Deep geothermal drilling in the resurgent Redondo dome of the Valles caldera has allowed us to define a consistent intracaldera stratigraphic sequence that differs in a number of respects from the temporally equivalent sequence outside the caldera. Above the deeply eroded, Pliocene Paliza Canyon Formation, felsic ash flows and sediments in the dome area form a complex sequence of undetermined age that we call the Lower Tuffs. An erosional interval separates these rocks from the overlying Otowi Member (1.4 m.y.) of the Bandelier Tuff. Another period of erosion, during which a tuffaceous sandstone was deposited, separates the Otowi from the overlying Tshirege Member (1.1 m.y.) of the Bandelier. Both the Otowi and Tshirege members, with maximum thicknesses of 833 and 1155 m, respectively, are substantially thicker within the caldera than outside, indicating simultaneous deposition and cauldron subsidence. Both are predominantly densely welded with distinctive interior zones of granophyric crystallization. Resurgent doming was initiated after emplacement of the Tshirege Member as evidenced by erosion of the upper portions of the cooling unit prior to deposition of overlying units. An isopach map of the sandstone deposited during this erosional period shows that streams draining the uplifting dome were localized along the present Redondo Creek trend. Subsequent volcanic activity resulted in the formation of at least three additional ash flow tuff cooling units prior to deposition of caldera fill and eruption of the Redondo Creek Member of the Valles Rhyolite. A numerical model applied to formation of the Redondo dome suggests that the top of the causative magma body is located at a depth of about 4700 m, 1458 m beneath the bottom of the deepest geothermal well in the dome. No wells have penetrated intrusives that could be related to this magma. We suggest that the locations of faults bounding the apical graben of the dome were influenced by older faults associated with the northeast trending Jemez lineament. These faults were active early in the uplift history of the dome and account for many of the structural differences between hypothetical dome development and reality.
Thermal fluids circulating in the active hydrothermal system of the resurgent Redondo dome of the Valles caldera have interacted with their diverse host rocks to produce well‐zoned alteration assemblages, which not only help locate permeable fluid channels but also provide insight into the system's thermal history. The alteration shows that fluid flow has been confined principally to steeply dipping normal faults and subsidiary fractures as well as thin stratigraphic aquifers. Permeability along many of these channels has been reduced or locally eliminated by hydrothermal self‐sealing. Alteration from the surface through the base of the Miocene Paliza Canyon Formation is of three distinctive types: argillic, propylitic, and phyllic. Argillic alteration forms a blanket above the deep water table in formerly permeable nonwelded tuffs. Beneath the argillic zone, pervasive propylitic alteration is weakly developed in felsic host rocks but locally intense in deep intermediate composition volcanics. Strong phyllic alteration is commonly but not invariably associated with major active thermal fluid channels. Phyllic zones yielding no fluid were clearly once permeable but now are hydrothermally sealed. High‐temperature alteration phases at Baca are presently found at much lower temperatures. We suggest either that isotherms have collapsed due to gradual cooling of the system, that they have retreated without overall heat loss due to uplift of the Redondo dome, that the system has shifted laterally, or that it has contracted due to a drop in the water table. The deepest Well (B‐12, 3423 m) in the dome may have penetrated through the base of the active hydrothermal system. Below a depth of 2440 m in this well, hydrothermal veining largely disappears, and the rocks resemble those developed by isochemical thermal metamorphism. The transition is reflected by temperature logs, which show a conductive thermal gradient below 2440 m. This depth may mark the dome's neutral plane, which separates an upper permeable zone of extensional fracturing from a lower, less permeable compressional regime. The Baca hydrothermal system is similar to those which have formed ore deposits in other calderas: particularly, Creede (Colorado) type epithermal silver base metal veins and stockworks. Recent scientific drilling has also intersected a deep zone of strong phyllic alteration and molybdenum mineralization in the Valles caldera's ring fracture system, a setting which localized a large stockwork molybdenite orebody in the nearby Questa caldera.
HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake River Plain (SRP). The SRP overlies a thermal anomaly, the Yellowstone-Snake River hotspot, that is thought to represent a deep-seated mantle plume under North America. The primary goal of this project is to document the volcanic and stratigraphic history of the SRP, which represents the surface expression of this hotspot, and to understand how it affected the evolution of continental crust and mantle. An additional goal is to evaluate the geothermal potential of southern Idaho.Project HOTSPOT has completed three drill holes.(1) The Kimama site is located along the central volcanic axis of the SRP; our goal here was to sample a long-term record of basaltic volcanism in the wake of the SRP hotspot.(2) The Kimberly site is located near the margin of the plain; our goal here was to sample a record of high-temperature rhyolite volcanism associated with the underlying plume. This site was chosen to form a nominally continuous record of volcanism when paired with the Kimama site. (3) The Mountain Home site is located in the western plain; our goal here was to sample the Pliocene-Pleistocene transition in lake sediments at this site and to sample older basalts that underlie the sediments.We report here on our initial results for each site, and on some of the geophysical logging studies carried out as part of this project.
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