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At least 523 records · Page 29Linked to original sources
USGS2023

Hydrologic compartmentalization and analytic-element groundwater-flow simulations for a draining mine tunnel

Draining mine tunnels contribute contaminants to groundwater and surface water, but remediation strategies may be hindered as hydrogeologic characterization and modeling of these heterogeneous features generally relies on sparse data sets. The Captain Jack mine site in Colorado, USA, presents a unique data set allowing for temporal evaluation of groundwater connectivity in the vicinity of an abandoned mine, where a hydraulic bulkhead is impounding water within the mine workings. This study applied statistical analysis of system pressure responses to bulkheading and used an analytic-element modeling approach to characterize heterogeneity and groundwater flow. Groundwater-level elevation data collected over a period of 4 years, both prior to and after bulkheading, indicate that the mine workings act as a sink to the local groundwater system. Despite groundwater flow being generally oriented towards the mine workings, there are also large vertical and horizontal hydraulic gradients which persist through time. Although the groundwater system is highly compartmentalized, statistical analysis using Kendall’s Tau indicates correlations between hydraulic head changes in the mine workings and several wells completed in crystalline bedrock, indicating the influence of fracture flow. An analytic-element model was parameterized to account for uncertainty in hydraulic conductivity, recharge, and discharge. Model results reproduced the range of observed hydraulic heads in the mine workings and adjacent igneous dikes but failed to closely simulate hydraulic heads in several wells located distal from the mine workings in granitic bedrock. The modeling approach shows potential promise, however, for conducting preliminary modeling to guide data collection at other similar mine sites.

Colorado↗
USGS2023

Attenuation of acid rock drainage by stimulating sulfur-reducing bacteria

Iron-sulfide minerals found in shale formations are stable under anaerobic conditions. However, in the presence of oxygen and water, acid-loving chemolithotrophic bacteria can transform the iron-sulfide minerals into a toxic solution of sulfuric acid and dissolved iron and minerals known as acid rock drainage (ARD). The objective of this study was to disrupt chemolithotrophic bacteria responsible for ARD using chemical treatments and to foster an environment favorable for competing microorganisms to attenuate the biologically induced ARD. Chemical treatments were injected into flow-through microcosms consisting of 501 g of pyrite-rich shale pieces inoculated with ARD bacteria. Three treatments were tested in the microcosms: (1) a sodium hydroxide-bleach mix, (2) a sodium lactate solution, and (3) a sodium lactate-soy infant formula mix. The effectiveness of the treatments was assessed by monitoring pH, dissolved iron, and other geochemical constituents in the discharge waters. The optimal treatment was a sequential injection of 1.5 g sodium hydroxide, followed by 0.75 g lactate and 1.5 g soy formula dissolved in 20 mL water. The pH of the discharge water rose to 6.0 within 10 days, dissolved iron concentrations dropped below 1 mg/L, the median alkalinity increased to 98 mg/L CaCO 3 , and sulfur-reducing and slime-producing bacteria populations were stimulated. The ARD attenuating benefits of this treatment were still evident after 231 days. Other treatments provided a number of ARD attenuating effects but were tempered by problems such as high phosphate concentrations, short longevity, or other shortcomings. The results of these laboratory microcosm experiments were promising for the attenuation of ARD. Additional investigations and careful selection of treatment methods will be needed for field application.

Tennessee↗
USGS2023

Interlaboratory comparison of testing hydraulic, elastic, and failure properties in compression: Lessons learned

Many geoscientific problems require us to exploit synergies of experimental and numerical approaches, which in turn lead to questions regarding the significance of experimental details for validation of numerical codes. We report results of an interlaboratory comparison regarding experimental determination of mechanical and hydraulic properties of samples from five rock types, three sandstone varieties with porosities ranging from 5% to 20%, a marble, and a granite. The objective of this study was to build confidence in the participating laboratories’ testing approaches and to establish tractable standards for several physical properties of rocks. We addressed the issue of sample-to-sample variability by investigating the variability of basic physical properties of samples of a particular rock type and by performing repeat tests. Compressive strength of the different rock types spans an order of magnitude and shows close agreement between the laboratories. However, differences among stress–strain relations indicate that the external measurement of axial displacement and the determination of system stiffness require special attention, apparently more so than the external load measurement. Furthermore, post-failure behavior seems to exhibit some machine-dependence. The different methods used for the determination of hydraulic permeability, covering six orders of magnitude for the sample suite, yield differences in absolute values and pressure dependence for some rocks but not for others. The origin of the differences in permeability, in no case exceeding an order of magnitude, correlate with the compressive strength and potentially reflect a convolution of end plug–sample interaction, sample-to-sample variability, heterogeneity on sample scale, and/or anisotropy, the last two aspects are notably not accounted for by the applied evaluation procedures. Our study provides an extensive data set apt for “benchmarking” considerations, be it regarding new laboratory equipment or numerical modeling approaches.

Environmental Earth Sciences↗
USGS2024

Ambient flow and transport in long-screened, sand-packed wells: Insights into cross contamination and wellbore flow

The presence of long-screened wells with a surrounding sand pack can have a major effect on the redistribution of contaminants in groundwater, particularly when the wells are set in low-hydraulic conductivity aquifers. Such redistribution, or cross contamination, can occur through vertical flow and advective transport or by in-well mixing via multiple non-advective transport processes. A multi-method approach, including the use of single borehole dilution tracer (SBDT) logging, was undertaken to estimate vertical transport of trichloroethylene (TCE) in 8 discontinued remedial extraction wells, all constructed with long screens (100 ft, or 30.6 m long) and surrounding sand packs, at Site 25, Edwards Air Force Base, California. The site is within an enclosed drainage basin that is underlain primarily by quartz-monzonite-granitic rocks in various states of weathering. Prior to this study, little information was available on the depths of fracture zones intersecting the wellbores. Results indicate that because of in-well mixing processes, a potential redistribution of TCE of up to 9 g/d per well occurs as a consequence of leaving the wells inactive (unpumped) and unsealed, as measured by SBDT logging. Simulations of flow made with a generic model of the site show that if the wells were to be sealed with well liners, with the intent of reducing vertical TCE transport but the sand pack left intact, TCE transport decreases by 53% overall compared to leaving the wells unlined.

Environmental Earth Sciences↗
USGS1990

The initial instability and finite-amplitude stability of alternate bars in straight channels

The initial instability and fully developed stability of alternate bars in straight channels are investigated using linearized and nonlinear analyses. The fundamental instability leading to these features is identified through a linear stability analysis of the equations governing the flow and sediment transport fields. This instability is explained in terms of topographically induced steering of the flow and the associated pattern of erosion and deposition on the bed. While the linear theory is useful for examining the instability mechanism, this approach is shown to yield relatively little information about well-developed alternate bars and, specifically, the linear analysis is shown to yield poor predictions of the fully developed bar wavelength. A fully nonlinear approach is presented that permits computation of the evolution of these bed features from an initial perturbation to their fully developed morphology. This analysis indicates that there is typically substantial elongation of the bar wavelength during the evolution process, a result that is consistent with observations of bar development in flumes and natural channels. The nonlinear approach demonstrates that the eventual stability of these features is a result of the interplay between topographic steering effects, secondary flow production as a result of streamline curvature, and gravitationally induced modifications of sediment fluxes over a sloping bed.

Earth-Science Reviews↗
USGS1967

Ground-water research in the U.S.A.

Ground-water reservoirs and the overlying unsaturated zone-collectively, the "subsurface"-have an enormous capacity to supply water to wells and useful plants, to store water to meet future needs for the same purposes, and, under suitable precautions, to accept wastes. This capacity can be exploited on a maximum scale, however, only on the basis of information one or more orders of magnitude greater than that available at present on the distribution, recoverability, and replenishability of subsurface water. Because usable water must be made available, and waste water must be disposed of, at costs of only a cent or a few cents per cubic meter, there is a critical need for research to devise methods of accomplishing these water-management tasks at reasonable cost. Among the chief target areas for research in subsurface hydrology are permeability distribution, including vertical permeability; prediction of the departure of the storage coefficient from the theoretically "instantaneous" property assumed in flow equations; theory of unsaturated flow based on fundamental soil characteristics that can be measured practicably; geochemical relations including the effects of injecting water of one composition into zones occupied by waters of different composition, generation of acid mine water, occurrence of saline water, and salt-fresh-water relations in coastal and other areas; prediction of the fate of wastes injected underground; geophysical techniques both surface and subsurface to extend, at low cost, information obtained by other means; and practical techniques of artificial recharge, especially through wells. ?? 1967.

Earth-Science Reviews↗
USGS1988

Dating shorelines of lakes in Patagonia, Argentina

Organic remains from elevated shorelines of two closed basins were radiocarbon dated and document lakes substantially larger than today between 19,500 and 15,000 years BP in northern Patagonia (lat. 41°S), but only for the last 4000 years in southern Patagonia (lat. 49°S). Such latitudinal difference in the timing of high lake levels is probably related to differences in past precipitation regimes, previously also suggested from the study of pollen records from these latitudes in Argentina.

Journal of South American Earth Sciences↗
USGS1989

Late neogene history of the Pacific-Caribbean gateway

Planktic foraminiferal provinces of Caribbean DSDP Hole 502A and East Pacific DSDP Hole 503A have been analyzed and compared with benthic and planktic isotope records, carbonate, hiatus events, and sea level changes. Four major events are evident in the closure history of the Pacific-Caribbean gateway, at 6.2, 4.2, 2.4 and 1.8 Ma. The faunal change at 6.2 Ma coincides with the δ 13 C shift and is primarily caused by upwelling in the western Caribbean. This suggests restricted circulation of intermediate water and deflection northeastward, strengthening the Gulf Stream as reflected in the first major erosion on Blake Plateau. The second faunal change, at 4.2 Ma, coincides with increased surface water salinity evident in δ 18 O data and indicates increasingly restricted surface water exchange. Divergence of faunal provinces beginning at 2.4 Ma is marked by increasing abundance of high salinity tolerant species ( Globigerinoides ruber ) in the Caribbean. This suggests that initial closure of the Pacific-Caribbean gateway and cessation of sustained surface current flow between the Pacific and Caribbean occurred as late as 2.4 Ma. Maximum divergence of faunal provinces begins at 1.8 Ma and continues to the present. This implies that at least incipient littoral-neritic leakage occurred across the Pacific-Caribbean gateway between 2.4 and 1.8 Ma, with final closure by 1.8 Ma.

Journal of South American Earth Sciences↗
USGS1991

Tertiary fission-track ages from the Bagua syncline (northern Peru): Stratigraphic and tectonic implications

The results of five zircon fission-track ages of volcanic tuffs intercalated within the continental deposits of the Bagua syncline (northern Peru) are reported. These 2500-meter-thick deposits overlie mid-Campanian to lower Maastrichtian fine-grained red beds (Fundo El Triunfo Formation). The disconformable fluvial conglomerates of the Rentema Formation are associated with a 54 Ma tuff (upper Paleocene-lower Eocene?) and would reflect the Inca-1 tectonic phase. The Sambimera Formation (Eocene to mid-Miocene) is a coarsening-upward sequence (from lacustrine to fluvial) that contains three volcanic tuffs of 31, 29, and 12 Ma, respectively. A probable stratigraphic gap, upper Eocene-lower Oligocene, would be related to the late Eocene Inca-2 phase. Neither deformation nor sedimentary discontinuity has been recognized so far. However, the lacustrine to fluvial transition could relate to the late Oligocene Aymara tectonic phase. The unconformable fanglomerates and fluvial deposits of the San Antonio Formation contain in their upper part a 9 Ma tuff (mid-to upper Miocene), and thier base records a major tectonic event (Quechua-2 phase?). The unconformable fanglomerates of the Tambopara Formation date the folding of the Bagua syncline, which could be ascribed to the latest Miocene Quechua-3 tectonics. These formations are correlative with comparable deposits in the sub-Andean basins, suggesting that these eastern areas underwent strong tectonic subsidence of the foreland basin type since mid-Miocene times.

Journal of South American Earth Sciences↗
USGS1994

Age and Sr isotopic composition of volcanic rocks in the Maricunga Belt, Chile: Implications for magma sources

K-Ar and 40 Ar/ 39 Ar dating of volcanic rocks from the Maricunga belt of north-central Chile indicate that igneous activity took place throughout most of Miocene time at various places in the 150 by 30 km belt. No migration patterns of volcanism appear in the Miocene rocks of the belt. Volcanic activity ceased by the end of the Miocene. All the Miocene volcanic rocks studied are calcic andesites to dacites with about 62% SiO 2 , 18% A1 2 O 3 , 4% Fe 2 O 3 (total), and 2% K 2 O. Initial 87 Sr/ 86 Sr (Sr i ) values fall into two groups, one of lower values around 0.7050 to the west and the other of higher values around 0.7060 to the east. It is postulated that the two Sr i groups reflect two adjoining coherent lower-crustal magma sources of possibly different age and subtly different composition that form part of the western edge of the South American craton.

Journal of South American Earth Sciences↗
USGS1988

The development of the continental margin of eastern North America-conjugate continental margin to West Africa

The continental margin of eastern North America was initiated when West Africa and North America were rifted apart in Triassic-Early Jurassic time. Cooling of the crust and its thinning by rifting and extension caused subsidence. Variation in amounts of subsidence led to formation of five basins. These are listed from south to north. (1) The Blake Plateau Basin, the southernmost, is the widest basin and the one in which the rift-stage basement took longest to form. Carbonate platform deposition was active and persisted until the end of Early Cretaceous. In Late Cretaceous, deposition slowed while subsidence persisted, so a deep water platform was formed. Since the Paleocene the region has undergone erosion. (2) The Carolina Trough is narrow and has relatively thin basement, on the basis of gravity modeling. The two basins with thin basement, the Carolina Trough and Scotian Basin, also show many salt diapirs indicating considerable deposition of salt during their early evolution. In the Carolina Trough, subsidence of a large block of strata above the flowing salt has resulted in a major, active normal fault on the landward side of the basin. (3) The Baltimore Canyon Trough has an extremely thick sedimentary section; synrift and postrift sediments exceed 18 km in thickness. A Jurassic reef is well developed on the basin's seaward side, but post-Jurassic deposition was mainly non-carbonate. In general the conversion from carbonate to terrigenous deposition, characteristics of North American Basins, occurred progressively earlier toward the north. (4) The Georges Bank Basin has a complicated deep structure of sub-basins filled with thick synrift deposits. This may have resulted from some shearing that occurred at this offset of the continental margin. Postrift sediments apparently are thin compared to other basins—only about 8 km. (5) The Scotian Basin, off Canada, contains Jurassic carbonate rocks, sandstone, shale and coal covered by deltaic deposits and Upper Cretaceous deeper water chalk and shale.

Journal of African Earth Sciences↗
USGS1990

Coal in sub-Saharan-African countries undergoing desertification

Coal has been reported in 11 of the 16 sub-Saharan countries discussed in this appraisal: Mauritania, Senegal, Mali, Niger, Benin, Nigeria, Cameroon, Central African Republic, Sudan, Ethiopia, and Somalia. No coal occurrences have been reported in Gambia, Togo, Burkina, Chad, and Djibouti but coal may be present within these countries because neighboring countries do contain coal-bearing rocks. Most of these countries are undergoing desertification or will in the near future. Wood, directly or in the form of charcoal, constitutes two-thirds of the fuel used in Africa. Destruction of forest and shrub lands for fuel is occurring at an increasing rate because of desertification and increasing energy demands. The decline in biological productivity, coupled with concentration of population in areas where water is available and crops may be grown, leads to increasing shortages of wood for fuel. Part of the present and future energy needs of the sub-Saharan region could be met by use of indigenous coal and peat. Nine sedimentary basins, completely or partially within the sub-Saharan region, have the potential of either coal and/or peat deposits of economic value: 1- Senegal Basin, 2- Taoudeni Basin and Gao Trough, 3- Niger Basin, 4- Chad Basin, 5- Chari Basin, 6- Benue Trough (Depression), 7- Sudan Trough, 8- Plateau and Rift Belt, and 9- Somali Basin. Niger and Nigeria are the only countries in sub-Saharan Africa in which coal is presently being mined as a fuel source for powerplants and domestic use. Peat occurs in the deltas, lower river, and interdunal basin areas of Senegal, Mauritania, and Sudan. Peat can be used as an alternate fuel source and is currently being tested as a soil amendment in the agricultural sector.

Journal of African Earth Sciences↗
USGS1996

Chemical and isotopic characteristics of fluids along the cameroon volcanic line, cameroon

Results of the chemical and isotopic analysis of the water and gases discharged from volcanic crater lakes and soda springs located along the Cameroon Volcanic Line were used to characterize and infer their genetic relationships. Variations in the solute compositions of the waters indicate the dominant influence of silicate hydrolysis. Na+ (40-95%) constitutes the major cation in the springs while Fe2+ + Mg2+ (70%) dominate in the CO2-rich lakes. The principal anion is HCO3 (>90%), except in the coastal springs where Cl predominates. Lakes Nyos and Monoun have Fe-Mg-Ca-HCO3 type signatures; the soda springs are essentially Na-HCO3 type, while all other lakes show similar ionic compositions to dilute surface waters. Dissolved gases show essentially CO2 (>90%), with small amounts of Ar and N2, while CH4 constitutes the principal component in the non-gassy lakes. Active volcanic gases are generally absent, except in the Lobe spring with detectable H2S. Stable isotope ratio evidence indicates that the bicarbonate waters are essentially of meteoric origin. CO2 (??13C = -2 to -8???) and He (3He/4He = 1 to 5.6Ra) infer a mantle contribution to the total CO2. CH4 has a biogenic source, while Ar and N2 are essentially atmospheric in origin, but mixing is quite common.

Journal of African Earth Sciences↗
USGS2004

Mars chronology: Assessing techniques for quantifying surficial processes

Currently, the absolute chronology of Martian rocks, deposits and events is based mainly on crater counting and remains highly imprecise with epoch boundary uncertainties in excess of 2 billion years. Answers to key questions concerning the comparative origin and evolution of Mars and Earth will not be forthcoming without a rigid Martian chronology, enabling the construction of a time scale comparable to Earth's. Priorities for exploration include calibration of the cratering rate, dating major volcanic and fluvial events and establishing chronology of the polar layered deposits. If extinct and/or extant life is discovered, the chronology of the biosphere will be of paramount importance. Many radiometric and cosmogenic techniques applicable on Earth and the Moon will apply to Mars after certain baselines (e.g. composition of the atmosphere, trace species, chemical and physical characteristics of Martian dust) are established. The high radiation regime may pose a problem for dosimetry-based techniques (e.g. luminescence). The unique isotopic composition of nitrogen in the Martian atmosphere may permit a Mars-specific chronometer for tracing the time-evolution of the atmosphere and of lithic phases with trapped atmospheric gases. Other Mars-specific chronometers include measurement of gas fluxes and accumulation of platinum group elements (PGE) in the regolith. Putting collected samples into geologic context is deemed essential, as is using multiple techniques on multiple samples. If in situ measurements are restricted to a single technique it must be shown to give consistent results on multiple samples, but in all cases, using two or more techniques (e.g. on the same lander) will reduce error. While there is no question that returned samples will yield the best ages, in situ techniques have the potential to be flown on multiple missions providing a larger data set and broader context in which to place the more accurate dates. ?? 2004 Elsevier B.V. All rights reserved.

Earth-Science Reviews↗
USGS2005

Interpretation of the Last Chance thrust, Death Valley region, California, as an Early Permian décollement in a previously undeformed shale basin

The Last Chance thrust, discontinuously exposed over an area of at least 2500 km 2 near the south end of the Cordilleran foreland thrust belt in the Death Valley region of east-central California, is controversial because of its poorly constrained age and its uncertain original geometry and extent. We interpret this thrust to be Early Permian in age, to extend throughout a sedimentary basin in which deep-water Mississippian shale overlain by Pennsylvanian and earliest Permian limestone turbidites accumulated, to represent about 30 km of eastward displacement, and to be related to convergence on a northeast-trending segment of the Early Permian continental margin. Last Chance deformation occurred between the times of the Antler and Sonoma orogenies of Late Devonian–Early Mississippian and Late Permian ages, respectively, and followed Early to Middle Pennsylvanian truncation of the continental margin by transform faulting. In the western part of the Mississippian shale basin in east-central California, the originally recognized exposures of the Last Chance thrust show Neoproterozoic and early Paleozoic strata above lower-plate Mississippian shale. Farther east, faults subparallel to bedding above, below, and within the Mississippian shale are interpreted to mark the thrust zone and to represent a regional décollement. At the eastern margin of the basin, upper-plate thrust slices of deep-water, late Paleozoic strata are interpreted to have piled up against the margin of the Mississippian carbonate shelf to form a large antiformal stack above the Lee Flat thrust, which we regard as the easternmost exposure of the Last Chance thrust. Thrust loading depressed the western part of the shelf, creating a new sedimentary basin in which about 3.5 km of younger Early Permian deep-water strata were deposited against the antiformal stack. Later, probably in the Late Permian, other thrusts, including the Inyo Crest thrust, which was subsequently overlapped by Early to Middle(?) Triassic marine strata, cut across the Last Chance thrust. We interpret the Last Chance thrust as similar in many ways to Appalachian-type décollements in which the zone of thrusting is localized along a shale interval. The Last Chance thrust, however, has been dismembered during later geologic events so that its original geometry has been obscured. Our model may have unrecognized analogs in other structurally complex shale basins in which the initial deformation was along a major shale unit.

Earth-Science Reviews↗
USGS2005

Structure and regional significance of the Late Permian(?) Sierra Nevada - Death Valley thrust system, east-central California

An imbricate system of north-trending, east-directed thrust faults of late Early Permian to middle Early Triassic (most likely Late Permian) age forms a belt in east-central California extending from the Mount Morrison roof pendant in the eastern Sierra Nevada to Death Valley. Six major thrust faults typically with a spacing of 15-20 km, original dips probably of 25-35??, and stratigraphic throws of 2-5 km compose this structural belt, which we call the Sierra Nevada-Death Valley thrust system. These thrusts presumably merge into a de??collement at depth, perhaps at the contact with crystalline basement, the position of which is unknown. We interpret the deformation that produced these thrusts to have been related to the initiation of convergent plate motion along a southeast-trending continental margin segment probably formed by Pennsylvanian transform truncation. This deformation apparently represents a period of tectonic transition to full-scale convergence and arc magmatism along the continental margin beginning in the Late Triassic in central California. ?? 2005 Elsevier B.V. All rights reserved.

Earth-Science Reviews↗
USGS2005

Macropolygon morphology, development, and classification on North Panamint and Eureka playas, Death Valley National Park CA

Panamint and Eureka playas, both located within Death Valley National Park, exhibit a host of surficial features including fissures, pits, mounds, and plant-covered ridges, representing topographic highs and lows that vary up to 2 m of relief from the playa surface. Aerial photographs reveal that these linear strands often converge to form polygons, ranging in length from several meters to nearly a kilometer. These features stand out in generally dark contrast to the brighter intervening expanse of flat, plant-free, desiccated mud of the typical playa surface. Ground-truth mapping of playa features with differential GPS (Global Positioning System) was conducted in 1999 (North Panamint Valley) and 2002 (Eureka Valley). High-resolution digital maps reveal that both playas possess macropolygons of similar scale and geometry, and that fissures may be categorized into one of two genetic groups: (1) shore-parallel or playa-interior desiccation and shrinkage; and (2) tectonic-induced cracks. Early investigations of these features in Eureka Valley concluded that their origin may have been related to agricultural activity by paleo-Indian communities. Although human artifacts are abundant at each locale, there is no evidence to support the inference that surface features reported on Eureka Playa are anthropogenic in origin. Our assumptions into the genesis of polygons on playas is based on our fortuitous experience of witnessing a fissure in the process of formation on Panamint Playa after a flash flood (May 1999); our observations revealed a paradox that saturation of the upper playa crusts contributes to the establishment of some desiccation features. Follow-up visits to the same feature over 2 yrs' time are a foundation for insight into the evolution and possible longevity of these features. ?? 2005 Elsevier B.V. All rights reserved.

Earth-Science Reviews↗
USGS2005

Miocene rapakivi granites in the southern Death Valley region, California, USA

Rapakivi granites in the southern Death Valley region, California, include the 12.4-Ma granite of Kingston Peak, the ca. 10.6-Ma Little Chief stock, and the 9.8-Ma Shoshone pluton. All of these granitic rocks are texturally zoned from a porphyritic rim facies, characterized by rapakivi textures and miarolitic cavities, to an equigranular aplite core. These granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type rapakivi granites from southern Finland. Chemical and isotope (Nd–Sr–Pb) data suggest that rapakivi granites of the southern Death Valley region were derived by partial melting of lower crustal rocks (possibly including Mesozoic plutonic component) with some mantle input as well; they were emplaced at shallow crustal levels (4 km) in an actively extending orogen.

California↗