geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

tectonic implications of rapid cooling of lower plate rocks from the buckskin rawhide metamorphic core complex west central arizona lower plate rocks exposed in the buckskin rawhide metamorphic core complex west central arizona underwent a pronounced increase in cooling rate 7 12 m y after the onset of extensional tectonic denudation at ca 27 ma a detailed 40 ar 39 ar study of lower plate rocks exposed near planet peak in the southwest part of the core complex indicates that the highest level mylonitic rocks exposed in the complex cooled below 350 c prior to 20 1 ma at an average rate of 15 c m y thereafter the cooling rate of these rocks increased progressively peaking at 280 80 c m y prior to initial exposure of the core between 15 1 and 13 3 ma published k ar 40 ar 39 ar and fission track ages for the most recently unroofed lower plate rocks exposed at the northeastern end of the core complex are consistent with a similar cooling history although the abrupt increase in cooling rate occurred as many as 5 m y later in this area because the increase in the cooling rate appears to have occurred progressively later toward the northeast it is unlikely to reflect changes in slip rate it appears to have coincided with cooling below 350 300 c and thus is interpreted to reflect steepening of the detachment system above the brittle ductile transition thermochronological data from lower plate rocks in the planet peak area together with constraints on their initial exposure at the surface are consistent with an increase in the dip of the detachment from le 5 at midcrustal depths where it was represented by a shear zone to at temperatures below 200 150 c lower plate rocks at both ends of the core complex probably cooled at rates 130 c m y which can not be reconciled with the average dip le 25 and slip rate 6 12 mm yr for the detachment fault given the likely geothermal gradient 25 c km during the early stages of extension this implies that cooling did not keep pace with denudation at upper crustal levels the very high cooling rates are interpreted to have primarily reflected the development of a pronounced thermal discontinuity across the detachment fault possibly 100 c as hot lower plate rocks were transported to shallow crustal levels rapid cooling of the lower plate at these levels may have been enhanced by interaction with fluids derived from the upper plate consistent with models that link the shallow level

the death valley turtlebacks reinterpreted as miocene pliocene folds of a major detachment surface determining the origin of extension parallel folds in metamorphic core complexes is fundamental to understanding the development of detachment faults an excellent example of such a feature occurs in the death valley region of california where a major undulatory detachment fault is exposed along the well known turtleback antiformal surfaces of the black mountains in the hanging wall of this detachment fault are deformed strata of the copper canyon formation new age constraints indicate that the copper canyon formation was deposited from 6 to 3 ma the formation was folded during deposition into a se plunging syncline with an axial surface coplanar with that of a synform in the underlying detachment this relation suggests the turtlebacks are a folded detachment surface formed during large scale extension in an overall constrictional strain field the present more planar black mountains frontal fault system may be the result of out stepping of a normal fault system away from an older detachment fa

detachment faults evidence for a low angle origin the origin of low angle normal faults or detachment faults mantling metamorphic core complexes in the southwestern united states remains controversial if sigma sub 1 is vertical during extension the formation of or even slip along such low angle normal faults is mechanically implausible no records exist of earthquakes on low angle normal faults in areas currently undergoing continental extension except from an area of actively forming core complexes in the solomon sea papua new guinea in light of such geophysical and mechanical arguments w r buck and b wernicke and g j axen proposed models in which detachment faults originate as high angle normal faults but rotate to low angles and become inactive as extension proceeds these models are inconsistent with critical field relations in several core complexes the rawhide fault an areally extensive detachment fault in western arizona propagated at close to its present subhorizontal orientation late in the tertiary extension of the region neither the wernicke and axen nor buck models predict such behavior in fact both models preclude the operation of low angle normal faults the authors recommend that alternative explanations or modifications of existing models are needed to explain the evidence that detachment faults form and operate with gentle dips

tertiary structure and thermal history of the harquahala and buckskin mountains west central arizona implications for denudation by a major detachment fault system the harquahala and buckskin mountains lie in the footwall of the whipple buckskin bullard detachment system in the harquahala mountains mesozoic fabric and structure are progressively more intensely overprinted by penetrative tertiary deformation toward the northeastern pan of the range tertiary mylonitic deformation is recognized by the presence of deformed miocene mafic dikes and characteristic textural features lineations in the mylonite trend 040 060 and megascopic kinematic indicators mostly indicate top to the northeast sense of shear the crest of an antiform defined by tertiary mylonitic fabric does not coincide with the topographic crest of the range reorientation of mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones as opposed to a single zone that was then bent the northeastern part of the brown s canyon granite acts as a large low strain lozenge in the southeast limb of the foliation arch mylonitic foliation along the southeast side of the range above this lozenge is strongly oblique to the trend of the detachment fault but the relative orientation is consistent with left oblique normal shear evidence of tertiary plastic deformation is absent southwest of sunset pass minor low angle normal faults in the southwestern harquahala mountains and little harquahala mountains dip northeast the low angle normal faults are cut by nw trending high angle right oblique faults in the central and southwestern harquahala mountains 40ar 39ar age spectra from k feldspar muscovite and hornblende and total gas ages from biotite indicate late cretaceous to early tertiary cooling to argon closure temperatures biotite and k feldspar from the area northeast of sunset pass record rapid early to middle miocene cooling reconstruction of the original geometry of the detachment system based on thermal differences indicated by contrasting cooling histories and the orientation of an early miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30 and 40 thus the harquahala mountains are a lilted block exposing of the order of a 10 km section through the pre tertiary crust heterogeneous proterozoic gneiss sparse paleozoic and mesozoic metasedimentary rocks and an oligocene plutonic complex are extensively overprinted by tertiary mylonitic fabrics in the buckskin mountains hornblende 40ar 39ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early tertiary time except in the vicinity of the oligocene plutonic complex feldspar and biotite 40ar 39ar cooling ages suggest that the footwall of the whipple detachment system experienced a more uniform cooling history in the buckskin mountains than in the harquahala mountains cooling ages between about 13 and 20 ma are recorded throughout the range with no consistent spatial pattern of ages the eagle eye detachment fault southeasternmost extension of the whipple buckskin bullard fault system becomes a transfer fault along the southeastern side of the harquahala mountains this fault separates two distinct extensional terranes on the northwest approximately 50 km of crustal extension has been accommodated on the whipple fault system while on the southeast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent big horn lake pleasant extended terrane as the deformation becomes more distributed maximum slip on any one normal fault decreases to the point where no single fault has enough slip to denude crustal levels at which tertiary plastic deformation has occurred

very high slip rates on continental extensional faults new evidence from u th he thermochronometry of the buckskin mountains arizona abstract new u th he in apatite ages suggest rapid slip rates on the tertiary buckskin rawhide detachment fault which crops out in the buckskin and rawhide mountains of western arizona single crystal u th he in apatite ages were determined for a suite of crystalline rock samples from the footwall to this detachment the samples were collected from a transect parallel to the fault slip direction and proximal to the fault zone they show a linear decrease in age with distance from 14 to 11 ma over 15 km the age versus distance relationship is interpreted to show a slip rate of 4 2 1 2 0 8 mm yr continuing for a period of 3 myr this rate is more or less in agreement with but more precise than previously published slip rates from this and other large offset extensional detachments such slip rates are alarmingly high being roughly 5 10 times faster than rates commonly observed on active structures at the earth s surface these rapid slip rates suggest that significant seismic hazard might be posed by some extensional structures and they raise questions regarding the relationship between such rapidly slipping large offset structures and the slower smaller offset faults that are more commonly observed at the earth s surface

the 40ar 39ar thermochronology of the eastern mojave desert california and adjacent western arizona with implications for the evolution of metamorphic core complexes mesozoic thickening and cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern mojave desert southeastern california and western arizona the application of 40ar 39ar thermochronology to rocks in this region provides information about the timing and nature of thrusting plutonism metamorphism denudation and detachment faulting the 40ar 39ar ages of 175 to 125 ma from the clipper piute turtle mohave bill williams and hualapai mountains are interpreted to be the result of a middle mesozoic thermal event s caused by crustal thickening the 40ar 39ar data from the clipper and piute mountains suggest that this thermal event was followed by a period of cooling at rates of 1 5 c m y orogenesis culminated during the late cretaceous when rocks exposed in the old woman piute chemehuevi and sacramento mountains attained temperatures 500 c which reset the k ar systems of minerals from proterozoic rocks high grade metamorphism in the old woman mountains area was caused by the intrusion of the old woman piute batholith at 73 1 ma cooling rates following batholith emplacement in the old woman mountains were 100 c m y between 73 and 70 ma and 5 10 c m y from 70 to 30 ma between 65 and 25 ma the entire eastern mojave desert underwent a period of cooling at a rate of 2 10 c m y by 30 ma rocks exposed in the old woman piute marble ship clipper and turtle mountains were below 100 c the 40ar 39ar ages from the sacramento mountains suggest that mylonitization caused by the onset of regional extension occurred at 23 1 ma when extension started in the chemehuevi mountains rocks exposed in the southwestern and northeastern portions of footwall to the chemehuevi detachment fault were at 180 c and 350 c respectively this suggests that the exposed part of the chemehuevi detachment fault initiated at a dip of 5 30 or as a series of higher angle faults that cut to a depth of 10 12 km and were later rotated to their present dip unroofing of the footwalls to detachment faults in the sacramento and chemehuevi mountains resulted in average cooling rates of 10 50 c m y between 22 and 15 ma

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

kinematics and timing of tertiary extension in the western lake mead region nevada explanation of the origin of the complex array of structures in some extensional terranes including folds and normal strike slip and reverse faults includes many models that implicitly assume kinematic compatibility between and contemporaneous operation of these structures we present new stratigraphic and age data from the highly extended western lake mead region nevada together with an analysis of fault kinematics technique of marrett and allmendinger 1990 to test the assumptions of kinematic compatibility and contemporaneity of structures in an area of excellent exposure and superb stratigraphic control our analysis indicates an overlapping but clearly distinct chronology of deformation early regional extension 18 13 5 ma is marked by development of a basin into which the middle miocene lower horse spring formation was deposited in the western lake mead region this basin was disrupted by more areally restricted post 13 ma normal and kinematically coupled right slip faulting along the las vegas valley shear zone kinematic analysis of faults indicates an average regional extension direction of nearly due west for the middle and late miocene extension and right slip faulting was followed by development of dominantly southvergent contractional structures including tight east plunging folds and east striking reverse faults these structures deform the post 8 5 ma muddy creek formation the muddy creek formation is not cut by the las vegas valley shear zone older faults including the eastern las vegas valley shear zone reactivated as south vergent reverse faults northeast striking left slip faults cut folds and reverse faults these observations show that north south shortening and left slip faulting postdate the major phase of extension and right slip faulting in the western lake mead area dynamic models that invoke either a single stress field or rotating stress fields to explain development of structures in the western lake mead area are inconsistent with the kinematic and age data similarly kinematic models that view all structures in the context of a single strain field are precluded by systematic cross cutting relationships that demonstrate at least partial diachroneity of deformational styles large magnitude extension south of the las vegas valley and lake mead fault zones appears to have been followed by north south contraction that was highly localized near the region of greatest extension we suggest that lateral pressure gradients arising from differential crustal thinning at the northern end of the colorado river extensional corridor may have provided the driving mechanism for localized contractional deformation

mesozoic and cenozoic extension recorded by metamorphic rocks in the funeral mountains california the funeral mountains metamorphic core complex like so many of these complexes in the north american cordillera contains early ductile structures related to mesozoic compressional tectonics that have been overprinted by structures related to later extension the core bounding detachment system formed in late miocene time and ductile extensional fabrics in its footwall have been viewed as evidence of a higher temperature stage in the same event integrated structural analysis and geochronology however indicate that ductile fabrics with a transport direction similar to the northwest direction of movement on the detachment are part of an older extensional event of late cretaceous age this event produced mylonitic fabrics a dominant foliation and well developed mineral and stretching lineations mesoscopic folds and a widespread crenulation lineation that developed during a late stage in this event also are consistent with northwest directed shear late cretaceous mylonitic fabrics are related to extensional shear zones within the core complex that have small stratigraphic offsets but may represent significant structural and metamorphic discontinuities rocks in the footwall of the shear zones experienced mid crustal depths that cannot be explained by the overburden available if the funeral mountains core rocks represent an intact block of mesozoic crust instead it appears that a significant section of mid crustal material may have been excised in late cretaceous time by the combined monarch canyon and chloride cliff shear zones as the result of gravitational instability of an overthickened crust in our interpretation this early extension was made possible by a waning of compression within the mesozoic thrust belt combined with a high regional heat flow due to the sierran arc this early extension took place 50 m y before development of the core bounding detachment system movement on which took place at low enough temperatures that deformation was predominantly brittle with some calc mylonite fabric development the youngest structures in the core are normal faults that cut the detachment and indicate a southwest transport direction orthogonal to those of both the earlier extensional events

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

development of extension parallel corrugations in the buckskin rawhide metamorphic core complex west central arizona extension parallel detachment fault corrugations are one of the defining features of metamorphic core complexes yet the origin of these corrugations remains disputed structural data from the buckskin rawhide metamorphic core complex in west central arizona demonstrate that prominent ne trending detachment fault corrugations are folds produced by extension perpendicular nw se shortening during the middle to late stages of core complex extension ca 18 10 ma kinematic data indicate that dominant ne directed slip on the buckskin rawhide detachment fault was locally overprinted by nw and se directed slip associated with corrugation folding the last motion recorded along several nw and se dipping flanks of the detachment fault is reverse slip directed toward the corrugation antiform hinges orientation patterns of upper plate bedding across the corrugations are compatible with folding about a ne trending axis extension perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding constriction and grain scale y axis shortening were most important during the later stages of mylonitization in the synextensional swansea plutonic suite which typically form l s tectonites upright meter and kilometer scale lower plate folds parallel the detachment fault corrugations and are best developed in well layered pre tertiary rocks most lower plate folding occurred by postmylonitic flexural slip that was coeval with detachment faulting the total amount of nw se shortening across the lower plate is 10 but the amount of nw se shortening recorded by the younger detachment fault is only 1 the relatively late stage development of corrugations in the buckskin rawhide metamorphic core complex suggests that the constriction and corrugation folding developed primarily in response to crustal thinning and tectonic denudation

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

miocene low angle normal faulting and dike emplacement homer mountain and surrounding areas southeastern california and southernmost nevada homer mountain and surrounding regions are within or adjacent to the western part of a broad region of low angle normal faults exposed within the lower colorado river trough during middle miocene time upper plate rocks in the homer sacramento dead and newberry mountains moved eastward or northeastward relative to the lower plate above single or multiple low angle normal faults deposition of coarse clastic sedimentary rocks occurred during extensional faulting and was accompanied by and closely followed by eruption of basaltic volcanics upper plate fault blocks of miocene volcanic and sedimentary rocks and older underlying crystalline rocks are tilted gently to steeply to the west or southwest low angle normal faults have complex sinuous traces due to the irregular form of fault surfaces antiformal warping and uplift of the lower plate about a north south trending axis due to tectonic denudation and isostatic rebound divided the regionally east thickening extensional allochthon into separate synformal and wedge shaped components the homer mountain area lies within or adjacent to the synformal component lower plate rocks are intruded by numerous middle miocene east west trending dikes which are in turn intruded by subhorizontal dikes both sets of dikes are cut and displaced by low angle normal faults except on the west flank of the newberry mountains where an east west trending dike cuts the basal low angle normal fault k ar geochronologic and field data establish the approximate synchroneity of dike emplacement and low angle faulting the state of stress in the lower plate as inferred from dike orientations differed radically from the state of stress in the upper plate as inferred from fault geometry low angle faulting and east northeast west southwest distension of upper plate rocks reflect regional reduction of compression in the east northeast west southwest direction and associated large scale east northeast west southwest crustal extension in contrast concave upward flexure of the lower plate in response to tectonic denudation and resultant isostatic uplift is inferred to have produced local subhorizontal compression at shallow crustal levels in the lower plate that overwhelmed the regional extensional stress and prevented emplacement of dikes oriented perpendicular to the direction of regional extension north south crustal extension during dike emplacement appears to have resulted from minor divergence of lower plate crustal blocks as they were displaced in a west southwest direction away from the colorado plateau

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

hurricane maximum intensity past and present hurricane intensity forecasting has lagged far behind the forecasting of hurricane track in an effort to improve the understanding of the hurricane intensity dilemma several attempts have been made to compute an upper bound on the intensity of tropical cyclones this paper investigates the strides made into determining the maximum intensity of hurricanes concentrating on the most recent attempts to understand the maximum intensity problem the theories of holland and emanuel are reviewed with the objective of assessing their validity in real tropical cyclones each theory is then tested using both observations and the axisymmetric hurricane numerical models of ooyama and emanuel it is found that ambient convective instability plays a minor role in the determination of the maximum intensity and that the emanuel model is the closest to providing a useful calculation of maximum intensity several shortcomings are revealed in emanuel s theory however showing the need for more basic research on the axisymmetric and asymmetric dynamics of hurricanes as an illustration of the importance of asymmetric vorticity dynamics in the determination of a hurricane s maximum intensity it is shown using ooyama s hurricane model that the maximum intensity of a tropical cyclone may be diminished by convectively generated vorticity anomolies excited outside the primary eyewall the vorticity anomolies are parameterized by adding a concentric ring of vorticity outside the primary eyewall that acts to cut off its supply of angular momentum and moist enthalpy it is suggested that the generation of vorticity rings or bands outside the primary eyewall is a major reason why tropical cyclones fail to attain their maximum intensity even in an otherwise favorable environment the upshot of this work points to the need for obtaining a more complete understanding of asymmetric vorticity processes in hurricanes and their coupling to the boundary layer and convection

mechanisms for the formation of domal and basinal detachment faults a three dimensional analysis detachment faults i e regional low angle normal faults in metamorphic core complexes of the north american cordillera commonly exhibit doubly plunging antiformal domal and synformal basinal geometries these geometries have been previously attributed to the superposition of two processes 1 the antiforms and synforms with axes parallel to the extension direction originated as primary fault undulations or corrugations developed coevally with fault slip and 2 the antiforms and synforms with axes perpendicular to the extension direction were produced by isostatic uplift due to tectonic denudation however the coaxial relationship between undulations of detachment faults and the sedimentary beds and metamorphic foliations in both the upper and lower plates of some detachment fault systems in the u s cordillera suggests that the synforms and antiforms with axes parallel to the extension direction may have formed as folds and that the domal and basinal geometry of detachment faults may have formed synchronously by a single process a three dimensional elastic thin plate model is developed to investigate possible mechanisms for the formation of domal and basinal detachment faults this model explores the interactions among the vertical horizontal and basal shearing forces during the formation of warped detachment faults the model considers the role of compression perpendicular to the extension direction and stress reduction parallel to the extension direction the results of the model suggest that the stress reduction itself in the extension direction is insufficient to cause buckling of a thin elastic crust two mechanisms are most likely to explain the formation of domal and basinal geometries of detachment faults 1 an upward pushing by undulatory crustal roots or buoyant synextensional plutons beneath the extensional belt and 2 buckling caused by a compression perpendicular to the extension direction both mechanisms are consistent with the geologic constraints during the development of cordilleran core complexes in addition to isostatic uplift due to tectonic denudation upward warping of detachment faults can also be caused by buckling basal shearing and buoyant forces

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

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

tertiary sedimentary rocks associated with the harcuvar core complex in arizona u s a insights into paleogeographic evolution during displacement along a major detachment fault system abstract geologic mapping and analysis of oligocene miocene sedimentary rocks in the upper plate of the buckskin rawhide detachment fault system west central arizona reveals a complex paleogeographic history during fault displacement involving shifting sediment source areas and multiple drainage reversals within the study area four upper plate fault blocks are capped by homoclinal sedimentary sections that display fanning dip relationships indicating concurrent tilting and sedimentation during deposition of these strata upper plate sedimentation was intermittently confined to separate half graben while at other times rapid rates of aggradation relative to fault displacement resulted in burial of ridges separating sub basins four sedimentary assemblages recognized within the study area can be correlated between fault blocks on the basis of lithologic similarity stratal position provenance paleocurrent data and sparse geochronologic constraints the basal assemblage consists of lacustrine rocks and interfingering fluvial strata composed of detritus derived from the granitic terrane surrounding the northern part of the study area these sediments were deposited during the earliest stages of tilting of upper plate fault blocks and inferred displacement along the buckskin rawhide detachment fault system the lower assemblage which directly overlies the basal assemblage consists of fine grained lacustrine deposits sandy conglomerate and breccia during lower assemblage deposition mass flow dominated alluvial fans were derived from source areas consisting mainly of mesozoic and paleozoic rocks exposed to the south of the study area these source areas may have been associated with the breakaway fault of the detachment system or with fault blocks stranded above structurally elevated parts of the footwall tilting of fault blocks continued during deposition of the lower assemblage and strongly affected dispersal patterns and lithofacies distributions lacustrine sediments were deposited mainly within large deep sub basins and with few exceptions lower assemblage streams followed half graben axes the middle assemblage consists of conglomerate and sandstone deposited by an extensive south directed stream system that probably flowed off undistended parts of the hanging wall across extended parts and locally onto the footwall tilting of upper plate fault blocks continued during middle assemblage deposition but rapid burial of escarpments by sediment permitted relatively unobstructed southward drainage the upper assemblage consists of sandy conglomerate deposited by a northeast directed system of broad shallow streams these deposits display a variety of clast types including tertiary mylonitic sedimentary and volcanic rocks that were eroded from the upwarped footwall of the core complex and overlying klippen upper assemblage streams maintained relatively consistent northeasterly flow even during the early stages of deposition when some fault blocks were still rotating

zircon u pb hf isotopes and whole rock geochemistry of gneissic granites from the jitang complex in leiwuqi area eastern tibet china record of the closure of the paleo tethys ocean abstract previous investigations have indicated that the changning menglian cmsz and longmu co shuanghu lssz suture zones represent the paleo tethys ocean of northern tibet and western yunnan of china respectively however our understanding of the paleo tethys suture zone in eastern tibet seems to be incomplete the north lancangjiang suture zone nlsz has been hypothesized to mark the paleo tethys ocean in eastern tibet the present study reports new zircon u pb ages and hf isotope data and whole rock major and trace element data from the gneissic granites from the jitang complex in the leiwuqi area of the nlsz in order to verify this hypothesis six samples of gneissic granites yielded weighted mean 206 pb 238 u ages of 249 237 ma the geochemical data show that the gneissic granites are characterized by high sio 2 70 79 77 75 wt k 2 o 2 00 6 13 wt low a cnk values hf t values from 2 7 to 2 5 and t c dm model ages 1447 1009 ma indicate that they were generated by the partial melting of the meso proterozoic juvenile crust these gneissic granites were interpreted to have formed in the post collisional tectonic setting related to the closure of the paleo tethys ocean by combining previously published and the new data we propose that the nlsz connects the lssz and cmsz in the tibetan plateau and marks a paleo tethys ocean basin in eastern tibet

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

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

development of active low angle normal fault systems during orogenic collapse insight from tibet active north trending rifts in tibet vary significantly in their character as a function of extension magnitude most rifts are characterized by internally drained basins bounded by high angle normal faults with paleogene or older rocks in the footwall however the central part of the yadong gulu rift near lhasa and the newly documented lunggar rift in west central tibet are bounded by low angle normal faults detachments with mylonitic rocks and miocene granites in the footwall and exhibit active basin incision and intrabasin topographic highs in areas of inferred maximum extension we suggest that tibetan rifts initiate as high angle normal fault and half graben or graben basin systems and evolve in response to increasing extension and footwall isostatic rebound into detachment systems that are active at uppermost crustal levels and above which rift basin fill is being uplifted and eroded

geologic continuous casting below continental and deep sea detachment faults and at the striated extrusion of sacsayhuaman peru in one common type of industrial continuous casting partially molten metal is extruded from a vessel through a shaped orifice called a mold in which the metal assumes the cross sectional form of the mold as it cools and solidifies continuous casting can be sustained as long as molten metal is supplied and thermal conditions are maintained i propose that a similar process produced parallel sets of grooves in three geologic settings as follows 1 corrugated metamorphic core complexes where mylonitized mid crustal rocks were exhumed by movement along low angle normal faults known as detachment faults 2 corrugated submarine surfaces where ultramafic and mafic rocks were exhumed by normal faulting within oceanic spreading centers and 3 striated magma extrusions exemplified by the famous grooved outcrops at the inca fortress of sacsayhuaman in peru in each case rocks inferred to have overlain the corrugated surface during corrugation genesis molded and shaped a plastic to partially molten rock mass as it was extruded from a moderate to high temperature reservoir

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

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

extension parallel folding in the scandinavian caledonides implications for late orogenic processes abstract structural analyses of the devonian basins and basement rocks in the scandinavian caledonides show the existence of large scale e w oriented folds which affect the para autochthonous western gneiss region of the baltic craton these folds mostly occur in areas of widespread and intense late orogenic extension chronology based on structural relationships both within the devonian sediments and caledonian rocks shows that folding has been active from the first stages of the ductile extension and ended after devonian basin sedimentation folding evolved through time towards crustal scale transcurrent left lateral faulting in the zones of maximum deformation such an evolution is explained by progressive unloading of the western gneiss region due to extension which involved a permutation of the maximum stress from a vertical position at the onset of extension to a horizontal position in the subsequent stages folding parallel to late orogenic extension as well as left lateral translation on east to northeast trending crustal scale faults suggest the occurrence of a regional direction of compression perpendicular to the direction of extension this in turn argues for an external force triggering the late orogenic extensional process in the scandinavian caledonides