In this paper, we merge more than 200 new apatite and zircon (U-Th)/He analyses and 21 apatite fission-track analyses from 71 new samples with previous published thermochronologic data using the same systems to understand the growth and large-scale kinematics of the central Andes between 21°S and 28°S. In general, minimum dates decrease and the total range of dates increases from west to east across the range. Large variations in thermochronometer dates on the east side reflect high spatial gradients in depth of recent erosional exhumation. Almost nowhere in this part of the Andes has Cenozoic erosion exceeded ~6–8 km, and in many places in the eastern half of the range, erosion has not exceeded 2–3 km, despite these regions now being 5–6 km above sea level. This means that west of the rapidly deforming and eroding eastern range front, uplift and erosion are largely decoupled as a result of meager precipitation, relatively low relief, internal drainage, and volcanic burial. We interpret the west-to-east pattern of decreasing minimum dates across the range as recording the time-transgressive eastward migration of a focused zone of deformation, erosion, and convergence between the South American plate and the eastern edge of the Andean orogenic plateau. At this scale, the thermochronologic data do not suggest major changes in rates of plateau propagation or shortening/convergence with time. We use the thermochronometer date-distance trend and a simple kinematic model to infer a rate of eastward propagation of deformation and plateau growth of 6–10 km/m.y. This plateau propagation model balances horizontal convergence, erosion, and crustal thickening and predicts rates of shortening and convergence between the Andes block and South American plate that are consistent with geologic and geodetic observations.

In the easternmost Himalaya and southeastern Tibet, the Namche Barwa–Gyala Peri massif and adjacent Lhasa block host some of the Earth’s most active geologic processes and extreme topography. Synthesis of U-Th/He and Ar-Ar thermochronology, anatectic history, seismicity, and structural geology shows the important role that surface processes have played in this region in both local and orogen-scale crustal dynamics. Basement rocks of the massif underwent an episode of metamorphism, partial melting, and focused deformation that began ca. 10 Ma and likely remains active due to thermally mediated feedbacks between these processes and erosion. Strong differential rock uplift at Namche Barwa established the immense Namche Barwa knickzone on the Yarlung Tsangpo River, which has been stabilized through coupling between erosion driven by high stream power and localized deformation. This knickzone has maintained a high secondary base level of ~3000 m for the upper Yarlung Tsangpo watershed and so has shielded a large region of southeastern Tibet from excavation by the river, which in turn could alter the morphology and so the dynamics of the eastern Himalayan orogenic wedge. The landscape evolution of the southeast Lhasa block involved slow regional unroofing or incision in the Neogene, a significant pulse of ~5 km of rapid exhumation from ca. 10 to 5 Ma, and since then a great reduction in exhumation started once the Namche Barwa knickzone on the Yarlung Tsangpo was established. The low-relief high-elevation surface in the area is a relatively young feature, developed after the rapid 10–5 Ma exhumation pulse.

The Lhasa and Qiangtang terranes of Tibet collided following Late Jurassic–Early Cretaceous consumption of oceanic lithosphere along the intervening Bangong suture zone. This continental collision led to the development of the south-directed, northern Lhasa thrust belt that is exposed ~1200 km along strike in central Tibet. We conducted geologic mapping and stratigraphic and geothermochronologic studies in the Duba region of the northern Lhasa terrane, located ~250 km northwest of the city of Lhasa. In the Duba region, granites were emplaced into the mid-crust between 139 and 121 Ma and subsequently exhumed and juxtaposed against Cretaceous strata between 105 and 90 Ma in the footwall of an interpreted passive roof thrust system. We suggest that this structural style dominates the Cretaceous–early Cenozoic evolution of the northern Lhasa thrust belt and provides an explanation for the scarcity of basement rock exposures in the Lhasa terrane despite >50% upper crustal shortening. Furthermore, we highlight similarities between the collision-related northern Lhasa and Tethyan Himalayan thrust belts, both of which are bound by sutures and associated with underthrusting of lower plate lithosphere.

Northeastern Tibetan Plateau growth illuminates the kinematics, geodynamics, and climatic consequences of large-scale orogenesis, yet only recently have data become available to outline the spatiotemporal pattern and rates of this growth. I review the tectonic history of range growth across the plateau margin north of the Kunlun fault (35°–40°N) and east of the Qaidam basin (98°–107°E), synthesizing records from fault-bounded mountain ranges and adjacent sedimentary basins. Deformation began in Eocene time shortly after India-Asia collision, but the northeastern orogen boundary has largely remained stationary since this time. Widespread middle Miocene–Holocene range growth is portrayed by accelerated deformation, uplift, erosion, and deposition across northeastern Tibet. The extent of deformation, however, only expanded ~150 km outward to the north and east and ~150 km laterally to the west. A middle Miocene reorganization of deformation characterized by shortening at various orientations heralds the onset of the modern kinematic regime where shortening is coupled to strike slip. This regime is responsible for the majority of Cenozoic crustal shortening and thickening and the development of the northeastern Tibetan Plateau.

The thermochronology for several suites of Mesozoic metamorphic and plutonic rocks collected throughout the northern Peninsular Ranges batholith (PRB) was studied as part of a collaborative isotopic study to further our understanding of the magmatic and tectonic history of southern California. These sample suites include: a traverse through the plutonic rocks across the northern PRB (N = 29), a traverse across a central structural and metamorphic transition zone of mainly metasedimentary rocks at Searl ridge (N = 20), plutonic samples from several drill cores (N = 7) and surface samples (N = 2) from the Los Angeles Basin, a traverse across the Eastern Peninsular Ranges mylonite zone (N = 6), and a suite of plutonic samples collected across the northern PRB (N = 13) from which only biotite 40Ar/39Ar ages were obtained. These geochronologic data help to characterize five major petrologic, geochemical, and isotopic zonations of the PRB (western zone, WZ; western transition zone, WTZ; eastern transition zone, ETZ; eastern zone, EZ; and upper-plate zone, UPZ). Apparent cooling rates were calculated using U-Pb zircon (zr) and titanite (sphene) ages; 40Ar/39Ar ages from hornblende (hbl), biotite (bi), and K-feldspar (Kf); and apatite fission-track (AFT) ages from the same samples. The apparent cooling rates across the northern PRB vary from relatively rapid in the west (zr-hbl ~210 °C/m.y.; zr-bio ~160 °C/m.y.; zr-Kf ~80 °C/m.y.) to less rapid in the central (zr-hb ~280 °C/m.y.; zr-bio ~90 °C/m.y.; zr-Kf ~60 °C/m.y.) and eastern (zr-hbl ~185 °C/m.y.; zr-bio ~180 °C/m.y.; zr-Kf ~60 °C/m.y.) zones. An exception in the eastern zone, the massive San Jacinto pluton, appears to have cooled very rapidly (zr-bio ~385 °C/m.y.). Apparent cooling rates for the UPZ samples are consistently slower in comparison (~25–45 °C/m.y.), regardless of which geochronometers are used. Notable characteristics of the various ages from different dating methods include: (1) Zircon ages indicate a progressive younging of magmatic activity from west to east between ca. 125 and 90 Ma. (2) Various geochronometers were apparently affected by emplacement of the voluminous (ETZ and EZ) La Posta–type plutons emplaced between 99 and 91 Ma. Those minerals affected include K-feldspar in the western zone rocks, biotite and K-feldspar in the WTZ rocks, and white mica and K-feldspar in rocks from Searl ridge. (3) The AFT ages record the time the rocks cooled through the AFT closure temperature (~100 °C in these rocks), likely due to exhumation. Throughout most of the northern traverse, the apatite data indicate the rocks cooled relatively quickly through the apatite partial annealing zone (PAZ; from ~110 °C to 60 °C) and remained at temperatures less than 60 °C as continued exhumation cooled them to present-day surface temperatures. The ages indicate that the western “arc” terrane of the WZ was being uplifted and cooled at ca. 91 Ma, during or shortly after intrusion of the 99–91 Ma La Posta–type plutons to the east. Uplift and cooling occurred later, between ca. 70 Ma and ca. 55 Ma, in the central WTZ, ETZ, and EZ rocks, possibly as upwarping in response to events in the UPZ. The UPZ experienced differential exhumation at ca. 50–35 Ma: Cooling on the western edge was taking place at about the same time or shortly after cooling in the younger samples in the ETZ and EZ, whereas on the east side of the UPZ, the rocks cooled later (ca. 35 Ma) and spent a prolonged time in the apatite PAZ compared to most northern traverse samples. Apparent cooling rates from Los Angeles Basin drill core samples of plutonic rocks show that four are similar to the WTZ thermal histories, and two are similar to the WTZ histories, indicating that the eastern part of the Los Angeles Basin area is underlain by mainly western zone PRB rocks. Thermal histories revealed by samples from Searl ridge indicate that the WTZ magmatism intruded the metasedimentary rocks prior to their deformation and metamorphism at ca. 97 Ma. Both low-grade schists and metasandstones of the western side of the ridge and high-grade gneisses of the eastern side of the ridge have thermal histories consistent with eastern zone rocks—suggesting a temporal/thermal relationship between the western transition zone and the eastern zones. Limited ages from six samples across the Eastern Peninsular Ranges mylonite zone (EPRMZ) indicate that this zone underwent cooling after emplacement of the youngest UPZ rocks at 85 Ma, suggesting that thrusting along the EPRMZ was either coeval with emplacement of the UPZ plutonic rocks or occurred shortly afterwards (~10–15 m.y.). Alternatively, the EPRMZ thrusting may have occurred at temperatures under ~180 °C at yet a later date. The geochronology presented here differs slightly from previous studies for similar rocks exposed across the middle and southern portions of the PRB, in that our data define a relatively smooth progression of magmatism from west to east, and the transition from western, oceanic-arc plutonism to eastern, continental arc plutonism is interpreted to have occurred at ca. 99–97 Ma and not at ca. 105 Ma.

Day 4 of the Field Forum is dedicated entirely to the magmatic evolution of the Tuolumne Intrusive Complex. The Tuolumne Intrusive Complex is mostly located in Yosemite National Park and is superbly exposed due to a series of glaciations that started 2–3 million years ago and ended ca. 10,000 yr B.P. one of the many reasons why it has become one of the most studied intrusions in the world. We will be driving from Mammoth Lakes to Yosemite National Park taking Hwy 395 north and then Hwy 120 west crossing the entire Tuolumne Intrusive Complex before we start off at its western margin. On this side, the Tuolumne Intrusive Complex intruded into the rocks of the Yosemite Valley Intrusive Suite, Sentinel, and Yosemite Creek granitoids (Day 3). Today we will make stops along Hwy 120 while traversing the Tuolumne Intrusive Complex from west to east ( Fig. 4-1) looking at exposures near the road and discussing field data, structural geology, geochronology, and geochemistry from the Tuolumne Intrusive Complex…

Paleoproterozoic supracrustal rocks in the region near Big Thompson Canyon, northern Colorado, have long been recognized as a spectacularly exposed example of regionally zoned metamorphism, preserving an apparently complete sequence from biotite- to migmatite-zones. Due to its location and relatively easy access, the Big Thompson Metamorphic Suite has also provided a valuable field-based educational experience for universities and colleges all along the Front Range and from elsewhere. In addition to a number of other studies, the pioneering work of William Braddock and graduate students from the University of Colorado resulted in more than a dozen M.Sc. and Ph.D. theses from the 1960s to the 1990s. Despite the volume of ground-breaking science conducted on these rocks in the past, there remain a number of fundamental questions regarding the metamorphic history and overall tectonic significance of many of the observable features. Several lines of evidence suggest there is potential for a complex tectonometamorphic history that likely spans from ~1.8 to 1.4 Ga. These include: thermochronologic and geochronologic data supporting multiple thermal and magmatic episodes, structural evidence for multiple deformation events, multiple generations of typical Barrovian minerals (e.g., staurolite), and the widespread occurrence of minerals not commonly associated with a classic Barrovian sequence (e.g., andalusite, cordierite). One purpose of this fieldtrip is to foster new ideas and stimulate new research directions that will utilize the Big Thompson Metamorphic Suite, and the Colorado Rockies in general, as field laboratories for better understanding fundamental orogenic processes.

This field trip examines contrasting lines of evidence bearing on the timing and structural style of Cenozoic (and perhaps late Mesozoic) extensional deformation in northeastern Nevada. Studies of metamorphic core complexes in this region report extension beginning in the early Cenozoic or even Late Cretaceous, peaking in the Eocene and Oligocene, and being largely over before the onset of “modern” Basin and Range extension in the middle Miocene. In contrast, studies based on low-temperature thermochronology and geologic mapping of Eocene and Miocene volcanic and sedimentary deposits report only minor, localized extension in the Eocene, no extension at all in the Oligocene and early Miocene, and major, regional extension in the middle Miocene.A wealth of thermochronologic and thermobarometric data indicate that the Ruby Mountains–East Humboldt Range metamorphic core complex (RMEH) underwent ~170 °C of cooling and 4 kbar of decompression between ca. 85 and ca. 50 Ma, and another 450 °C cooling and 4–5 kbar decompression between ca. 50 and ca. 21 Ma. These data require ~30 km of exhumation in at least two episodes, accommodated at least in part by Eocene to early Miocene displacement on the major west-dipping mylonitic zone and detachment fault bounding the RMEH on the west (the mylonitic zone may also have been active during an earlier phase of crustal extension). Meanwhile, Eocene paleovalleys containing 45–40 Ma ash-flow tuffs drained eastward from northern Nevada to the Uinta Basin in Utah, and continuity of these paleovalleys and infilling tuffs across the region indicate little, if any deformation by faults during their deposition. Pre–45 Ma deformation is less constrained, but the absence of Cenozoic sedimentary deposits and mappable normal faults older than 45 Ma is also consistent with only minor (if any) brittle deformation. The presence of ≤1 km of late Eocene sedimentary—especially lacustrine—deposits and a low-angle angular unconformity between ca. 40 and 38 Ma rocks attest to an episode of normal faulting at ca. 40 Ma.Arguably the greatest conundrum is how much extension occurred between ca. 35 and 17 Ma. Major exhumation of the RMEH is interpreted to have taken place in the late Oligocene and early Miocene, but rocks of any kind deposited during this interval are scarce in northeastern Nevada and absent in the vicinity of the RMEH itself. In most places, no angular unconformity is present between late Eocene and middle Miocene rocks, indicating little or no tilting between the late Eocene and middle Miocene. Opinions among authors of this report differ, however, as to whether this indicates no extension during the same time interval. The one locality where Oligocene deposits have been documented is Copper Basin, where Oligocene (32.5–29.5 Ma) conglomerates are ~500 m thick. The contact between Oligocene and Eocene rocks in Copper Basin is conformable, and the rocks are uniformly tilted ~25° NW, opposite to a normal fault system dipping ~35° SE. Middle Miocene rhyolite (ca. 16 Ma) rests nonconformably on the metamorphosed lower plate of this fault system and appears to rest on the tilted upper-plate rocks with angular unconformity, but the contact is not physically exposed. Different authors of this report interpret geologic relations in Copper Basin to indicate either (1) significant episodes of extension in the Eocene, Oligocene, and middle Miocene or (2) minor extension in the Eocene, uncertainty about the Oligocene, and major extension in the middle Miocene.An episode of major middle Miocene extension beginning at ca. 16–17 Ma is indicated by thick (up to 5 km) accumulations of sedimentary deposits in half-graben basins over most of northern Nevada, tilting and fanning of dips in the synextensional sedimentary deposits, and apatite fission-track and (U-Th)/He data from the southern Ruby Mountains and other ranges that indicate rapid middle Miocene cooling through near-surface temperatures (~120–40 °C). Opinions among authors of this report differ as to whether this period of extension was merely the last step in a long history of extensional faulting dating back at least to the Eocene, or whether it accounts for most of the Cenozoic deformation in northeastern Nevada. Since 10– 12 Ma, extension appears to have slowed greatly and been accommodated by highangle, relatively wide-spaced normal faults that give topographic form to the modern ranges. Despite the low present-day rate of extension, normal faults are active and have generated damaging earthquakes as recently as 2008.

The Lake Mead region of northwest Arizona and southeast Nevada contains exceptional exposures of extensional basins and associated normal and strike-slip faults of mainly Miocene age. The Salt Spring Wash Basin is located within the hanging wall of a major detachment fault in the northern White Hills in northwest Arizona, the South Virgin–White Hills detachment fault. The basin is the focus of a detailed basin analysis designed to investigate its three-dimensional structural and stratigraphic evolution in order to determine how a major reentrant in the detachment fault formed. Geochronology and apatite fission-track thermochronology from other studies constrain movement on this detachment fault system to ca. 18–11 Ma, while our study suggests faulting from ca. 16.5 to 11 Ma. Salt Spring Wash Basin consists of variably tilted proximal rock avalanche and alluvial-fan deposits shed from uplifting hanging-wall and predominantly footwall blocks. The basinal strata were deformed during early to middle Miocene faulting on the detachment fault, normal faults, and a faulted rollover fold within the basin. New and existing 40Ar/39Ar ages on tilted volcanic tuffs and basalt lava flows within the basin strata constrain deposition of these deposits from 15.19 to 10.8 Ma. An apparent lag between the initiation of footwall uplift at 18–17 Ma (based on thermochronology) and basin subsidence at 16.5–16 Ma in the eastern Lake Mead region may be explained by the influences of preexisting paleotopography, or it may be an artifact of lack of exposure of the base of the basin. An early phase of faulting and basin sedimentation from 16.5–16 to 14.6 Ma generated the relief to produce a 500+-m-thick lower section of megabreccia (landslide) and conglomerate (debris flows). Salt Spring Wash Basin experienced relatively high sedimentation rates of 200–600 m/m.y. during its early history. A 14.64 Ma basalt lies at a facies change to 650 m of conglomerate of the middle sequence that was deposited in an alluvial-fan to braid-plain setting. Changes in basin geometry included the development of the reentrant in the northern Salt Spring Wash Basin with the rollover fold at its southern margin. The middle sequence records a significant decrease in sedimentation rates from hundreds of meters per million years to ~60–30 m/m.y., major facies changes, and decreased rate of uplift of footwall rocks. The upper sequence of the basin includes ca. 11–8 Ma basalts interbedded with conglomerate. The ca. 6 Ma lacustrine Hualapai Limestone caps the section and indicates a profound change in sedimentation. The history of the Salt Spring Wash Basin indicates that there was a step-over geometry in the detachment fault that was linked across the southern margin of the reentrant in the basin during deposition of the middle sequence.

Three major low-angle normal faults in the eastern Lake Mead area, Nevada and Arizona, are segments of a regional, 55-km-long, detachment fault. This fault, the South Virgin–White Hills detachment, consists of the Lakeside Mine, Salt Spring, and Cyclopic Mine fault segments. All three segments dip gently west and record top-to-the-west displacement. Based on apatite fission-track and apatite and titanite (U-Th)/He thermochronology of footwall rocks, tilt relations, and 40Ar/39Ar dates on tuffs and basalts within hanging-wall synextensional sedimentary sequences, significant extension along the South Virgin–White Hills detachment occurred between 16.5 and 14 Ma. Minor extension continued until ca. 8 Ma. Displacement on the South Virgin–White Hills detachment decreases from a maximum of ~17 km at the Gold Butte block in the north to 5–6 km at the Cyclopic Mine in the south. The along-strike, southward decrease in displacement is accompanied by a change in type of fault rock from mylonite along the Lakeside Mine fault (northern segment), to chloritic cataclasite along the Salt Spring fault (central segment), to unconsolidated fault breccia along the Cyclopic Mine fault (southern segment). Differences in fault rock may reflect decreasing exhumation of footwall rocks as a result of decreased displacement to the south. About 40% of the displacement gradient can be accommodated along a series of left-slip faults in the upper plate of the detachment. The Golden Rule Peak lineament, an east-trending alignment of structural and topographic features, may be a transverse structure that accommodates differential displacement between the Salt Spring and Cyclopic Mine faults. The trace of the South Virgin–White Hills detachment is highly sinuous in map view and is marked by three prominent salients that define west-plunging antiformal warps in the detachment surface. We interpret the corrugations in the South Virgin–White Hills detachment to have formed by a process of linkage of originally separate en echelon fault segments followed by eastward tilting of the footwall. Depositional patterns, particularly between the Lakeside Mine and Salt Spring segments, support this interpretation. The Grand Wash fault forms the present-day physiographic boundary between the Colorado Plateau and the Basin and Range Provinces; however, based on greater amount of displacement and exhumation, we suggest that the South Virgin–White Hills detachment is the principal structure accommodating regional extension in the eastern Lake Mead extensional domain.

Structural, stratigraphic, and thermochronologic studies provide insight into the formation of basement-cored uplifts within the Colorado Plateau–Basin and Range transition zone in the Lake Mead region. Basement lithologic contacts, foliations, and ductile shear zones preserved in the core of the Virgin Mountain anticline parallel the trend of the anticline and are commonly reactivated by brittle fault zones, implying that basement anisotropy exerted a strong influence on the uplift geometry of the anticline. Potassium feldspar 40Ar/39Ar thermochronology indicates that basement rocks cooled from ≥250–325 °C to ≤150 °C in the Mesoproterozoic and remained at shallow crustal levels (<5–7 km) until they were exhumed to the surface. Apatite fission-track ages and track length measurements reveal a transition from slow cooling beginning at 30–26 Ma to rapid cooling at ca. 17 Ma, which we interpret to mark the change from regional post-Laramide denudational cooling to rapid extension-driven exhumational cooling by ca. 17 Ma. Middle Miocene conglomerates (ca. 16–11 Ma) flanking the anticline contain locally derived basement clasts with ca. 20 Ma apatite fission-track ages, implying rapid exhumation rates of ≥500 m m.y.−1. The apparently complex geometry of the anticline resulted from the superposition of first-order processes, including isostatic footwall uplift and extension-perpendicular shortening, on a previously tectonized and strongly anisotropic crust. A low-relief basement-cored uplift may have formed during the Late Cretaceous–early Tertiary Laramide orogeny; however, the bulk of uplift, exhumation, and deformation of the Virgin Mountain anticline occurred during middle Miocene crustal extension.

This paper reevaluates the geometry and processes of extension in the boundary zone between the western Colorado Plateau and the Basin and Range Province. Based on new mapping of extensional detachment faults, restored cross sections, and 40Ar/39Ar K-feldspar thermochronology, we present an alternative to the previously published model that the Gold Butte block is a tilted 15–18-km-thick intact basement crustal section. Mapping of windows of crystalline basement at 1:12,000 scale delineates a bedding-parallel detachment fault system that parallels the Great Unconformity in the Tramp Ridge block, just north of the Gold Butte block. Above this detachment fault, extensional allochthons containing Upper Paleozoic through Tertiary (>18 Ma) rocks exhibit tilting due to westward translation and tilting. We project this geometry above the Gold Butte block itself based on restoration of slip across the Gold Butte fault. This reconstruction suggests that the detachment system extended over lateral distances of >1000 km2, helping define a region of relatively modest extension (~25% for cover; 10% for basement) within the Nevada transition zone between the Colorado Plateau and Basin and Range. In agreement with previously published mapping and structural cross sections, our restored cross sections suggest that extensional deformation initiated with formation of hanging-wall anticlines above a listric Grand Wash fault system and evolved via a combination of both listric faulting and domino-block translation and tilting. New data presented in this paper document that extension was also facilitated by slip on bedding-subparallel detachment zones in the Bright Angel Shale, along the basement unconformity, and along other zones of weakness, such that the extended Paleozoic cover was partly decoupled from less-extended basement. This detachment system ramps down into basement to merge with the South Virgin–White Hills detachment at the west end of Gold Butte, the principal extensional detachment of the region. Our mapping and structural model suggest that movement on these detachment faults initiated at low angle. Further, using the geometry from restored cross sections, we infer that the deepest rocks now exposed in the western Gold Butte block resided at depths of ~4 km below the Great Unconformity (~8 km below the surface) rather than the previously published 15 km below the unconformity (~19 km below the surface). New 40Ar/39Ar K-feldspar thermochronology from the Gold Butte block, added to a compilation of published thermochronologic data, is used to help evaluate alternative models. K-feldspar multiple diffusion domain (MDD) modeling suggests that rocks throughout all but the westernmost part the block had cooled through 150–200 °C before the Phanerozoic and resided at temperatures <200 °C prior to onset of rapid Miocene extension at 17 Ma. Pre-extensional (pre–17 Ma) 100 °C and 200 °C isotherms were located near the east and west ends of the basement block, respectively. Muscovite, biotite, and K-feldspar from a 70 Ma Laramide pluton deep in the block give 40Ar/39Ar ages of 70, 50, and 30 Ma, respectively. MDD modeling of K-feldspar from this sample is compatible with cooling the westernmost part of the block from 225 °C to 150 °C between 17 and 10 Ma. Available thermochronology can be explained by either structural model: our model requires pre-extensional geothermal gradients of ~25 °C/km, rather than 15–20 °C/km as previously published.

The eastern Lake Mead region, to the north of the belt of metamorphic core complexes that define the Colorado River extensional corridor, underwent large-magnitude extension in the middle to late Miocene. We present two speculative new models for extension in this area that resolve several puzzling and paradoxical relations. These models are based on new field mapping and structural, geochronologic, and thermochronologic data from the northern White Hills, Lost Basin Range, and south Wheeler Ridge. The Meadview fault, a previously underappreciated structure, is an east-side-down normal fault that separates the northern Lost Basin Range to the west from south Wheeler Ridge to the east. Proterozoic crystalline rocks of the northern Lost Basin Range yielded an apatite fission-track (AFT) age of 15 Ma, whereas 2 km to the east, across the Meadview fault, crystalline rocks of south Wheeler Ridge yielded a 127 Ma AFT age. Similarly, at the south end of the Lost Basin Range, crystalline rocks with ca. 15 Ma AFT ages lie within 5 km of crystalline rocks of Garnet Mountain that yielded a 68 Ma AFT age across the Grand Wash fault. Neither of these relations can be explained by existing tilted crustal section or tilt-block models. In our “classic” metamorphic core complex model, the Grand Wash fault (breakaway), the Meadview fault, and the South Virgin–White Hills detachment represent different structural levels of a single, regional detachment that was active between ca. 16 and 11 Ma. The hanging wall of the detachment consists of rocks at south Wheeler Ridge, the Paleozoic ridges, and possibly part of the crystalline basement of the Gold Butte block, sedimentary and volcanic rocks in the hanging walls of the Salt Spring and Cyclopic Mine faults, and possibly stranded tilt blocks beneath the Grand Wash Trough supradetachment basin. The footwall, exhumed by subvertical simple shear and characterized by middle Miocene AFT ages, includes the central and western Gold Butte block, Hiller Mountains, and crystalline rocks of the White Hills and the Lost Basin Range. The east-dipping Meadview fault bounds the crystalline core on the east; the west-dipping South Virgin–White Hills detachment bounds the core on the west. Therefore, the Grand Wash fault represents the structurally highest part of the detachment, and the South Virgin–White Hills detachment represents the structurally deepest exposed part of the detachment. In the modified core complex model, the Grand Wash, Meadview, and South Virgin–White Hills detachment faults are separate structures, and the Grand Wash Trough is a “trailing-edge” basin bound on the east by the Grand Wash fault and on the west by the Meadview fault. The South Virgin–White Hills detachment is the main detachment along which extension was accommodated, and the Meadview fault is a major antithetic normal fault that facilitated exhumation of the core at the trailing edge of the detachment system.

Geomorphic, thermochronologic, geochemical, structural, and geophysical data all lend support to the hypothesis that the Colorado Rocky Mountains are an example of dynamic topography that has responded variably to broad epeirogenic uplift since the late Miocene. Our view is that this epeirogenic uplift is primarily related to mantle buoyancy and to a lesser extent, on isostatic adjustments caused by regional denudation. Neogene uplift components were superimposed on earlier (Laramide and mid-Tertiary) uplift events such that the present-day high topography of the Colorado Rocky Mountains reflects a composite uplift history.Newly recognized gravels of the ancestral Colorado River located beneath lava flows of Grand Mesa suggest that the Colorado River flowed west from the Rocky Mountains out onto the Colorado Plateau by 11 Ma. Radiometric dating of late Miocene basalt flows and thermochronologic data for western Colorado indicate that regional denudation and river incision became much more rapid ca. 9.5–6.0 Ma, prior to integration of the Colorado River through Grand Canyon. Post-10 Ma river incision rates and magnitudes are variable, and we interpret these variations to reflect differential uplift of discrete areas of the Rocky Mountain region during the late Cenozoic. For instance, in areas such as Grand Mesa, the Flat Tops, and the Park Range, incision rates and magnitudes are generally ~100–150 m/m.y. and 1.0–1.5 km, respectively. For comparison, these values are 2–3 times larger than incision rates and magnitudes over the past 5–10 Ma in the upper Green River basin.Differential incision across major structural boundaries indicates that post-10 Ma river incision has been driven by Neogene rock uplift. Additional evidence of post-Laramide deformation includes warping of the Oligocene to early Miocene pre-Browns Park Formation erosion surface. This surface has up to 1.8 km of structural relief that has been produced since the Miocene. For basalt-capped regions such as Grand Mesa and the Flat Tops, Neogene rock uplift of 1.0–1.5 km has probably been accompanied by a commensurate amount of surface uplift, resulting in increased local relief (England and Molnar, 1990).Flexural isostatic modeling suggests that isostatic adjustments account for only 10%–40% of the post-10 Ma rock uplift recorded in western Colorado, and that 850–1500 m of probable post-10 Ma rock uplift cannot be accounted for by the isostatic response to denudation. Areas such as Grand Mesa and the Flat Tops are associated with the largest magnitudes of post-10 Ma rock uplift, and generally overlie areas of anomalously low P-wave velocities (the largest of which is the Aspen Anomaly), which suggests that mantle buoyancy could have driven Neogene uplift of the Colorado Rocky Mountains. 3He/4He ratios strongly suggest that there are direct mantle-to-surface interactions of neotectonics and surface waters. In summary, we conclude that the Rocky Mountains are continuing to evolve in response to neotectonic events that accelerated in the late Miocene, and which are continuing to drive base-level changes and drainage integration of the modern upper Colorado River system. The combined data are best explained by broad epeirogenic Neogene uplift of the Colorado Rocky Mountains driven by mantle flow and buoyancy.