The Valles caldera, located in the heart of the Jemez Mountains in north-central New Mexico, is the world's premier example of a resurgent caldera, a giant circular volcano with an uplifted central floor and a near-perfect ring of roughly 15 postcaldera lava dome and flow eruptions. This new Valles caldera map and cross sections represent the cumulative research efforts of countless geologists over the past 40 years, and several state and federal agencies. GM-79 compiles detailed geologic mapping completed in the past eight years from parts of the nine 7.5-min USGS topographic quadrangles that encompass the caldera. More than 150 map units are described in detail. Also incorporated are new geochronologic data and recent refinements to nomenclature.
Many previous workers have recognized that the Tijeras–Canoncito fault system is a longlived fault system. The number, timing, and kinematics of different deformation events, however, have been dif ficult to resolve due to a lack of piercing points and stratigraphic limitations. We propose a method for resolving the kinematics and minimum number of faulting events and use fission track and 40Ar/39Ar dating techniques together with geologic constraints to evaluate the timing of these events. Our approach is to use minor faults as a record of strain in the damage zone of the Tijeras fault and to assume that strain is compatible with fault slip. Thus, for example, the absence of evidence for crustal thickening in the damage zone is taken to indicate that the fault did not experience reverse motion. This work indicates that faultzone structural elements largely record dextral strikeslip motion with a component of northsidedown normal motion. This is the earliest faulting event, which ther mochronologic and geologic constraints indicate initiated during the Laramide orogeny. The fault may have been reactivated during the transition in stress states between the Laramide orogeny and Rio Grande rifting and was definitely active during extension associated with the Rio Grande rift. Reactivation involved leftlateral strikeslip motion and, at least locally, further northsidedown nor mal movement.
The Rocky Mountain erosion surface, also known as the late Eocene or subsummit surface, forms the gently undulating topography along the eastern two-thirds of the central Front Range and broadens to both the north and south on more easily eroded granitic terrain. The surface is overlain in the southern Front Range and adjoining areas by the 36.7 Ma Wall Mountain Tuff. Present elevation of the Rocky Mountain surface varies from 2000 to 3000 m, due to the original slope and to post-Eocene faulting within the Front Range and along its rifted western margin. Apatite fission-tmck 0 sample traverses along Clear Creek canyon, the north fork of the South Platte River, and on Pikes Peak show that the Rocky Mountain surface bevels Proterozoic rocks with AFI' cooling ages of 70 to 50 Ma. AFT cooling ages are older than 100 Ma along the east edge of the range, which argues against major post-Laramide uplift of the Front Range relative to the High Plains. The Rocky Mountain surface developed during the unusually warm and equable Eocene climate in which deep chemical weathering and the preponderance of small storm events smoothed the topography. The surface formed by coalescence of pediments and was protected from dissection by a combination of volcanic cover and the alluvial aprons that bordered the range. Change to a significantly wetter, stormier climate beginning in early Pliocene led to excavation of the protecting piedmont sediments and allowed incision of the deep canyons that now cross the surface. Use of the two structural datums provided by the Rocky Mountain surface and the base of the AFI' partial annealing zone, tosther with modeling of the track-length data, indicates that total denudation in the Pikes Peak area is about 3.7 km, of which 0.8 to 1.2 km was stripped in early Laramide, 2.2 to 2.6 km was eroded during late Laramide and canring of the Rocky Mountain surface, and only about 0.3 km was removed in late Cenozoic.
Heat flow, bottom-hole temperature (BHT), and thermal conductivity data are used to evaluate the present thermal conditions in the Anadarko basin. Heat flow values decrease from 54-62 mWm{sup {minus}2} in the northern part of the basin to 39-53 mWm{sup {minus}2} in the southern portion of the basin. The variation in the regional conductive heat flow is controlled by basin geometry and by the distribution of radiogenic elements in the basement. The heat flow, thermal conductivity, and lithologic information were combined to construct a 3-D model of the temperature structure of the Anadarko basin. The highest temperatures sedimentary rocks older than Pennsylvanian are offset 35 km north-northwest of the deepest part of the basin. This offset is related to the regional increase in heat flow to the north and to the presence of high thermal conductivity granite wash adjacent to the Wichita Mountains. A plot of the temperature difference between the equilibrium temperatures estimated from the model and the measured BHTs as a function of depth is remarkably similar to the published correction curve for BHTs for wells in Oklahoma. Vitrinite reflectance and apatite fission-track (FT) data are used to estimate the paleogeothermal conditions in the basin. Published vitrinite reflectance valuesmore » are consistent with a past geographic temperature distribution comparable to the observed distribution with the maximum values offset from the basin axis. FT analysis of sandstones from wells in the southeastern portion of the basin indicates that subsurface temperatures were at least 30C higher than at present, suggest the possibility of substantial erosion in this area.« less
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