effect of lithospheric thickness on the formation of high and low angle normal faults it is accepted that extension of a homogeneous brittle layer should produce high angle normal faults the rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness new model calculations indicate that a fault cutting lithosphere thicker than 10 20 km can build up only a few kilometres horizontal throw before a new fault replaces it such a small offset leaves the fault little rotated from its initial high dip angle for thinner lithosphere the fault offset may be much larger leading to rotation of the upper inactive parts of a high angle fault to a low dip angle in continents this can happen where thin upper crustal lithosphere is decoupled from mantle lithosphere by weak lower crust the calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low angle fault structures this prediction is consistent with the occurrence of low angle faults in areas with higher than normal heat flow

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

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

geotechnical properties of rock masses their control on slope form and mechanisms of change along the napier range western australia abstract slope form and the mechanisms of change which slope profiles exhibit are frequently subject to scrutiny by geomorphologists however the majority of studies do not consider fundamental material properties and rock mass geotechnical characteristics the results presented here highlight the importance of synthesisistandard geomorphological site investigation techniques with quantifiable rock geotechnical parameters in order to understand slope form and development field research has been undertaken along the napier range of the kimberly region western australia the napiers are an extensive upraised devonian limestone reef along which a number of characteristic slope profiles can be identified laboratory studies of yield strength stress strain characteristics elastic properties and discontinuity parameters have been conducted on material sampled at sites representative of each slope profile type the results suggest that highly concave slopes have formed in limestone which exhibits little deformation before yield has a high modulus of elasticity and few irregular fractures convexo concave slopes on the other hand are characteristic of material which displays a greater ability to strain below yield a relatively low modulus of elasticity and a pronounced discontinuity pattern it is the combination of field investigation and laboratory study which most successfully explains variations in slope form

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

the comparison of topographic long profiles of gullies on earth to gullies on mars a signal of water on mars the topographic signature of a landform can give important clues as to its formation process here we have used topographic long profiles to study the process of gully formation on mars we studied topographic long profiles of gullies on earth to 1 confirm that previously published generalisations of how long profile shape varies with process also applies at the kilometre scale of martian gullies and 2 use as a direct comparison with the martian data we have compared 24 fluvial and 22 debris flow long profiles of terrestrial gullies derived from laser altimeter and gps measurements to 78 long profiles of a range of gullies on mars derived from a stereo photogrammetry point matching technique we have confirmed that this manual point matching technique is reliable for the martian data by comparison with full digital elevation models we used nine different characteristics of the long profiles including slope and curvature parameters to perform a canonical discriminant analysis which allowed us to identify the variables most important for differentiating between fluvial and debris flow gullies on earth in agreement with published literature for larger scale features we found that terrestrial debris flow gullies tend to be steeper and less concave than fluvial gullies we have found that gully long profiles on mars can resemble long profiles of terrestrial gullies formed by either fluvial or debris flow processes with slightly more affinity to fluvial systems gullies on mars can only be weakly separated from those on earth they can be separated from terrestrial fluvial gullies on curvature parameters and from terrestrial debris flow gullies by slope parameters in addition we have found that different alcove types identified from planview morphology are also distinctive in terms of their long profile morphology gullies which incise back into the bedrock are more similar to terrestrial debris flows whereas polar pit gullies are most similar to terrestrial fluvial gullies our findings suggest that the presence of a bedrock alcove promotes debris flow behaviour in gullies on mars

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

supradetachment basin evolution during continental extension the aegean province of western anatolia turkey we present a detailed structural and stratigraphic record of a neogene quaternary supradetachment sedimentary succession in the aegean extensional province of western anatolia and we compare its tectonic features and evolution to those of other well documented supradetachment basins around the world the sedimentary fill of the alasehir basin records the uplift and exhumation of a core complex in the footwall of a detachment fault within the central menderes massif accumulation of footwall derived clastic sediments in this basin started ca 20 ma shortly after the initiation of the approximately e w trending alasehir detachment and its shear zone and continued until ca 2 ma major sedimentary facies types include fluvial and alluvial fan deposits debris flow and mass flow deposits and locally developed lacustrine rocks these sedimentary units were accumulated largely in distal depocenters within the extending basin as the low angle 15 28 detachment faulting created little accommodation space near the basin margins while producing high back shed topography in the uplifted menderes core complex the drainage system was dominated mainly by extension parallel transverse streams during the main phases of basin evolution extension parallel scissor hinge faulting produced differential uplift and subsidence in the adjacent fault blocks changed the direction of sediment transport and drainage patterns over short distances and resulted in the local uplift of the older basin strata these processes led to the development of subbasins with lateral variations in basement topography strata thickness and sedimentary facies distribution and generated a segmented basin architecture high angle synthetic and antithetic faults that formed extensively after 3 ma caused back tilting of the sedimentary strata formation of half grabens with their own axial drainage systems and development of angular unconformities with the onset of this crustal scale block faulting the detachment fault ceased to operate and the quaternary gediz graben started to develop at the northern end of the alasehir supradetachment basin our comparative evaluation of select basins shows a maximum sediment thickness of 3 km average extension rates of 6 to 8 9 mm yr 1 and accumulation rates of 0 1 0 2 mm yr 1 uncorrected for compaction in supradetachment basins in general the rates and amounts of extension the geometry of detachment faulting the rates of footwall uplift and the kinematics and interplay of different fault systems are the most important factors controlling three dimensional structural architecture and evolution of supradetachment basins

the influence of hillslope gradient regolith texture stone size and stone position on the presence of a vesicular layer and related aspects of hillslope hydrologic processes a case study from the australian arid zone abstract a near surface layer of vesicles may be found in many dryland soils often below a stony desert pavement during and after rain the uppermost part of the regolith develops a surface seal trapping air which is compressed and which may also subsequently expand as a result of solar heating this results in the deformation of the soft regolith and the formation of a vesicular layer just below the surface the work reported here was undertaken to determine the influence of slope gradient regolith texture stone size and depth of stone embedment on the presence of a vesicular layer in arid australia this appears to be the first investigation of the role of topography in the formation of soil vesicularity the four factors listed typically vary systematically with position on a hillslope the data collected indicate that the development of a vesicular layer is inversely related to hillslope gradient and size of the stone positively related to the amount of silt and clay in the regolith and weakly related to the depth to which the stone is embedded field evidence indicates that the vesicular layer becomes much more extensive on the lower parts of hillslopes in the study area and thus so too must the choke on water penetration that such layers create the spatial distribution of vesicular layers thus constitutes a factor that has traditionally been overlooked in studies of dryland hillslope hydrology

surface stone cover on desert hillslopes parameterizing characteristics relevant to infiltration and surface runoff surface stones are a common feature of desert hillslopes they influence slope processes ranging from the interception and partitioning of rainwater to the dislodgement and transport of regolith particles the aspects of a stone mantle which relate to these various influences are diverse and include surface cover fraction dimensions of the infiltration annulus and geometric and packing effects analyses of the characteristics of stone cover at two study sites were made using photographic methods and are used to evaluate different means of reporting mean stone sizes it is found that mean diameter based on weight as conventionally employed in sieve analysis is insensitive to important stone cover characteristics and may be influenced greatly by uncommon large stones similarly mean diameters based on count become inappropriate if large numbers of small stones dominate the calculated mean concealing other site to site differences in stone cover as a more appropriate parameterization of stone geometry the use of mean diameters weighted by stone surface area and perimeter length are proposed these weighting factors have the advantage that they relate closely to stone characteristics relevant to slope hydrology and erosion processes

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

relation between infiltration and stone cover on a semiarid hillslope southern arizona abstract a knowledge of the spatial distribution of infiltration rates is essential to the application of realistic distributed rainfall runoff models in semiarid areas one method of determining this distribution is to develop a predictive equation based on a readily observable surface property such as stone cover previous studies of the relation between infiltration and stone cover on shrub covered semiarid hillslopes have yielded both positive and negative correlations it is suggested that positive correlations result where infiltration measurements are confined to areas between large shrubs negative correlations on the other hand reflect pronounced shrub intershrub differences in infiltration and stone cover and are found where both shrub and intershrub areas are sampled in this study of a semiarid hillslope in southern arizona both shrub and intershrub areas are sampled and a negative correlation is obtained between infiltration and stone cover this correlation arises from the fact that under shrubs fine sediments have accumulated primarily as a result of differential splash whereas between shrubs such sediments have been selectively removed by a combination of rainsplash and overland flow leaving behind a gravel lag consequently infiltration rates are higher under shrubs than between them owing to the higher percentage of sand in the surface soil the larger quantity of organic matter both within the soil and on its surface and the greater digging and burrowing by animals in addition simulated rainfall experiments imply that shrubs with moderately dense canopies are more effective than adjacent surface gravels in dissipating the kinetic energy of raindrops and thereby reducing surface sealing and promoting infiltration inasmuch as the field site is typical of gentle shrub covered piedmont hillslopes over much of the sonoran desert inverse relations between infiltration rate and stone cover are probably characteristic of such hillslopes and may be used to predict the spatial distribution of infiltration rates for inclusion in rainfall runoff models

boulder weathering and erosion associated with a wildfire sierra ancha mountains arizona an april may 2000 coon creek fire burned 37 5 km 2 of the sierra ancha mountains 32 3 km miles north of globe az including 25 sandstone and 19 diorite boulders surveyed in 1989 and resurveyed after the burn after the summer 2000 monsoon season and after the winter 2001 season when viewed from the perspective of cumulative eroded area both sandstone and diorite displayed bimodal patterns with 79 of sandstone boulder area and 93 of diorite boulder area undergoing either no fire induced erosion or fire induced erosion 76 mm when stretched over cumulative boulder areas erosion due to the fire averaged 26 mm for sandstone and 42 mm for diorite post fire erosion from thunderstorm summer rains averaged i mm for 5 diorite and i mm for 10 sandstone boulders while only a single diorite boulder eroded an average of 1 2 mm after the winter winter erosion removed an average of 5 5 mm from 14 sandstone boulders thus fire appears to increase a rock s susceptibility to post fire weathering and erosion processes as predicted by goudie et al earth surf process landf 17 1992 605 in contrast to experimental research indicating the importance of size in fire weathering no statistically significant relationship exists between erosion and boulder height or boulder surface area a result similar to zimmerman et al quat res 42 1994 255 these data exclude 12 original sites and 85 boulders at sites impacted by the fire that could not be relocated with a reasonable cause for the lack of relocation being boulder obliteration by the fire given evidence from 1 0 be and 2 6 al cosmogenic nuclides earth planet sci lett 186 2001 269 supporting the importance of boulders in controlling evolution of nonglaciated bouldered landscapes geol soc amer bull 76 1965 1165 fire obliteration of boulders could be an important process driving drainage evolution in nonglaciated mountains

effects of rock fragments on soil erosion by water at different spatial scales a review this paper reviews the various effects of rock fragments on soil erosion by water since these effects are scale dependent they are investigated at three different nested spatial scales the microplot 4 10 6 100m2 the mesoplot 10 2 102 m2 and the macroplot 101 104 m2 for each scale the corresponding process mechanisms are discussed particular attention is paid to the effects of rock fragment cover on the intensity of soil erosion processes at the mesoplot scale i e on interrill areas rock fragments at the soil surface can have negative as well as positive effects on sediment yield these ambivalent effects are conditioned by the type of fine earth porosity soil surface slope vertical position and size of rock fragments and by the occurrence of horseshoe vortex erosion at the microplot scale i e the soil surface area which is covered by a single rock fragment and at the macroplot scale i e upland areas where both interrill and rill erosion takes place rock fragments at the soil surface have a negative effect on sediment yield in these two scales rock fragments can thus be considered as natural soil surface stabilizers at the macroplot scale the mean decrease of relative interrill and rill sediment yield with rock fragment cover can be expressed by an exponential decay function the scatter of the data indicates that a given rock fragment cover can have different efficiencies in reducing interrill and rill sediment yield depending on the varying intensities of the hydrological and erosion subprocesses these findings have implications for erosion modelling and soil conservation

timing of detachment faulting in the bullfrog hills and bare mountain area southwest nevada inferences from 40ar 39ar k ar u pb and fission track thermochronology crustal extension in the bullfrog hills and bare mountain area of southwest nevada is associated with movement along a regional detachment fault normal faulting in the upper plate and rapid cooling denudation of the lower plate were coeval with miocene silicic volcanism and with west northwest transport along the detachment fault a west northwest progression of tilting along upper plate normal faults is indicated by ages of the volcanic rocks in relation to angular unconformities near the breakaway tilting in the upper plate occurred between 12 7 and 11 6 ma continued less strongly past 10 7 ma and was over by 8 2 ma ten to 20 km west of the breakaway tilting occurred between 10 7 and 10 33 ma continued less strongly after 10 33 ma and was over by 8 1 ma the cooling histories of the lower plate metamorphic rocks were determined by thermochronologic dating methods k ar and ar 40 39 a on muscovite biotite and hornblende ar 40 39 a on k feldspar u pb on apatite zircon and sphene and fission track on apatite zircon and sphene lower plate rocks 10 km west of the breakaway cooled slowly from early cretaceous lower amphibolite facies conditions through 350 50 degrees to 300 50 degrees c between 57 and 38 ma then cooled rapidly from 205 50 degrees to 120 5o degrees c between 12 6 1 6 and 11 1 1 9 ma lower plate rocks 20 km west of the breakaway cooled slowly from early cretaceous upper amphibolite facies conditions through 500 50 degrees c at 78 67 ma passed through 350 50 degrees to 300 50 degrees c between 16 3 0 4 and 10 5 0 3 ma then cooled rapidly from 285 50 degrees to 120 50 degrees c between 10 2 and 8 6 ma upper plate tilting and rapid cooling denudation of the lower plate occurred simultaneously in the respective areas the early slow cooling part of the lower plate thermal histories was probably related to erosion at the earth s surface which stripped off about 9 km of material in 50 to 100 m y the results indicate an initial fault dip greater than or equal to 30 degrees and a 12 mm yr 1 west northwest migration of the locus of rapid tilting in the upper plate

miocene synextension sedimentation patterns whipple mountains southeastern california implications for the geometry of the whipple detachment system a sedimentary basin formed in the southern and eastern whipple mountains during early to middle miocene detachment faulting stratigraphic relations and facies patterns indicate that the shape and size of the sedimentary basin was controlled both by fault activity and by sedimentary processes high angle normal faults formed the southwestern and perhaps also the northeastern boundary of the half graben whereas a transfer fault probably formed the southeastern boundary the transfer fault influenced the earliest synextension sedimentation in the basin the location of this fault and similar faults in the mohave mountains and aubrey hills of arizona is inferred to have been controlled by the position of primary irregularities or corrugations of the detachment fault shifts in facies patterns particularly a shift in the position of the basin depocenter reflect changes in basin geometry comparison with adjacent areas is necessary to distinguish between tectonic and nontectonic causes a shift in depocentral position parallel to regional transport direction in the whipple mountains basin is interpreted as resulting from a change in the location of the high angle normal fault forming the southwestern margin of the basin whereas a shift perpendicular to transport direction probably resulted from overtopping of topographic barriers within a fault block the most active phase of faulting occurred prior to deposition of stratigraphic units younger than the 18 5 ma peach springs tuff relative uplift of the core of the whipple mountains probably as a result of isostatic rebound following tectonic denudation influenced sedimentation patterns after deposition of the peach springs tuff

middle eocene sedimentary and volcanic infilling of an evolving supradetachment basin white lake basin south central british columbia the middle eocene white lake and skaha formations in the white lake basin british columbia record the sedimentary and volcanic infilling of a supradetachment basin that developed during the latter stages of shuswap metamorphic core complex exhumation the 1 1 km thick white lake formation is characterized by volcanogenic sediment gravity flow fluvial and sheetflood facies interbedded with volcanic deposits facies relations suggest white lake strata accumulated on coalesced west sloping alluvial fans that drained an active volcanic center the overlying 0 3 km thick skaha formation records increased tectonism and mass wasting pervasively shattered skaha avalanche slide and sheetflood deposits accumulated on alluvial fans shed from hanging wall and footwall sources exposed along the okanagan valley fault clast compositions of the white lake and skaha formations record alluvial and tectonic stripping that locally eliminated hanging wall blocks mylonite clasts in upper skaha beds imply significant okanagan valley fault footwall uplift during the middle eocene and syntectonic erosion of the shuswap metamorphic core complex the syntectonic sedimentary record preserved within the white lake basin elucidates the relations and timing between core complex exhumation and extensional tectonism in this region the white lake and skaha formations are the apparent age equivalent of the klondike mountain formation of northern washington usa white lake basin strata however are more complexly interstratified post depositionally disrupted and contain a more complete record of core complex unroofing variations in the spatial distributions and textural and compositional character of middle eocene strata in this area underscore the need to exercise care when developing regional scale sedimentary tectonic volcanic models

relationship between mesozoic and cenozoic tectonic features in west central arizona and adjacent southeastern california the maria fold and thrust belt mftb is a narrow belt of mesozoic crustal shortening that trends east west across west central arizona and adjacent southeastern california it is characterized by generally south vergent folds and thrusts that commonly displace proterozoic crystalline rocks over deformed and metamorphosed paleozoic and mesozoic strata the mftb is cut by a south to southeast trending belt of mid tertiary extensional deformation extension was characterized by large displacements on low angle normal faults known as detachment faults and by isostatic uplift of mylonitic midcrustal rocks now exposed in metamorphic core complexes the geometry and style of extensional deformation change along the extensional belt and reveal the influence of the mftb several extensional features are spatially coincident with the root zone of mftb thrusts 1 an areally extensive west northwest trending belt of denuded tertiary mylonitic fabrics in the whipple and harcuvar metamorphic core complexes 2 a style of extension characterized by minimum extensional dismemberment of the upper plate and maximum denudation and uplift of deep seated lower plate rocks 3 an abrupt bend in the belt of arched uplifted rocks below detachment faults and 4 an abrupt bend in the trend of the breakaway zone of the detachment faults and an associated lateral ramp in the detachment fault system the first two features may be the result of isostatic uplift of a crustal root downward protruding moho bulge inherited from crustal thickening within the mftb and the other two features may reflect at the time of detachment fault initiation the influence of stresses in the crust associated with flexural support of the buoyant mesozoic crustal root

characteristics of stone covers on the surface of basalt flows in arid northeast jordan variations in the stone covers on desert surfaces have implications for the operation of hydrological processes some recent attention has been given to the difficulties of identifying and quantifying relevant characteristics of stone covers investigation of how such characteristics vary in relation to geological and topographical controls are limited results are presented of a field study of the characteristics of stone covers on four lithologies in the late tertiary to quaternary basalt plateau of northeast jordan marked variations occur in clast dimensions and the characteristics of sorting between lithologies which are likely to affect hydrological response a more subtle control relating to slope conditions is superimposed on the lithological differences r and q mode factor analysis identifies groupings within the data which demonstrate the distinctiveness of different lithologies characteristics of the stone covers on sites affected by human occupation are markedly different from undisturbed sites on the same lithology

the initiation and growth of gullies in madagascar are humans to blame abstract lavaka madagascar s unusual gullies require thick lateritic regolith on smoothly convex hills and they are initiated locally by a variety of cultural and natural proximate causes such as paths roads defensive trenches around hillforts steep fields unchanneled to slightly concentrated storm run off springs mass movements etc growth continues by rain attack incision by run off earthfalls caused by wetting and drying cycles and especially basal sapping by diffuse groundwater flow above bedrock somewhat in that order lavaka are less directly permitted and promoted by 1 hardening of exposed laterite surfaces which creates a hard soft hard profile from bedrock to laterite which favors incision relative to lateral erosion 2 the superimposition of concave run off profiles onto convex hills and locally 3 re equilibration of watersheds after stream piracy and faulting run off and hardening are interrelated with devegetation because hardening results from exposure following devegetation because both hardening and devegetation increase run off relative to infiltration and because hardening rain attack and run off impede vegetation devegetation is certainly worsened by harmful human activities such as range burning forest cutting and or overgrazing but tectonism and natural climatic aridification are probably ultimately more important given that many lavaka are simply a natural part of the landscape s evolution and that some lavaka clearly predate primary uncut rain forest

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

detached strata in a tertiary low angle normal fault terrane southeastern california a sedimentary record of unroofing breaching and continued slip miocene sedimentary strata exposed in the eastern chemehuevi mountains southeastern california record development of an evolving low angle normal fault system the sequence includes more than 1 km of conglomerate and sandstone with rare interbedded monolithologic breccia and volcanic flows clasts of peach springs tuff in basal units indicate that this succession is younger than 18 ma these rocks have been displaced by a regionally extensive low angle normal fault the chemehuevi detachment and are folded and faulted structural reconstructions and the character of associated fault rocks indicate that the chemeheuvi fault was initiated at a low angle and that the footwall was progressively unloaded through thinning and displacement of its cover during extensional deformation the syntectonic sedimentary rocks described here provide evidence that movement continued on the gently dipping 15 sup 0 fault even after part of the fault was breached and the footwall eroded the conglomerates and sandstones were deposited by stream flow and debris flow on alluvial fans synsedimentary faulting is suggested by angular discordance below one monolithologic breccia bed and by local coarsening upward sequences clast types reveal progressive unroofing of hanging wall rocks to exposer the chemehuevi fault zone from which chloritic brecciated granite clasts were derived clasts weremore then derived from both the hanging wall and the footwall footwall debris being dominant high in the section distinctive clasts show that late displacement on this evolving fault system was on the order of 1 to 5 km less

large rock avalanche deposits eastern basin and range utah emplacement diagenesis and economic potential large rock avalanche deposits are a common component of the basin fill within the extensional tectonic terrain of the basin and range these deposits recently have been interpreted to host oil and gas within the railroad valley area of eastern nevada large blocks of brecciated bed rock are a primary component of these avalanche deposits and are potentially excellent oil and gas reservoirs our work provides further insight into the emplacement and economic potential of these deposits exposed large rock avalanche deposits of the miocene oak city formation on the western margin of the canyon range utah contain coherent breccia blocks up to 3 5 km long 1 km wide and 200 m thick these deposits were derived from the near vertical dipping bed rock of the adjacent canyon range and now are exposed as much as 5 5 km from the range front within the sevier desert basin emplacement was relatively rapid as indicated by three well developed breccia facies within the carbonate breccia blocks stratigraphically from the base the facies include 1 matrix rich breccia 2 jigsaw breccia and 3 crackle breccia the deposits were cut and segmented by a series of syndepositional normal faults that developed during late miocene and post miocene extension primary porosity w s reduced by cement soon after burial cathodoluminescence cement patterns indicate that initially the basinward breccia blocks were more deeply buried relative to the water table than the breccia blocks proximal to the canyon range after initial cementation the basinward blocks were uplifted relative to the water table secondary porosity approaches 8 in the carbonate blocks and is greater than 14 within the jigsaw breccia the size and porosity of these breccia blocks indicate their potential as reservoir targets

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

the role of attenuation in the formation of the railroad valley structural basin east central nevada detachment control of petroleum reservoirs the eastern boundary of railroad valley has long been considered a large displacement steep normal fault by mapping a low angle fault called the white pine detachment wpd from the ranges into the valley we show that steep normal faulting was not a significant process in the formation of railroad valley the wpd or its equivalent extends throughout the white pine and grant ranges as shown by mapping of previous authors well data show that it is also present in railroad valley our structure contour map of the wpd based on surface and subsurface data shows that the wpd dips uniformly from the ranges into railroad valley and is not displaced significantly by high angle faulting even if high angle faulting were assumed the maximum displacement consistent with the data would be 2000 ft 610 m much smaller than the total structural relief of at least 16 000 ft 4880 m between the grant range and the adjacent valley structural elevations in the valley are low because the paleozoic section has been greatly attenuated a related contribution to structural relief is uplifting of the ranges caused by diapiric emplacement of plutons in response to tectonic unloading the wpd is only one of many detachments in railroad valley we propose that most or all of the oil reservoirs are in attenuated brittle blocks between detachments seals are provided by detachments or by the discontinuous nature of associated fracturing attenuation by juxtaposing hot infrastructure rocks with rocks that normally occur thousands of feet above the granitic and metamorphic basement may have contributed to maturation of source rocks begin page 1154

structural and stratigraphic development of the whipple chemehuevi detachment fault system southeastern california implications for the geometrical evolution of domal and basinal low angle normal faults recent debates on the development of detachment faults during continental extension have been centered on whether they were active at low angles and initiated as relatively planar faults in addition intense investigations have concentrated on how the detachment fault geometry evolved in time and space and how the extension was accommodated in their upper and lower plates the superb exposure of the mid tertiary whipple mountains detachment fault system in southeastern california provides an unusual opportunity to address these questions analysis of the stratigraphy and structures of upper plate rock units constrains the three dimensional kinematic evolution of the mid tertiary whipple chemehuevi detachment fault system in southeastern california tertiary volcanic strata in the northwestern whipple mountains and central chemehuevi valley consist of similar rock types that have undergone about 15 extensional strain in contrast a different succession of tertiary volcanic and sedimentary rocks underlies the northeastern whipple mountains along the structural trend of central chemehuevi valley these strata have been extended by 40 differences in magnitudes of extension and mid tertiary stratigraphies between these two areas argue for the presence of a synformal gently east dipping normal fault above the whipple fault in the central chemehuevi valley this fault accommodates 15 km of the total 40 km extension along the whipple chemehuevi detachment fault system in addition tilting patterns of upperplate strata and the presence of an areally extensive syn extension landslide 70 km2 show that 1 the whipple detachment fault was active at low angles 70 km2 from the north side of the whipple mountains its emplacement occurred before the end of detachment faulting

the bouse formation and bracketing units southeastern california and western arizona implications for the evolution of the proto gulf of california and the lower colorado river mio pliocene sediments of the lower colorado river area represent the northernmost well documented extent of the proto gulf of california a tectonically enigmatic marine incursion that occupied much of what is now the gulf of california region including the lower colorado river area and the salton trough as much as 8 m y prior to the onset of spreading and transform related subsidence in that area fanglomerate and volcanic rocks informally referred to here as the osborne wash strata interfinger with the conformably overlying bouse formation the bouse formation includes carbonate and coarse terrigenous clastic basin margin deposits that interfinger laterally with basin fill material basin fill strata comprise a basal estuarine carbonate unit overlain by fine grained terrigenous clastic deltaic deposits the upper portion of the bouse interfingers with overlying cobble conglomerates here referred to as the colorado river gravels syndepositional folding of the bouse formation and bracketing units is believed to reflect slumping on oversteepened slopes perhaps exacerbated by episodic tectonic activity syndepositional faults show both normal and reverse separation outcrop relations allow but do not prove a strike slip component of motion on some structures contemporaneous minor faults seem to reflect mutually incompatible stress orientations this suggests that they record either localized stress fields or localized anomalous responses to regional stresses alternatively they may reflect extension in two directions in the sediment pile overlying the subsiding basin floor the bouse formation and bracketing units record four stages in the evolution of the northern proto gulf lower colorado river area 1 dissection of preexisting detachment fault controlled topography and localized interior drainage alluvial deposition about 14 9 ma 2 regional subsidence and proto gulf transgression perhaps as early as about 8 ma not later than about 5 5 ma 3 progradation of ancestral colorado river delta into the northern end of the proto gulf basin prior to about 4 3 ma and 4 arrival of throughgoing colorado fluvial channel prior to about 3 5 4 ma outcrop relations between the osborne wash strata and the bouse formation and the relationship of these units to modern landforms indicate that topography in the lower colorado river area has not changed significantly since middle miocene time subsidence of the bouse basin is believed to have occurred via broad regional downwarping which may have represented a sag formed as the locus of active proto gulf extension propagated northward into the lower colorado river area from the block faulted southern portion of the proto gulf timing suggests that incipient proto gulf extension in the lower colorado river area was arrested by the approximately 5 ma shift to the modern transtensional regime

finite extension and low angle normal faulting andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes in contrast low angle normal faults are sometimes observed on which tens of kilometers of slip has occurred and moderate angle faults with slip of 5 to 15 km are common a simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults the formation of low angle faults may require inhomogeneities in stress or material properties but once slip begins on a low angle fault it is likely to continue whereas slip on high angle faults is likely to end after modest slip to be replaced by the formation of new high angle faults

late stage slip history of the buckskin rawhide detachment fault and temporal evolution of the lincoln ranch supradetachment basin new constraints from the middle miocene sandtrap conglomerate

unusually strong vertical motions in a caribbean hurricane abstract unusually strong updrafts and downdrafts in the eyewall of hurricane emily 1987 during its rapidly deepening phase are documented by both in situ aircraft measurements and a vertically pointing doppler radar updrafts and downdrafts as strong as 24 and 19 m s 1 respectively were found mean updrafts and downdrafts were approximately twice as strong as those found in other hurricanes updrafts had approximately the same width as downdrafts the most vigorous updrafts were located in the front quadrants of the storm and most of the strongest downdrafts were found in the rear quadrants the downdrafts could not be explained in terms of evaporative or melting cooling or precipitation drag evidence is presented that moist symmetric instability initiated by precipitation loading may have been responsible for the strong downdrafts

slope form and associations with ground boulder cover in arid environments northeast jordan abstract many studies of slope form and sediment transfer dynamics use either bounded field plots or laboratory physical hardware models to study associations between morphology and process results are presented here of a study which examines spatial variations in slope form changing ground surface boulder cover between sites and at different points along slope profiles and evidence for the movement of fine grained sediments which underlie the boulder cover the study site is located in northeast jordan an arid landscape characterised by a late tertiary to early quaternary basalt plateau spreading across the foot slopes of the druz mountains basalt age ranges from approximately 8 9 ma to recent scoracious deposits from local eruptive centres which are no more than 100 000 years old the study considers whole slope profiles rather than a representative section of a slope and accounts for form differences by quantifying slope shape using a length height integral slope form differs with basalt age gradually changing from concave to convex shapes the boulder cover can be used to examine the mobility and redistribution of underlying fine grained sediment by establishing the degree of clast burial at points along slope profiles twenty five slope profiles were surveyed and five plots distributed between crest and toe located at similar points on thirteen the axial dimensions of boulders within each plot were recorded sorting and the preferential movement of smaller clasts is a characteristic of the steepest part of slopes while mean clast size increases downslope for older basalts and decreases downslope for younger basalts the degree of wadi network evolution and the development of sediment pans known as qa and marab varies with flow age links can be established between the age and physical nature of individual lava flows differences in slope form and their spatial distribution associations also exist between changes in boulder dimensions basalt type and the extent of clast burial between topographic highs and lows

use of multiparameter relative age methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont eastern mojave desert california numerical and calibrated age determinations of the late quaternary alluvial fan deposits of the soda mountains piedmont in the mojave desert provide an opportunity to study the utility of the multiparameter relative age ra method for distinguishing and mapping geomorphic surfaces on a desert piedmont most ra parameters could not discriminate between deposits of holocene age although pavements have formed over locally significant parts of surfaces as young as middle holocene several parameters including lithologic composition particle size soil development and varnish cover permit distinguishing between holocene surfaces and late pleistocene surfaces statistically significant differences in initial particle size and lithology of the deposits inferred to be the result of complex interaction among hillslope alluvial fan and eolian processes and climatic change create conditions unfavorable for use of most ra techniques in contrast soil profile development and varnish cover data are successful in discrimination among deposits of holocene and pleistocene age this is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan

effects of rock fragment covers on erosion and transport of noncohesive sediment by shallow overland flow rock fragments on soil surfaces can have a variety of contrasting effects on the hydraulics of overland flow and soil erosion this makes modeling soil erosion difficult systematic studies are needed to assess the effects of rock fragments on the hydraulics of overland flow and to specifically relate these hydraulics to sediment yield therefore flume experiments were conducted in which overland flow was applied to a highly erodible sediment surface covered by various percentages of rock fragments high local turbulence was caused by horseshoe vortex erosion around individual rock fragments for a given discharge and slope the dimension of the scour was affected by the space between the rock fragments for low cover percentages the scour can develop undisturbed and sediment yield increases with percent of rock fragment cover up to about 20 sediment yield then declines as percent cover increases above 20 beyond a cover threshold value of 20 the larger opportunity for scour hole development is counteracted by limited inter rock fragment space that does not allow scour holes to develop to their full extent as sediment yield varies nonmonotonically with rock fragment cover percent sediment yield cannot be predicted by spatially averaged flow hydraulics

palaeomagnetic dating of fault rocks evidence for permian and mesozoic movements and brittle deformation along the extensional dalsfjord fault western norway summary palaeomagnetic and structural studies of the dalsfjord nappe western norway show that the basal ow angle detachment dalsfjord fault is a long lived fault zone and that the most important phase of faulting was of devonian extension probably nucleated on an earlier silurian scandian thrust fault rocks produced during subsequent movements indicate that the dalsfjord fault underwent periods of brittle low angle extensional reactivation during the permian 250 260 ma and upper jurassic lower cretaceous c 150 ma corresponding with stages of major extensional movements on the continental shelf palaeomagnetic studies may be of great importance for dating faults and major movement stages in long lived fault systems the particular importance of the results is that they show that low angle normal faults can operate in a brittle upper crustal regime

properties of various sediment sampling procedures several issues regarding the sampling and analysis of bed material from gravel bed streams are addressed in this paper the need to convert surface samples into volumetric equivalents and methods to do so are explained it is found that most surface samples are unique and thus are not directly comparable particular attention is paid to areal sampling with clay and with wax the areal to volumetric conversion of samples removed from different sediment structures is also considered criteria are proposed for determining the minimum dimensions of unbiased volumetric samples these criteria can be used to determine the minimum area of an areal sample the minimum depth of a volumetric sample is shown to be larger than the depth of most surface layers thus a surface layer can not be sampled volumetrically a technique is presented for sampling a wide size range of underwater surface particles this technique has been tested in the field and found to provide good results