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USGS1989

Seismic methods for determining earthquake source parameters and lithospheric structure

The seismologic methods most commonly used in studies of earthquakes and the structure of the continental lithosphere are reviewed in three main sections: earthquake source parameter determinations, the determination of earth structure using natural sources, and controlled-source seismology. The emphasis in each section is on a description of data, the principles behind the analysis techniques, and the assumptions and uncertainties in interpretation. Rather than focusing on future directions in seismology, the goal here is to summarize past and current practice as a companion to the review papers in this volume. Reliable earthquake hypocenters and focal mechanisms require seismograph locations with a broad distribution in azimuth and distance from the earthquakes; a recording within one focal depth of the epicenter provides excellent hypocentral depth control. For earthquakes of magnitude greater than 4.5, waveform modeling methods may be used to determine source parameters. The seismic moment tensor provides the most complete and accurate measure of earthquake source parameters, and offers a dynamic picture of the faulting process. Methods for determining the Earth’s structure from natural sources exist for local, regional, and teleseismic sources. One-dimensional models of structure are obtained from body and surface waves using both forward and inverse modeling. Forward-modeling methods include consideration of seismic amplitudes and waveforms, but lack the formal resolution estimates obtained with inverse methods. Two- and three-dimensional lithospheric models are derived using various inverse methods, but at present most of these methods consider only traveltimes of body waves. Controlled-source studies of the Earth’s structure are generally divided by method into seismic refraction/wide-angle reflection and seismic reflection studies. Seismic refraction profiles are usually interpreted in terms of two-dimensional structure by forward modeling of traveltimes and amplitudes. The refraction method gives excellent estimates of seismic velocities, but relatively low resolution of structure. Formal resolution estimates are not possible for models derived from forward modeling, but informal estimates can be obtained by perturbing the best-fitting model. Inversion methods for seismic refraction data for one-dimensional models are well established, and two- and three-dimensional methods, including tomography, have recently been developed. Seismic reflection data provide the highest resolution of crustal structure, and have provided many important geological insights in the past decade. The acquisition and processing of these data have been greatly advanced by the hydrocarbon exploration industry. However, reliable crustal velocity control is generally lacking, and the origin of deep crustal reflections remains unclear, resulting in nonunique interpretations. A new form of lithospheric seismology has recently emerged that combines the advantages of seismic refraction and seismic reflection profiles, and the distinction between the two methods is steadily diminishing. Major challenges for future work will be the collection of data that are more densely sampled in space, and the development of interpretation methods that provide quantitative estimates of the uncertainties in the calculated models.

GSA Memoirs↗
USGS1989

Regional crustal structure and tectonics of the Pacific Coastal States: California, Oregon, and Washington

The Pacific Coastal States form a complex geologic environment in which the crust and lithosphere have been continuously reworked. We divide the region tectonically into the southern transform regime of the San Andreas fault and the northern subduction regime, and summarize the geophysical framework with contour maps of crustal thickness, lithospheric and seismicity cross sections, and results from site-specific geophysical studies. The uniformity of crustal thickness (30 ± 2 km) in southern California is remarkable, and appears to be primarily the result of crustal extension in the Mojave Desert and ductile shear of the lower crust along the plate transform boundary. Southern California seismicity defines a broad zone of deformation that extends from the Borderland to the Mojave Desert (about 300 km). The geophysical framework of central and northern California records magmatism and accretion associated with the Mesozoic and Cenozoic subduction, late Cenozoic transform faulting, and in the Basin and Range to the east, extension. The crust thickens from about 20 km at the coast to as much as 55 km in the Sierra Nevada, and thins to about 30 km in the Basin and Range. Cross sections of the crust show that seismic velocities and densities vary significantly over short distances perpendicular to the coast, reflecting processes that include the accretion of oceanic sediments and igneous crust, and significant lateral motion of crustal blocks. Maximum hypocentral depths in central California become deeper as the crust thickens to the west, but seismicity is low beneath the Great Valley and Sierra Nevada, which together appear to form a relatively undeforming block. The lower crust of the Pacific Coastal States has a high average seismic velocity (6.7 km/sec or greater), which probably is the product of tectonic underplating of oceanic crust and/or magmatic underplating by a basaltic melt.

California, Oregon, Washington↗
USGS1989

Isostatic residual gravity and crustal geology of the United States

A new isostatic residual gravity map of the conterminous United States presents continent-wide gravity data in a form that can be readily used, with geologic information and other geophysical data, in studies of the composition and structure of the continental crust. This map was produced from the gridded gravity data on which the recently released Gravity Anomaly Map of the United States is based. About 1 million onland and 0.8 million offshore gravity observations interpolated to a 4- by 4-km grid serve as the basis for both maps. The Airy-Heiskanen model of isostatic compensation of topography applied to topographic and bathymetric data averaged over 5- by 5-min compartments was used to remove, to first order, the large, long-wavelength Bouguer gravity anomalies caused by deep density distributions that support topographic loads. The parameters used in the Airy-Heiskanen model were topographic density, 2.67 g/cm 3 ; sea-level crustal thickness, 30 km; and density contrast across the base of the model crust, 0.35 g/cm 3 . Many of the conspicuous short-wavelength anomalies (widths less than several hundred kilometers) on the isostatic residual gravity map correlate with mapped or near-surface geologic features, and primarily reflect shallow-density distributions rather than any departures from isostatic equilibrium. In general, gravity highs occur over (1) mafic igneous bodies emplaced in rift or magmatic arc settings or as isolated intrusions controlled by structures; (2) accreted slices of mafic oceanic, island-arc, or transitional crust; and (3) uplifted crystalline basement. Gravity lows are found over (1) thick bodies of felsic intrusive or extrusive rocks; (2) sedimentary deposits in extensional, convergent, or transform settings; and (3) depressed crystalline basement. Anomalies with widths as much as 1,000 km or more also appear to reflect crustal properties in many cases—several broad gravity highs are associated with crust having a high average seismic wave velocity, and comparable broad gravity lows occur over areas of low average seismic velocity. Alternative ways of viewing the isostatic residual gravity data provide additional information about density distributions in the crust. The first-vertical derivative map accentuates gravity anomalies over shallow, abrupt density changes at the expense of those resulting from deeper or more gradual density transitions. The maximum horizontal gradient map contains information about the locations of pronounced density boundaries. Two-dimensional spectral analysis of the gravity data provides a quantitative means for identifying dominant fabrics in the gravity field and for distinguishing various terranes from each other. Neither Bouguer nor isostatic residual gravity anomalies are particularly well suited for practical modeling of deep structure in conjunction with deep seismic information. However, a scheme in which the entire Earth outside the area of interest is approximated by laterally homogeneous layers and isostatically compensated topography, and in which the area of interest is modeled using the seismic constraints applied in a two-and-one-half-dimensional geometry, holds promise for exploiting useful features of both the Bouguer and isostatic residual gravity anomalies.

conterminous United States↗
USGS1989

Crustal studies using magnetic data

The magnetic method plays an important role in mineral, petroleum, and geothermal exploration. It also has made important contributions to geologic mapping, structural geology, and plate-tectonic theory. In particular, magnetic measurements using aircraft provide a relatively inexpensive way to trace magnetic rock units beneath covered areas, to reveal the shape of subsurface magnetic bodies, and to interpolate subsurface geologic information between widely spaced seismic data and other localized geophysical measurements. Computerized interpretation procedures currently fall into two categores: techniques designed to enhance the data, which include various display and filtering procedures, and modeling experiments, which may be either forward (trial-and-error) or inverse in nature.

GSA Memoirs↗
USGS1989

Geophysical framework of the continental United States: Progress, problems, and opportunities for research

Significant progress has been made over the past five decades in determining the geophysical framework of the continental United States. Highlights include detailed maps of gravity and aeromagnetic anomalies, heat flow, crustal thickness, seismicity, state of stress, and paleomagnetic pole positions. Important tectonic insights have come from earthquake studies, and from knowledge of lithospheric structure derived from seismic reflection, refraction/wide-angle reflection, surface-wave, and teleseismic data. Additional major advances in lithospheric geophysics will depend on four key factors: the reduction of uncertainties in the measurement and interpretation of geophysical data, the widespread application of coincident geophysical methods in concert with geological investigations, the collection of a more uniform continent-scale data base for all geophysical measurements, and the investigation of topical geophysical questions regarding the physical state and properties of the lithosphere. The impracticality of repeating most geophysical field measurements introduces poorly known, but likely large, uncertainties. Since most measurements are not repeated, high priority must be given to the reduction and quantification of uncertainties in measurements and interpretations. The most productive future investigations, in terms of resolution and minimum uncertainties in interpretation, will be those that apply different geophysical methods along identical profiles or areas, and that include geological investigations as a vital ingredient. Important gaps remain in our knowledge of the geophysical framework of the United States on a continent-wide scale, including the deep conductivity structure, the nature of the Moho discontinuity, the structure of the subcrustal lithosphere, and the depth of the lithosphere/asthenosphere boundary. Most transition zones separating geologic or physiographic provinces are poorly studied, yet these zones are likely to be the locations of the most profound changes in the physical properties of the lithosphere. Application of coincident geophysical techniques is needed to study these transition zones. Several topical geophysical questions warrant special emphasis in the future. These questions include the rheology of the crust and subcrustal lithosphere; the distribution, composition, and abundance of fluids in the crust; the genesis and evolution of the Moho; the origin of crustal conductivity zones and deep crustal reflections; the evidence for seismic anisotropy; and the short-term prediction of earthquakes.

GSA Memoirs↗
USGS1990

Chapter 14: Middle Cretaceous silicic metavolcanic rocks in the Kings Canyon area, central Sierra Nevada, California

Metamorphosed silicic volcanic and hypabyssal rocks of middle Cretaceous (110 to 100 Ma) age occur in two roof pendants in the Kings Canyon area of the central Sierra Nevada. The metavolcanic remnants are similar in age to or are only slightly older than the voluminous enclosing batholithic rocks. Thus, high to surface levels of the batholith are implied for this region. This is interesting considering that deep-level (∼25 km) batholithic rocks of the same age as the metavolcanic rocks occur at the southern end of the range. Apparent structural continuity between these two regions suggests that the southern half of the range offers an oblique section through young (˜100 Ma) sialic crust. The middle Cretaceous ages of the two volcanic sequences are indicated by U/Pb zircon and Rb/Sr bulk-rock isochron data. The two isotopic systems agree very closely with one another. Some of the U/Pb systems within the Boyden Cave pendant are discordant due to the inheritance or entrainment of Proterozoic zircon. This is a common phenomenon in volcanic or plutonic rocks erupted or emplaced within the Kings sequence metamorphic framework, a belt of distinct pendants with abundant continent-derived sedimentary protoliths. In conjunction with other petrochemical parameters, lavas and magmas of this framework domain are shown to be contaminated with sedimentary admixtures. The contaminated domain of the batholith reflects the bounds of the Kings sequence framework, which along its eastern margin probably represents a major pre-batholith to early batholith tectonic break. The middle Cretaceous metavolcanic sequences were apparently built on two distinctly different early Mesozoic substrates separated by a major tectonic break. In the Boyden Cave pendant, the substrate may be represented by the shallow to deep-marine Kings sequence; to the east in the Oak Creek pendant, the substrate consists of a thick silicic ignimbrite sequence. In both areas the middle Cretaceous rocks and adjacent sequences share intense ductile deformation fabrics. Earlier views that considered these fabrics as an expression of Jurassic orogenic deformation are in error. Structural and age relations indicate that the fabrics developed between 105 and 100 Ma and during the medial phases of Cretaceous composite batholith growth.

California↗
USGS1992

Chapter 4: Cretaceous thrusting and Neogene block rotation in the northern Portneuf Range region, southeastern Idaho

The Putnam thrust has long been recognized as an important Mesozoic structure in the northern Portneuf Range, southeastern Idaho. At most localities, the thrust places Ordovician rocks above Permian and Pennsylvanian rocks, although near its southeastern extent, it ramps laterally downsection to the southeast. At its southeasternmost exposures, Cambrian rocks are juxtaposed above Mississippian rocks. New work indicates that the hanging wall of the Putnam thrust contains three imbricate thrust slices or subplates, which are, from structurally lowest to highest (and generally from north to south), the Lone Pine subplate, the Narrows subplate, and the Bear Canyon-Toponce subplate. The steeply south-dipping, east-trending Narrows thrust overlies the Lone Pine subplate, underlies the Narrows subplate, and is a lateral ramp that merges eastward into the Putnam thrust. Where exposed, the Narrows thrust places Late Proterozoic quartzite of the Brigham Group over Ordovician and Cambrian rocks. The Bear Canyon thrust overlies the Narrows subplate and underlies the Bear Canyon-Toponce subplate, dips eastward along the west side of the Portneuf Range, and places lower Brigham Group quartzite above Cambrian limestone and Cambrian and Late Proterozoic upper Brigham Group quartzite and argillite. At its northern extent, the Bear Canyon thrust curves to the east, where it merges with the Putnam thrust. On the east side of the range, the intensely folded Toponce thrust places upper Brigham Group quartzite above Ordovician rocks; the Toponce is believed to be an eastward extension of the Bear Canyon thrust. East-dipping rocks within the Lone Pine subplate were not strongly deformed during Cretaceous thrusting, in contrast to rocks within the Narrows subplate, where east-vergent recumbent folds, cleavage directions that fan about northerly strikes, and tectonic thickening and thinning of beds indicate intense, thrust-parallel shear. The deformation and thrust geometry within the Narrows subplate suggest that the Narrows subplate actually consists of several horses within a foreland-dipping duplex. Late Miocene and younger basin deposits occur in north-trending valleys adjacent t o the northern Portneuf Range and, to the west, the Bannock and Pocatello ranges. At most places, the Neogene deposits dip to the east by as much as 35°, indicating that late Miocene and younger extension and down-to-the-east rotation occurred along mostly west-dipping listric faults that are inferred to merge on at least one regional detachment. Although range-bounding faults account for a large component of extension and rotation, an additional large component was contributed by numerous, relatively small-displacement normal faults within mountain ranges.

Idaho↗
USGS1948

Batholith and associated rocks of Corona, Elsinore, and San Luis Rey quadrangles southern California

The batholith of Southern and Lower California is exposed continuously from near Riverside, California, southward for a distance of about 350 miles. In central Lower California it is covered in part by younger rocks, but discontinuous bodies extend to the southern end of Lower California, and hence the batholith is probably over 1000 miles long. Its width is about 60 miles. A strip across the northern part of the batholith about 70 miles wide has been studied; the western half was mapped in detail, and the eastern half was covered in rapid reconnaissance. In the area studied the batholith intrudes Triassic sediments and Jurassic(?) volcanic rocks along its western border and Paleozoic sediments along its eastern border. Screens and roof pendants are common within the batholith. The Triassic rocks are mildly metamorphosed in the western part of the area but become progressively more coarsely crystalline toward the east. The Paleozoic rocks are rather coarsely crystalline. The metamorphism in large part preceded the intrusion of the batholith, and only locally was there appreciable contact metamorphism. The batholith and older rocks are overlain by Upper Cretaceous and younger sediments. Small bodies of andesite and basalt are associated with the Tertiary sediments, and small bodies of nepheline basalt of Quaternary age are present in the area. The batholith was intruded in early Upper Cretaceous time. The batholith in the area studied was emplaced by over 20 separate injections. Most of the resulting rock types are found in only one or a few small bodies which are confined to a small area. In the area studied in detail (Pl. 1) five types are present in many large, widely separated bodies, making up about 88 per cent of the area underlain by the batholith. In the eastern half of the batholith three more widespread types are present. In the western half of the body the rocks range fro a gabbro to granite, but in the eastern half several tonalites constitute nearly the whole of the mass. The gabbro is composed of many related rocks. Some have hornblende, some pyroxene; in some the plagioclase is anorthite, in others it is as sodic as andesine-Iabradorite. Some of the tonalites contain abundant inclusions that have been almost completely reworked by the magma and have been softened and stretched into thin discs. These inclusions are well oriented and near the contacts with older rocks they parallel the contacts, but elsewhere they strike about N. 30° W. and dip steeply to the east. One tonalite, whose feldspar is andesine, has scattered crystals with cores of bytownite, and has well-crystallized hornblende with cores of pale uralitic hornblende and remnants of augite. Hornblende and biotite are the predominant mafic minerals of the tonalites and granodiorites. The iron content of the mafic minerals of the gabbros is moderate, and it increases as the rocks become richer in silica. The norms and the modes are shown on a variation diagram (Figs. 11, 12). The chemical analyses of the rocks fall near smooth variation curves (Fig. 4). The general strike of the structures of the area have been about N. 30° W. from Paleozoic to the present time. The Paleozoic and Triassic sediments, the orientation of the inclusions and other structures of the batholith, the elongation of the batholith and the mountain ranges, and the strike of the major faults are in about the same direction. In the batholith and the older sediments the dips are steep to the east. The batholith must have been emplaced by stoping and not by forceful injection. Calculations show that the cooling of a large batholith is chiefly through the roof and not through the walls. Crystallization to a depth of 3 kilometers takes place in about half a million years. The different rocks of the batholith were formed from the intermediate gabbro by crystal differentiation and assimilation in depth. In early Upper Cretaceous time diastrophism folded the older rocks and formed, in depth, a strip of gabbroic magma about 1000 miles long. A small amount of this magma was intruded nearly to the surface. The deep magma differentiated quietly until its upper part attained the composition of a tonalite. Earth movements then occurred at least five times in rapid succession and caused the injection of the different tonalites. Some of these carry abundant inclusions, indicating a widespread shattering of the wall rock shortly before final emplacement. From time to time local movements caused the injections of the different granodiorites. When the deep-seated magma reached the composition of a light-colored granodiorite, widespread diastrophism moved the main granodiorite upward. Further local movement caused the emplacement of the many local granodiorites and granites.

California↗
USGS1954

Stratigraphy and history of the Moenkopi formation of Triassic age

The Moenkopi formation of Triassic age is composed of a series of deposits that form a wedge thinning eastward from a maximum of about 2000 feet in western Utah and southern Nevada to the vanishing point along an irregular margin in western Colorado, northeastern Arizona, and western New Mexico. Partly marine and partly continental in the thick western sections, it is entirely continental in the east. Invertebrate faunas indicate that deposition began either during or preceding the middle of the Early Triassic (Meekoceras zone) and continued into late Early Triassic (Tirolites zone) and probably into Medial Triassic time. Vertebrate faunas also indicate an Early Triassic and probably, in part, a Medial Triassic age . Studies of the deposits indicate three major transgressions and three regressions across southern Utah and northern Arizona. Analysis of sedimentary rock types and original structures in them suggests a complex mixture of environments involved in the development of the formation : stream beds, lagoons, playas, flood plains or tidal flats, shallow sea floors, and others. Some types are clear cut and readily demonstrated; others are open to question. Evidence from flora, fauna, and sediments indicates a semiarid to arid climate. Except for uplift in the Uncompahgre region of Colorado indicated by conglomeratic beds in the Moenkopi near by, the entire region probably remained very low and flat during Moenkopi deposition.

Colorado, Arizona, New Mexico↗
USGS1957

Chapter 22: Environments and facies of existing bays on the central Texas coast

The estuaries and neritic waters along the central Texas coast are characterized by broad ranges of environmental factors, providing conditions favorable to temperate organisms in winter and tropical organisms in summer. Dredging in the coastal waters revealed a number of distinct faunal facies, the distribution of which appears to be controlled primarily by salinity. Characteristic assemblages are found in the following areas: closed bays, polyhaline bays, a highly saline lagoon, passes, beaches on the open Gulf, and shallow waters of the open gulf. In the closed bays three subfacies are recognized-one in the bay heads, one on the reefs, and one in the interreef areas; in the open gulf the assemblages living close to shore are differentiated from those found 2-4 miles off shore. In most stations each facies is characterized by an abundance of certain living forms. Dead shells and other skeletal parts are much more widely distributed, but many such strays may be recognized as transported elements by their rarity or by their eroded surfaces. Each group of organisms has one or more centers of maximum development in numbers of species. The Foraminifera, for example, attain their greatest development in the polyhaline bays and near the heads of passes, the ostracodes near the bay heads, mollusks in the polyhaline bays and in the Gulf, and diatoms under normal marine conditions in the Gulf. The reef assemblage of the closed bays is poor in numbers of species, but the individuals are so numerous that they form extensive deposits.

Texas↗
USGS1957

Physical and ecologic features of the Sagadahoc Bay Tidal Flat, Georgetown, Maine

Sagadahoc Bay is open to the ocean at the south and has no significant fresh-water stream entering it. The intertidal zone is roughly a mile long by half a mile wide; most of it is made up of medium to fine sand, but organic-rich mud characterizes the head of the flat and the protected coves. Refraction-seismograph surveys showed that the bedrock surface lies 30 to 200 feet below the surface of the tidal flat and that it is irregular and fluted longitudinally. Repeated surveys indicate that the intertidal flat builds up and cuts down but is apparently in equilibrium with the present sea level. The sediment that fills the bay came from the sea. Waves and tidal currents tend to move it landward; storms accelerate this, or reverse the direction of movement, depending on the characteristics of the storm. Tidal- and wave-generated currents 0.1 foot above the bottom range in velocity from 0.35 to 0.82 foot per second on incoming tides and from 0.20 to 0.58 foot per second on ebbing tides. Incoming tides float large quantities of sand landward; ebbing tides never carry floating sand. Two distinctive and extensive Mya shell-pavement layers were found at depths of roughly 2 and 3 feet below the present surface of the tidal flat. The lower layer is approximately 1,000 years old according to a radiocarbon age determination of its Mya shells. It is suggested that these shell layers formed by sluicing away of a layer of sand about 2 feet thick, which had been thrown into loose packing by an earthquake at high tide. The earth shock induced a submarine slide of the sand in the outer part of the bay, which oversteepened the profile of the sand headward nearly to the head of the bay. Mya and other shells settled through the layer of quicksand while the sand was running out seaward. Living in the intertidal zone is the usual assemblage of clams, gastropods, crustaceans, worms, and seaweeds found on most northern New England tidal flats. The Mya arenaria population is decreasing, but in general myas are more numerous in the muddy areas than in the sandy areas. They grow more rapidly in the sand, though in the past decade there has been no significant renewal of the Mya population in the sandy part of the flat. Macoma balthica inhabits the muddy areas, whereas Ensis, Spisula, and Arctica are restricted to the low-tide zone and the shallow water below. Gemma gemma grows in great abundance in the sandy part of the flat but is rare in the muddy parts. Small shrimp and green crabs are common. The calcareous shells of these animals are all potential fossils, but the shrimp and crab exoskeletons are not, for their tests are rapidly decomposed in this environment. Other potential fossils are wood and bark, acorn caps, conifer cones, leaves of deciduous trees, seeds, and occasionally even grass stems and pieces of eel grass. All these are reasonably well preserved in the constant reducing environment that prevails an inch or two below the surface. An inverse relationship exists between the abundances of Mya arenaria and Gemma gemma. Cores and test pits show that gemmas are more numerous on the Sagadahoc flat now than they have been in the recent past (estimated 10–100 years). Gemmas are the dominant mollusk in the sandy part of the flat now that the myas are so extremely rare. The speculation is that gemmas became dominant largely because the Mya population was greatly reduced by intensive digging during and just after the last war and through depredations by green crabs. Possibly the warming climate has favored the gemmas selectively. The writer infers that the gemmas are a serious competitor of the myas and that the gemmas now starve out Mya spat, which is known to be carried into the bay each spring and fall. Two recommendations are made: (1) determining under controlled laboratory conditions the food requirements of Gemma and the density of Gemma population that will permit survival of Mya larvae from set through a stage that will assure maturation; and (2) killing off a large percentage of the Gemma population and observing whether or not a natural set of Mya occurs. Gemmas can be killed quickly under a flame shield such as is used to soften asphalt-sand mixtures in street paving. Inasmuch as gemmas are ovoviviparous they should not repopulate the flat rapidly.

Maine↗
USGS1945

American old and middle tertiary larger foraminifera and corals

The scleractinian coral fauna of the Eocene Upper Scotland formation of Barbados comprises 27 species and varieties belonging to 23 genera and subgenera. There is a mingling of hermatypic and ahermatypic forms suggesting a tropical shallow-water, non-littoral environment at depths at or even beyond the lower limits of temperature for vigorous reef-coral growth. Most (16) of the species and 2 of the genera are new and generally show relations with the middle Eocene Claibornian coral fauna of somewhat similar environment in the United States. A few species are either the same as or have affinity with Claibornian forms; one or two are allied to western European or Mediterranean Tertiary species. Of particular interest is the occurrence of Endopachys maclurii. Two corals are described from the Miocene of Martinique. One is a new species of the odd subgenus of Sphenotrochus, Eusthenotrochus, heretofore known only from one recent species and another from the Eocene of the Paris Basin. The other is Dominicotrochus dominicensis (Vaughan), a turbinolian previously known from a single Dominican specimen of uncertain origin and horizon.

Memoir of the Geological Society of America↗
USGS1985

Age of biostratigraphic horizons within the Ordovician and Silurian systems

Three samples that have a bearing on the age of horizons within the Ordovician and Silurian systems, two previously dated by the conventional K-Ar method and one by the 40 Ar/ 39 Ar total-fusion method, have been reanalysed using the 40 Ar/ 39 Ar age-spectrum method. Conventional K-Ar and total-fusion 40 Ar/ 39 Ar ages can always be questioned because of the relative ease with which the K-Ar system can be disturbed, either thermally or chemically (i.e. Dalrymple & Lanphere 1969; Clauer et al. 1982). The 40 Ar/ 39 Ar age-spectrum method has the potential for identifying disturbed K-Ar systems (i.e. Berger 1975; Harrison & McDougall 1980). The authors feel that the age-spectrum data from these samples are significant because the previous results for these samples have been questioned in recently proposed Palaeozoic time-scales because of a possible disturbance of the K-Ar isotopic system (i.e. Gale et al. 1979, 1980; Gale 1982).

Geological Society Memoir↗
USGS1999

Chapter 1: Previous research

Santorini has fascinated and stimulated explorers and scholars since ancient times. Jason and the Argonauts were apparently visitors to the islands and described a giant called Talos. Molten metal flowed from his feet and he threw stones at them. The island is perhaps best known for the paroxysmal eruption that took place in the Late Bronze Age at the height of the Minoan civilization that dominated Crete and the Aegean region. The legend of Atlantis, in which a whole city sank beneath the sea in a single day and night, is plausibly based on the effects of this eruption on the Minoan Civilization. The geographer Strabo described the eruption of 197 bc in the following way: ... for midway between Thera and Therasia fires broke forth from the sea and continued for four days, so that the whole sea boiled and blazed, and the fires cast up an island which was gradually elevated as though by levers and consisted of burning masses... This introduction gives a brief synopsis of research on the volcano since this dramatic chronicle was written. Research on Santorini has contributed substantial advances not only in the understanding of Santorini itself but to general principles in volcanology and petrology. three centres for lava eruptions on Therasia and northern Thera: the Peristeria, Simandiri and Skaros-Therasia Volcanoes.

Santorini↗
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↗