Resurgent Dome Research Articles

Depositional environments and paleolimnology of an ancient caldera lake: Oligocene Creede Formation, Colorado

The Oligocene Creede Formation represents an exceptionally well-preserved intracaldera sedimentary sequence within a large, resurgent caldera. The tuffaceous, epiclastic, and limestone deposits observed in surface exposures and Continental Scientific Drilling Program (CSDP) core provide a record of depositional processes and basin evolution following caldera collapse. The basal part of the Creede Formation is characterized by interbedded caldera wall–derived debris-flow breccias and ephemeral lake deposits. This unit is succeeded by deep-water lacustrine beds that constitute the bulk of the Creede Formation. Interbedded fallout tuffs from intracaldera volcanic eruptions significantly affected lacustrine sedimentation patterns and provide a means of basinwide correlation. Prominent sublacustrine travertine accumulations circumscribe the margins of the moat basin and probably outline the structural margin of the caldera. Deposition in a cool, montane climate is indicated by the presence of pseudomorphs after ikaite in the travertine and lake beds, and corroborative paleofloral analyses. Distinctive lowstand (shallow lake) and highstand (deep lake) facies-association distributions were deposited. Changes in the proportion and character of lowstand versus highstand sequences imply an evolution from a narrow, deep, steep-walled basin to a broad, shallow, incised basin through time. Carbonate minerals were deposited in the lake as travertine at spring-orifices, and as suspension-fallout laminae across the bottom. Little evidence for shelly or fish fauna is preserved in the lake beds. The character of the laminated strata and abundance of depositional carbonate suggest that the lake evolved into a permanently stratified, bicarbonate lake. The volume and mineralogy of depositional carbonate changed through time, but evidence for development of hypersaline, alkaline lake-water compositions is not observed. The level of basin-fill and geomorphic considerations preclude control of lake level by a hydrologic sill; thus, the basin hydrology appears to have been controlled through a combination of climate and groundwater seepage. The deposits of the Creede Formation closely resemble those in modern and ancient caldera and crater lake basins. The effects ofvolcanism on detrital lacustrine sedimentation are profound; however, the influences on lake chemistry and biology are less clear. The extensive travertine in the Creede Formation appears to be related to the volcanic setting, although the origin is as of yet unclear.

Rhyolite intrusions in the intracaldera Bishop Tuff, Long Valley Caldera, California

Drilling of the Long Valley Exploratory Well on the resurgent dome in the Long Valley Caldera revealed > 300 m cumulative thickness of granophyric intrusions within the 1180-m-thick, 760 ka intracaldera Bishop Tuff. The intrusions are aphyric to sparsely plagioclase-phyric, high-silica, high-barium and low-strontium rhyolites. They resemble the lavas of the Early Rhyolite, the first phase of post-caldera volcanism. A mean 40Ar/39Ar age of 590 ± 17 ka from a part of a shallow intrusion is coeval with Early Rhyolite volcanism. A second mean age of 454 ± 17 ka from the same intrusion may reflect either younger Early Rhyolite activity with no external equivalent or hydrothermal resetting of the argon system. Hydrothermal alteration of the intrusions is characterized by introduction of quartz, calcite and pyrite and formation of illite/smectite. High CO2 content of fluids apparently inhibited zeolite formation. Alteration varies locally within intrusions and intrusive groups and does not vary systematically with depth. Oxygen shows consistent depletion of the 18O isotope from an initial magmatic composition of + 6.0 to + 8.5‰ to values ranging from +1.4 to −0.4‰. The constant oxygen isotope depletion most likely reflects alteration of intrusions due to local emplacement-induced hydrothermal circulation rather than a caldera-scale hydrothermal system. In contrast, 18O depletion of the host Bishop Tuff increases regularly with depth (except at an intrusive contact). A pre-Early Rhyolite geothermal gradient of approximately 70 °C/km was inferred. This is substantially higher than the current gradient but substantially lower than expected for the case of a conductive regime over a shallow residual magma chamber. Either the intrusions were fed from a deep chamber, or a cool hydrologic recharge regime was established early in caldera history.The age, thickness and suspected lateral extent of these shallow intrusions are such that emplacement of the intrusions, rather than inflation of a shallow chamber, is responsible for resurgence of the central Long Valley Caldera. Similar intrusions occur in another well on the resurgent dome ( LV13-21 ) but not in wells located off the dome.

Los Azufres silicic center (Mexico): inference of caldera structural elements from gravity, aeromagnetic, and geoelectric data

Los Azufres geothermal field is located within a silicic volcanic complex in central Mexico. The complex is one of the major silicic centers in the Trans-Mexican Volcanic Belt (TMVB). Pradal and Robin (1985) first suggested the existence of the Los Azufres caldera, and Ferrari et al. (1991) recognized the existence of a collapse structure. According to Pradal and Robin this is a caldera of resurgent type. This geophysical study aims to contribute to the knowledge of the structure of the Los Azufres area. Gravity, aeromagnetic, magnetotelluric (MT) and d.c. vertical electric-resistivity soundings were analyzed. Results show that Los Azufres is a very structurally complex setting with relatively thin crust caused by the extensional tectonics characterizing this central sector of the TMVB. Faults belonging to the E-W to NE-SW (extensional neotectonics) and NW-SE (Basin and Range province) systems are observed to affect the geologic units of Los Azufres. According to our study, the Los Azufres geothermal field is located in a structural high located in the middle of a sub-circular depression delimited to the north-northeast by the Santa Ines Range, and to the southwest by the Mil Cumbres formation. The larger depression consists of two narrow, deep depressions that correspond to La Venta and to the Valley of Juarez. They are separated by the above mentioned structural high. These sub-depressions are believed to be the sites of a maximum caldera collapse, and the structural high is interpreted to be at least in part the caldera's resurgent dome. Geoelectric structure of the caldera derived from d.c. resistivity indicates that the brines of the Los Azufres geothermal system ascend along faults, both bounding and internally disrupting the structural high/resurgent dome. A reasonable correlation is observed between gravity and aeromagnetic data.

Shallow and peripheral volcanic sources of inflation revealed by modeling two‐color geodimeter and leveling data from Long Valley Caldera, California, 1988–1992

We refined the model for inflation of the Long Valley caldera near Mammoth Lakes, California, by combining both geodetic measurements of baseline length and elevation changes. Baseline length changes measured using a two‐color geodimeter with submillimeter precision revealed that the resurgent dome started to reinflate in late 1989. Measurements between late 1989 and mid‐1992 revealed nearly 13 cm of extension across the resurgent dome. Geodetic leveling surveys with approximately 2‐mm precision made in late 1988 and in mid‐1992 revealed a maximum of about 8 cm of uplift of the resurgent dome. Two ellipsoidal sources satisfy both the leveling and two‐color measurements, whereas spherical point sources could not. The model's primary inflation source is located 5.5 km beneath the resurgent dome with the two horizontal axes being nearly equal in size and the vertical axis being 4 times the length of the horizontal axes. A second ellipsoidal source was added to improve the fit to the two‐color measurements. This secondary source is located at a depth between 10 and 20 km beneath the south moat of the caldera and has the geometry of an elongated ellipsoid or pipe that dips down to the northeast. In addition, the leveling data suggest dike intrusion beneath Mammoth Mountain during the 1988–1992 interval, which is likely associated with an intense swarm of small earthquakes during the summer of 1989 at that location. Our analysis shows the dike intrusion to be the shallowest of the three sources with a depth range of 1–3 km below the surface to the top of the intrusion.

Response of Long Valley Caldera to the Mw = 7.3 Landers, California, Earthquake

Of the many sites in the western United States responding to the June 28, 1992, Landers earthquake (Mw = 7.3) with remotely triggered seismicity, only Long Valley caldera is monitored by both seismic and continuous deformation networks. A transient strain pulse and surge in seismicity recorded by these networks began within tens of seconds following arrival of the shear pulse from Landers. The cumulative strain and number of triggered earthquakes followed the same exponentially decaying growth rate (time constant 1.8 days) during the first 6 days following Landers. The strain transient, which was recorded on a borehole dilatometer at the west margin of the caldera and a long‐base tiltmeter 20 km to the east, peaked on the sixth day at ≈0.25 ppm and gradually decayed over the next 15–20 days. The absence of a clear strain signal exceeding 0.4 ppm in data from the two‐color geodimeter deformation lines, which span the central section of the caldera, indicates that the strain transient cannot be due solely to pressure changes in the concentrated pressure source 7 km beneath the central part of the caldera that accounts for most of the uplift of the resurgent dome since 1980. The triggered seismicity occupied the entire seismogenic volume beneath the caldera. The focal mechanisms, the frequency‐magnitude distribution, and the spatial distribution of the triggered earthquakes are typical of other swarms in Long Valley caldera. The cumulative seismic moment of the triggered earthquakes through the first 2 weeks after the Landers earthquake corresponds to a single M = 3.8 earthquake, which is too small by nearly 2 orders of magnitude to account for the 0.25‐ppm peak amplitude of the observed strain transients. Evidently, the strain transient represents the dominant response mode, which precludes direct triggering of local earthquakes by the large dynamic stresses from Landers as the dominant process. Conditionally viable models for the triggering process beneath the caldera include (1) the transient pressurization of magma bodies beneath the resurgent dome and Mammoth Mountain by the advective overpressure of rising bubbles, (2) a surge in fluid pressure within the seismogenic zone due to upward cascading failure of isolated compartments containing superhydrostatic pore fluids, (3) relaxation (fluidization) of a partially crystallized magma body or dike intrusion in the deep crustal roots of Long Valley magmatic system, or (4) aseismic slip on midcrustal faults. Either the deep, relaxing‐magma body or lower crustal dike intrusion satisfy all the strain observations with a single deformation source. The latter model admits the possibility that large, regional earthquakes can trigger the episodic recharge of the deep roots of crustal magmatic systems.

Effects of geothermal development on deformation in the Long Valley Caldera, eastern California, 1985–1994

Long Valley caldera in east central California has been the site of crustal unrest in the form of seismicity and ground deformation in response to magmatic inflation since 1980. Uplift of the resurgent dome has totaled ∼0.6 m over the 1975–1992 period. Within this region of uplift, and near the southwestern edge of the resurgent dome, is the Casa Diablo area, which experienced relative subsidence of 0.17 m between 1985 and 1992. Geothermal fluid production began at Casa Diablo in 1985 to supply a binary electric power plant; currently, three such plants generate a total of about 40 MW of electricity. The plants are supplied by 170°C water pumped from depths near 150m; all the produced water is reinjected at cooler temperatures at depths near 600 m. Analyses of data from (1) regional leveling lines and a network of bench marks in the geothermal well field area, (2) tilt observations from an L‐shaped array of bench marks, and (3) calculations relating pressure and temperature changes to subsidence indicate that relative subsidence at Casa Diablo results mainly from reductions in pressure in the shallow production reservoir and overlying formations and reductions in temperature in the underlying injection reservoir. Secondary effects on deformation are caused by offsets along the Taylor‐Bryant fault bounding the west side of the well field.

S wave attenuation structure in Long Valley Caldera, California, from three‐component S‐to‐P amplitude ratio data

Using S‐to‐P amplitude ratios measured from three‐component seismograms we tomographically imaged spatial variations in S wave attenuation in parts of Long Valley caldera. Data from seismograms of local earthquakes in the Long Valley caldera region that occurred during 1984 were employed with the S‐to‐P amplitude ratio technique with corrections for source radiation pattern to determine QS. Resolution is limited throughout most of the model; however, significant resolution and acceptable error in parts of the model allow meaningful interpretation of the results. The primary result is the image of a low‐QS anomaly at 7–8 km depth beneath the resurgent dome. This is independent evidence for high temperatures and possible magma accumulation at this location. Other regions of anomalously low QS are marginally significant or poorly resolved; of these low QS at 4–5 km depth beneath the Inyo Domes has the most potential significance. The similarity of the results from the three‐component data and from the vertical‐component data implies that vertical‐component data, which are generally more abundant, can be used with confidence.

Transient deformation during triggered seismicity from the 28 June 1992 Mw = 7.3 Landers earthquake at Long Valley volcanic caldera, California

Continuous records from a borehole strainmeter and a long baseline tiltmeter in the Long Valley caldera provide critical insights into the origin of at least one episode of minor seismicity in volcanic regions triggered by the 28 June 1992, M L 7.3 Landers, California, earthquake. A strain transient reaching a peak of 0.25 microstrain occurred in the few days following the Landers event and decayed over the next 20 days. A tilt perturbation during the same time reached a peak amplitude of 0.2 microradians. These signals correspond approximately in time to the primary seismic moment release across a 50 km 2 region of the south part of the caldera at depths between 2 and 10 km. Corresponding strain transients in 5-km geodetic lines across the south caldera are not apparent above the 95% confidence limits of about 0.4 microstrain in daily sampled data during this same period. These data rule out models involving single localized inflation sources within the upper crust beneath the caldera, including that responsible for the current rapid inflation of the resurgent dome. They also preclude models involving aseismic slip on single strike-slip or normal faults in the caldera. A single source in the form of a relaxing magma body at a depth of 50 km beneath the caldera can account for the deformation data, but whether the small stress changes are sufficient to drive the triggered seismicity is not clear. An alternate possibility involves distributed deformational sources triggered by the passage of the 10 microstrain peak amplitude surface waves from the earth- quake. This distributed deformational source could result either from rupturing of overpressured fluid or gas chambers commonly encountered in volcanic regions or from advective gas overpressure during release of gas bubbles in hydrothermal or magmatic fluids.

Seismological evidence for magmatic and hydrothermal structure in Long Valley Caldera from local earthquake attenuation and velocity tomography

We compare the results of six different tomographic inversions for the three‐dimensional seismic attenuation and velocity structure in Long Valley caldera, California, in order to constrain interpretations on the location and character of magmatic and geothermal structure in the caldera crust. P and S wave amplitudes and arrival times obtained in previous studies from seismograms of local earthquakes recorded on vertical‐ and three‐component seismographs were inverted for QSV−1, QSH−1, ΔQS−1, ΔQP−1, and ΔVP/VS in the upper 9 km of the caldera crust. Detailed comparisons of the results of the inversions suggest significant locations of high temperatures and perhaps magma and hydrothermal zones in the caldera. A region of low Qs with locally variable QP and VP/VS is imaged in the 7–9 km depth range beneath the resurgent dome. Overlying and adjacent to this anomaly are significant low‐QP and low‐VP/VS anomalies as well as smaller zones of low QS and high VP/VS. Together, these suggest zones of high temperature/magma below and adjacent to zones of supercritical hydrothermal fluids. At 4–5 km depth along the southern rim of the caldera just east of Mammoth Mountain a significant anomaly with relatively low‐QP and relatively low‐VP/VS is imaged and is interpreted as a supercritical hydrothermal zone. Poorly resolved relatively low QP and relatively high VP/VS are seen at 4–5 km depth beneath the Inyo craters, suggesting possible high temperatures and perhaps magma. Because of lack of data in the individual inversions the resultant images have spatially variable error and resolution. However, by comparing multiple images of different seismic parameters we have improved our ability to determine the significance of and constrain interpretation of seismic structure anomalies.

Simulated annealing inversion of teleseismic P‐wave slowness and azimuth for crustal velocity structure at Long Valley Caldera

Simulated annealing, a non‐linear global search algorithm, is used to invert teleseismic P‐wave slowness and azimuth data for crustal velocity structure. Synthetic tests show that simulated annealing is able to locate low P‐wave velocity zones with reasonable accuracy in space, and recovers almost 70% of the target velocity perturbations. Testing suggests that significant results can be obtained with as few as two stations. Inversion of ray direction data from Long Valley caldera finds an asymmetrical low velocity zone beneath the resurgent dome at 8km depth, having a perturbation of −25% from the background. This zone extends to the east and south to depths of at least 24km, with a perturbation of about −10%. These features generally agree with previous results from teleseismic travel‐time inversion and forward modeling of ray direction, and are probably associated with the residual Long Valley caldera magma chamber.

Inflation of Long Valley Caldera from one year of continuous GPS observations

A permanent Global Positioning System receiver at Casa Diablo Hot Springs, Long Valley Caldera, California was installed in January, 1993, and has operated almost continuously since then. The data have been transmitted daily to the Jet Propulsion Laboratory for routine analysis with data from the Fiducial Laboratories for an International Natural sciences Network (FLINN) by the JPL FLINN analysis center. Results from these analyses have been used to interpret the on going deformation at Long Valley, with data excluded from periods when the antenna was covered under 2.5 meters of snow and from some periods when Anti Spoofing was enforced on the GPS signal. The remaining time series suggests that uplift of the resurgent dome of Long Valley Caldera during 1993 has been 2.5±1.1 cm/yr and horizontal motion has been 3.0±0.7 cm/yr at S53W in a no‐net‐rotation global reference frame, or 1.5±0.7 cm/yr at S14W relative to the Sierra Nevada block. These rates are consistent with uplift predicted from frequent horizontal strain measurements. Spectral analysis of the observations suggests that tidal forcing of the magma chamber is not a source of the variability in the 3 dimensional station location. These results suggest that remotely operated, continuously recording GPS receivers could prove to be a reliable tool for volcano monitoring throughout the world.

Quakes, eruptions and other geologic cataclysms

The dynamic Earth: plate interactions seafloor spreading mantle convection. Earthquakes: major quakes areas at risk faults causes damage seismic sea waves. Volcanic eruptions: violent volcanoes fire below activity gas explosions hazards. Earth movements: landslides rockslides mudflows submarine slides soil erosion. Catastrophic collapse: sinking earth ground failures subsidence resurgent calderas. Floods: hydrologic cycle and mapping flood-prone areas types drainage basins. Dust storms: desert regions dust bowls sand dunes. Glaciers: polar ice caps causes glacial surge rising sea levels continental glaciers. Impact cratering: rates and events meteorites rogue asteroids impact effects. Mass extinction: historic causes effects modern extinctions. (Part contents).

Long-lived magmatic phases at Los Azufres volcanic center, Mexico

The Los Azufres volcanic complex has had a long eruptive history of ∼ 1.5 Myr or more (less than 3 Myr), subsequent to the building of a large Miocene and Pliocene andesitic complex. Since about 1.5 Ma, its development consists of two main periods of volcanic activity. The first period, from ∼1.5 to 0.8 Ma, has two magmatic cycles, about 200,000 years in duration, characterized by acidic, followed basaltic volcanism. Such a magmatic evolution suggests successive emptyings of a shallow reservoir in which differentiation previously occurred. About 0.6 Ma, major deep magma supplies were probably responsible for the resurgent doming which led to uplift of the southern part of the caldera. From ∼0.6 Ma to Present, the second period exposes volcanic products grading from basalts to rhyolites. Considering the recent age of the last ignimbrites (26,000 to 29,000 years), one can assume that the volcanic activity related to a voluminous differentiated magma body at shallow depth has not yet ended, especially in the southern area, in and around the resurgent zone occupied by the geothermal field.

Volcanotectonic faulting, soft-state deformation, and rheomorphism of tuffs during development of a piecemeal caldera, English Lake District

The Scafell caldera, an Ordovician ensialic andesite-rhyolite caldera in the English Lake District, is probably the best-exposed piecemeal caldera so far documented. Intra-caldera volcanotectonic faults indicate whole-sale fragmentation of the caldera floor and were reactivated repeatedly during a major subaerial caldera-collapse episode. Such piece-meal collapse is in contrast to calderas in which the floor has undergone coherent piston-like ( en bloc ), downsag, or trapdoor-like subsidence. Mesobreccias, large slide blocks, complex growth-fault geometries, and abrupt local thickness and facies variations in the caldera-fill ignimbrites show that the caldera floor repeatedly developed intricate and rapidly changing topographies of high relief, owing to formation of close-spaced volcanotectonic faults and their frequent reactivation, including movements of reversed sense. At the onset of caldera collapse, volcanotectonic faulting and associated seismicity caused widespread (>260 km 2 ) and ubiquitous subaerial gravity sliding and spreading of an extensive, thick (>30 m), phreatoplinian ash layer. This is recorded as abundant low-angle soft-state slide surfaces, with compressional and extensional deformation off slipped material, including numerous close-spaced ramifying high-angle faults and zones of disaggregation. Particularly intense soft-state deformation is localized at steeply inclined faults that define numerous caldera-floor fault blocks, 0.1-2 km across, and at these faults, the style and geometry of the small-scale soft-state deformation show the sense of fault displacement. Intracaldera silicic ignimbrites overlie the disturbed ash layer and are cut by numerous steep zones of ductile (rheomorphic), brittle, and hybrid deformation structures that formed in response to faulting before the hot tuffs cooled. The deformation zones are characterized by rheomorphic folds, steep high-attenuation rheomorphic shear fabrics, and a facies termed domain breccia that is interpreted as fused rheomorphic breccia. The style of this hot-state deformation is variable and reflects differences in competence, strain rate, and timing of faulting relative to the emplacement and cooling history of each ignimbrite. The deformation zones separate relatively undeformed ignimbrite that underwent dip-slip displacements of as much as 400 m and angular rotations of as much as 20°. The zones can be traced downward into steeply dipping faults that cut caldera-floor rocks. Overlying lacustrine (caldera lake) sediments display slumps and a depositional architecture that record declining persistence of the fault activity. There is no evidence of largescale resurgent doming.

Inversion for P and S wave attenuation structure, Long Valley caldera, California

P and S wave amplitude data from digital, three‐component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three‐dimensional attenuation structure in the upper crust of Long Valley caldera, California. Differential attenuation (∂Q−1) values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios. The spectral ratios are inverted for ∂Q−1 using the LSQR algorithm. Zones of high, positive P wave ∂Q−1 (greater P wave attenuation) are observed at a depth of 4–5 km beneath the east flank of Mammoth Mountain and at 6–8 km beneath the southwestern edge of the resurgent dome. A region of positive S wave ∂Q−1 is seen at 7–8 km beneath the southern edge of the resurgent dome, whereas at 6–7 km, the S wave results are near average. The region of positive S wave ∂Q−1 at 7–8 km beneath the resurgent dome may indicate the roof of a magmatic system. The positive P wave ∂Q−1 at 4–5 km beneath Mammoth Mountain and 6–8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water. These zones may represent some of the source regions for the Long Valley geothermal system.

Mechanisms of crustal uplift and subsidence at the Yellowstone caldera, Wyoming

Leveling surveys in 1923, 1976, and each year from 1983 to 1993 have shown that the east-central part of the Yellowstone caldera, near the base of the Sour Creek resurgent dome, rose at an average rate of 14±1 mm/year from 1923 to 1976 and 22±1 mm/year from 1976 to 1984. In contrast, no detectable movement occurred in the same area from 1984 to 1985 (-2±5 mm/year), and from 1985 to 1993 the area subsided at an average rate of 19±1 mm/year. We conclude that uplift from 1923 to 1984 was caused by: (1) pressurization of the deep hydrothermal system by fluids released from a crystallizing body of rhyolite magma beneath the caldera, then trapped beneath a self-sealed zone near the base of the hydrothermal system; and (2) aseismic intrusions of magma into the lower part of the sub-caldera magma body. Subsidence since 1985 is attributed to: (1) depressurization and fluid loss from the deep hydrothermal system, and (2) sagging of the caldera floor in response to regional crustal extension. Future intrusions might trigger renewed eruptive activity at Yellowstone, but most intrusions at large silicic calderas seem to be accommodated without eruptions. Overpressurization of the deep hydrothermal system could conceivably result in a phreatic or phreatomagmatic eruption, but this hazard is mitigated by episodic rupturing of the self-sealed zone during shallow earthquake swarms. Historical ground movements, although rapid by most geologic standards, seem to be typical of inter-eruption periods at large, mature, silicic magma systems like Yellowstone. The greatest short-term hazards posed by continuing unrest in the Yellowstone region are: (1) moderate to large earthquakes (magnitude 5.5–7.5), with a recurrence interval of a few decdes; and (2) small hydrothermal explosions, most of which affect only a small area (<0.01 km2), with a recurrence interval of a few years.

Middle Cretaceous ash-flow tuff and caldera-collapse deposit in the Minarets Caldera, east-central Sierra Nevada, California

A 2.3-km section of ash-flow tuff and associated caldera-collapse deposit, representing the extrusive facies of part of the Sierra Nevada batholith, is totally exposed from its floor to its top in the Minarets Caldera, east-central Sierra Nevada. Rapid burial and subsequent hornblende hornfels facies metamorphism resulted in remarkable preservation of primary textures and structures, despite the development of cleavage domains in parts of the caldera fill; late Tertiary uplift and Quaternary erosion have produced a rugged terrain where every meter of section is available for study. Large-scale caldera-filling eruption of ash-flow tuff was interrupted by emplacement of a wedge-shaped mass of caldera-collapse deposit as much as 2 km thick, whose volume exceeded 70 km 3 . Individual clasts in the caldera-collapse deposit range to as much as 1.8 km across and include a wide variety of andesitic to rhyoliic lavas and related volcaniclastic rocks, remnants of a precaldera volcanic field that was probably much more extensive than the caldera itself. The caldera-fill sequence rests with angular unconformity on a rugged surface eroded into older volcanic rocks; the sequence is capped by bedded volcaniclastic rocks, including delicately laminated tuffs of probable caldera lake origin. The total aerial extent of the Minarets Caldera is not known, but the area studied, plus scattered pendants of ash-flow tuff and associated volcaniclastic rocks to the west, defines a 30- x 22-km elliptical area that may approximate its original shape. The caldera fill is invaded by a body of quartz monzonite porphyry, locally miarolitic,that was probably emplaced during an episode of caldera resurgence.

Preliminary geologic studies of Sierra El Aguajito (Baja California, Mexico): a resurgent-type caldera

Geologic field studies conducted in the Quaternary volcanic field of Tres Virgenes (State of Baja California Sur, Mexico) revealed the existence of a resurgent caldera. The caldera's eruptive products, the Aguajito sequence, overlie the products of the nearby Reforma caldera (Reforma sequence) whose youngest products have already been dated as Quaternary. The rim of the Aguajito caldera is inferred by the existence of an arcuate alignment of rhyolitic domes. The mean diameter of this subcircular feature is 10 km. The volume of its mapped acidic products is a minimum of 10 km 3. Several horizons within the sequence contain shells. K/Ar dates of the ignimbrites and domes of El Aguajito formation confirm that the unit are Pleistocene. The detailed stratigraphy also shows the evolution of a marine regression partly related to the caldera.

Velocity structure of the Long Valley Caldera from the inversion of local earthquake P and S travel times

A high‐resolution tomographic study of the Long Valley region began in 1989 with the installation of a special fan array of eight three‐component borehole (80–160 m depths) receivers on the northwestern rim of the caldera to provide the data necessary for a joint inversion for hypocenters and the three‐dimensional P and S wave velocity structure in and below the caldera. The experiment specifically targeted the subsurface location of the previously inferred magma chamber beneath the Resurgent Dome. Additional coverage was provided by existing U.S. Geological Survey and University of Nevada, Reno seismographic networks. We employed the progressive inversion scheme of Thurber (1983), with cubic spline interpolation after Michelini and McEvilly (1991). The final model is based on data from 280 well‐distributed local earthquakes. Some 6900 arrival times were used, including about 700 S wave times from the three‐component stations. The resulting velocity structure reflects the known geology, defining the low‐velocity caldera fill to 2 km depth, contrasting markedly with the surrounding higher‐velocity Sierran block and highland terrane. No isolated distinct low‐velocity anomalies are revealed beneath the caldera floor, although a diffuse zone of reduced velocity persists to a depth of about 8 km. These lower velocities may be related to hydrothermal alteration and/or extensive fractures. The Vp/Vs structure contains significant lateral variation within and beneath the caldera to about 8 km depth, and these variations can be related to accepted models of the active geothermal system. The lack of a significant S wave velocity anomaly, along with the normal or low values of the Vp/Vs ratio, argues against the presence of a sizable and distinct magma body at shallow to midcrustal depth beneath Long Valley caldera, although a very low percentage of partial melt cannot be precluded.

The Selbaie Zn-Cu-Ag deposits, Quebec, Canada; an example of evolution from subaqueous to subaerial volcanism and mineralization in an Archean caldera environment

The Selbaie deposits of western Quebec are hosted in calc-alkalic pyroclastic rocks of the Abitibi subprovince. Rocks range upward from subaqueous debris flows through pumiceous tuff to densely welded ignimbrite; they form an intracaldera sequence over 1,200 m thick which records evolution from subaqueous to subaerial deposition. Mineralization occurred within this sequence on the southwest flank of the Brouillan tonalite, a central intrusion and possibly a resurgent dome, and includes regionally extensive and temporally early exhalite, local massive sulfide, and later epithermal veins. These veins were emplaced by open-space filling along synvolcanic structures and make up the bulk of the economic resource.Mineralization and alteration show similarities to both massive sulfide and epithermal deposits. Cu/Zn increases with depth but also increases laterally toward veins and toward the intrusion. Ag/Cu in veins is higher in the upper levels of the system. Whole-rock mass balance calculations indicate enrichment in Si, Fe, Mg, and Mn with proximity to mineralization and depletion in Ca, K, and Na. Na metasomatism, common in host rocks to Archean massive sulfide deposits, is absent in subaerially emplaced tuff at Selbaie and K feldspar is preserved.Selbaie is unusual (1) as an Archean example of a subaerial caldera that evolved from subaqueous to subaerial volcanism, (2) in representing economically important, Archean epithermal mineralization, and (3) in demonstrating the link between subaqueous volcanogenic massive sulfide deposits and subaerial volcanogenic epithermal veins. It is, perhaps, unique in preserving so well all three of these aspects in a single mineralized system. The rarity of similar, mineralized subaerial volcanic settings in the Abitibi subprovince reflects its primitive crustal development, but may also be due to incomplete preservation of subaerial environments.