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USGS1999

Chapter 2: Geological and tectonic setting of Santorini

Santorini lies in an area of complex extensional and subduction-related tectonics in a continental environment (Jackson 1994). The region has had a long geological history with large changes occurring during the Alpine orogeny and in the Late Tertiary as a consequence of post-collisional extension and major re-adjustments of the plate boundaries. The Aegean region is thought to be moving towards the southwest where the Aegean microplate overrides the Eastern Mediterranean sea floor. The collision has created the Hellenic trench to the south of Crete where the eastern Mediterranean sea floor is subducting beneath the Aegean Sea at 5-6 cm a-1. Santorini is one of several Quaternary volcanoes that define the present day active volcanic arc related to this subduction zone (Fig. 2.1). We here provide a synopsis of the broader geological and tectonic context of Santorini within the Aegean area. The geology, isotope geochemistry and geochronology of the crystalline basement of the southern Aegean is of particular relevance as it is likely that the Santorini magmas have interacted extensively with the continental crust. Geochemical evidence presented in Chapters 6 and 7 confirms this prediction. We also give a synopsis of the Quaternary Aegean Island Arc, of which Santorini is the most active centre.

Santorini↗
USGS1999

Chapter 3: Development of the Santorini volcanic field in space and time

Santorini is one of the largest Quaternary volcanic centres of the Aegean Region. The caldera cliffs preserve well-exposed sequences of lavas and pyroclastic deposits, which record the long development of the volcano in space and time. These include the products of 12 major explosive eruptions and the dissected remains of several ancient lava shields, stratovolcanoes, and lava-dome complexes. The former existence of multiple eruptive centres scattered over the present-day islands shows that Santorini is best considered as a volcanic field , which probably also continues under the sea (Heiken & McCoy 1984). Santorini is best known for the Minoan eruption of the late Bronze Age (Bond & Sparks 1976; Heiken & McCoy 1984; Sparks & Wilson 1990), but some of the previous explosive eruptions may have been as large (Druitt et al . 1989). The occurrence of repeated explosive eruptions has triggered formation of at least four large calderas, such that the present-day caldera is a composite structure (Druitt & Francaviglia 1992). Santorini is potentially one of the most dangerous volcanoes in Europe, having had numerous eruptions in historic times, some of them with significant explosive components (Fytikas et al . 1990 a ).

Santorini↗
USGS1999

Chapter 4: Compositional zoning and petrology of the Thera pyroclastics

Compositional zoning is a common feature of pyroclastic deposits erupted from calderas worldwide (Smith 1979; Hildreth 1981; Bacon & Druitt 1988; De Silva 1991; Feeley & Davidson 1994). Compositionally zoned tuffs provide a geologically instantaneous snapshot of the vertical stratigraphy and density stratification in the chamber immediately before eruption. Magma bodies are commonly zoned in density, with light, relatively cool, silicic magma overlying hotter, more mafic magma. Vertical zonation can arise by a number of processes, including crystallization and convective fractionation on the chamber.

Santorini↗
USGS1999

Chapter 5: Cumulate nodules in the Thera pyroclastics

Gabbroic and dioritic nodules are common in some horizons of the Thera pyroclastics. These nodules contain pyroxene as the main ferromagnesian phase in contrast to the early Akrotiri centre, which contain nodules with abundant hornblende (Nicholls 1971a; Pichler & Kussmaul 1972). Calcsilicate nodules and gabbroic nodules have also been recognized in Santorini lavas (Fouqu6 1879; Nicholls 1971a). Previous studies of Santorini volcanic rocks indicate that crystal fractionation has played an important role in generating the compositional range basalt-andesite-dacite-rhyodacite (Nicholls 1971a; Mann 1983; Huijsmans 1985; Barton & Huijsmans 1986). This interpretation is also consistent with much of the petrological data presented in Chapter 4 and the geochemical data and models presented in Chapter 6. However, disequilibrium phenocryst assemblages in some lavas (Huijsmans 1985; Chapter 4), banded pumice clasts in many pyroclastic deposits (Druitt et al. 1989; Chapters 4), isotopic disequilibria in phenocrysts (Pyle et al. 1988; Chapters 6 and 7) and complex zoning patterns in plagioclase crystals (Stamatelopoulou-Seymour et al. 1990) indicate that magma mixing has also occurred. In addition, trace element and isotopic abundances can be interpreted in terms of assimilation of continental crust during fractionation (Barton et al. 1983; Stamatelopoulou-Seymour et al. 1990; Chapter 6). The nodules provide information on petrogenetic processes that have influenced the evolution of Santorini magmas. Their petrology is documented in this chapter and compared with that of associated juvenile ejecta. The nodules are identified as cumulates from andesitic and dacitic magmas, con- firming the importance of fractional crystallization in the petrogenesis of Santorini magmas. They also demonstrate that mafic plutonic rocks can be formed from intermediate to silicic magmas.

Santorini↗
USGS1999

Chapter 7: Studies of short-lived radionuclides in Santorini volcanics

Disequilibria between the short-lived radioactive nuclides 226 Ra, 230 Th and 238 U in young volcanic rocks can be used both for dating the time of crystallisation of a rock, and as isotopic tracers of the sources of magmas and the chemical evolution of volcanic systems. The principles of these methods have been reviewed extensively elsewhere (Condomines et al . 1988; Gill et al . 1992; Gill & Condomines 1992; Macdougall 1995). In this chapter, we present new 238 U- 230 Th- 226 Ra disequilibria data for the Thera pyroclastics and for selected lavas. These data complement the radiogenic isotopic data presented in Chapter 6 on the compositional evolution through time of the pyroclastic ejecta. In particular, we find that the ( 230 Th/ 232 Th) isotopic ratio corrected to the time of eruption varies in a systematic way with time through the second cycle of the Thera pyroclastics and variations in this ratio are correlated with the variations of 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and Pb isotopes.

Santorini↗
USGS1999

Chapter 8: The evolution of Santorini

There now follows a synthesis of the main results of this study. We interpret the observations in terms of the interplay between magmatic, tectonic and volcanic processes and place these processes within a global context. Santorini has displayed a wide variety of igneous and volcanic phenomena over a period of at least 650 000 years of almost continuous and focused volcanism. Perhaps the most intriguing issues are the causes of volcanic and magmatic cyclicity, the secular variations in magma composition and the way in which magmatic processes have influenced the style of volcanism.

Santorini↗
USGS1999

Appendix 1: Analytical methods and errors

Thirty eight K-Ar and eight 40Ar/39Ar high-precision age determinations were made at the US Geological Survey, Menlo Park, on a total of 22 rocks from the entire volcanic field. Duplicate or triplicate determinations were carried out on 14 samples in order to improve analytical precision. All ages were measured on whole-rock samples selected after thin-section examination. Most of the samples meet the usual criteria for whole-rock dating (Mankinen © Dal-rymple 1972), but some contain minor amounts of glass and a few samples are very glassy. The samples selected for dating were crushed to 0.5-lmm (-18 to +35 mesh). For K-Ar dating aliquots weighing c. 25 g were used for the Ar measurements. A 10 g aliquot was ground to -200 mesh and splits of the powder were used for K20 measurements, which were made in duplicate on each of two separate splits of sample powder by flame photometry after lithium metaborate fusion and dissolution (Ingamells 1970). Ar analyses were by isotope-dilution mass spectrometry using a high-purity (>99.9%) 38Ar tracer and techniques and equipment described previously (Dalrymple & Lanphere 1969). All samples for Ar extraction were baked overnight at 280°C. Mass analyses were done on a 22.68 cm radius, multiple-collector mass spectrometer with a nominal 90° sector magnet, using automated data collection (Stacey etal 1981). Errors given for the calculated K-Ar ages of individual measurements are estimates of the standard deviation of analytical precision. The errors were calculated using formulae derived by Cox & Dalrymple (1967) and Dalrymple & Lanphere (1969).

Geological Society, London, Memoirs↗
USGS1999

Appendix 2: Tabulated analytical data

Details of analytical methods and errors are given in Appendix 1. bd means below detection and na not analysed. Samples numbers which begin with the letter S are from Druitt (1983) and those that begin with LS are from Edwards (1994). Sample numbers which begin with the number 8 are either from Mellors (1988) or Pyle (1990 b ). Subunits of each major tuff of the Thera pyroclastics are denoted by upper case letters (A to D). These are listed in Table 3.4. In the case of the minor sequences (Appendix 2.3), the sequence abbreviations (M6 to M8) are shown in Fig. 3.17 and the individual unit abbreviations (CD1 to CD10) are from Edwards (1994).

Geological Society, London, Memoirs↗
USGS2003

Current controlled deposition on the Wilkes Land continental rise, Antarctica

Turbidite, contourite and hemipelagic deposition are the main components of Wilkes Land continental rise sedimentation above the regional unconformity WL2. On the continental shelf, unconformity WL2 marks the start of shelf progradation, which is interpreted to correspond with the onset of glacial conditions in this segment of the east Antarctic margin. Unusually large (i.e. up to 900 m relief and 18 km between levee crests) channel-levee deposits, and high relief (up to 490 m) mounded contourite-style deposits develop above unconformity WLlb. Unconformity WLlb overlies unconformity WL2 and is interpreted to have formed under a fully continental glacial regime where ice streams reached the palaeo-continental shelf edge. Based on an analysis of multichannel seismic profiles and sediment cores, we differentiate three phases in the development of the sedimentary unit between WLlb and the present seafloor. From older to younger these are: Phase 1, dominated by turbidite deposition; Phase 2, dominated by turbidite and contourite deposition with significant mound building; and Phase 3, dominated by turbidite and contourite deposition without active mound building. We hypothesize that building of the mounds during Phase 2 corresponded with times of expansion of the Antarctic ice-sheet when vast amounts of sediment were eroded from the continent and continental shelf. The large amount of unsorted glacial sediment supplied to the outer shelf apparently travelled down the slope canyons and rise channels as turbidity current flows to feed the usually large continental rise channel-levee complexes. The suspended fines of the turbidity flows were then entrained in a palaeo-nepheloid layer and carried by the westward flowing palaeo-contour currents until their deposition in the mounds. During Phase 3, sediment supply to the continental rise, although important in volume and capable of turbidite and contour-current deposition, was insufficient to support further building of the mounds. We believe the decrease in sediment supply to the continental rise from Phase 2 to Phase 3 could be the result of a change on sediment depocentres, with most of the sediment supplied to the margin during Phase 3 being trapped on the continental shelf. We believe that ultimately these changes are related to the stage of glacial evolution of the continent.

Geological Society, London, Memoirs↗
USGS2011

Chapter 50 Geology and tectonic development of the Amerasia and Canada Basins, Arctic Ocean

Amerasia Basin is the product of two phases of counterclockwise rotational opening about a pole in the lower Mackenzie Valley of NW Canada. Phase 1 opening brought ocean–continent transition crust (serpentinized peridotite?) to near the seafloor of the proto-Amerasia Basin, created detachment on the Eskimo Lakes Fault Zone of the Canadian Arctic margin and thinned the continental crust between the fault zone and the proto-Amerasia Basin to the west, beginning about 195 Ma and ending prior to perhaps about 160 Ma. The symmetry of the proto-Amerasia Basin was disrupted by clockwise rotation of the Chukchi Microcontinent into the basin from an original position along the Eurasia margin about a pole near 72°N, 165 W about 145.5–140 Ma. Phase 2 opening enlarged the proto-Amerasia Basin by intrusion of mid-ocean ridge basalt along its axis between about 131 and 127.5 Ma. Following intrusion of the Phase 2 crust an oceanic volcanic plateau, the Alpha–Mendeleev Ridge LIP (large igneous province), was extruded over the northern Amerasia Basin from about 127 to 89–75 Ma. Emplacement of the LIP halved the area of the Amerasia Basin, and the area lying south of the LIP became the Canada Basin.

Geological Society, London, Memoirs↗
USGS2016

Submarine glacial landforms on the Bay of Fundy–northern Gulf of Maine continental shelf

The Bay of Fundy–northern Gulf of Maine region surrounds the southern part of Nova Scotia, encompassing, from west to east, the Bay of Fundy, Grand Manan Basin, German Bank, Browns Bank, Northeast Channel and northeastern Georges Bank (Fig. 1a, b). During the last glacial maximum ( c. 24–20 14 C ka BP), the SE margin of the Laurentide Ice Sheet (LIS) occupied the study area, the rest of the Gulf of Maine and the continental Scotian Shelf off Atlantic Canada (see Dyke et al. 2002 , fig. 1; Shaw et al. 2006 , fig. 8; Hundert & Piper 2008 , fig. 16). Early mapping of the glaciated region on the Scotian Shelf using side-scan sonar imagery and seismic-reflection profiles revealed topographic features interpreted to be recessional moraines indicative of retreat of the LIS ( King et al. 1972 ; King 1996 ). Subsequently, multibeam sonar seafloor mapping of local-scale glacial landforms on the inner Scotian Shelf off Halifax, Nova Scotia (Fig. 1b) provided further information on the dynamics of the advance and retreat of the ice sheet ( Loncarevic et al. 1994 ). Interpretation of seismic-reflection profiles across Georges Bank revealed that the surficial sediment is a veneer of glacial debris transported to Georges Bank by the LIS during the late Pleistocene from continental areas to the north ( Shepard et al. 1934 ; Knott & Hoskins 1968 ; Schlee 1973 ; Twichell et al. 1987 ; Fader et al. 1988 ). Recent high-resolution multibeam sonar surveys of German Bank and the Bay of Fundy mapped a complex of ice-advance and ice-retreat features attributed to the activity of the LIS ( Todd et al. 2007 ; Todd & Shaw 2012 ).

Geological Society, London, Memoirs↗
USGS2016

Pockmarks in Passamaquoddy Bay, New Brunswick, Canada

Pockmarks are seafloor depressions associated with fluid escape ( Judd & Hovland 2007 ). They proliferate in the muddy seafloors of coastal Gulf of Maine and Bay of Fundy, where they are associated with shallow natural gas likely of biogenic origin ( Ussler et al. 2003 ; Rogers et al. 2006 ; Wildish et al. 2008 ). In North America, shallow-water pockmark fields are not reported south of Long Island Sound, despite the abundance of gassy, muddy estuaries. The absence of pockmarks south of the limit of North American glaciation suggests that local and regional heterogeneities, possibly related to glacial or sea-level history or bedrock geology, influence pockmark field distribution. In shallow-water embayments, such as Passamaquoddy Bay, New Brunswick, pockmarks can be large (>200 m diameter) and number in the thousands.

New Brunswick↗
USGS2004

Deciphering multiple Mesoproterozoic and Paleozoic events recorded in zircon and titanite from the Baltimore Gneiss, Maryland: SEM imaging, SHRIMP U-Pb geochronology, and EMP analysis

The Baltimore Gneiss , exposed in antiforms in the eastern Maryland Piedmont, consists of a suite of felsic and mafic gneisses of Mesoproterozoic age. Zircons from the felsic gneisses are complexly zoned, as shown in cathodoluminescence imaging ; most zircon grains have multiple overgrowth zones, some of which are adjacent and parallel to elongate cores. Sensitive high-resolution ion microprobe ( SHRIMP ) analyses of oscillatory-zoned cores indicate that the volcanic protoliths of the felsic gneisses crystallized at ca. 1.25 Ga. These rocks were subsequently affected by at least three Mesoproterozoic growth events , at ca. 1.22, 1.16, and 1.02 Ga. Foliated biotite granite intruded the Baltimore Gneiss metavolcanic sequence at ca. 1075 Ma. The Slaughterhouse Granite (renamed herein) also is Mesoproterozoic , but extremely discordant U-Pb data from high-U, metamict zircons preclude calculating a precise age. The 1.25 Ga rocks of the Baltimore Gneiss are coeval with rocks emplaced in the Grenville Province during the Elzevirian orogeny, and the 1.22 Ga zircon overgrowths are coincident with a later stage of this event . Younger zircon overgrowths formed during the Ottawan phase of the Grenville orogeny. Backscattered electron imaging of titanites from felsic gneisses and foliated biotite granite reveals that many of the grains contain cores, intermediate mantles, and rims. Electron microprobe traverses across zoned grains show regular variations in composition. SHRIMP ages for titanite from the foliated biotite granite are 374 ± 8, 336 ± 8, and 301 ± 12 Ma. The ca. 374 Ma age suggests growth of titanite during a thermal event following the Acadian orogeny, whereas the late Paleozoic titanite growth ages may be due to greenschist-facies replacement reactions associated with Alleghanian metamorphism and deformation.

Maryland↗
USGS2007

Geology of the Yucca Mountain site area, southwestern Nevada

Yucca Mountain in southwestern Nevada is a prominent, irregularly shaped upland formed by a thick apron of Miocene pyroclastic-flow and fallout tephra deposits, with minor lava flows, that was segmented by through-going, large-displacement normal faults into a series of north-trending, eastwardly tilted structural blocks. The principal volcanic-rock units are the Tiva Canyon and Topopah Spring Tuffs of the Paintbrush Group, which consist of volumetrically large eruptive sequences derived from compositionally distinct magma bodies in the nearby southwestern Nevada volcanic field, and are classic examples of a magmatic zonation characterized by an upper crystal-rich (>10% crystal fragments) member, a more voluminous lower crystal-poor (<5% crystal fragments) member, and an intervening thin transition zone. Rocks within the crystal-poor member of the Topopah Spring Tuff, lying some 280 m below the crest of Yucca Mountain, constitute the proposed host rock to be excavated for the storage of high-level radioactive wastes. Separation of the tuffaceous rock formations into subunits that allow for detailed mapping and structural interpretations is based on macroscopic features, most importantly the relative abundance of lithophysae and the degree of welding. The latter feature, varying from nonwelded through partly and moderately welded to densely welded, exerts a strong control on matrix porosities and other rock properties that provide essential criteria for distinguishing hydrogeologic and thermal mechanical units, which are of major interest in evaluating the suitability of Yucca Mountain to host a safe and permanent geologic repository for waste storage. A thick and varied sequence of surficial deposits mantle large parts of the Yucca Mountain site area. Mapping of these deposits and associated soils in exposures and in the walls of trenches excavated across buried faults provides evidence for multiple surface-rupturing events along all of the major faults during Pleistocene and Holocene times; these paleoseismic studies form the basis for evaluating the potential for future earthquakes and fault displacements. Thermoluminescence and U-series analyses were used to date the surficial materials involved in the Quaternary faulting events. The rate of erosional downcutting of bedrock on the ridge crests and hillslopes of Yucca Mountain, being of particular concern with respect to the potential for breaching of the proposed underground storage facility, was studied by using rock varnish cation-ratio and 10 Be and 36 Cl cosmogenic dating methods to determine the length of time bedrock outcrops and hillslope boulder deposits were exposed to cosmic rays, which then served as a basis for calculating long-term erosion rates. The results indicate rates ranging from 0.04 to 0.27 cm/k.y., which represent the maximum downcutting along the summit of Yucca Mountain under all climatic conditions that existed there during most of Quaternary time. Associated studies include the stratigraphy of surficial deposits in Fortymile Wash, the major drainage course in the area, which record a complex history of four to five cut-and-fill cycles within the channel during middle to late Quaternary time. The last 2-4 m of incision probably occurred during the last pluvial climatic period, 22-18 ka, followed by aggradation to the present time. Major faults at Yucca Mountain-from east to west, the Paintbrush Canyon, Bow Ridge, Stagecoach Road, Solitario Canyon, Fatigue Wash, Windy Wash, and Northern and Southern Crater Flat Faults-trend predominantly north, are spaced 1-5 km apart, have bedrock displacements ranging from 125 m to as much as 500 m, and exhibit Quaternary movements of several centimeters to a few meters. Displacements are predominantly down to the west, and bedrock/alluvium contacts commonly are marked by fault-line scarps. The predominant northerly fault trend changes to a more northeasterly trend in adjacent areas south

Nevada↗
USGS2021

Global biotic events evident in the Paleogene marine strata of the eastern San Francisco Bay area, California

Paleogene marine strata in the eastern San Francisco Bay area are exposed in discontinuous outcrops in the various tectonic blocks. Although there are many missing intervals, the strata were previously thought to span most of the Paleocene and Eocene. Revision of biochronology and calibration to the international time scale as well as to the global oxygen isotope curve and sea-level curves indicate that the strata are latest Paleocene through middle Eocene in age and contain faunal changes that are linked to the overall global climate trends and hyperthermals of that time. The Paleocene-Eocene thermal maximum, third Eocene thermal maximum, early Eocene climatic optimum, and middle Eocene climatic optimum are all identified in the eastern San Francisco Bay marine strata. The dominance of smoothly finished, dissolution-resistant agglutinated benthic foraminiferal species corresponds with a rapid shoaling and rapid deepening (overcorrection) of the calcium compensation depth associated with the Paleocene-Eocene thermal maximum. The benthic foraminiferal extinction event was a dramatic turnover of benthic foraminiferal species that occurred shortly after the onset of the Paleocene-Eocene thermal maximum. Opportunistic species such as Bulimina , which indicate environmental stress and lower oxygen conditions, are commonly associated with the Paleocene-Eocene thermal maximum. Environmental changes similar to those observed during the Paleocene-Eocene thermal maximum also characterize the third Eocene thermal maximum, based on the agglutinated and opportunistic species. The early Eocene climatic optimum is noted by the presence of foraminiferal assemblages that indicate a stable, warmer water mass, abundant food, and an influx of terrigenous material. The onset and end of the middle Eocene climatic optimum are recognized by the dominance of siliceous microfossils. This research updates the age and environmental interpretations of the Paleogene formations occurring in the vicinity of Mount Diablo, eastern San Francisco Bay area. The revised interpretations, which are based on foraminifers and calcareous nannoplankton, make it possible to identify various global climatic and biotic events.

California↗
USGS1968

Overlapping plutonism, volcanism, and tectonism in the boulder batholith region, western Montana

It is well known that the Boulder batholith region experienced intensive plutonism, volcanism, and tectonism that all began in Late Cretaceous time, after at least 700 m.y. of structural and igneous inactivity except for sporadic epeirogeny. Recent stratigraphic, structural, paleontologic, arid, especially, radiometric evidence makes it possible to date these dynamic events rather closely. The time relations that are revealed do not form a simple sequence of volcanism-folding-thrusting-batholith emplacement, as has often been supposed, but involve an intertwined complex. Significant volcanism began ∼ 85 m.y. ago in late Coniacian or early Santonian time, with deposition of the thick, local tuffaceous Slim Sam Formation. Volcanism climaxed from 77 to 79 m.y. ago, in early Campanian time, when the region was buried under at least 10,000 feet of calc-alkalic volcanic and volcaniclastic rocks, which included many sheets of welded tuff - the Elkhorn Mountains Volcanics -, and a vast amount of contemporaneous ash was airborne beyond the region. Major volcanism ceased ∼ 73 m.y. ago, late in the Campanian, not to recur until early Eocene time, ∼ 50 m.y. ago. The bulk of the batholith was emplaced beneath and within the volcanic edifice in early and middle Campanian time, during a 6 m.y. span from 78 to 72 m.y. ago, and some leucocratic masses were intruded during the next few million years, so that the whole batholith was emplaced within about 10 m.y. Folding at and near the site of the batholith began in late Coniacian or Santonian time and culminated before middle Campanian time; the main folding north and east of the batholith was post-Campanian, probably Maestrichtian. Thrusting began before middle Santonian time, and recurred intermittently well into the Maestrichtian, or even a little later. Thus volcanism, plutonism, folding, and thrusting began and ended within a few million years of each other, during the last 20 m.y. of the Cretaceous. Major folding, thrusting, and volcanism started about the same time, though not always at the same places, and a little earlier than plutonism. In any given locality, volcanism ended before major folding; the climax of plutonism followed the climax of volcanism; thrusting preceded and accompanied plutonism near the batholith, but followed plutonism farther away; thrusting ended a little later than folding. These dynamic processes so closely related in time and space must also be genetically related in the Boulder batholith region. Gilluly's (1965) conclusion that the orogeny which produced the great Cretaceous thrusts of Montana was "essentially without plutonic associations" is not tenable.

Montana↗
USGS1968

Resurgent cauldrons

Resurgent cauldrons are defined as cauldrons (calderas) in which the cauldron block, following subsidence, has been uplifted, usually in the form of a structural dome. Seven of the best known resurgent cauldrons are: Valles, Toba, Creede, San Juan, Silverton, Lake City, and Timber Mountain. Geologic summaries of these and Long Valley, California, a probable resurgent caldera, are presented. Using the Valles caldera as a model, but augmented by information from other cauldrons, seven stages of volcanic, structural, sedimentary, and plutonic events are recognized in the development of resurgent cauldrons. They are: (I) Regional tumescence and generation of ring fractures; (II) Calderaforming eruptions; (III) Caldera collapse; (IV) Preresurgence volcanism and sedimentation; (V) Resurgent doming; (VI) Major ring-fracture volcanism; (VII) Terminal solfatara and hot-spring activity. These stages define the terminal cycle of resurgent cauldrons, which in the Valles caldera spanned more than 1 million years. The known and inferred occurrence of the seven stages in the eight cauldrons discussed, together with some time control in four cauldrons, indicates that resurgent doming is early in the postcollapse history; hence, it seems part of a pattern and not fortuitous. Doming of the cauldron block by magma pressure is preferred to doming by stock or laccolithic intrusion, although these processes may be subsidiary. Magma rise that produces doming may be explained in several ways, but the principal cause is not known. Nor is it known why some otherwise similar calderas do not have resurgent domes, although size and thickness of the cauldron block and the degree to which it was deformed during caldera collapse may be factors. All known resurgent structures are larger than 8 miles in diameter and are associated with silicic and, presumably, high-viscosity magmas. Genetically, resurgent cauldrons belong to a cauldron group in which subsidence of a central mass takes place along ring fractures and is related to eruption of voluminous ash flows, thereby differing from Kilauean-type calderas. It is proposed that typical Krakatoan-type calderas differ in that collapse is chaotic and ring fractures are not essential to their formation. Krakatoan calderas typically occur in the andesitic volcanoes of island arcs or the eugeosynclinal environment, and their sub-volcanic analogues are not known, whereas resurgent and related Glen Coe-type cauldrons are more common in cratonic or post-orogenic environments as are their sub-volcanic analogues - granitic ring complexes. Granitic ring complexes, such as Lirue, Sande, Ossipee, and Alnsj0, are probably the closest sub-volcanic analogues of resurgent calderas. The source areas of most of the ash-flow sheets of western United States and Mexico are yet to be found. It is suggested that many of them will prove to be resurgent structures. Present evidence suggests that ore deposits are more commonly associated with resurgent cauldrons than with other cauldron types.

Memoir of the Geological Society of America↗