laramide exhumation of the bighorn mountains wyoming an apatite u th he thermochronology study we report u th he apatite ages from the crystalline core of the bighorn mountains and compare the exhumation history derived from those ages with the exhumation history determined from sedimentary rocks in the adjacent powder river and bighorn basins our u th he apatite ages range from 62 to 369 ma and represent a pre laramide he partial retention zone that was deformed and uplifted at ca 65 5 ma the geometry of the he partial retention zone indicates that the basement in the bighorn mountains is deformed into a doubly plunging anticline the preservation of a pre laramide partial retention zone in the upper few hundred meters of precambrian basement indicates that in general the temperature at the cambrian unconformity did not exceed the apatite he closure temperature this is difficult to reconcile with evidence from adjacent basins for thick sequences of sedimentary rocks 3 4 km prior to 65 ma and normal modern geothermal gradients either the range was never deeply buried 2 3 km its geothermal gradient has been low 20 c since at least the mesozoic or our apatites have higher u th he closure temperatures 80 90 c than those measured for other apatites

experimental assessment of the infiltration properties of a coarse soil medium in a dielectric infiltration test an accurate determination of the hydrological characteristics of porous media such as the values of the porosity and effective porosity are essential this is important to understand the transport processes of infiltration and the movement of water and contaminants in the porous media in this study a laboratory soil column experiment to estimate the porosity and effective porosity of toyoura standard sand samples using a dielectric method termed the frequency domain reflectometry with vector network analyzer fdr v was performed the fdr v device uses high frequency microwaves ranging from 0 1 to 3 ghz to measure the complex dielectric constants of the sample from the measured complex dielectric constant the two parameters of the soil samples then were derived using a proposed dielectric mixture model and tracer concentration model the effective porosity of the soil sample is measured at 0 311 with the fdr v dielectric method and 0 345 by the soil tracer column test comparing this with the calculated porosities of the soil sample the ratio of effective porosity is approximately 78 for the dielectric method and approximately 86 for the tracer concentration method these different values can be explained to be within the measurement range with regard to the measurement volume for the soil column these results indicate that measurement by the dielectric method using an fdr v device is an efficient and useful tool for estimating the hydrological parameters of porous media because of the dielectric response of earth materials

uranium thorium helium dating of apatite experience with samples from different geochemical environments abstract u he dating which during its history has been very problematic might find special application in the field of hydrothermal mineralizations where other methods generally fail the mineral apatite is a potential candidate for this method in order to shed light on various aspects of apatite dating by the u th he method two samples each from different geochemical environments plutonic pneumatolytic and hydrothermal were probed isotope dilution techniques were used for concentration analyses from which 4he model dates also called helium indices after n b keevil were calculated 4he diffusion measurements were carried out for comparison of the 4he retentivities between the samples comparisons of the apatite he indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the u th he dating method applied to apatite yields an age of cooling rather than an emplacement age the calculated closure temperatures are similar to those obtained by the fission track method 100 c our results suggest that the high temperature apatites plutonic and pneumatolytic may be more suitable for he dating than hydrothermal apatite important criteria for the dating of apatite by u th he are the presence of moderate radionuclide concentrations tens of ppm and a minimum abundance of inclusions

soil pipe distribution and hydrological functioning within the humid tropics a synthesis some of the most responsive runoff systems in the world are found within the humid tropics these runoff responses are likely to be affected by the presence of natural pipes within the soil this study provides a synthesis of the hydrological aspects of these phenomena within the humid tropics of the studies reporting the presence of soil piping within the humid tropics most are associated with ultisol soils and locally most pipe outlets are observed on the lower sections of hillslopes while the drainage role of pipes has been observed providing faster and slower components of stream hydrographs the mechanism of their recharge remains less clear in part this is because their spatial extent is poorly mapped within the hillslopes of the humid tropics further studies quantifying the length and recharge of soil pipe networks within the humid tropics are needed

surface hydrology iv flow in sloping layered soil in a nonuniform soil with rotational symmetry the properties change along a coordinate n that forms an angle ia n with the vertical during infiltration there will be a flow component in the parallel direction s downstream which is proportional to the rain itself and the slope the horizontal flow is proportional to the vertical flow the slope and the coefficient of anisotropy the last has been calculated for a soil made up distinct layers in the limiting case of very thin layers or near the saturation point the vertical and horizontal conductivities are respectively the expected arithmetic and harmonic means familiar from saturated flow the coefficient of anisotropy increases with the rate of vertical flow to a power greater than unity the horizontal flow component can be many times the vertical one it finds application in pollution problems and in the study of the underground water regime

fission track lengths in the apatite annealing zone and the interpretation of mixed ages abstract measurements of confined fission tracks in apatites from deep boreholes show that their mean length is reduced and the length distribution becomes progressively broader through the fission track annealing zone at subsurface temperatures of around 100 c and above the length distribution characteristically becomes very broad without a pronounced peak the lengths of the longest tracks remain essentially constant at all stages of natural annealing observed in the boreholes this pattern is similar to that found in laboratory annealing of spontaneous fission tracks in apatite from an outcrop sample of the same formation these observations show that confined track lengths can be used as important indicators of the type of thermal history that a sample has experienced in the temperature zone of increasing track stability the length distributions reported here provide a basis for interpretation of fission track ages that might otherwise be ambiguous an apatite age which results from a uniform slow cooling history will have a broad negatively skewed length distribution while a bimodal distribution provides clear evidence of a two stage history involving partial annealing by a discrete thermal event the apparent fission track age associated with a bimodal length distribution will be a mixed age intermediate between the original age and that of the later event

temperatures and depth dependent heat flow in western north sea bottom hole temperatures and lithologic information from 167 released wells between 52 and 58 n in the british sector of the north sea have been studied and measurements of thermal conductivity have been conducted on many samples from all the main stratigraphic units although large errors may be associated with the temperature information it is sufficiently reliable to establish that heat flow is not conserved with depth over much of the western north sea in the central graben and anglo dutch basin heat flow in the upper 1 2 km 3 300 6 600 ft is around 50 mw m 2 increasing downward to about 100 mw m 2 farther west on the mid north sea high and on the east midlands shelf the situation is reversed with shallow heat flows of about 80 mw m 2 decreasing downward to about 50 mw m 2 these variations are not readily explained either by heat flow refraction or by tectonic activity they are most probably attributable to the deep and continuing circulation of water the circulation may well be on a very large scale and must be largely fault controlled

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

hillslope hydrology in tropical rainforest steeplands in brunei many remaining areas of tropical rainforest in south east asia are located on landscapes dominated by deep valleys and very steep slopes now that logging activities are extending into these steeplands it is essential to understand how the natural rainforest system behaves if any kind of realistic assessment of the effects of such disturbance is to be made this paper examines the hydrological behaviour of an undisturbed rainforest system on steep topography in the temburong district of brunei north west borneo the physical and hydrological properties of the regolith material are generally typical of tropical residual soils the regolith has a clay texture and a low dry bulk density beneath a superficial litter organic horizon the infiltration capacity of the surface soil was several hundred mm h 1 that of the exposed mineral subsoil was an order of magnitude less similar to the saturated hydraulic conductivity ksat of around 180 mm h 1 at a depth of 150 cm there was no indication that ksat reduced with depth except very near the bedrock interface soil tensions were measured using a two dimensional array of tensiometers on a 30 slope during dry season conditions infiltrating rain water contributes to soil moisture and drying of the soil is dominated by transpiration losses during wet season conditions perched water tables quickly develop during heavy rainfall giving rise to the rapid production of return flow in ephemeral channels no infiltration excess or saturation overland flow was observed on hillslopes away from channel margins subsurface storm flow combined with return flow produce stream flow hydrographs with high peak discharges and very short lag times storm event runoff coefficients are estimated to be as high as 40 it is concluded that the most distinctive feature of the hydrology of this steepland rainforest is the extremely flashy nature of the catchment runoff regime produced by the combination of thin but very permeable regolith on steep slopes copyright 2000 john wiley sons ltd

ring permeametry design operation and error analysis assessment of slope stability soil management or contaminant transport problems usually requires numerous yet accurate point measurements of permeability this technical note describes a new method for the rapid field assessment of permeability in near surface soils or unconsolidated sediments the procedure is known as ring permeametry and is an ex situ core based method giving measurements which can be guaranteed to be stratum specific unlike measurements from some in situ techniques the potential sources of precision and bias error within the method are quantified and their effect on the uncertainty of permeability estimates is illustrated 1997 john wiley sons ltd

hydrological characteristics of slope deposits in high tropical mountains cordillera real bolivia abstract the hydrological processes within slope deposits are not well known particularly for high tropical mountains with climatic conditions giving seasonal contrasts in the hydrological cycle measurements have been taken in a high andean valley of the cordillera real bolivia with the aim to determine the role of the slope deposits in the framework of a hydraulic modelling different transfer time measurements combined with morphometrical and sedimentological analysis were carried out on three talus slopes and two moraines during the rainy season after injection of salt the electrical conductivity of the water in slope deposits springs is monitored to estimate transfer velocities the hydrological behaviour of these slope deposits depends on the season in the dry season all the water infiltrates and there is no visible outlet during the wet season one part soaks in and gives springs down slope the lag times estimated by the measurement of tracer velocity demonstrate that these slope deposits delayed the flow depending on the type of deposit the delay is at least 24 h in the case of the talus slopes and more than 48 h in the case of lateral moraines

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

exhumation of the inyo mountains california implications for the timing of extension along the western boundary of the basin and range province and distribution of dextral fault slip rates across the eastern california shear zone 1 new geologic mapping tectonic geomorphologic 10be terrestrial cosmogenic nuclide and u th he zircon and apatite thermochronometric data provide the first numerical constraints on late cretaceous to late quaternary exhumation of the inyo mountains and vertical slip and horizontal extension rates across the eastern inyo fault zone california the east dipping eastern inyo fault zone bounds the eastern flank of the inyo mountains a prominent geomorphic feature within the western basin and range province and eastern california shear zone u th he zircon and apatite thermochronometry yield age patterns across the range that are interpreted as indicating 1 two episodes of moderate to rapid exhumation associated with laramide deformation during the late cretaceous early tertiary 2 development of a slowly eroding surface during a prolonged period from early eocene to middle miocene 3 rapid cooling exhumation and initiation of normal slip along the eastern inyo fault zone accommodated by westward tilting of the inyo mountains block at 15 6 ma and 4 rapid cooling exhumation and renewed normal slip along the eastern inyo fault zone at 2 8 ma fault slip continues today as indicated by fault scarps that cut late pleistocene alluvial fan surfaces the second episode of normal slip at 2 8 ma also signals onset of dextral slip along the hunter mountain fault yielding a pliocene dextral slip rate of 3 3 1 0 mm a where a is years summing this dextral slip rate with estimated dextral slip rates along the owens valley death valley and stateline faults yields a net geologic dextral slip rate across the eastern california shear zone of 9 3 2 2 1 4 to 9 8 1 4 1 0 mm a

single grain u th he ages from phosphates in acapulco meteorite and implications for thermal history the cooling history of the acapulco meteorite for 400 c is well established using various chronometers suggesting extremely fast cooling 1000 c ma in contrast the thermal history for low temperatures 400 c is poorly understood because of large uncertainties in the chronometers applicable to this temperature range to better constrain the cooling history for the low temperature range we applied u th he dating techniques to individual phosphate grains one whitlockite and 11 apatite grains yielded u th he ages ranging from 1272 22 1 analytical error only ma to 4584 51 ma with tight clustering at 4 55 ga the weighted mean of the five oldest ages 4538 32 ma 1 uncertainty including systematic error is suggested to be the minimum age representing primary cooling of the acapulco body passing through 120 c although it is impossible to precisely quantify the effects of energetic particle ejection from the outermost 20 m of the phosphates petrographic evidence suggests that most dated samples are fragments likely derived from the interior of larger grains thus greatly reducing this source of error indeed the five oldest samples cannot have suffered substantial ejection since the uncorrected ages are identical with the crystallization age of the acapulco meteorite the new u th he data suggest rapid cooling of acapulco down to 120 c this evidence suggests that the younger 40 ar 39 ar age 4507 9 1 ma obtained from acapulco plagioclase which should reflect cooling through 300 c is spuriously young due to systematic errors i e decay constants and or standard data in the 40 ar 39 ar method as suggested by comparison between high precision 40 ar 39 ar and u pb ages for terrestrial volcanic rocks the scattered he age distribution 4 0 ga implies very heterogeneous thermal disturbances after the primary cooling of the body

multi scale permeability estimation for a tropical catchment physically based and spatially distributed modelling of catchment hydrology involves the estimation of block or whole hillslope permeabilities invariably these estimates are derived by calibration against rainfall runoff response rarely are these estimates rigorously compared with parameter measurements made at the small scale this study uses a parametrically simple model topmodel and an uncertainty framework to derive permeability at the catchment scale the utility of expert knowledge of the internal catchment dynamics i e extent of saturated area in constraining parameter uncertainty is demonstrated model derived estimates are then compared with core based measurements of permeability appropriately up scaled the observed differences between the permeability estimates derived by the two methods might be attributed to the role of intermediate scale features natural soil pipes an alternative method of determining block permeabilities at the intermediate or hillslope scale is described this method uses pulse wave tests and explicitly incorporates the resultant effects of phenomena such as soil piping and kinematic wave migration the study aims to highlight issues associated with parameterizing or validating distributed models rather than to provide a definitive solution the fact that the permeability distribution within the borneo study catchment is comparatively simple assists the comparisons the field data were collected in terrain covered by equatorial rainforest combined field measurement and modelling programmes are rare within such environments

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

polyphase exhumation in the western qinling mountains china rapid early cretaceous cooling along a lithospheric scale tear fault and pulsed cenozoic uplift the western sector of the qinling dabie orogenic belt plays a key role in both late jurassic to early cretaceous yanshanian intracontinental tectonics and cenozoic lateral escape triggered by india asia collision the taibai granite in the northern qinling mountains is located at the westernmost tip of a yanshanian granite belt it consists of multiple intrusions constrained by new late jurassic and early cretaceous u pb zircon ages 156 3 ma and 124 1 ma applying various geochronometers 40ar 39ar on hornblende biotite and k feldspar apatite fission track apatite u th sm he along a vertical profile of the taibai mountain refines the cooling and exhumation history the new age constraints record the prolonged pre cenozoic intracontinental deformation as well as the cooling history mostly related to india asia collision we detected rapid cooling for the taibai granite from ca 800 to 100 c during early cretaceous ca 123 to 100 ma followed by a period of slow cooling from ca 100 ma to ca 25 ma and pulsed exhumation of the low relief cretaceous peneplain during cenozoic times we interpret the early cretaceous rapid cooling and exhumation as a result from activity along the southern sinistral lithospheric scale tear fault of the recently postulated intracontinental subduction of the archean palaeoproterozoic north china block beneath the alashan block a late oligocene to early miocene cooling phase might be triggered either by the lateral motion during india asia collision and or the pacific subduction zone late miocene intensified cooling is ascribed to uplift of the tibetan plateau

constraining the exhumation and burial history of the safod pilot hole with apatite fission track and u th he thermochronometry 1 the san andreas fault observatory at depth safod pilot hole traverses the upper 2 km of a site 1 8 km west of the san andreas fault saf near parkfield california in order to evaluate the burial and exhumation history of the site and its relationship to the kinematics and mechanics of the saf we use 15 apatite fission track ft and 5 u th he analyses from pilot hole samples to document their thermal history sample ages decrease with depth ft and u th he ages range from 60 and 31 ma respectively in the upper 800 m of the hole to 3 and 1 ma at the base of the hole 2 2 km depth 93 c thermal modeling of the distribution of ft lengths indicates three events in the last 80 ma 1 cooling and exhumation of 60 c that culminated at 30 ma 2 reheating of 50 c from 30 to 8 4 ma probably as the result of basin subsidence and burial by 1 1 5 km of sediments and 3 cooling of 30 c and estimated coast range exhumation of 1 km since 8 4 ma

mesozoic and cenozoic tectonic history of the central chinese tian shan reactivated tectonic structures and active deformation 1 the present day topography of the tian shan range is considered to result from crustal shortening related to the ongoing india asia collision that started in the early tertiary in this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event apatite fission track analysis and u th he dating on apatite and zircon indicate that inherited paleozoic structures were reactivated in the late paleozoic early mesozoic during a cimmerian orogenic episode and also in the late cretaceous paleogene around 65 60 ma these reactivations could have resulted from the accretion of the kohistan dras arc or lithospheric extension in the siberia mongolia zone activity resumed in the late mesozoic prior to the major tertiary orogenic phase finally the ongoing deformation which again reactivates inherited tectonic structures tends to propagate inside the endoreic basins that were preserved in the range leading to their progressive closure this study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas

comparison of methodological uncertainties within permeability measurements permeability measurements are critical to the calculation of water flow within hillslopes despite this errors in permeability measurements are often ignored and can be very large particularly in disturbance sensitive gley soils this work compares the uncertainties associated with six field methods of permeametry applied to a gleyed soil in upland britain slug tests constant head borehole permeametry and falling head borehole permeametry were undertaken on established piezometers additionally ring permeametry and two types of trench tests were evaluated method related uncertainty due to proximity of impeding layers of high sorptivity soils produces under and over estimates of permeability by a factor of up to 0 2 and 5 respectively this uncertainty band is smaller than the observed effects of anisotropy and temporal variability had smearing and soil ring leakage errors not been minimized the methodological uncertainties would have been so large that they would have distorted the true spatial field of permeability and its estimated impact on the balance of vertical and lateral flow

contrasting flow pathways within tropical forest slopes of ultisol soils there are very few experimental studies identifying hydrological pathways within rain forest slopes such knowledge is however necessary to understand why forest disturbance affects rainfall riverflow response and nutrient migration this study examines flow pathways within lowland rain forest slopes comprising udults of the ultisol soil order experimentation was conducted on four se asian hillslope units each 5 5 m in plan in the bukit timah catchment singapore island and in the w8s5 catchment sabah borneo island the flow pathways were identified by artificial tracer experiments we evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns the tracer work indicated much faster subsurface flows at bukit timah than w8s5 for the storms studied some explanation of the greater subsurface waterflows at bukit timah in comparison to w8s5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries vertical flow of the tracer through the upper 1 m of soil predominated 90 per cent of percolation in the bukit timah slopes in some contrast a major component approximately 60 per cent of the tracer percolation was directed laterally within the w8s5 slopes the flow vectors calculated using the hydrometric methods did however grossly under estimate the degree of lateral deflection of waterflow generated at w8s5 and to a lesser extent over estimated it at bukit timah in part these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small natural soil pipes present within both catchments in conclusion the study indicates that marked variations in flow vectors exist within the udult great group of se asian soils and hydrometric calculations may be poor predictors of these dominant pathways copyright 2005 john wiley sons ltd

age of 360 m reef terrace hawaii and the rate of late pleistocene subsidence of the island observations from a manned submersible vehicle indicate that the 360 m reef terrace northwest of the island of hawaii is a drowned coral reef the preferred uranium series age of coralline algae collected from the reef face is 120 5 ka this age agrees with the notion that the reef was drowned during the sea level rise following the major lowstand of the sea that occurred at 145 ka oxygen isotope stage 6 this drowning pattern is similar to the previously determined radiocarbon age of 13 ka for drowning of the 150 m reef off west hawaii which drowned during the sea level rise following the last major lowstand of the sea at 18 ka oxygen isotope stage 2 estimated average subsidence of the reef site off northwest hawaii is 2 7 mm yr since final drowning of the 360 m reef

subsidence history of australia s southern margin constraints on basin models the subsidence history of australia s southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation the first phase is characterized by rapid tectonic subsidence about 50 m m y which lasted for about 25 30 m y during the early cretaceous the period of rapid subsidence represents the rifting phase of margin development this early stage is correlated with associated rifting phenomena faulting and extension of the brittle upper crust formation of grabens and half grabens and the rapid accumulation of volcanogenic detritus otway group the amount of extension 20 50 is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin the model includes uniform extension and thinning of the continental lithosphere a prolonged extensional event followed by simple cooling horizontal and vertical conduction of heat and radioactive heat production by the crust the rift phase may have begun earlier but the most rapid and widespread subsidence occurred during the early cretaceous the end of rapid and widespread subsidence occurred during the early cretaceous the end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly about 90 ma adjacent to the continent ocean boundary cob the second stage of margin development is the post rift or drift phase a period of much slower rates of subsidence and relative tectonic quiescence subsidence during the post rift stage is modeled as cooling and contraction of extended continental lithosphere estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the cob estimates of extension from the geometry of basement blocks near the cob do not agree with the analysis from subsidence modeling upper crustal extension seems to record only a small proportion of total lithospheric extension alternative or supplementary mechanisms of extension are explored with emphasis on implications from a model that includes localized shear along a lithospheric detachment system

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

sources of suspended sediment within a tropical catchment recovering from selective logging quantification of the source of suspended sediments generated by selective forestry activities is central to the development of sustainable forestry guidelines this assessment is hampered by a dearth of available studies particularly from the tropics this study involved the monitoring at 10 s intervals of surface discharge and turbidity from 15 contributory areas of a 44 ha catchment the catchment is located within lowland dipterocarp rain forest on borneo island in a region recovering from the first episode of selective timber removal that took place some 5 years previously both the within storm dynamics of the sediment flux and the time integrated sediment yields were analysed to link the source landforms to the catchment behaviour a 10 year rainfall event of 167 mm occurred during the monitoring period and triggered a debris slide and several log culvert collapses along the area s main timber haulage road the sampling design captured this event s dynamics and allowed lumped catchment response to be traced to the new landforms during the 1 day period of the 10 year event some 33 t of suspended sediment were transported from one debris slide comprising a significant proportion of the 105 t discharging from the whole catchment which itself constituted 40 of the annual yield of 592 t km 2 the contributory areas with only ephemeral waterflows including former haulage roads and tracks generated relatively little sediment during this 10 year event or in other storms this work suggests that though some sediment sources recover from the impacts of forest road construction and harvesting collapse of roadfill materials or more local log culvert failure persists for several years after harvesting sustainable forestry guidelines that do not focus on ameliorating these persistent instabilities may not significantly mitigate the geomorphic impacts of conventional selective harvesting copyright 2004 john wiley sons ltd

runoff generation in a steep tropical montane cloud forest catchment on permeable volcanic substrate 1 most studies to date in the humid tropics have described a similar pattern of rapid translation of rainfall to runoff via overland flow and shallow subsurface stormflow however study sites have been few overall and one particular system has received very little attention so far tropical montane cloud forests tmcf on volcanic substrate while tmcfs provide critical ecosystem services our understanding of runoff generation processes in these environments is limited here we present a study aimed at identifying the dominant water sources and pathways and mean residence times of soil water and streamflow for a first order tmcf catchment on volcanic substrate in central eastern mexico during a 6 week wetting up cycle in the 2009 wet season total rainfall was 1200 mm and storm event runoff ratios increased progressively from 11 to 54 with the increasing antecedent wetness conditions our isotope and chemical based hydrograph separation analysis showed increases of pre event water contributions to the storm hydrograph from 35 to 99 stable isotope based mean residence times estimates showed that soil water aged only vertically through the soil profile from 5 weeks at 30 cm depth to 6 months at 120 cm depth a preliminary estimate of 3 years was obtained for base flow residence time these findings all suggest that shallow lateral pathways are not the controlling processes in this tropical forest catchment rather the high permeability of soils and substrate lead to vertical rainfall percolation and recharge of deeper layers and rainfall runoff responses appeared to be dominated by groundwater discharge from within the hillslope

cenozoic thermal evolution of the central sierra nevada california from uth he thermochronometry apatite u th he cooling ages are reported for igneous apatite samples from the central sierra nevada and compared to published apatite fission track ages and track length data from the same mineral separates helium ages are youngest at low elevations and increase systematically toward higher elevations ranging from 43 to 84 ma at yosemite valley 32 to 74 ma at kings river canyon and 23 to 75 ma at mt whitney helium ages from high elevation samples are generally concordant with corresponding fission track ages while lower elevation helium ages are substantially younger cooling histories inferred from present laboratory derived fission track annealing and helium diffusion models do not match well suggesting that either helium diffusion rates or fission track annealing rates are miscalibrated at temperatures below about 60 c for geologic exposure periods unlike the fission track results the helium data do not indicate a very low geothermal gradient in the sierra nevada during early to middle tertiary time

an evaluation of low temperature apatite uth he thermochronometry abstract three studies have been conducted to test the usefulness of apatite u th he thermochronometry the first study compared u th he and fission track ages from quickly cooled plutonic rocks having concordant zircon and apatite fission track ages several samples analyzed show good helium retention the second experiment assessed the effect of compositional variations in apatite on closure temperature calculated closure temperatures agree with existing data and show no statistical dependence on composition arrhenius plots for all of the samples show linear trends at low temperatures representing volume diffusion that roll over to more shallowly sloping non linear paths at higher temperatures the roll over temperature appears to increase with increasing cl content in the apatite the third test focused on fluorapatites from the ktb borehole in germany to compare laboratory estimates with diffusion behavior in a natural setting the ktb apatites show a fair amount of scatter and record u th he ages that are younger than or equal to apatite fission track ages indicating a closure temperature less than or equal to that for the apatite fission track method closure temperatures predicted from the scattered data range from 80 120 c 1 c ma cooling rate or 95 135 c 10 c ma cooling rate the low end of this range agrees with our laboratory diffusion estimates for fluorapatite

l apatite de durango mexique analyse d un mineral standard pour la datation par traces de fission abstract durango apatite is analyzed by four analysts using three different dating techniques both the annealing technique and the external detector technique are applied to basal and prismatic sections cut from single crystals the population technique is applied to mounts of randomly oriented grains provided an individual determination of the geometry factor is performed all ages are indistinguishable from the reference age of 31 4 0 5 ma there is no noticeable influence of either the dating technique or the crystallographic orientation of the dated section on the ft age based on the projected lengths of surface tracks spontaneous track length reduction varies from 4 to 10 mean 6 depending on the analyst two simple corrections are applied to eliminate the net effect of the missing short tracks on the mean projected length after correction the amount of shortening of the spontaneous tracks is found to be slightly larger mean 8 but is still dependent on the analyst 4 to 12 the mean is in fairly good agreement with the value obtained from confined track length measurements 9 finally an isochronal 1 h plateau annealing procedure was applied to a separate set of basal and prismatic sections cut from the same single crystals annealing temperatures up to 300 c do not affect the ft age erratic ages at 350 c are ascribed to the occurrence of unetchable gaps it is argued that the absence of a distinct plateau up to temperatures where it could be expected from theoretical considerations indicates that the ft age of durango apatite needs no correction

uplift and erosion of the san bernardino mountains associated with transpression along the san andreas fault california as constrained by radiogenic helium thermochronometry apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the san andreas fault by documenting aspects of the low temperature 40 100 c thermal history of the tectonic blocks of the san bernardino mountains in southern california we have placed new constraints on the magnitude and timing of uplift old helium ages 64 21 ma from the large big bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the late cretaceous period these ages imply that the surface of the plateau may have been exposed in the late miocene and was uplifted only 1 km above the mojave desert in the last few myr by thrusting on the north and south a similar range in helium ages 56 14 ma from the higher san gorgonio block to the south suggests that its crest was once contiguous with that of the big bear block and that its greater elevation represents a localized uplift that the big bear plateau did not experience the structure of the san gorgonio block appears to be a gentle antiform based on the geometry of helium isochrons and geologic constraints young ages 0 7 1 6 ma from crustal slices within the san andreas fault zone indicate uplift of a greater magnitude than blocks to the north these smaller blocks probably experienced 3 4 km of uplift at rates 1 5 mm yr in the past few myr and would stand 2 5 km higher than the big bear plateau if erosion had not occurred the greater uplift of tectonic blocks adjacent to and within the san andreas fault zone is more likely the result of oblique displacement along high angle faults than motion along the thrust fault that bounds the north side of the range we speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike slip system transpression thus appears to have been accommodated by both vertical displacement within the san andreas fault zone and thrusting on adjacent structures

flowpaths source water contributions and water residence times in a mexican tropical dry forest catchment summary runoff in forested tropical catchments has been frequently described in the literature as dominated by the rapid translation of rainfall to runoff through surface and shallow subsurface pathways however studies examining runoff generation in tropical catchments with highly permeable soils have received little attention particularly in tropical dry forests we present a study focused on identifying the dominant flowpaths water sources and stream water residence times in a tropical dry forest catchment near the pacific coast of central mexico during the wet season pre event water contributions to stormflow ranged from 72 to 97 with the concentrations of calcium magnesium sodium and potassium closely coupling the geochemistry of baseflow and groundwater from the narrow riparian near stream zone baseflow from the intermittent stream showed a strongly damped isotopic signature and a mean baseflow residence time of 52 110 days was estimated these findings all suggest that instead of the surface and near surface subsurface lateral pathways observed over many tropical catchments runoff is generated through vertical flow processes and the displacement and discharge of stored water from the saturated zone as the wet season progressed contributions from the saturated zone persisted however the stormflow and baseflow geochemistry suggests that the contributing area of the catchment increased our results show that during the early part of the wet season runoff originated primarily from the headwater portion of the catchment as the wet season progressed and catchment wetness increased connectivity among sub basin was improved resulting in runoff contributions from across the entire catchment

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

thermochronological data from sudan in the frame of the denudational history of the nubian red sea margin low temperature thermochronology provides information on the timing of rifting and denudation of passive margins and the red sea with its well exposed young rift margins is a suitable setting for its application here we present new apatite fission track aft data from sudan northern hinterland and red sea coastal areas from the former region we obtained ages between 270 2 ma ad 253 53 ma and from the coastal belt between 83 8 ma and 39 7 ma these data prompted a review and comparison with low temperature thermochronological data from the whole nubian red sea margin and a discussion on their implication in assessing the margin evolutionary style aft data are available for egypt and eritrea as well as apatite u th he ahe ages for two transects transversal to the margin in eritrea both in egypt and eritrea aft data record a cooling event at about 20 25 ma early miocene and an earlier more local cooling event in egypt at about 34 ma early oligocene the thermal modeling of the sudan samples provides an indication of a rapid cooling in miocene times but does not support nor rules out an early oligocene cooling phase the re assessment of new and existing thermochronological data within the known geological framework of the nubian and conjugate arabian margins favours the hypothesis that early rifting stages were affecting the whole gulf of suez red sea gulf of aden system since the oligocene these precocious more attenuated phases were followed by major extension in miocene times as to the mode of margin evolution aft age patterns both in egypt and eritrea are incompatible with a downwarp model the distribution of ahe ages across the eritrean coastal plain suggests that there the escarpment was evolving predominantly by plateau degradation copyright 2009 john wiley sons ltd

controls on the spatial distribution of natural pipe outlets in heavily degraded blanket peat abstract natural soil pipes are recognised as a common geomorphological feature in many peatlands and they can discharge large quantities of water and sediment however little is known about their morphological characteristics in heavily degraded peat systems this paper presents a survey of pipe outlets in which the frequency and extent of natural soil pipes are measured across a heavily gullied blanket peat catchment in the peak district of northern england over a stream length of 7 71 km we determined the occurrence and size of 346 pipe outlets and found a mean frequency of 22 8 km 1 gully bank topographic position was an important control on the size and depth of pipe outlets aspect had a large influence on pipe outlet frequency with southwest and west facing gully banks hosting more than 43 of identified pipe outlets pipe outlets on streambanks with signs of headward retreat were significantly larger and closer to the peat surface compared to pipe outlets that issued onto uniform streambank edges we suggest that larger pipe frequencies are observed on gully banks that are more susceptible to desiccation cracking and propose that future peatland restoration works could prioritise mitigating against pipe formation by revegetating and reprofiling south and west facing gully banks

assessment of grass root effects on soil piping in sandy soils using the pinhole test abstract soil piping is an important land degradation process that occurs in a wide range of environments despite an increasing number of studies on this type of subsurface erosion the impact of vegetation on piping erosion is still unclear it can be hypothesized that vegetation and in particular plant roots may reduce piping susceptibility of soils because roots of vegetation also control concentrated flow erosion rates or shallow mass movements therefore this paper aims to assess the impact of grass roots on piping erosion susceptibility of a sandy soil the pinhole test was used as it provides quantitative data on pipeflow discharge sediment concentration and sediment discharge tests were conducted at different hydraulic heads i e 50 mm 180 mm 380 mm and 1020 mm results showed that the hydraulic head was positively correlated with pipeflow discharge sediment concentration and sediment discharge while the presence of grass roots expressed as root density was negatively correlated with these pipeflow characteristics smaller sediment concentrations and sediment discharges were observed in root permeated samples compared to root free samples when root density exceeds 0 5 kg m 3 piping erosion rates decreased by 50 compared to root free soil samples moreover if grass roots are present the positive correlation between hydraulic head and both sediment discharge and sediment concentration is less pronounced demonstrating that grass roots become more effective in reducing piping erosion rates at larger hydraulic heads overall this study demonstrates that grass roots are quite efficient in reducing piping erosion rates in sandy soils even at high hydraulic head 1 m as such grass roots may therefore be used to efficiently control piping erosion rates in topsoils

an empirical test of helium diffusion in apatite borehole data from the otway basin australia we have analyzed helium ages of apatites from several boreholes in the otway basin australia to evaluate whether laboratory helium diffusivity can be accurately extrapolated to conditions relevant in nature downhole apatite helium ages define a broad swath of values from 78 71 ma at the surface 15 c to nearly zero at depths corresponding to ambient temperatures of 80 c the width of the swath results from uncertainties in corrected borehole temperatures differences in the thermal history experienced by the various boreholes and possibly from slightly different helium diffusivities among the detrital apatite grains studied in the eastern otway basin the shape and position of the helium age profile is in good agreement with predictions based on the extrapolation of laboratory diffusivity data for durango apatite coupled with published thermal histories for this part of the basin in contrast helium ages are much younger than predicted in the western otway basin based on measured ages from otway sediments which have been essentially isothermal over the last few million years an empirical diffusivity d 2 of 2 10 15 s 1 can be assigned to apatites residing at downhole temperatures of 67 97 c this empirical diffusivity is consistent with laboratory diffusion measurements demonstrating that such measurements are reasonably accurate and can be applied with confidence to natural geologic settings given this confirmation of the laboratory diffusivity data the discrepancy between the observed and modeled helium age profiles in the western otway basin suggests that these sediments recently experienced higher temperatures than presently supposed

new evidence on the neogene uplift of south tianshan constraints from the u th he and aft ages of borehole samples of the tarim basin and implications for hydrocarbon generation we identified a neogene rapid uplift denudation event of the south tianshan based on apatite u th he and apatite fission track aft ages in tertiary rocks of the tarim basin using borehole samples the u th he thermochronology can be used to reveal the tectono thermal events with lower temperature than that of aft thermochronology and has not been used previously to study the uplift of the tianshan mountain using these data we show the relationship between the uplift of the south tianshan and the subsidence deposition of the northern tarim basin during the neogene the apatite helium ages reveal the migration of uplift erosion and deposition in the northern tarim basin a rapid uplift of the south tianshan during the miocene and a corresponding rapid subsidence in the northern tarim basin occurred however in the pliocene the kuqa depression and south tianshan uplifted and eroded at the same time and in turn provided the detrital source rocks for the northern uplift of the tarim basin in contrast to earlier studies we arrive at the conclusion that the south tianshan experienced rapid uplift in the miocene based on u th he data of apatite obtained from borehole samples collected in the tarim basin itself and not from the bordering mountain chain combined apatite u th he and fission track thermochronometry enables reconstruction of thermal histories of sedimentary rocks between 40 and 120 c and this has implications for the generation of liquid hydrocarbon in the 65 120 c range in the basin thermal and burial histories of typical samples were also modelled to show the rapid uplift in our study our works not only provide a new evidence for the south tianshan uplift but also indicate that there is a coupling between uplift and subsidence in the south tianshan and adjacent northern part of the tarim basin which controlled hydrocarbon accumulation in the kuqa depression and northern uplift of the tarim basin

assessing the role of orogen parallel faulting in post orogenic exhumation low temperature thermochronology across the norumbega fault system maine as a part of a regional effort to determine the extent of low temperature thermochronological discontinuities across major orogen parallel faults in northern new england 41 apatite fission track

accommodation of transpressional strain in the arabia eurasia collision zone new constraints from u th he thermochronology in the alborz mountains north iran the alborz range of n iran provides key information on the spatiotemporal evolution and characteristics of the arabia eurasia continental collision zone the southwestern alborz range constitutes a transpressional duplex which accommodates oblique shortening between central iran and the south caspian basin the duplex comprises nw striking frontal ramps that are kinematically linked to inherited e w striking right stepping lateral to obliquely oriented ramps new zircon and apatite u th he data provide a high resolution framework to unravel the evolution of collisional tectonics in this region our data record two pulses of fast cooling associated with sw directed thrusting across the frontal ramps at 18 14 and 9 5 7 5 ma resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half u shaped geometry uniform cooling ages of 7 6 ma along the southernmost e w striking oblique ramp and across its associated nw striking frontal ramps suggests that the ramp was reactivated as a master throughgoing n dipping thrust we interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from ne to n nne the reduction in the obliquity of thrusting may indicate the termination of strike slip faulting and possibly thrusting across the iranian plateau which could have been triggered by an increase in elevation furthermore we suggest that 7 6 m y old s directed thrusting predated inception of the westward motion of the south caspian basin

faunal burrows as indicators of paleo vegetation in eastern washington usa abstract the palouse loess deposits in the pacific northwestern united states contain buried paleosols with distinctive biological fabrics the washtucna soil formed 40 000 15 000 yr ago during the last full glacial contains up to 90 by volume distinctive cylindrical pedotubules that are 1 2 cm in diameter formed by burrowing fauna the overlying post glacial loess and modern surface soils that support perennial grasslands lack these cylindrical burrows we investigated the paleoecological significance of the burrows observations of several types of burrowing fauna under native vegetation indicated that nymphs of cicadas homoptera cicadidae are responsible for cylindrical back filled burrows that match those in the washtucna soil we measured the abundance of active cicada burrows in soils in four native vegetation zones soil sites establishing that cicada burrows comprise 19 of the rooting zone volume in sagebrush steppe but just 1 3 of the rooting zone volume in bunchgrass steppe meadow steppe and coniferous forest this suggests strongly that abundant cicada burrows in paleosols are a proxy for the geographic extent of plant communities that contained sagebrush to assay the activity of cicada nymphs through time we measured burrowed volume by depth from the soil surface down through the washtucna soil i e from the present to ca 40 000 yr bp at five stratigraphic research sites paleosol sites cicada burrowed volume is generally less than 5 in the rooting zone of the surface soils and throughout holocene loess but is up to 94 in the rooting zone of the washtucna soil at paleosol sites that currently support bunchgrass steppe outside the present day sagebrush steppe zone cicada host preferences in vegetation zones today suggest that sagebrush was the dominant shrub as the washtucna soil formed from this reconstruction combined with information from studies of pollen and plant opal we infer that periglacial steppe plant communities dominated by sagebrush spread from 40 000 to 15 000 yr bp into areas of the columbia plateau that support bunchgrass steppe meadow steppe and even some coniferous forest today trace fossils of soil fauna in paleosols are indicators of fluctuations in biological activity in response to climatic changes of the quaternary period in palouse loess deposits

thermochronological constraints on the multiphase exhumation history of the ivrea verbano zone of the southern alps abstract the ivrea verbano zone of the western southern alps nw italy exposes a well preserved tilted section across the lower continental crust making it a key region for studying deep crustal and exhumation processes this paper refines the cooling and exhumation history of the ivrea verbano zone using k ar dating of mica and illite rich fault gouges as well as zircon fission track and u th he thermochronology the adjacent strona ceneri zone sesia lanzo zone and lower penninic nappes are included in the study to derive a broader picture of the low temperature history of the area in the strona profile of the ivrea verbano unit the biotite k ar zircon fission track and u th he geochronometers show well preserved but unusually wide partial retention zones the youngest ages representing the formerly deepest position are situated along the insubric line the main foliation of the ivrea verbano zone is vertical the exhumation of the ivrea verbano zone which section has a horizontal position on the surface now took place in three steps during jurassic time the ivrea verbano zone was exhumed to a shallow to mid crustal position by continental scale extension in this displacement the pogallo line probably played a dominant role the studied section occupied an oblique position with a calculated angle of ca 15 to 23 in the jurassic later the ivrea verbano zone experienced a minor cooling event in the late eocene 38 ma zircon fission track ages that was probably related to thrusting and erosion the final exhumation towards the surface took place in the mid miocene as documented by the ca 14 ma zircon u th he ages and a 12 8 ma k ar fault gouge age the magnitude and the high rate of final exhumation suggest orogen parallel extension as a driving force which is widespread in the alps in the lower to middle miocene and is most probably connected to orogenic collapse

thermochronometrically constrained anatomy and evolution of a miocene extensional accommodation zone and tilt domain boundary the southern wassuk range nevada 1 apatite ahe and zircon zhe u th he thermochronometric data from the southern wassuk range wr coupled with 40ar 39ar age data from the overlying tilted tertiary section are used to constrain the thermal evolution of an extensional accommodation zone and tilt domain boundary ahe and zhe data record two episodes of rapid cooling related to the tectonic exhumation of the wr fault block beginning at 15 and 4 ma extension was accommodated through fault block rotation and variably tilted the southern wr to the west from 60 70 in the central wr to 15 35 in the southernmost wr and pine grove hills and minimal tilting in the anchorite hills and along the mina deflection to the south middle miocene geothermal gradient estimates record heating immediately prior to large magnitude extension that was likely coeval with the extrusion of the lincoln flat andesite at 14 8 ma geothermal gradients increase from 19 4 c km to 65 20 c km toward the mina deflection suggesting that it was the focus of middle miocene arc magmatism in the upper crust the decreasing thickness of tilt blocks toward the south resulted from a shallowing brittle ductile transition zone postmagmatic middle miocene extension and fault block advection were focused in the northern and central wr and coincidentally moderated the large lateral thermal gradient within the uppermost crust

40ar 39ar thermochronometry of k feldspar from the ktb borehole germany abstract the great depth and relatively steep geothermal gradient 29 c km of the ktb deep crustal borehole located in the bavarian region of germany provides the basis for our study of diffusive argon loss in k feldspar the observed borehole temperature at 9 km is 265 c which reaches well into the argon closure interval for k feldspar 125 350 c four microcline and two adularia samples distributed relatively evenly down the 9 1 km deep borehole were analyzed by the 40ar 39ar method the thermal histories of the shallow samples are consistent with existing thermochronologic data from the borehole after correction for chlorine correlated excess 40ar the deepest two samples result in age spectra that record zero apparent ages over the first few percent of 39ar released which is predicted by diffusion theory where a range of diffusion volumes are present inverse modeling of the argon kinetics of these two deep samples reveals that the present elevated geothermal gradient in the borehole has existed for only the last approximately one million years recent volcanism within 30 km of the borehole is likely the source of this present day thermal pulse

thermochronologic evaluation of postcollision extension in the anatolide orogen western turkey 1 to better understand the driving mechanisms behind the transition from collision to extension in a convergent orogen data from multiple low temperature thermochronometers were obtained from the simav detachment fault sdf the earliest developed major extensional structure recognized in the western anatolide orogen of western turkey twenty two zircon fission track ft 26 apatite ft 12 apatite u th he ages and 26 apatite ft track length analyses are presented the data establish that the sdf was a major extensional fault active between 25 and 19 ma the coincidence in timing of magmatism with cessation of sdf activity implies the detachment became locked owing to doming induced by magma emplacement zircon ft ages away from the influence of miocene magmatism record rapid footwall cooling between 25 and 21 ma at a slip rate of up to 15 km myr and demonstrate that active ductile deformation along the sdf migrated northward with time apatite ft ages from the footwall of the sdf are spatially invariant over 100 km and consistently 2 3 myr younger than the zircon fission track ages from the same samples these data are consistent with a regional but relatively rapid erosion linked cooling phase that removed 2 3 km of overburden following cessation of sdf activity but before deposition of sedimentary rocks on the detachment surface at 16 4 ma postcollision extension of the anatolide orogen with development of the sdf can be best explained as the result of postcollision magmatism and thermal weakening of the crust inducing the extensional reactivation of an earlier major out of sequence thrust

helium and argon thermochronometry of the gold butte block south virgin mountains nevada one of the largest exposures of precambrian crystalline rock in the basin and range province of the southwestern usa is the gold butte block of the south virgin mountains about 15 km west of the colorado plateau it has been interpreted as a largely continuous crustal cross section about 15 20 km thick that was exhumed by a deeply penetrating normal fault during miocene extension to test this interpretation as well as the use of the newly developed titanite u th he thermochronometer we examined the low temperature thermal history of the gold butte block with the apatite and titanite u th he and muscovite 40 ar 39 ar thermochronometers apatite he ages average 15 2 1 0 2 ma throughout the block indicating that the entire section was warmer than 70 c prior to miocene exhumation titanite he ages increase from 18 6 1 5 ma near the paleobottom west end of the block to 195 15 ma near the paleotop east end a rapid change from mid tertiary to increasingly older titanite he ages to the east is observed at about 9 3 km paleodepth and is interpreted as a fossil he partial retention zone for titanite assuming a pre exhumation geotherm of 20 c km consistent with earlier apatite fission track work this depth would have corresponded to 196 c prior to exhumation indicating that laboratory derived he diffusion characteristics for titanite that yield a closure temperature of about 200 c are applicable and correct muscovite 40 ar 39 ar ages are 1 0 1 4 ga near the paleotop of the block and 90 ma near the paleobottom together with 207 pb 206 pb ages on apatite and titanite and an earlier apatite fission track transect across the gold butte block our data indicate that the continental crust at the western edge of the colorado plateau resided at moderate geothermal gradients and slowly declined in temperature from 1 4 ga to about 100 200 ma a 90 ma cooling event clearly affected the mid crust deepest portions of gold butte which may reflect accelerated cooling or a brief heating and cooling cycle at this time after which gradients returned to about 20 c km prior to rapid exhumation in the miocene this work thus supports previous structural and thermochronologic studies that suggest that the gold butte block is the thickest largely continuous cross section of crust exposed in the southwestern usa

solving the production diffusion equation for finite diffusion domains of various shapes part i implications for low temperature u th he thermochronology abstract we propose an accurate fast and easy to use method to derive numerical solutions for production diffusion equations for finite diffusion domains of various shapes and arbitrary cooling histories previous studies provide solutions for spheres infinite cylinders and infinite sheets we extend this range and provide solutions for finite bodies i e finite cylinders and rectangular blocks of any aspect ratio this approach is important as recently it has become clear that for example the physical grain is the diffusion domain for he diffusion in apatite and titanite j geophys res 105 2000 2903 geochim cosmochim acta 63 1999 3845 we discuss the use of the new approach for forward modelling u th he production diffusion in apatite taking results with finite cylinders as a good approximation for apatite crystals it is found that approximating instead with spheres or infinite cylinders having the same radius yields differences in calculated ages that can easily be as large as 20 35 the relative differences are most pronounced in thermal histories that spend significant time at or near the closure temperature on the other hand reasonable agreement is found with spheres having the same surface to volume ratio

topographic controls on overland flow generation in a forest an ensemble tree approach summary overland flow is an important hydrological pathway in many forests of the humid tropics its generation is subject to topographic controls at differing spatial scales our objective was to identify such controls on the occurrence of overland flow in a lowland tropical rainforest to this end we installed 95 overland flow detectors ofds in four nested subcatchments of the lutzito catchment on barro colorado island panama and monitored the frequency of overland flow occurrence during 18 rainfall events at each ofd location temporal frequency for each such location we derived three non digital terrain attributes and 17 digital ones of which 15 were based on digital elevation models dems of three different resolutions these attributes then served as input into a random forest ensemble tree model to elucidate the importance and partial and joint dependencies of topographic controls for overland flow occurrence lutzito features a high median temporal frequency in overland flow occurrence of 0 421 among ofd locations however spatial temporal frequencies of overland flow occurrence vary strongly among these locations and the subcatchments of lutzito catchment this variability is best explained by 1 microtopography 2 coarse terrain sloping and 3 various measures of distance to channel with the contribution of all other terrain attributes being small microtopographic features such as concentrated flowlines and wash areas produce highest temporal frequencies whereas the occurrence of overland flow drops sharply for flow distances and terrain sloping beyond certain threshold values our study contributes to understanding both the spatial controls on overland flow generation and the limitations of terrain attributes for the spatially explicit prediction of overland flow frequencies

natural age dispersion arising from the analysis of broken crystals part ii practical application to apatite u th he thermochronometry abstract we describe a new numerical inversion approach to deriving thermal history information from a range of naturally dispersed single grain apatite u th he ages the approach explicitly exploits the information about the shape of the 4he diffusion profile within individual grains that is inherent in the pattern of dispersion that arises from the common and routine practice of analysing broken crystals additional dispersion arising from differences in grain size and in u and th concentration of grains and the resultant changes to helium diffusivity caused by differential accumulation and annealing of radiation damage is explicitly included in this approach we calculate the ingrowth and loss due to both thermal diffusion and the effects of ejection of helium over time using a finite cylinder geometry broken grains are treated explicitly as fragments of an initially larger crystal the initial grain lengths l0 can be treated as unknown parameters to be estimated although this is computationally demanding a practical solution to the problem of solving for the unknown initial grain lengths is to simply apply a constant and sufficiently long l0 value to each fragment we found that a good value for l0 was given by the maximum fragment length plus two times the maximum radius of a given set of fragments currently whole crystals and fragments with one termination are taken into account a set of numerical experiments using synthetic fragment ages generated for increasingly complex thermal histories and including realistic amounts of random noise 5 15 are presented and show that useful thermal history information can be extracted from datasets showing very large dispersion these include experiments where dispersion arises only from fragmentation of a single grain length 400 m and radius 75 m c 6 50 dispersion including the effects of grain size variation for spherical equivalent grain radii between 74 and 122 m c 10 70 dispersion and the combined effects of fragmentation grain size and radiation damage for eu between 5 and 150 ppm c 10 107 dispersion additionally we show that if the spherical equivalent radius of a broken grain is used as a measure of the effective diffusion domain for thermal history inversions then this will likely lead to erroneous thermal histories being obtained in many cases the viability of the new technique is demonstrated for a real data set of 25 single grain u th he apatite ages obtained for a gabbro sample from the bk 1 bierkraal borehole drilled through the bushveld complex in south africa the inversion produces a well constrained thermal history consistent with both the u th he data and available fission track analysis data the advantage of the new approach is that it can explicitly accommodate all the details of conventional schemes such as the effects of temporally variable diffusivity zonation of u and th and arbitrary grain size variations and it works equally effectively for whole or broken crystals and for the most common situation where a mixture of both are analysed for the routine application of the apatite u th he thermochronometry technique with samples where whole apatite grains are rare our experiments indicate that 15 20 single grain analyses are typically required to characterise the age dispersion pattern of a sample the experiments also suggest that picking very short crystal fragments as well as long fragments or even deliberately breaking long crystals to maximise the age dispersion in some cases would ensure the best constraints on the thermal history models the inversion strategy described in this paper is likely also directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size