Two Oligocene conglomeratic units, one primarily nonvolcaniclastic and the other volcaniclastic, are preserved on the west side of the Jemez Mountains beneath the 14 Ma to 40 ka lavas and tuffs of the Jemez Mountains volcanic field. Thickness changes in these conglomeratic units across major normal fault zones, particularly in the southwestern Jemez Mountains, suggest that the western margin of the Rio Grande rift was active in this area during Oligocene time. Furthermore, soft-sediment deformation and stratal thickening in the overlying Abiquiu Formation adjacent to the western boundary faults are indicative of syndepositional normal-fault activity during late Oligocene–early Miocene time. The primarily nonvolcaniclastic Oligocene conglomerate, which was derived from erosion of Proterozoic basement-cored Laramide highlands, is exposed in the northwestern Jemez Mountains, southern Tusas Mountains, and northern Sierra Nacimiento. This conglomerate, formerly called, in part, the lower member of the Abiquiu Formation, is herein assigned to the Ritito Conglomerate in the Jemez Mountains and Sierra Nacimiento. The clast content of the Ritito Conglomerate varies systematically from northeast to southwest, ranging from Proterozoic basement clasts with a few Cenozoic volcanic pebbles, to purely Proterozoic clasts, to a mix of Proterozoic basement and Paleozoic limestone clasts. Paleocurrent directions indicate flow mainly to the south. A stratigraphically equivalent volcaniclastic conglomerate is present along the Jemez fault zone in the southwestern Jemez Mountains. Here, thickness variations, paleocurrent indicators, and grain-size trends suggest north-directed flow, opposite that of the Ritito Conglomerate, implying the existence of a previously unrecognized Oligocene volcanic center buried beneath the northern Albuquerque Basin. We propose the name Gilman Conglomerate for this deposit. The distinct clast composition and restricted geographic nature of each conglomerate suggests the presence of two separate fluvial systems, one flowing south and the other flowing north, separated by a west-striking topographic barrier in the vicinity of Fenton Hill and the East Fork Jemez River in the western Jemez Mountains during Oligocene time. In contrast, the Upper Oligocene–Lower Miocene Abiquiu Formation overtopped this barrier and was deposited as far south as the southern Jemez Mountains. The Abiquiu Formation, which is derived mainly from the Latir volcanic field, commonly contains clasts of dacite lava and Amalia Tuff in the northern and southeastern Jemez Mountains, but conglomerates are rare in the southwestern Jemez Mountains.

We investigated a Plio-Pleistocene alluvial succession in the Albuquerque Basin of the Rio Grande rift in New Mexico using geomorphic, stratigraphic, sedimentologic, geochronologic, and magnetostratigraphic data. New 40 Ar/ 39 Ar age determinations and magnetic-polarity stratigraphy refine the ages of the synrift Santa Fe Group. The Pliocene Ceja Formation lies on the distal hanging-wall ramp across much of the Albuquerque Basin. The Ceja onlapped and buried a widespread, Upper Miocene erosional paleosurface by 3.0 Ma. Sediment accumulation rates in the Ceja Formation decreased after 3.0 Ma and the Ceja formed broad sheets of amalgamated channel deposits that prograded into the basin after ca. 2.6 Ma. Ceja deposition ceased shortly after 1.8 Ma, forming the Llano de Albuquerque surface. Deposition of the Sierra Ladrones Formation by the ancestral Rio Grande was focused near the eastern master fault system before piedmont deposits (Sierra Ladrones Formation) began prograding away from the border faults between 1.8 and 1.6 Ma. Widespread basin filling ceased when the Rio Grande began cutting its valley, shortly after 0.78 Ma. Although the Albuquerque Basin is tectonically active, the development of through-going drainage of the ancestral Rio Grande, burial of Miocene unconformities, and coarsening of upper Santa Fe Group synrift basin fill were likely driven by climatic changes. Valley incision was approximately coeval with increased northern- hemisphere climatic cyclicity and magnitude and was also likely related to climatic changes. Asynchronous progradation of coarse-grained, margin-sourced detritus may be a consequence of basin shape, where the basinward tilting of the hanging wall promoted extensive sediment bypass of coarse-grained, margin-sourced sediment across the basin.

Abstract The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km 3 ) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.

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 40 Ar/ 39 Ar 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.

The Lost Basin Range in the eastern Lake Mead domain consists of Proterozoic rocks that bound the west side of the Grand Wash Trough. Exhumation of the Proterozoic rocks of the Lost Basin Range occurred from ca. 18 to 15 Ma based on seven apatite fission-track ages that range from 20 to 15 Ma. The Lost Basin Range fault lies along the west side of the Lost Basin Range and steps to the east to the southern end of the Wheeler fault, which then runs north for 60 km, where it joins the Grand Wash fault. The geometry of the southern Wheeler–Lost Basin Range fault system is that of a relay ramp between two, west-dipping, high-angle normal faults. The intervening area of the fault step over, Gregg Basin, is interpreted as a relay ramp basin. New interpreted ages from stratigraphic units on the north and east sides of the Lost Basin Range integrated with existing structural data from the eastern Lake Mead domain reveal that faulting, sedimentation, and tilting of hanging-wall and footwall blocks along the southern Wheeler–Lost Basin Range fault system began by 15.3 Ma. Sedimentation continued until after 13 Ma along the southeastern Lost Basin Range, while the age of continuing sedimentation in Gregg Basin is poorly constrained. A paleocanyon in the footwall of the southern Wheeler fault filled with conglomerate and minor breccia between ca. 15.3 and ca. 14 Ma and then overtopped to the south to cover the Paleozoic rocks of south Wheeler Ridge. The Paleozoic strata of the south Wheeler Ridge area tilted east 20°–30° more than the Miocene strata that overlie them, and therefore this tilting occurred before ca. 14 Ma. Upward-decreasing (fanning) bedding attitudes in the overlapping Miocene conglomerate indicate that Paleozoic strata were being tilted along with the Miocene strata by ca. 14 Ma. Gentle (5° and less) east dips in the lower beds of the Hualapai Limestone above and east of the paleocanyon suggest that most tilting in the western Grand Wash Trough ceased by ca. 11 Ma. The lower conglomerate of Gregg Basin lies below, and interfingers with, the limestone of Gregg Basin, which is undated but correlates with the 11–7 Ma Hualapai Limestone in the adjacent Grand Wash Trough. The syncline in upper Gregg Basin strata is linked spatially to the Wheeler and Lost Basin Range faults and indicates that these faults were likely active at 11–7 Ma. The two faults appear to cut the Gregg Basin limestone, and therefore post–7 Ma fault activity at lower rates is likely.

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.

Paleomagnetic data from three regionally extensive Oligocene ignimbrite sheets, two sequences of Miocene andesite flows, and ten sequences of Upper Miocene to Pliocene basaltic andesite flows in the Candelaria Hills and adjacent areas, west-central Nevada, provide further evidence that, since the late Miocene, and possibly between latest Miocene and earliest Pliocene time, the broad region that initially facilitated Neogene displacement transfer between the Furnace Creek and central Walker Lane fault systems experienced some 20° to 30° of clockwise vertical-axis rotation. The observed sense and magnitude of rotation are similar to those previously inferred from paleo-magnetic data from different parts of the Silver Peak Range to the south. We propose that clockwise rotation within the transfer zone formed in response to horizontal components of simple and pure shear distributed between early-formed, northwest-striking right-lateral structures that initiated in mid- to late Miocene time. Notably, the spatial distribution of the early-formed transfer zone is larger and centered south of the presently active stepover, which initiated in the late Pliocene and is characterized by a trans-tensional deformation field and slip on east-northeast–oriented left-oblique structures that define the Mina deflection. The sense and magnitude of rotation during this phase of deformation, which we infer to be of pre–latest Pliocene age, are inconsistent with the geodetically determined regional velocity field and seismologically determined strain field for this area. As a consequence, the longer-term kinematic evolution of the stepover system, and the adjoining parts of the Furnace Creek and Walker Lane fault systems, cannot be considered as a steady-state process through the Neogene.