Caldera Margin Research Articles

La Pacana caldera, N. Chile: a re-evaluation of the stratigraphy and volcanology of one of the world's largest resurgent calderas

La Pacana caldera in the Central Andes of northern Chile is one of the largest and best exposed resurgent calderas in the world. The caldera had previously been recognised as the source of the regionally-extensive Atana ignimbrite, but additional field and stratigraphic evidence, along with new K–Ar age determinations and geochemical data have led to a revision of the geology and development of this major Andean caldera. In particular, this information allows more realistic estimates of eruptive volumes and has implications for the style of ignimbrite eruption. Two major ignimbrites appear to have originated from La Pacana caldera, based on their thickness variations, lateral distributions and stratigraphic relations: the crystal-poor, rhyolitic Toconao ignimbrite (4–5 Ma) and the crystal-rich, dacitic Atana ignimbrite (4 Ma). Following caldera collapse and formation of the resurgent Atana block, several crystal-rich dacitic–rhyolitic domes formed along the margin of the resurgent block. New K–Ar ages show that this post-caldera volcanism continued from 4 to 2 Ma, indicating that the La Pacana magmatic system was active for at least 2 Ma after the main eruption. The Atana ignimbrite extends west, south and east of La Pacana caldera. Our work shows that the ignimbrite sequence northeast of the caldera, formerly mapped as Atana outflow, represents two new units which we name the upper and lower Tara ignimbrites. The distribution of the Tara ignimbrites points to a source to the north. The upper Tara ignimbrite comprises four flow units with interbedded surge and fall deposits and a characteristic, heterogeneous pumice population. It occurs in the La Pacana moat and onlaps the resurgent block. These field relations and a new K–Ar age of 3.8 Ma show convincingly that this ignimbrite erupted after formation of La Pacana caldera. The lower Tara ignimbrite is a single extensive flow unit, and has an age of 5.6 Ma. Two outcrops of lag breccia occur adjacent to the caldera topographic margin and these are interpreted as vent areas for the Atana ignimbrite. This indicates that the structural caldera margin does not coincide with the resurgent block as previously thought, but is close to the topographic margin. Combining this redefinition of caldera geometry with the revised distribution of the Atana ignimbrite implies an overall trap-door-like configuration of caldera collapse, with a hinge to the north and maximum subsidence in the south. The new volume estimate of 2500 km 3 for the caldera-forming Atana ignimbrite, including intracaldera and outflow facies, places it among the most voluminous ignimbrites of the world.

TEMPORAL EVOLUTION OF ARC MAGMATISM AND HYDROTHERMAL ACTIVITY, INCLUDING EPITHERMAL GOLD VEINS, BOROVITSA CALDERA, SOUTHERN BULGARIA

Plutonic, volcanic, and hydrothermal activity in magmatic arcs is commonly focused spatially, leaving behind complex records of multiple ore-forming processes that can be appreciated in detail only if the timing of igneous and hydrothermal systems is ascertained. The Spahievo ore district, Bulgaria, is adjacent to the Oligocene Borovitsa caldera, one of the largest in Europe. Juxtaposition of porphyry mineralization, barren advanced argillic alteration, and low-sulfidation epithermal alteration and mineralization zones along the caldera margin provides an uncommon opportunity to examine the temporal development of an evolved magmatic-hydrothermal system. 40 Ar/ 39 Ar ages determined using a CO 2 laser to fuse or incrementally heat minerals from fresh igneous and altered rocks indicate a rapid succession of (1) pre-caldera intermediate magmatism including monzonitic intrusion and coeval porphyry-style mineralization and advanced argillic alteration between 32.9 and 32.3 Ma; (2) rhyolitic ash-flow tuffs, dikes, and domes, with adularia-sericite alteration accompanied by epithermal precious metal mineralization at 32.1 Ma; and (3) nearly synchronous collapse of a nested caldera and intrusion of a ring-fault rhyolite dome and intracaldera dikes at 31.8 Ma. These ages link monzonitic intrusion, Cu-Mo mineralization, and advanced argillic alteration, and indicate that rhyolitic magmatism and adularia-sericite alteration with associated base-metal plus Au mineralization is slightly younger, about 100 to 500 k.y. Deposition of Au ore ca. 300 k.y. prior to caldera collapse distinguishes this ore district from many well-known deposits in which ore formation accompanied andesitic or rhyolitic intrusions along ring fractures that postdate the collapse.

Arcuate pyroclastic conduits, ring faults, and coherent floor at Kumano caldera, southwest Honshu, Japan

At the 41×23 km Miocene Kumano caldera in southwestern Japan, a large arcuate pyroclastic breccia unit is interpreted as the dissected conduit for voluminous explosive eruptions. The pyroclastic breccia and associated granite porphyry occur along the southern margin of the caldera. These rocks, collectively as much as 22 km long and 800 m wide, intrude the arcuate fault bounding the southern caldera margin. The breccia is interpreted as the vent facies of ash-flow tuff adjacent to the caldera, based on similar lithologies, phenocryst modes of juvenile clasts, and matrix character. Cataclasites along the major faults include both country rocks and pyroclastic breccia, yet some cataclasite blocks are contained within the pyroclastic breccia. These geometric and textural relations suggest that the faulting took place during caldera-forming eruption. Prevolcanic sedimentary rocks enclosed by the arcuate faults are interpreted as the dissected coherent floor of the caldera, which were subsided several hundreds to a thousand meters. No large-scale piecemeal disruption of the caldera floor is evident. Orientations of striations with the cataclasites and altitudes of the base of the tuff suggest the asymmetric trap-door subsidence of this caldera. The overall caldera geometry is the nested trap-door piston-cylinder subsidence, each 21×15 and 25×23 km in diameter, in contrast to previous interpretation of a funnel shape.

Volcanic geology of Furnas Volcano, São Miguel, Azores

Furnas is the easternmost of the three active central volcanoes on the island of São Miguel in the Azores. Unlike the other two central volcanoes, Sete Cidades and Fogo, Furnas does not have a well-developed edifice, but consists of a steep-sided caldera complex 8×5 km across. It is built on the outer flanks of the Povoação/Nordeste lava complex that forms the eastern end of São Miguel. Constructive flanks to the volcano exist on the southern side where they form the coastal cliffs, and to the west. The caldera margins tend to reflect the regional/local tectonic pattern which has also controlled the distribution of vents within the caldera and areas of thermal springs. Activity at Furnas has been essentially explosive, erupting materials of trachytic composition. Products associated with the volcano include plinian and sub-plinian pumice deposits, ignimbrites and surge deposits, phreatomagmatic ashes, block and ash deposits and dome materials. Most of the activity has occurred from vents within the caldera, or on the caldera margin, although strombolian eruptions with aa flows of ankaramite and hawaiite have occurred outside the caldera. The eruptive history consists of at least two major caldera collapses, followed by caldera infilling. Based on 14 C dates, it appears that the youngest major collapse occurred about 12,000–10,000 years BP. New 14 C dates for a densely welded ignimbrite suggest that a potential caldera-forming eruption occurred at about 30,000 years BP. Recent eruptions (<5000 years old) were mainly characterised by alternating episodes of magmatic and phreatomagmatic activity of plinian and sub-plinian magnitude, forming deposits of interbedded ash and lapilli. An historical eruption is documented in 1630 AD; new evidence suggests that another occurred during the early occupation of the area at about 1440 AD.

Correlation of deposits and vent locations of the proximal Campanian Ignimbrite deposits, Campi Flegrei, Italy, based on natural remanent magnetization and anisotropy of magnetic susceptibility characteristics

Correlation of the distal deposits of the Campanian Ignimbrite with their proximal equivalents in the Campi Flegrei caldera is complicated by a lack of medial exposures, complex and limited proximal stratigraphic sections, and large lateral facies changes. Paleomagnetic data from 10 sites in and near the Campi Flegrei yield natural remanent magnetizations (NRM) that are statistically indistinguishable from the distal Campanian Ignimbrite. In addition, their virtual geomagnetic pole (VGP) yields a possible correlation with Lac du Bouchet, France, secular variation data that indicate an age of approximately 32,850 14 C years. The secular variation curve at this age was only briefly at this VGP, and did not return to it for >10,000 years, so the paleomagnetic correlation of proximal and distal deposits is unique and robust. The date is consistent with 14 C dates from the Campanian Ignimbrite, but younger than 39 Ar/ 40 Ar dates for the same rocks. This suggests that a better correction factor for cosmic flux for this time period is needed to calibrate older 14 C dates. Anisotropy of magnetic susceptibility (AMS) data show that the proximal deposits have an oblate (disk-shaped), poorly lineated magnetic fabric. The distal deposits are much better lineated. The difference may be due to chaotic depositional currents in the proximal areas, in which particles were not well aligned. With greater distance of travel, and loss of energy, particles within the flow became aligned and developed stronger AMS lineations. Early eruptions of the Piperno Tuff were from a central vent north of Pozzuoli, whereas later tuffs that underlie the Breccia Museo may have been emplaced by flows associated with ring vents located on the northern and southern caldera margins.

Intracaldera structure and megabreccias at Dorobu caldera, northeastern Honshu, Japan

In the Pliocene Dorobu caldera, 13 by 6 km in diameter, on northeastern Honshu, Japan, a number of occurrences of a distinctive megabreccia block facies is found within voluminous ash-flow tuffs, the Dorobu ash-flow tuffs A and B. Dorobu tuff A is believed to be the deposit of the initial caldera-forming eruption. An isopach map of tuff B and orientation of narrow dikes indicate that an internal large depression occurred inside the present caldera margin. This and locations of post-caldera conduits and near vent facies of Dorobu tuff C suggest that the caldera has a nested collapse structure, and possible conduits of the tuff B are within the internal depression. Annular collapses that sequentially moved towards the caldera center occurred during eruption of tuffs A and B. The overall structure of Dorobu caldera is nested, and it possibly has step-like floor, as described elsewhere from piecemeal calderas. Megabreccia block facies consists of polyhedral massive blocks composed of Miocene to Pliocene country rocks with large diameters (up to several hundreds meters). The block facies as a whole does not consist of sheets or piles of debris, as described in the San Juan Mountains and Stillwater Range, USA, but as isolated blocks set in the matrix of Dorobu tuffs A and B. In previous studies of caldera development, the mode of emplacement of caldera-collapse breccias has been considered to be by rock fall or rock slide during the caldera forming eruption. A paleo-magnetic study of the block facies shows two significant characteristics. Predominant variations of the magnetic directions of blocks indicate that they were emplaced with rotations, caused by rock slide, rock fall, or a toppling motion. In contrast, magnetized directions of one megabreccia block almost coincide with that of the caldera country rock, suggesting that this block must have kept its original magnetic direction. Our findings suggest that it is not a rock slide deposit derived from caldera walls, but rather a segment of subsided caldera floor. The mode of emplacement of the megabreccia block facies at Dorobu caldera is mostly similar to the other megabreccias, however, additionally, it is necessary to distinguish segments of caldera floor from rolled or toppled megabreccia blocks.

The Bishop Tuff: New Insights From Eruptive Stratigraphy

The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1‐F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1‐F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2‐F8), Lack phenocrystic pyroxenes, and contain few or no Glass Mountain‐derived rhyolite lithic fragments. Earlier packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in Glass Mountain‐derived rhyolite or other lithologies exposed on the northern caldera rim. Recognition of the intraplinian nature of Ig1 east of the caldera and use of the fall deposit chronometry yields accumulation estimates of ca. 25 hrs for an earlier, less‐welded subpackage and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to ≥1 mm/s of dense‐welded massive ignimbrite, equivalent to ≥2.5 mm/s of non‐welded material. Comparisons of internal stratification in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in ≥35 hrs. Identifiable vent positions migrated from an initial site previously proposed in the south‐central part of the caldera (F1‐8, Ig1) in complex fashion; one vent set (for eastern Ig2) migrated east and north toward Glass Mountain, while another set (for northern Ig2) opened from west to east across the northern caldera margin. Vent locations for Ig1 and Ig2 southwest of the caldera have not been identified. The new stratigraphic framework shows that much of the Bishop ignimbrite is intraplinian in nature, and that fall deposits and ignimbrite units previously inferred to be sequential are largely or wholly coeval. Fundamental reassessment is therefore required of all existing models for the eruption dynamics and the nature and causes of pre‐eruptive zonations in trace elements, volatiles, and isotopes in the parental magma chamber.

Non-explosive, constructional evolution of the ice-filled caldera at Volcán Sollipulli, Chile

A radar and gravity survey of the ice-filled caldera at Volcan Sollipulli, Chile, indicates that the intra-caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km3 of ice. Reconnaissance geological observations show that the volcano has erupted compositions ranging from olivine basalt to dacite and have identified five distinct volcanic units in the caldera walls. Pre- or syn-caldera collapse deposits (the Sharkfin pyroclastic unit) comprise a sequence which evolved from subglacial to subaerial facies. Post-caldera collapse products, which crop out along 17 of the 20 km length of the caldera wall, were erupted almost exclusively along the caldera margins in the presence of a large body of intra-caldera ice. The Alpehue crater, formed by an explosive eruption between 2960 and 2780 a. BP, in the southwest part of the caldera is shown to post date formation of the caldera. Sollipulli lacks voluminous silicic pyroclastic rocks associated with caldera formation and the collapse structure does not appear to be a consequence of a large-magnitude explosive eruption. Instead, lateral magma movement at depth resulting in emptying of the magma chamber may have generated the caldera. The radar and gravity data show that the central part of the caldera floor is flat but, within a few hundred metres of the caldera walls, the floor has a stepped topography with relatively low-density rock bodies beneath the ice in this region. This, coupled with the fact that most of the post-caldera eruptions have taken place along the caldera walls, implies that the caldera has been substantially modified by subglacial marginal eruptions. Sollipulli caldera has evolved from a collapse to a constructional feature with intra-caldera ice playing a major role. The post-caldera eruptions have resulted in an increase in height of the walls and concomitant deepening of the caldera with time.

Incremental collapse and stratocone growth within a funnel-shaped caldera, Guayabo, Costa Rica

Correlations between the surface geology and detailed borehole stratigraphy/density logs, particularly in the geothermally active area, illustrate that the present morphology and structure of the Guayabo caldera is the product of at least four subsidence events. Much of the sub-caldera tuff-lava sequence, also exposed in the topographic caldera walls, is the product of several ca. 2-Ma-old overlapping stratocones formerly present in the area, but later subjected to subsidence and edifice collapse. The V-shaped gravity anomaly across the caldera is due to a funnel-shaped cross section, developed as follows. The earliest and innermost (now concealed) down-faulted zone was filled by a low-density pumice unit which extended southwest along an early arc-normal graben. At least two subsequent subsidence events produced the present scarps and outflow tuff-breccias, but the erupted volume is markedly lower than the subsidence volume. The missing magma may have been diverted to the east, where, at the same time, further stratocone buildup was occurring. The gravity signature over this eastern caldera margin suggests the presence of a large, dense sub-volcanic intrusive complex. Further edifice collapse and graben formation, due to a combination of gravitational failure and arc-normal strain, are relatively recent and continuing processes.The evolution of the Guayabo caldera therefore involved a series of subsidence events punctuated by stratocone buildup and collapse. The resolution of concentric, ring-faulted block structures does not support a chaotic collapse model but, instead, indicates the incremental development by widening of a funnel-shaped caldera.

Evolution of volcanic and tectonic features in caldera settings and their importance in localization of ore deposits

Many calderas are located along regionally important fault zones that are intermittently active before and after the caldera cycle. In mineralized calderas, the ore deposits are controlled by structures developed during caldera formation and by regional faults which intersect and reactivate the caldera-related structures. In the earliest phase of the caldera cycle, tumescence occurs over the shallowly emplaced magma chamber and normal faults develop within the structural dome above the magma chamber. This early phase of faulting is disrupted by caldera formation but some of the faults are preserved outside the caldera margin and may become important ore-controlling structures such as the Ruja mercury deposit in the McDermitt caldera complex, Nevada, and the precious metal deposits in the Atlanta district, Nevada, outside the Indian Peak caldera. Caldera-related structures such as ring fracture faults are commonly the most important in controlling the emplacement of resurgent magma, as well as postcollapse intrusions and associated ore deposits. During the resurgence phase of the caldera, faults related to an apical graben, and radial and ring faults and fractures form and may extend outside the collapse structure. Radial and ring faults and fractures provided the structural control for gold alunite-type deposits in the Rodalquilar caldera complex, Spain.At the end of the caldera cycle, the area over the batholith related to the caldera-forming magma chambers is uplifted as isostatic compensation occurs, or subsides if the volcanic field is developed within a rapidly subsiding graben. Volume change resulting from crystallization will also contribute to subsidence. Regional subsidence of the Lake Owyhee volcanic field in southeastern Oregon at the end of the caldera cycle is reflected by arkosic sedimentation in the caldera basins and throughout the eastern part of the volcanic field. Crystallization of the underlying batholith permits the reestablishment of brittle deformation along the regional faults which initially may have controlled magma emplacement. This tectonic activity may reactivate caldera structures and these typically become important for localization of hydrothermal activity. Hot spring-type gold deposits in the Lake Owyhee volcanic field formed when a regional northwest-trending structure reactivated the caldera ring fracture fault zone of the Three Fingers caldera. The formation of hydrothermal ore deposits at the end of the caldera cycle is often related to tectonic reactivation of caldera structures by regional faults and emplacement of small volumes of magma as stocks and vent complexes along these structures.

A 12,000‐year record of vertical deformation across the Yellowstone caldera margin: The shorelines of Yellowstone Lake

The 600 ka Yellowstone caldera exhibits several signs of unrest, the most evident of which is historic ground deformation including both uplift and subsidence. We document deformation in the area of the southeastern caldera across ∼12,000 years using the postglacial shoreline terraces of Yellowstone Lake. Raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces, with an accuracy of +/−0.5 m. Of about 11 recognizable terraces, the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns. Shoreline ages are based on minimum‐ and maximum‐limiting radiocarbon and obsidian‐hydration dates. Each terrace is interpreted as representing an episode of uplift (∼1 kyr−1) of the caldera interior and subsequent subsidence, with little net volume change. This cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization, and/or episodic trapping and release of magmatic fluids (evolved during cooling and crystallization) in a self‐sealing reservoir, as hypothesized by Dzurisin et al. (1990). Early postglacial shoreline deformation reveals substantial intracaldera subsidence, possibly reflecting cooling and loss of trapped hydrothermal fluids. Extension along north‐south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin, including downwarping and local faulting of hot, weak intracaldera crust. Net deformation over the past ∼3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim, relative to the extracaldera region. This uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode. Subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence. The cause of this trend is undetermined but is most likely related to the effects of regional extension and long‐term cooling within the Yellowstone caldera.

Large-scale rheomorphic shear deformation in Miocene peralkaline ignimbrite E, Gran Canaria

the single ignimbrite cooling unit E (average thickness, 28 m; volume, ca. 30 km3) forms the uppermost member of the Miocene Upper Mogan Formation on Gran Canaria. It is strongly chemically zoned from basal, first-erupted comendite (peralkaline rhyolite) to late-erupted trachyte, and, apart from an upper trachytic zone, it is densely welded. E was emplaced onto a surface inclined ca. 2–5° from the source caldera. Detailed mapping of key sections, up to 300 m long, exposed in barranco walls, ca. 10 km from the caldera margin, reveals structures that are interpreted to have been produced by rheomorphic deformation of the ignimbrite along shear zones. The shear zones formed within the lower-viscosity comenditic tuff. Extensional structures include mega-boudinage and ‘decapitated sequences’ and compression resulted in sequence repitition by overthrusting. Mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs (compaction, lateral creep, folding, vertical density-driven diapirism) cannot account for these features, which reflect lateral (post-compactional) rheomorphic movement locally in excess of 800 m. We suggest the following sequence of events: emplacement of the several flow units; compaction, with little lateral movement; rheomorphic deformation. During and after compaction, layers of secondary porosity developed within the comenditic tuff, possibly where upward escape of gas was prevented by overlying, relatively impermeable layers of densely compacted ignimbrite. These structurally weak layers of high porosity subsequently acted as shear zones.

Post-collapse volcanic history of calderas on a composite volcano: an example from Roccamonfina, southern Italy

Roccamonfina, part of the Roman Potassic Volcanic Province, is an example of a composite volcano with a complex history of caldera development. The main caldera truncates a cone constructed predominantly of this caldera may have been associated with one of the ignimbritic eruptions of the Brown Leucitic Tuff (BLT) around 385 000 yr BP. The Campagnola Tuff, the youngest ignimbrite of the BLT, however, drapes the caldera margin and must postdate at least the initial stages of collapse. During the subsequent history of the caldera there were several major explosive eruptions. The largest of these was that of the Galluccio Tuff at about 300 000 yr BP. It is likely that there was further collapse within the main caldera associated with these eruptions. It is of note that despite these subsequent major explosive eruptions later collapse occurred within the confines of the main caldera. Between eruptions caldera lakes developed producing numerous lacustrine beds within the caldera fill. Extensive phases of phreatomagmatic activity generated thick sequences of pyroclastic surge and fall deposits. Activity within the main caldera ended with the growth of a large complex of basaltic trachyandestite lava domes around 150 000 yr BP. Early in the history of Roccamonfina sector collapse on the northern flank of the volcano formed the northern caldera. One of the youngest major events on Roccamonfina occurred at the head of this northern caldera with explosive activity producing the Conca Ignimbrite and associated caldera. There is no evidence that there was any linkage in the plumbing systems that fed eruptions in the main and northern calderas.

Morphology of a postglacial fault scarp across the Yellowstone (Wyoming) caldera margin and its implications

Morphology of a postglacial fault scarp across the Yellowstone (Wyoming) caldera margin and its implications

Facies analysis of an ancient, dismembered, large caldera complex and implications for intra-arc subsidence: Middle Jurassic strata of Cobre Ridge, southern Arizona, USA

The Middle Jurassic (∼ 170 Ma) Cobre Ridge caldera is an elongate caldera complex that formed during the eruption of the tuff of Pajarito, a crystal-rich rhyodacite ignimbrite with an estimated volume > 1000 km 3. The caldera subsided in two subequal blocks: to the southeast, caldera-forming ignimbrite is as much as 3000 m thick, whereas the northwest half of the caldera subsided to lesser depths during the initial eruption, but was the locus of subsidence during subsequent eruptions, providing a depocenter for a > 1.5 km-thick section of secondary- and post-collapse volcanic and sedimentary strata. These strata record moat formation and filling and waning volcanism, including: (1) areally restricted ignimbrites up to several tens of meters thick that may have been ponded between caldera margins and/or fault scarps; (2) a ⩾ 600 m-thick ignimbrite whose eruption probably caused further collapse of the caldera or enlargement of the caldera margins; (3) a localized debris apron deposit up to 500 m thick that represents material reworked from intra-caldera pyroclastic deposits and possibly caldera walls; and (4) eolian and fluvial sandstones and water-lain tuffs. Megabreccia blocks as large as 0.5 km along strike probably demarcate approximate caldera margins, suggesting that the composite caldera was approximately 50 km long by 25 km wide. The preserved thickness of the strata of Cobre Ridge ( > 4500 m) is greater than many ancient continental volcanic sequences, suggesting external (i.e. tectonic) controls on subsidence, but preservation of this great thickness of strata is apparently due entirely to volcanic subsidence.

The Hechiceros Caldera: A recently identified Mid‐Tertiary caldera in eastern Chihuahua, Mexico

Geologic mapping of Tertiary volcanic rocks in the Sierra Hechiceros, 50 to 80 km south of Big Bend National Park has delineated a 20 by 25 km Mid‐Tertiary caldera, herein named the Hechiceros Caldera. This caldera was formed by the eruption of the Palomas tuff which has a minimum volume of approximately 170 km3, and is distributed radially around the caldera. The caldera was filled with more than 500 meters of postcaldera rhyolitic lavas which overflowed the caldera margin and caused additional subsidence. The rocks of the Sierra Hechiceros are subalkalic and include rhyolites and dacites. The chemistry of these rocks is transitional between the calc‐alkalic province of the Sierra Madre Occidental and the alkalic rocks of the eastern Trans‐Pecos magmatic province. The potassium argon ages both the Palomas tuff and the postcaldera El Jobero rhyolite are about 29±0.6 Ma.

Groundwater dilution and residence times, and constraints on chloride source, in the Mokai Geothermal System, New Zealand, from chemical, stable isotope, tritium, and 36Cl data

The Mokai geothermal system, New Zealand, is located within a volcanic collapse structure comprised of flat‐lying silicic rocks. The deep chloride fluid rises in the south, inside the caldera margin; near the surface the fluid flows laterally more than 12 km to the north, because of the hydraulic gradient caused by 300 m of relief. The maximum measured temperature of the production wells is near 323°C at 2500 m depth, whereas temperatures in two wells on the northern margin of the upflow are &lt;200°C, with thermal inversions at 350 and 1000 m indicating lateral flow. These two marginal wells have about 50% of the chloride of the deep fluid, indicating dilution by cold ground water. Chloride‐bearing springs discharge 6 km north of the zone of upflow and are even further diluted by cold groundwater than the wells on the margin of the system. The tritium data in this hydrologically rather simple system indicate that there is a mean residence time of greater than 100 years for the groundwater diluent; also, older water is mixed with &lt;25% of young (thermonuclear, post‐1955) water before the mixture dilutes outflowing chloride fluid from the deep system. The deep chloride fluid has a 36Cl/Cl ratio of 2.6×10−15 and a 36Cl concentration of 106 atoms per 10−6 kg. This fluid has a lower ratio (by a factor of 90) and a higher absolute concentration (by a factor of 7) than present‐day stream water. However, the marginal wells have been diluted by groundwater with at least 500 36Cl atoms per 10−6 kg, 30 times richer in 36Cl than present‐day stream waters. This spike is most likely due to circulation of precipitation formed during thermonuclear testing to depths of 1000 m (though the component with the 36Cl spike only makes a moderate, &lt;25%, contribution to the groundwater diluent; most of the diluent is much older, pre‐1955). The amount of 36Cl in the deep fluid at Mokai is 5–10 times less than that predicted from a calculation of the neutron fluxes in the silicic reservoir and underlying greywacke basement rocks. This indicates that the chloride was not derived from host rock leaching in the upper 3 km; rather, the fluids have interacted with more primitive rocks below the drilled depths of the Taupo Volcanic Zone, with the chloride possibly derived from a magmatic brine.

Evidence of active ground deformation on the mid‐ocean ridge: Axial Seamount, Juan de Fuca Ridge, April‐June 1988

Since September 1987 a precision bottom pressure recorder (BPR) has been deployed within the summit caldera of Axial Seamount on the central Juan de Fuca Ridge. The instrument is capable of measuring pressure to 1 mbar resolution and recording these measurements at 64 samples per hour for up to 15 months. After removal of oscillatory signals due to tidal and oceanographic effects and linear trends caused by sensor drift, any significant change in the pressure record should indicate a change of depth associated with vertical ground movement. In subaerial volcanic systems, such ground movements commonly indicate active inflation or deflation of underlying magma bodies and are frequently coincident with rift eruptions. Axial Seamount was selected for this pilot study for three primary reasons: (1) its tectonic setting, (2) the presence of a well‐formed summit caldera, and (3) observational evidence of geologically recent volcanic activity. Results from the first 9 months of the BPR deployment revealed a significant change in pressure, which is interpreted to represent a 15‐cm subsidence of the caldera floor during two 2‐ to 3‐week periods in April–June 1988. Also during these periods, an anomalous decline in temperature at the site was recorded that is correlated with an apparent increase in current velocity at the Axial Seamount Hydrothermal Emissions Study (ASHES) vent field, suggesting vigorous advection of cold water into the caldera. Concurrent oceanographic data from Geosat and from current meter arrays do not indicate any large‐scale oceanographic phenomena capable of generating these simultaneous events. One mechanism to explain simultaneous ground subsidence and temperature decline at the caldera center and increased bottom current at the caldera margin is the generation of a buoyant parcel of heated water in response to the intrusion or the eruption of magma associated with volcanic deflation. Such a parcel must have been sufficiently buoyant and proximal to induce horizontal entrainment currents capable of producing the bottom water conditions that were observed at both sites. Similar volcanic events also may have generated large midwater plumes that have been described previously along the southern Juan de Fuca Ridge.

Geochemistry of intrusive rocks associated with the Latir volcanic field, New Mexico, and contrasts between evolution of plutonic and volcanic rocks

Plutonic rocks associated with the Latir volcanic field comprise three groups: 1) ∼25 Ma high-level resurgent plutons composed of monzogranite and silicic metaluminous and peralkaline granite, 2) 23–25 Ma syenogranite, and alkali-feldspar granite intrusions emplaced along the southern caldera margin, and 3) 19–23 Ma granodiorite and granite plutons emplaced south of the caldera. Major-element compositions of both extrusive and intrusive suites in the Latir field are broadly similar; both suites include high-SiO2 rocks with low Ba and Sr, and high Rb, Nb, Th, and U contents. Moreover, both intermediateto siliciccomposition volcanic and plutonic rocks contain abundant accessory sphene and apatite, rich in rare-earth elements (REE), as well as phases in which REE's are essential components. Strong depletion in Y and REE contents, with increasing SiO2 content, in the plutonic rocks indicate a major role for accessory mineral fractionation that is not observed in volcanic rocks of equivalent composition. Considerations of the rheology of granitic magma suggest that accessory-mineral fractionation may occur primarily by filter-pressing evolved magmas from crystal-rich melts. More limited accessory-mineral crystallization and fractionation during evolution of the volcanic magmas may have resulted from markedly lower diffusivities of essential trace elements than major elements. Accessory-mineral fractionation probably becomes most significant at high crystallinities. The contrast in crystallization environments postulated for the extrusive and intrusive rocks may be common to other magmatic systems; the effects are particularly pronounced in highly evolved rocks of the Latir field. High-SiO2 peralkaline porphyry emplaced during resurgence of the Questa caldera represents non-erupted portions of the magma that produced the Amalia Tuff during caldera-forming eruption. The peralkaline porphyry continues compositional and mineralogical trends found in the tuff. Amphibole, mica, and sphene compositions suggest that the peralkaline magma evolved from metaluminous magma. Extensive feldspar fractionation occurred during evolution of the peralkaline magmas, but additional alkali and iron enrichment was likely a result of high halogen fluxes from crystallizing plutons and basaltic magmas at depth.

The mechanism of intrusion of the Inyo Dike, Long Valley Caldera, California

We analyze the intrusion of the 11‐km‐long Inyo Dike at the margins of Long Valley caldera, eastern California. The dike trends N07°W and is divided into at least three segments which are rotated by as much as 25° with respect to the main trend. The dike seems affected primarily by the regional stress field of right‐lateral shear of the western United States and by the local thermal conditions of the crust; the dike seems unaffected by the preexisting caldera margins and Sierra‐Nevada frontal fault system. The high heat flow in Long Valley caldera implies that crustal rocks below 3–4.5 km deform by steady state creep under tectonic strain rate and support low to vanishing tectonic shear stresses. The upper rocks, above 3–4.5 km, deform by frictional slip along fractures and may support tectonic shear stresses as high as 24 MPa. We demonstrate that depth variations of tectonic stresses may have a profound effect on the segmentation and rotation of dikes, both at Long Valley and in other areas of high heat flow. The analysis places constraints on several tectonic conditions. The Inyo Dike intruded under a tectonic stress state with a horizontal maximum compression oriented N07°W. The maximum extensional fracture strength of the host rocks is 1–2.5 MPa, and the pressure drop within the propagating Inyo Dike was about 0.55 MPa/km. The volatile overpressure in the magma chamber was about 15 MPa during eruption of rhyolitic lavas at the Inyo Domes.