folsom drought and episodic drying on the southern high plains from 10 900 10 200 14c yr b p the paleoenvironments of late pleistocene and early holocene time on the southern high plains have been studied for decades but regionally extensive or long term easily recoverable proxy climate indicators are difficult to find the stratigraphy of valley fill and upland eolian deposits and stable carbon isotope data in addition to geographically limited paleontological data now provide clues to the environment during this time which includes the earliest or paleoindian period 11 200 8000 14c yr b p of human occupation during the clovis occupation 11 200 10 900 14c yr b p valleys contained perennial streams this was followed in folsom time 10 900 10 200 14c yr b p by an abrupt change to lakes and ponds with water levels fluctuating between several meters depth and no surface water and marshes and accumulation of sheet sands on uplands starting the earliest phase of construction of the regional dune fields these changing conditions indicate a shift from relatively wetter to relatively drier conditions with episodic drought stable c isotopes further indicate that warming characterized the clovis folsom transition during the rest of the paleoindian period the environment was relatively cool but fluctuated between wetter and drier conditions with an overall trend toward drying that resulted in further enlargement of the dune fields and culminated in the warm dry altithermal beginning 8000 14c yr b p clovis time probably was the wettest of any paleoindian period in terms of runoff and spring discharge the folsom period was drier and was the earliest episode of regional wind erosion and eolian deposition and may have been the warmest of paleoindian times evidence of a previously hypothesized clovis drought in this region is sparse

magma transport of heat on io a mechanism allowing a thick lithosphere models indicate that the high heat flow from io would result in a very thin approximately 5 km silicate lithosphere overlying a molten interior if all heat was transported through the lithosphere via conduction however the presence of mountains with relief in excess of 10 km would seem to demand a thick lithosphere at least locally a significant fraction of io s heat flow may be transported via advection through volcanoes advective heat transfer permits a thicker lithosphere than does pure conduction possibly reconciling io s high heat flow with the rugged topography

a reexamination of late pleistocene boreal forest reconstructions for the southern high plains previous paleoecological research on the southern high plains resulted in development of a late quaternary chronology of pollen analytical episodes and the proposal that boreal forest existed in the region in the late pleistocene these interpretations are now considered untenable because 1 a number of the radiocarbon ages are questionable 2 there are serious problems of differential pollen preservation and reproducibility of the pollen data 3 there is an absence of supporting geological or paleontological data and 4 the soils of the area contain none of the distinctive pedological characteristics produced by a boreal forest available data suggest that the region was primarily an open grassland or grassland with some nonconiferous trees through most of the quaternary

conjectures on the thermal and tectonic evolution of the earth abstract the tectonic modes operating at any given time in the earth are intimately related to the thermal evolution since tectonics is driven by heat removal from the earth s interior conversely the viability of a proposed tectonic mode depends on its ability to remove heat from the interior as well as on its inferred consistency with geological evidence on this basis it seems that plate tectonics may have been dominant only in the later part of earth history and that proposed earlier modes involving only a subcrustal thermal boundary layer may never have been dominant unless the effects of the basalt eclogite transition or of latent heat removal were able to enhance their heat transport efficiency more generally the tectonic mode driven by the cool thermal boundary layer at the top of the mantle may have depended very sensitively on the effects of composition and latent heat on density calculations indicate that plumes could have operated through most of earth history at about the present level of activity unless heat conduction from the core into the mantle has been inhibited in later times in which case they would have been hotter and more active in earlier times plumes could not have substituted for plate tectonics because plumes and plates are driven by different thermal boundary layers that operate largely independently

rolling hinge or fixed basin a test of continental extensional models in death valley california united states two models are typically offered to explain severe continental extensional tectonism low angle detachment faults with alloch thonous sedimentary basins above a crustal rolling hinge or differential displacement on systems of distinct deeply rooted normal faults with in place sedimentary basins structural and stratigraphic relationships between the badwater turtleback and the adjacent furnace creek basin in death valley provide a test of these contrasting models there ductile extensional deformation of basement rock at the badwater turtleback coincided with brittle extensional deformation and local uplift in the adjacent furnace creek basin between 13 and 18 ma in the furnace creek basin crustal extension of this age is evident as 1 greater fault density and offset in pre 13 ma rocks relative to post 13 ma rocks and 2 the presence of laterally variable sedimentary breccias and conglomerates which indicate a syndepositional paleotopographic high this high is approximately on line with the likely projection of an early badwater turtleback fault the surface expression of the turtleback mylonites these relationships indicate that the badwater turtleback and furnace creek basin were contiguous prior to 13 ma and that the basin formed in place relative to the black mountains basement these findings support arguments for extension in the death valley area by distinct deeply rooted fault zones rather than by a rolling hinge detachment fault

three dimensional model of the late cenozoic history of the death valley region southeastern california the accumulation of a large database on the timing and kinematics of late cenozoic deformation in the death valley region of southeastern california indicates a complex three dimensional history on the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull apart between two conjugate strike slip faults the garlock and furnace creek faults and evolved into a system characterized by distributed transtension related to the eastern california shear zone our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region the model may resolve 1 a long standing problem of the eastern termination of the garlock fault which is explained here as a complex system of splays that initially terminated in the pull apart between the furnace creek and garlock systems and 2 the complex architecture of the black mountains which is explained here in terms of initial extreme attenuation between the garlock and furnace creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension this model suggests much of the displacement field is taken up in rotations and translations and the actual crustal thinning in our model is relatively small 50 66 of original thickness

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

abrupt climate oscillations during the last deglaciation in central north america evidence from stable isotopes and a variety of proxies from two ontario lakes demonstrate that many of the late glacial to early holocene events that are well known from the north atlantic seaboard such as the gerzensee killarney oscillation also known as the intra allerod cold period younger dryas and preboreal oscillation also occurred in central north america these results thus imply that climatic forcing acted in the same manner in both regions and that atmospheric circulation played an important role in the propagation of these events

late quaternary vegetation and climate changes in central texas based on the isotopic composition of organic carbon stable carbon isotope analysis of organic carbon in alluvial deposits and soils of three streams in central texas reveals significant shifts in the ratio of c3 to c4 plant biomass production during the past 15 000 yr these temporal changes in vegetation appear to be in response to changes in climate during the late pleistocene c4 plants comprised only about 45 to 50 of the vegetative biomass in this area suggesting that conditions were cooler and wetter than at any time during the past 15 000 yr the time between 11 000 and 8000 yr b p is interpreted as transitional between late pleistocene conditions and warmer and drier holocene conditions based on a slight increase in the abundance of c4 plant biomass during the middle holocene between approximately 6000 and 5000 yr b p mixed c3 c4 plant communities were replaced almost completely by c4 dominated communities indicating prairie expansion and warmer and drier climatic conditions by 4000 yr b p the abundance of c4 plant biomass decreased to levels similar to the early holocene transitional period suggesting a return to cooler and wetter climatic conditions no significant shift in the ratio of c3 to c4 productivity has occurred during the last 4000 yr except for a slight increase in the abundance of c4 plant biomass around 2000 yr b p the results of this investigation correlate well with other regional late quaternary climatic interpretations for central and north texas the southern plains region and with other portions of the great plains

high resolution tephra and u pb chronology of the 3 33 3 26 ga mendon formation barberton greenstone belt south africa abstract the mendon formation in the barberton greenstone belt of south africa marks the boundary between the onverwacht and fig tree groups these groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation respectively this transition marks the end of komatiitic volcanism in the barberton greenstone belt and is accompanied by numerous impact related spherule layers this study characterizes the upper mendon formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary a suite of whole rock and handheld x ray fluorescence analyses are presented in conjunction with textural information stratigraphic relationships and u pb ages to create a temporal and chemostratigraphic framework for the mendon formation local and regional stratigraphic variations including absence of distinctive layers and variation in layer thickness seen across the mendon preclude ascription of a single stratigraphy that accurately describes the 1 2 km of section present in this formation these variations indicate diachronous deposition of the mendon formation over a wide areal extent and into multiple basins or sub basins by more than one magmatic source 204 pb corrected 206 pb 238 u and 207 pb 235 u concordia model ages of 3279 9 1 ma and 3287 3 2 9 ma for two samples from upper portions of the mendon formation provide temporal context for deposition two samples from the basal 10 m of the fig tree group above the s2 spherule bed that marks the boundary between the onverwacht and fig tree groups give model ages of 3267 8 6 9 ma and 3261 18 ma these ages provide added constraints for the onverwacht fig tree boundary and confirm that the weltevreden formation is roughly age correlative with the uppermost mendon formation while the mendon and weltevreden formations are in part age correlative and have similar lithologies they do not appear to be genetically related the dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the mendon and weltevreden formations indicate that the primary mode of crustal formation was likely plume related magmatic accretion and not subduction the relatively sharp transition within the barberton greenstone belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact related spherule layers which suggest that major impacts may have played a role in the evolution of early tectonics to more modern subduction related styles

the black mountains turtlebacks rosetta stones of death valley tectonics the black mountains turtlebacks expose mid crustal rock along the western front of the black mountains as such they provide keys to understanding the tertiary structural evolution of death valley and because of the outstanding rock exposure they also provide valuable natural laboratories for observing structural processes there are three turtlebacks the badwater turtleback in the north the copper canyon turtleback and the mormon point turtleback in the south although important differences exist among them each turtleback displays a doubly plunging antiformal core of metamorphic and igneous rock and a brittle fault contact to the northwest that is structurally overlain by miocene pleistocene volcanic and or sedimentary rock the turtleback cores contain mylonitic rocks that record an early period of top southeastward directed shear followed by top northwestward directed shear the earlier formed mylonites are cut by and locally appear concurrent with 55 61 ma pegmatite we interpret these fabrics as related to large scale basement involved thrust faults at the turtlebacks now preserved as areally extensive metamorphosed basement over younger cover contacts the younger and far more pervasive mylonites record late tertiary extensional unroofing of the turtleback footwalls from mid crustal depths available geochronology suggests that they cooled through 300 8c at different times 13 ma at badwater 6 ma at copper canyon 8 ma at mormon point at mormon point and copper canyon turtlebacks these dates record cooling of the metamorphic assemblages from beneath the floor of an 11 ma tertiary plutonic complex collectively these relationships suggest that the turtlebacks record initiation of ductile extension before 14 ma followed by injection of a large plutonic complex along the ductile shear zone ductile deformation continued during extensional uplift until the rocks cooled below temperatures for crystal plastic deformation by 6 8 ma subsequent low angle brittle fault slip led to final exposure of the igneous and metamorphic complex the turtleback shear zones can constrain models for crustal extension from map view as well as cross sectional perspectives in map view the presence of basement involved thrust faults in the turtlebacks suggest the black mountains were a basement

structural geology and kinematic history of rocks formed along low angle normal faults death valley california several late cenozoic low angle normal or detachment faults on the western flank of the black mountains are each characterized by the following in descending structural order 1 a hanging wall of upper tertiary to quaternary sedimentary and volcanic strata that displays little evidence for fault related damage other than widely spaced planar or listric normal faults 2 a sharp planar and locally striated principal slip plane forming the lower boundary of the hanging wall 3 an upper zone zone i of very fine grained fault generated rocks composed dominantly of gouge 4 a lower zone zone ii of coarser grained fault rocks consisting chiefly of foliated breccia and 5 a variably damaged footwall consisting of partly mylonitic precambrian units or tertiary plutonic rocks that has been exhumed from depths of 10 12 km since late miocene time the fault rocks which were mostly derived from the footwalls preserve evidence for cataclastic and particulate flow fault rocks contain authigenic minerals but lack the cyclically deformed mineral filled syntectonic veins that are abundant in some other late cenozoic high angle and strike slip faults meso and microscale fabrics in zones i and ii indicate that finite shear strains increase progressively upward toward the principal slip plane in a conceptual kinematic model of a shear box displacements of the hanging wall produced a shearing flow in the fault rocks below some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones i and ii since 770 ka during the latest stages of incremental deformation in the brittle shear zones distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes in this respect the detachment faults differ from inactive segments of the san andreas system along which displacements were progressively and irreversibly localized onto a single principal slip surface the strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry strain gradients in the black mountains are notably asymmetric they are present only below the principal slip planes not above

review secular tectonic evolution of archean continental crust interplay between horizontal and vertical processes in the formation of the pilbara craton australia the archean pilbara craton contains five geologically distinct terranes the east pilbara karratha sholl regal and kurrana terranes all of which are unconformably overlain by the 3 02 to 2 93 ga de grey superbasin the 3 53 3 17 ga east pilbara terrane ep represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3 53 3 43 3 35 3 29 and 3 27 3 24 ga each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3 72 ga and melting of crust to generate first tonalite trondhjemite granodiorite ttg and then progressively more evolved granitic magmas in each case plume magmatism was accompanied by uplift and crustal extension the combination of conductive heating from below thermal blanketing from above and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust these mantle melting events caused severe depletion of the subcontinental lithospheric mantle making the ep a stable buoyant unsubductable continent by c 3 2 ga extension accompanying the latest event led to rifting of the protocontinent margins at between 3 2 and 3 17 ga after 3 2 ga horizontal tectonic forces dominated over vertical forces as revealed by the geology of the three terranes karratha sholl and regal of the west pilbara superterrane the c 3 12 ga whundo group of the sholl terrane is a fault bounded 10 km thick volcanic succession with geochemical characteristics of modern oceanic arcs including boninites and evidence for flux melting that indicate steep archean subduction at 3 07 ga the 3 12 ga sholl terrane 3 27 ga karratha terrane and c 3 2 ga regal terrane accreted together and onto the ep during the prinsep orogeny this was followed by development of the de grey superbasin an intracontinental sag basin and widespread plutonism 2 99 2 93 ga as a result of orogenic relaxation and slab break off craton wide compressional deformation at 2 95 2 93 ga culminated with 2 91 ga accretion of the 3 18 ga kurrana terrane with the ep this compression caused amplification of the dome and keel structure in the ep final cratonization was effected by emplacement of 2 89 2 83 ga post tectonic granites

earth science a resolution of the archaean paradox a model of early earth in which heat from the interior is carried to the surface through volcanic heat pipes suggests that our planet 4 billion years ago had more in common with jupiter s moon io than with today s earth see article p 501 the hadean era of earth s history 4 5 3 8 billion years ago is the dark age of geology illuminated by occasional scraps of data on the basis of a geodynamical model formerly applied to jupiter s volcanically active moon io william moore and alexander webb propose a heat pipe model to explain heat transport and lithospheric dynamics of an early earth in transition between the early magma ocean and the era of plate tectonics the model suggests that a cold thick single plate lithosphere developed as a result of frequent volcanic eruptions that cycled surface materials downwards the model s predictions compare well with the geological record and indicate that declining heat sources over time could have precipitated an abrupt transition to plate tectonics

archaeological geology of the lubbock lake site southern high plains of texas the lubbock lake site is a well stratified archaeological locality in yellowhouse draw on the southern high plains of texas it has a cultural record that spans the past 11 000 yr that is contained within a thick sequence of sediments and soils the sequence at lubbock lake is one of the most closely integrated records of late quaternary human occupation sedimentation and soil formation documented in north america stratum 1 11000 yr b p and older was deposited by a meandering stream and contains clovis age cultural remains stratum 2 is composed of lacustrine sediments 11000 to 10000 yr b p and overlying marsh deposits 10000 to 8500 yr b p with paleoindian material in both units the firstview soil 8500 to 6300 yr b p formed in the top of stratum 2 stratum 3 6300 to 5000 yr b p contains early archaic material eolian and marsh deposits suggestive of warm arid conditions and the yellowhouse soil indicating a brief period of landscape stability stratum 4 5000 to 4500 yr b p is primarily an eolian unit that marks the second of two periods of severe drought in the middle holocene the lubbock lake soil formed in stratum 4 beginning 4500 yr b p coincides with the late archaic and ceramic periods and documents landscape stability and a return to climatic conditions similar to those of today stratum 5 contains late ceramic protohistoric and historic archaeological material within eolian and slopewash sediments and the apache and singer soils which indicate several shifts toward aridity within the past 1 000 yr

interactions between low angle normal faults and plutonism in the upper crust insights from the island of elba italy the late miocene to early pliocene zuccale low angle normal fault on the island of elba italy was one of the principal structures active during extensional collapse of the apennine fold and thrust belt we investigate the relationships among the zuccale fault subsidiary footwall fault networks and igneous bodies that were intruded into the immediate footwall of the zuccale fault both brittle and ductile kinematic indicators found in association with fault zones and igneous bodies yield a consistent wnw ese extension direction suggesting that faulting and intrusion overlapped in time structure contour analysis indicates that the zuccale fault has a regional domal morphology the dimensions and spatial location of the dome correlate with the likely subsurface position of the porto azzurro pluton originally intruded at 6 km depth we propose that doming of the zuccale fault may have been related in part to emplacement of the porto azzurro pluton as a tabular intrusion involving some component of vertical inflation and roof uplift the immediate footwall of the zuccale fault is everywhere crosscut by a complex linked network of high and low angle extensional faults with observed displacements of

rapid extension recorded by cooling age patterns and brittle deformation naxos greece the metamorphic core complex exposed as the island of naxos in the aegean sea records an unusually complete sequence of structures developed as a result of continental extension the structures formed during miocene rise and cooling from ductile upper amphibolite facies and anatectic conditions to brittle near surface conditions beneath the naxos detachment fault top to the north ductile fabrics in the footwall which initially developed during amphibolite facies prograde metamorphism were overprinted by a succession of north directed normal sense lower temperature brittle structures as the footwall was tectonically unloaded and unroofed pseudotachylite and cataclasite formation brittle faulting alteration and erosion of the footwall occurred during continued slip and tectonic denudation neogene conglomerate and megabreccia in part derived from exhumation of this footwall lie structurally above the peripheral naxos fault published k ar and 40ar 39ar ages for hornblende white mica and biotite in the footwall decrease northwestward apparent ages 17 50 ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained published ages 16 10 ma in higher grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the nne direction of extension assuming this is the direction of unroofing we interpret the rate of this younging as the fault slip rate as the footwall rocks moved 20 km ssw relative to their hanging wall along the base of the naxos detachment fault the calculated rates of slip average 5 8 mm yr comparable to maximum rates reported in the basin and range province

fabric development in syn tectonic intrusive sheets as a consequence of melt dominated flow and thermal softening of the crust the temperature dependence of ductile deformational processes suggests that the thermal anomaly associated with a cooling syn tectonic intrusion will produce a crustal low viscosity horizon that will concentrate deformation in the vicinity of the pluton during cooling this effect should be most prominent when magma is present but should also occur after solidification of the magma when flow is by solid state processes this paper evaluates this hypothesis for sheet like intrusions using one dimensional time dependent thermal models and accompanying predictions of viscosity vs time history based on experimental flow laws these models predict that for systems with an initial normal geothermal gradient e g extensional systems strike slip systems or thrust systems with low displacement rates the base of a large plutonic sheet cools more slowly than the top and fabric development should be most pronounced on the floor of the pluton in contrast in megathrust systems where displacements are sufficiently rapid to produce temperature inversions cooling is also upside down and fabric development should preferentially occur along the top of plutonic sheets moreover when a pluton is emplaced within a zone of inverted isotherms the heat may be trapped within the inversion a natural system characterized by this history should show a sharp thermal front coincident with the top of a paleo temperature inversion if a pluton is weaker than its country rock it will form a weak horizon in the crust throughout its cooling history and the plutonic sheet should take up the bulk of the deformation throughout its cooling history if a pluton is stronger than its country rock under conditions of solid state flow however models predict a two phase deformational history prior to solidification the deformation should be concentrated in the pluton with the magma representing a weak horizon in the crust but upon crystallization the pluton should become relatively rigid with respect to its country rock and deformation should be concentrated in the wall rocks these two cases predict markedly different structural histories and the history could easily be misinterpreted in the first case static mineral growth might be prominent in the wall rocks and these textures could be misinterpreted as evidence for post tectonic emplacement in the second case the extensive high t deformation of the wall rocks could be readily ascribed to non tectonic emplacement related deformation rather than a consequence of syn tectonic emplacement the model predictions are tested with three examples of well exposed plutonic sheets a neogene extensional system in death valley california and two megathrust systems in southern alaska border ranges fault system and mclaren metamorphic belt structural observations from these systems are broadly consistent with the model predictions and lend support to first order characteristics of the models nonetheless more sophisticated 2 d models of the coupled thermal mechanical system are needed to thoroughly test the hypothesis

variation in radiocarbon ages of soil organic matter fractions from late quaternary buried soils radiocarbon dating of three organic matter fractions total humic acid and residue isolated from late quaternary buried soils of the central great plains reveals that there often are considerable differences among but no consistent order to the ages of fractions for late holocene soils the residue fraction or the total fraction generally produces the oldest age for late pleistocene soils however no fraction was consistently the oldest the absence of a consistent sequence of fraction ages is attributed to postburial contamination of soils when bulk samples from the same soil were split and sent to two laboratories different radiocarbon ages were usually obtained the variability in radiocarbon ages of soil organic matter confirms that caution should be taken when using radiocarbon ages obtained from different laboratories to make regional stratigraphic correlations

brittle faulting induced by ductile deformation of a rheologically stratified rock sequence badwater turtleback death valley california field relations in the footwall of the badwater turtleback fault provide evidence that macroscopically ductile deformation was directly responsible for early stages of brittle faulting during extensional denudation footwall rocks which are typically mylonitie consist of dominantly pegmatite quartzofeldspathic gneiss and dolomite and calcite marble because of the ductile deformation these rocks are widely variable in thickness bodies of rock that exceed 10 m in thickness may pinch out to zero thickness in distances of less than 100 m the mylonitic rocks also display crosscutting relations that indicate that onset of bulk brittle behavior occurred at different stages of uplift for different rock types calcite marble was the last to deform by predominantly crystal plastic mechanisms and therefore accommodated most of the shear strain during later stages of lower plate ductile deformation where the calcite marble pinched out into adjacent stronger rocks the high strains caused brittle faulting in the adjacent rocks the process described here of ductile deformation leading to extreme thickness variations and consequent brittle faulting directly links mid crustal ductile deformation with upper crustal brittle deformation it further demonstrates that brittle faulting can initiate at depths where ductile deformation accommodates the bulk strain

are turtleback fault surfaces common structural elements of highly extended terranes abstract the death valley region of the u s a contains three topographic surfaces resembling the carapace of a turtle these three surfaces are well exposed along the black mountain front and are named the badwater copper canyon and mormon point turtlebacks it is widely accepted that the turtlebacks are also detachment surfaces that separate brittlely deformed cenozoic volcanic and sedimentary rocks of the hanging wall from the strongly mylonitic ductilely deformed pre cenozoic rocks of the footwall we have found a turtleback like detachment surface along the southern margin of the alasehir gediz graben in western anatolia turkey this surface qualifies as a turtleback fault surface because it a is overall convex upward and b separates brittlely deformed hanging wall cenozoic sedimentary rocks from the ductilely to brittlely deformed strongly mylonitic pre cenozoic footwall rocks the surface named here horzum turtleback contains striations that overprint mylonitic stretching lineations indicating top to the ne sense of shear this suggests that the northeasterly directed cenozoic extension in the region resulted in a ductile deformation at depth and as the crust isostatically adjusted to the removal of the rocks in the hanging wall of the detachment fault the ductilely deformed mylonitic rocks of the footwall were brought to shallower depths where they were brittlely deformed the turtleback surfaces have been considered unique to the death valley region although detachment surfaces rollover folds and other extensional structures have been well observed in other extended terranes of the world the presence of a turtleback fault surface in western anatolia turkey suggests that the turtleback faults may be common structural features of highly extended terranes

relation of deformation and multiple intrusion in the death valley extended region california with implications for magma entrapment mechanism the crystalline core of the black mountains crustal section death valley california exposes a tremendous volume of miocene plutonic rock intruded at a depth of 10 13 km few plutons were intruded above 10 km prior to unroofing by tectonic denudation in the eastern part of the core a brittle detachment fault separates predominantly miocene hanging wall strata from the midcrustal miocene 11 6 and 8 7 ma plutonic complex and early proterozoic basement in the west mylonitic lineations and foliations locally well developed within the 11 6 ma intermediate mafic willow spring pluton are cut by dikes which emanate from an 8 7 ma silicic plutonic complex the younger silicic complex throughout the crystalline core exhibits few ductile deformational structures published thermal and barometric studies from both plutonic bodies indicate similar midcrustal 10 13 km emplacement depths at ambient temperatures just above 300 c the significant difference in densities of these magmas argues against a density control for magma entrapment also the country rock above and below the plutonic complex contains no apparent differences from field observations that would suggest a change in density the depth of entrapment corresponds well with the expected depth for the crustal strength maximum determined from laboratory experiments the similar emplacement depths but contrasting styles of deformation of the two plutonic bodies further suggests that entrapment may have been controlled by a high strength barrier represented by the brittle ductile transition late hypabyssal intrusions and associated volcanism are linked to diachronous rapid unroofing of the range block all show a northwest progression paralleling the regional extension direction thus when migration of magma through the high strength barrier did occur it was apparently related to increased strain rates which allowed magma ascent by fracture exploitation rheological stratification of the crust may have played an important if not major role in trapping magmas in the middle crust in this area

grassland vegetation in the southern great plains during the last glacial maximum abstract new pollen records from white lake in the southern high plains and from friesenhahn cave on the southeastern edwards plateau of texas indicate that the glacial age vegetation of the southern great plains was a grassland the high plains was a treeless artemisia grassland and the edwards plateau at the south edge of the great plains was a grassland with pinyon pines and deciduous trees in canyons and riparian habitats the glacial age grasslands differ from modern shortgrass and tallgrass prairies and may have no modern analog the dominance of prairie vegetation during the last glacial maximum is compatible with late pleistocene mammalian faunas and late glacial grassland pollen records from the region earlier interpretations of a pine spruce forest on the high plains were based on pollen assemblages that are here shown to have been altered by postdepositional deterioration resulting in differential preservation of conifer pollen grains accordingly the tahoka pluvial and other climatic episodes defined by high plains pollen records are abandoned

plate tectonics on super earths equally or more likely than on earth abstract the discovery of extra solar super earths has prompted interest in their possible mantle dynamics and evolution and in whether their lithospheres are most likely to be undergoing active plate tectonics like on earth or be stagnant lids like on mars and venus the origin of plate tectonics is poorly understood for earth likely involving a complex interplay of rheological compositional melting and thermal effects which makes it challenging to make reliable predictions for other planets nevertheless as a starting point it is common to parameterize the complex processes involved as a simple yield stress that is either constant or has a byerlee s law dependence on pressure because the simplifying assumptions made in developing analytical scalings may not be valid over all parameter ranges numerical simulations are needed one numerical study on super earths finds that plate tectonics is less likely on a larger planet o neill and lenardic 2007 in apparent contradiction of an analytical scaling study valencia et al 2007 to try and understand this we here present new calculations of yielding induced plate tectonics as a function of planet size focusing on the idealized end members of internal heating or basal heating as well as different strength profiles and compared to analytical scalings in the present study we model super earths as simple scaled up versions of earth i e assuming constant physical properties keeping the ratio of core mantle radii constant and applying the same temperature difference between top bottom boundaries and the same internal heating rate effects that originate outside of the planet such as tidal forces meteor impacts and intense surface heating from a nearby star are not considered we find that for internally heated convection plate tectonics is equally likely for terrestrial planets of any size whereas for basally heated convection plate tectonics becomes more likely with increasing planet size this is indicated both by analytical scalings and the presented numerical results which agree with each other when scalings are adjusted to account for increasing mean density with increasing planet size plate tectonics becomes more likely with increasing planet size for all scenarios the influence of the pressure variation of viscosity thermal expansivity and conductivity may however act in the opposite sense and needs to be determined in future studies at least in the simplest case factors other than planet size such as the presence of surface water are likely most important for determining the presence or absence of plate tectonics

human responses to middle holocene altithermal climates on the north american great plains abstract the climate of the great plains during the middle holocene varied considerably but overall it was marked by a north south gradient of increasingly warmer and drier conditions with a reduction in effective moisture surface water and resource abundance and an increase in resource patchiness sediment weathering erosion and aeolian activity pronounced drought conditions were most evident on the southern high plains understanding the human responses to middle holocene climates is complicated by a lack of archaeological data which is partly a result of geomorphic processes that removed or deeply buried sites of this age and by the varying adaptive responses of hunter gatherers during this period on the southern high plains where drought was most severe surface and groundwater sources dried and bison populations were diminished prompting substantial adaptive changes including local abandonment well digging to tap underground water and a widening of the diet breadth to incorporate higher cost lower return seed and plant resources sites of this age on the central and northern plains also show a possible increase in diet breadth with the incorporation of plant foods in the diet and perhaps changes in settlement mobility including possible shift into higher elevation areas or mapping on to extant rivers and springs but linking those changes to middle holocene drought is less straightforward