margin of the baja california peninsula kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting continental rifting and seafloor spreading have overlapped in time for the past 3 6 myr which also is likely attributed to the highly oblique kinematics of rifting

seismic structure of the southern gulf of california from los cabos block to the east pacific rise 1 multichannel reflection and coincident wide angle seismic data collected during the 2002 premier experiment sea of cortez addressing the development of oblique rifting pescador experiment provide the most detailed seismic structure to date of the southern gulf of california multichannel seismic mcs data were recorded with a 6 km long streamer 480 channel aboard the r v maurice ewing and wide angle data was recorded by 19 instruments spaced every 12 km along the transect the mcs and wide angle data reveal the seismic structure across the continent ocean transition of the rifted margin typical continental and oceanic crust are separated by a 75 km wide zone of extended continental crust dominated by block faulted basement little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust suggesting a constant rate of magmatic productivity since seafloor spreading began oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature 1300 c and passive mantle upwelling at the early stages of seafloor spreading the crustal thickness width of extended continental crust and predicted temperature conditions all indicate a narrow rift mode of extension on the basis of upper and lower crust stretching factors an excess of lower crust was found in the extended continental crust total extension along transect 5w is estimated to be 35 km following crustal extension new oceanic crust 6 4 km thick was formed at a rate of 48 mm a 1 to accommodate plate separation

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

on the plate boundary forces that drive and resist baja california motion the driving forces of microplate transport remain one of the major unknowns in plate tectonics our hypothesis postulates that the baja california microplate is transported along the north america pacific plate boundary by partial coupling to the pacific plate and low coupling to the north america plate to test this idea we use numerical modeling to examine the interplate coupling on a multiple earthquake cycle time scale along the baja california pacific plate boundary and compare the modeled velocity field with the observed geodetic motion of the baja california microplate we find that when the strain can localize along a weak structure surrounding microplate faults high interplate coupling produced by frictional tectonic stresses can reproduce the observed kinematics of the baja california microplate as seen from geodetic rigid plate motions we also find that the northward motion of baja california can influence the fault slip partitioning of the major faults in the north america pacific plate boundary region north of baja california

seismicity and state of stress in the la paz los cabos region baja california sur mexico we present the results of an analysis of seismic data collected for seven years in the la paz los cabos region the data set includes earthquakes with magnitudes of up to 3 6 and focal depths mostly between 2 and 14 km the results show that some located epicenters correlated with known faults of the region but others did not suggesting faults that have not yet been recognized in addition even though earthquakes occurred all over the study area zones of stress concentration were identified in such zones the seismicity often developed as earthquake swarms composite fault plane solutions were prepared by using data of those areas the resulting mechanisms for events of the northern part of the study area indicated normal faulting east side down in such case the p axis had a mean vertical angle of 55 in the n12 w direction whereas the t axis was nearly horizontal and with a n68 e average trend this part of the study area is found to be under a predominant tensional stress regime to the southwest of the area the focal mechanisms showed predominant components of strike slip faulting there the t axes were still subhorizontal but the p axes were more horizontal resulting in a mean plunge of about 20 in the n38 w direction on the other hand highly consistent directions of p and t axes from 13 representative events of the gulf of california fault system led to average axes that are close approximations to the principal stresses that drive such a fault system a comparison of these axes with average p and t axes of our region shows that the latter are about 16 to 23 degrees more westerly than the former we propose then either that the earthquakes in the study area occur more in response to local tectonic forces or that our inferred stress axes are only approximations to the principal stresses the complex tectonic situation of the region makes the first possibility feasible but the fact that the earthquakes occurred on pre existing faults favors the second one future seismic data of the region will help to elucidate this uncertainty finally the microearthquakes studied were not generated by the gulf of california fault system that forms the boundary between the pacific and north america plates our results then are evidence of a wide zone of deformation across which subsidiary faulting accommodates part of the relative plates motion

marine incursion synchronous with plate boundary localization in the gulf of california volcanic strata on southwest isla tiburon define the age of interstratified marine rocks and through revision of existing correlations the age of the proto gulf of california marine incursion a 5 7 0 2 ma ash flow was emplaced at the base of the marine section a rhyodacite dike and its related lava flow dated as 11 2 1 3 ma 3 7 0 9 ma and 4 2 1 8 ma intrudes and overlies respectively the marine rocks the 11 2 ma age which was the core datum for a middle miocene proto gulf of california origin for the underlying rocks is discordant with all other isotopic and microfossil ages an alternative interpretation utilizing all available geologic and geochronologic data except this discordant age is that marine strata on isla tiburon are latest miocene to early pliocene age reinterpretation of these strata supports a simplified history of marine incursion into the gulf of california marine rocks as old as 8 2 ma in the southern gulf of california indicate an early marine incursion perhaps flooding a region of more intense proto gulf of california continental extension flooding of the entire basin by 6 5 6 3 ma correlates to the sudden onset of significant pacific north american plate boundary motion within the gulf of california

oblique rifting and segmentation of the ne gulf of aden passive margin 1 the gulf of aden is a young obliquely opening oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid oceanic ridge it is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading the northeastern margin of the gulf of aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin on either side of the socotra fracture zone exhibiting distinct morphologic stratigraphic and structural features the western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits though the morphostructural features of the margin segments are different the stress field analysis provides coherent results all along the margin four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses 2 and 3 the two dominant directions of extension n150 e and n20 e are perpendicular to the mean trend of the gulf of aden n75 e and parallel to its opening direction n20 e n30 e respectively unlike another study in the western part of the gulf our data suggest that the n150 e extension stage is older than the n20 e extension stage these conflicting chronologies which are nowhere unambiguously established suggest that the two extensions coexisted during the rifting on land data are compared with offshore data and are interpreted with reference to oblique rifting the passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field which reveals the asymmetry of the rifting process the first order segmentation of the sheba ridge is inherited from the prior segmentation of the passive margin

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

paleomagnetic evidence of localized vertical axis rotation during neogene extension sierra san fermin northeastern baja california mexico paleomagnetic data from sierra san fermin in the gulf of california extensional province indicate that localized clockwise rotations about vertical axes occurred during pliocene through recent extension and dextral shear relative declination discordances in upper miocene and pliocene ash flow tuffs indicate a net clockwise rotation of 30 16 clockwise rotation between 12 5 and 6 ma is statistically insignificant 11 17 structural observations and geochronological data suggest that rotations in this area began post 6 ma comprising uniform sense block rotations oblique divergence associated with extension and dextral slip in the northwest striking boundary between the pacific and north american plates northeast striking sinistral slip faults and north striking normal faults accommodate distributed dextral shear in this area allowing fault blocks to rotate in a clockwise sense a model for oblique divergence predicts 21 km of shear in the direction of relative plate motion and 20 7 km ene directed extension perpendicular to the main gulf escarpment a broad region of northeastern baja california may have undergone similar distributed shear two possible dynamic models may explain this shear in one model rotation accumulates above a deep subhorizontal basal shear zone rotating blocks may extend downward to a detachment beneath the extensional province either a low angle eastward continuation of the san pedro martir fault or to a basal shear surface on top of a subducted remnant of the farallon plate alternatively distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults in either case shear may have transferred northward onto faults west of the san andreas fault contributing to late miocene to recent clockwise rotation of the western transverse ranges this shear is not accounted for in the 300 km of dextral slip computed from cross gulf geologic tie points

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

extensional segmentation of the main gulf escarpment mexico and united states periodic reversal of upper plate transport direction on miocene and younger extensional fault systems occurs in the northwestern gulf extensional province baja california and adjacent united states with segments in which the peninsular ranges are in either footwall or hanging wall positions accommodation zones separate these segments and are defined by changes of structural style and topography in the peninsular ranges and or the gulf extensional province these and other characteristics allow speculative definition of segments 50 150 km long along the baja california peninsula

middle miocene extension in the gulf extensional province baja california evidence from the southern sierra juarez new geologic mapping structural studies and geochronology of miocene volcanic and sedimentary rocks in the southern sierra juarez baja california shed light on the extensional history of the gulf extensional province prior to sea floor spreading in the gulf of california the southern sierra juarez is underlain by lower middle miocene rocks including fluvial strata intermediate composition volcanic deposits basalt lava flows and cinder cones and dacite pyroclastic deposits and lavas that nonconformably overlie the cretaceous peninsular ranges batholith the 40 ar 39 ar geochronology indicates that basaltic rocks are 16 90 0 05 ma and dacite pyroclastic deposits are between 16 69 0 11 ma and 15 98 0 13 ma these strata were subsequently cut by two generations of faults first generation faults comprise a dominant set of north south striking west dipping normal faults a secondary set of north south striking east dipping normal faults and a lesser set of variably oriented strike slip faults all three fault sets are temporally and spatially related and were produced by east west extension the dominant west dipping faults which are antithetic to and oblique to the east dipping main gulf escarpment may have been a precursor or an early phase accommodation zone along the escarpment west dipping normal faults are cut by a 10 96 0 05 ma dacite hypabyssal intrusion thus bracketing the age of east west extension between 15 98 0 13 ma and 10 96 0 05 ma hence this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated miocene extension within baja california second generation faults which are comprised of east west striking strike slip faults that cut first generation faults and associated northwest striking northeast dipping normal faults may be related to early development of the transpeninsular strike slip province global plate reconstructions suggest that transtensional motion between the north american and pacific plates along the western margin of baja california began during middle miocene time coeval with east west extension in the southern sierra juarez this observation supports a hypothesis that middle miocene transtensional plate motion was partitioned into two components a strike slip component parallel to active faults along the western offshore margin of baja california and an extensional component normal to the margin but located in what is now the gulf extensional province hence the onset of extension within the circum gulf region was in response to plate boundary processes

crustal thickness variations in the margins of the gulf of california from receiver functions receiver functions rfs from teleseismic events recorded by the nars baja array were used to map crustal thickness in the continental margins of the gulf of california a newly forming ocean basin although the upper crust is known to have split apart simultaneously along the entire length of the gulf little is known about the behaviour of the lower crust in this region the rfs show clear p to s wave conversions from the moho beneath the stations the delay times between the direct p and p to s waves indicate thinner crust closer to the gulf along the entire baja california peninsula the thinner crust is associated with the eastern peninsular ranges batholith prb crustal thickness is uncorrelated with topography in the prb and the moho is not flat suggesting mantle compensation by a weaker than normal mantle based on seismological evidence the approximately w e shallowing in moho depths is significant with extremes in crustal thickness of 21 and 37 km similar results have been obtained at the northern end of the gulf by lewis et al who proposed a mechanism of lower crustal flow associated with rifting in the gulf extensional province for thinning of the crust based on the amount of pre pliocene extension possible in the continental margins if the lower crust did thin in concert with the upper crust it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin in this case we suggest that when breakup occurred the lower crust in the margins of the gulf was still behaving ductilely alternatively the lower crust may have thinned after the gulf opened the implications of these mechanisms are discussed

chaotic topography mantle flow and mantle migration in the australian antarctic discordance oceanic crust formed over the past 30 million years at the australian antarctic discordance aad is characterized by chaotic sea floor topography reflecting a weak magma supply from an unusually cold underlying mantle during the past 3 4 million years however a source of increased magma supply coinciding with the known indian pacific mantle isotopic boundary has propagated into the eastern aad displacing the chaotic terrain and replacing it with normal sea floor pacific mantle reached the eastern boundary of the aad at least 7 million years ago but it was not until 3 4 million years ago that lavas derived from pacific mantle were first erupted within the aad this long hiatus combined with the ridge transform geometry across the aad boundary constrains the locus of mantle migration to a narrow relatively shallow region directly beneath the spreading axis of the southeast indian ridge

gently dipping normal faults identified with space shuttle radar topography data in central sulawesi indonesia and some implications for fault mechanics abstract space shuttle radar topography data from central sulawesi indonesia reveal two corrugated domal landforms covering hundreds to thousands of square kilometers that are bounded to the north by an abrupt transition to typical hilly to mountainous topography these domal landforms are readily interpreted as metamorphic core complexes an interpretation consistent with a single previous field study and the abrupt northward transition in topographic style is interpreted as marking the trace of two extensional detachment faults that are active or were recently active fault dip as determined by the slope of exhumed fault footwalls ranges from 4 to 18 application of critical taper theory to fault dip and hanging wall surface slope and to similar data from several other active or recently active core complexes suggests a theoretical limit of three degrees for detachment fault dip this result appears to conflict with the dearth of seismological evidence for slip on faults dipping less than 30 the convex upward form of the gently dipping fault footwalls however allows for greater fault dip at depths of earthquake initiation and dominant energy release thus there may be no conflict between seismological and mapping studies for this class of faults

transtensional rifting in the proto gulf of california near bahia kino sonora mexico continental rifts require focused strain to rupture and form an ocean basin in oblique rifts such as the gulf of california focused transtensional strain associated with strike slip faulting may serve as a catalyst for rupture to test this hypothesis we analyzed structural mapping geochronology paleomagnetism and fault kinematics of pre and synrift rocks exposed in an 200 km 2 coastal mountain belt flanking the eastern rift margin of the northern gulf of california this coastal sonora region hosts the onshore portion of the transform boundary between the upper tiburon and adair tepoca marine basins two early formed oblique rift segments extension commenced here between 11 5 ma and 7 ma resulting in 25 40 of e ne tilting initiation of clockwise vertical axis rotation of fault bounded blocks and minor basin sedimentation rates of deformation prior to 7 ma are unconstrained due to a lack of exposed syntectonic deposits deformation after 7 ma was associated with rapid tilting and the majority of observed clockwise vertical axis rotation and strike slip faulting nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older tilted strata by 5 6 ma deformation in coastal sonora must have largely ceased and migrated westward into the upper tiburon marine basin we document up to 120 total extension and total clockwise block rotations up to 53 in portions of the study area extension and rotation were supplanted by strike slip faulting as deformation proceeded we develop a tectonic model for this coastal sonora fault zone which is bounded by major nw striking transform faults with 10 km of displacement internal to the coastal sonora fault zone the majority of an estimated 6 2 1 1 km of dextral deformation associated with up to 5 7 km of wnw directed extension occurred over the final 1 2 m y of its life span at a strain rate approaching 10 14 s 1 this activity occurred as the plate boundary localized along nascent pull apart basins in the northern gulf of california consistent with the hypothesis that late proto gulf of california dextral shear zones such as the coastal sonora fault zone acted to focus lithospheric scale strain and promoted continental rupture in the wide rift setting of the mexican basin and range

principles of drainage basin analysis from multisource data application to the structural analysis of the zaire basin abstract classifications which attempt to combine descriptive and genetic aspects of types of drainage basins must take into account the application of new techniques satellite imagery high precision topographic maps and digital terrain models in addition the use of new concepts such as neotectonics and drainage basin history will lead to a reassessment of classification schemes after a review of current classifications we propose a genetic scheme that is based on the development of drainage networks and which includes integration of local and regional scale perturbations state of the art methods of drainage basin analysis are applied to the zaire basin to provide new information on deep structures the influence of basin and block tectonics is revealed in the context of major late proterozoic shear faults which have been rejuvenated up to the present day this is demonstrated from an analysis of the drainage network in the light of geological and geophysical observations

dating sediment burial with in situ produced cosmogenic nuclides theory techniques and limitations abstract dating sediment burial over million year time scales is crucial in many areas of the earth sciences and archeology but is often difficult using traditional techniques sediment burial can be dated by the radioactive decay of cosmogenic nuclides provided that the sediment was exposed to cosmic rays prior to burial dating calculations are straightforward if sediment is buried deeply and rapidly enough to prevent cosmogenic nuclide production after burial however the analysis can be complicated by postburial production if sediment is insufficiently shielded from secondary cosmic ray nucleons and muons this paper discusses how buried sediments can be dated over timescales up to 5 myr using 26 al and 10 be in quartz

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

large magnitude oligo miocene extension in southern sonora implications for the tectonic evolution of northwest mexico new structural and 40ar 39ar data from the santa rosa area southeastern sonora shed light on the magnitude and timing of extensional tectonism episodic magmatic activity from the late cretaceous to the miocene is recorded by a 3 4 km thick structural stratigraphic section that includes a composite laramide granodiorite batholith and its andesitic wall rocks early to middle tertiary rhyolite ignimbrites and silicic domes and late oligocene to middle miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate the older units are complexly faulted and steeply tilted by several generations of normal faults palinspastic reconstructions indicate cumulative extension of 90 early gently dipping nw striking normal faults document major extension oriented n50 e angular unconformities and growth fault relations within oligo miocene sequences bracket this extension between 26 and 20 ma younger widely spaced ns to nnw trending high angle normal faults cut the previously faulted and tilted sections and produced modest 10 15 e w extension that is largely bracketed between 20 and 17 ma the magnitude of neogene extension in this region is greater and the timing is older than previously recognized reconnaissance of other areas between the sierra madre occidental and the coast at this latitude suggests that most extension in sonora occurred between 27 and 12 ma while remnants of the farallon plate were still being subducted plate tectonic models that predict substantial extension in sonora during proto gulf 10 5 ma transtensional deformation may need to be reevaluated an alternative model presented here is that baja california began moving with pacific plate motion shortly after the 11 ma termination of subduction and that northwest motion of baja relative to mainland mexico might total 500 km distributed across the previously extended oligo miocene magmatic arc

pre miocene palaeogeography of the los cabos block baja california sur geochronological and palaeomagnetic constraints the los cabos block lcb is located at the southern end of the baja california peninsula and is composed mainly of intrusive rocks that were emplaced into pre cretaceous heterogeneous metasediments and are partly covered by miocene volcanic and volcaniclastic rocks field observations provide evidence that the undeformed homogeneous gabbronorite of the sierra el novillo in the ne part of the lcb is the oldest crystalline unit which is intruded by undeformed to slightly deformed tonalitic quartz dioritic granitoids for these rocks an rb sr intrusion age of 129 15 ma and a biotite whole rock cooling age of 116 2 ma were obtained low initial 87sr 86sr and high e nd values of about 0 7036 and 5t o 6 respectively characterise the intrusives as primitive mantle derived granites further to the southeast and south granitoids are more silicic with abundant deformed sequences orthogneisses diatexites and migmatites among others from the southern part of the lcb an rb sr intrusion age for the undeformed granites of 115 4 ma and a biotite cooling age of 90 2 ma were determined initial 87sr 86sr was determined to be 0 7054 and e nd values fall between 2 and 0 the isotopic data do not clearly allow us to characterise the deformed granitoids of the lcb as being intruded by the undeformed unit deformation could have been partly due to syn intrusive tectonics palaeomagnetic data suggest minor if any northward displacement of the lcb with respect to continental mexico corresponding to the rifting in the gulf of california since the late miocene and significant 35 45 clockwise rotations possible tilting evects which may have occurred given the dip of tertiary volcanics on top of the intrusives would modify these values for rotation and to a lesser degree of northward displacement geochemical isotopic and palaeomagnetic results and the cooling history of the lcb are similar to those from the puerto vallarta batholith whereas other mexican continental margin plutons further to the se display lower intrusion ages more rapid cooling rates and generally more primitive magma compositions in the light of the data presented a common magmatic evolution of the lcb and the puerto vallarta batholith is strongly suggested 2000 elsevier science b v all rights reserved

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

analog models of oblique rifting in a cold lithosphere new lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution basin segmentation and mantle exhumation occurring during rift localization we present two models one with a preexisting oblique lithospheric weakness model b and another with no weakness zone model a both oblique rifts have an obliquity of about 40 the main results are as follows 1 the fault populations especially during the early stages of deformation are composed of faults that in strike are largely intermediate between rift parallel and perpendicular to displacement this fault population is characteristic of oblique rifts observed in previous studies 2 in later stages faults parallel to the rift become numerous in both models buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift parallel faults 3 during the final stages of extension in model b the crust is deformed by rift parallel faults while in the basins the small scale deformation pattern is composed of displacement normal faults however in model a displacement normal faults tend to accommodate most of the extension controlling its final stages they probably also control the formation of the ocean continent transition any possible mantle exhumation as well as the geometry of oceanic accretion centers these results provide an insight into the possible evolution of the gulf of aden conjugate margins which developed in an oblique context and most probably without any preexisting rift parallel localizing heterogeneity in the lithosphere