Low-density Rocks Research Articles

Abundance and species richness in natural aquatic microcosms: a test and refinement of the Niche-Limitation Hypothesis

The Energy-Limitation Hypothesis (ELH) predicts that species richness is an increasing function of abundance. In contrast, the Niche-Limitation Hypothesis (NLH) predicts that high abundances become easier to attain as species richness increases. We tested the NLH and ELH using aquatic invertebrate communities of tropical rock pools. These rock pools span a gradient from very low density pools (N <200 individuals/liter) to very high density pools (N ≯ 5001). Only in low density rock pools were species richness and abundance positively related. In intermediate abundance rock pools no relationship between species richness and abundance was observed, and in high abundance rock pools the relationship between species richness and abundance was strongly negative. These patterns are inconsistent with the predictions of the ELH, but not with predictions of the modified NLH which adds carrying capacity to the model.

Le filtrage des anomalies gravimetriques; une cle pour la comprehension des structures tectoniques du Boulonnais et de l'Artois (France)

Abstract The geology of the Boulonnais has been well studied since the early part of the last century [Gosselet and Bertaut, 1873; Olry, 1904; Pruvost and Delepine, 1921]. Extensive coal exploration added substantially to the general understanding of the geology of the region but as outcrop is poor, many questions remain. Gravity methods used in the analysis of geological structures have had a long and successful history in helping to study the earth's crust for scientific and applied objectives. Regional gravity data are particularly useful in mapping geographic distribution and configuration of density contrast of rocks. Previous gravity research shows the main trends of the structure. In most cases the regional Bouguer gravity hides the relationship between the geology and the shape of the anomaly caused by the perturbing body. New information can be obtained by filtering the maps. The purpose of filtering a map is to remove unwanted characteristics and enhance desirable characteristics that are diagnostic for the geology. Because of their simple mathematical forms, most potential field filters are in the spectral domain. It is advisable to transform the original unfiltered field to the spectral domain, apply the filter, then transform the filtered map back to the spatial domain for use in the interpretation. Several spectrally filtered versions of the original gravity map are used in this regional interpretation. In the case of the Boulonnais the most useful filters have been the horizontal component and the first vertical derivative. In the first instance computing the horizontal gradients of the gravity field permits us to localise the limit of the blocks and then the fault positions. The gravimetric field above a vertical contact of rock with different density shows a low on the side of the low density rocks and a high on the side of the high density rocks. The inflection point is located just on the contact of the two types of rocks. This contact can be outlined by locating the maxima of the horizontal gradient. In the case of a low dipping contact maxima stay close to the contact, but are displaced down dip. In the second instance the first vertical derivative acts as a booster for the short wavelength; this attenuates or destroys the effect of the regional field. The resulting map shows a better structure because in complex areas they give a better definition of the different bodies by separating their effects. In the case of the Boulonnais the first vertical derivative allows us to distinguish the depressed region from the uplifted one. The structural evolution of the Boulonnais-Artois area includes two main extensional events in the late Palaeozoic-early Cretaceous interval and an inversion in mid-late Palaeocene time. The new gravity data in combination with recent field and published data have provided a new insight into the structure of the Boulonnais-Artois area and a new interpretation is proposed. -- Fault patterns are oriented 110N and 040N in the Boulonnais and 140N in Artois areas. -- The linkage between the faults shows a relay geometry with transfer zones [cf. Morley et al., 1990 and Pea-cock and Sanderson, 1994]. The best example is located between Sangatte (near the tunnel) and Landrethun faults where overlapping synthetic faults with a relay ramp are imaged. -- There is no major continuous fault zone but a complex en echelon fault system. -- Linkage between Boulonnais and Artois fault is not well constrained. An important discontinuity between the two regions is apparent. This model underlines the importance of overlapping fault tips with the generation of transfer zones. These structures are also known in the Wessex and Weald basins [Stoneley, 1982; Chadwick, 1993] where heritage and inversion are significant.

Structure and tectonic evolution of the transitional region between the central Appalachian foreland and interior cratonic basins

The transitional region between the central Appalachian foreland basin and Michigan and Illinois interior cratonic basins is characterized by first and second order structures related to plate convergent processes at the Laurentian plate margin. The first order structures are the Waverly and Cincinnati arches believed to be forebulges associated with the Taconic and Alleghanian orogenies, respectively. The second order structures are basement faults that exhibit a protracted history of recurrent displacements. First and second order structures are linked in that the basement faults have had a major effect in controlling the location and uplift history of the arches. The Waverly and Cincinnati arches share many similarities that suggest a common process of arch development. The Waverly arch developed over the low density rocks in the Central Metasedimentary belt of the Grenville province near its structural contact with the Central Gneiss belt. Uplift on the arch commenced during passive margin development in the Cambrian, but reached its maximum development at the end of the Middle Ordovician Taconic orogeny. Reactivation of faults along the arch continued as late as the Pennsylvanian. The Cincinnati arch as expressed in the Precambrian basement surface is coincident with the underlying Grenville front on a regional scale. The front is a major crustal suture which served to localize and control the history of the Cincinnati arch. Local deviations between the location of the arch and front are explained by the presence of Proterozoic rift basins filled with low density sedimentary rocks, by interactions between the developing Appalachian, Michigan, and Illinois basins, and by a northward decrease in Alleghanian tectonic loading. In a pattern similar to the Waverly arch, the Cincinnati arch began its history with a period of regional uplift as early as the Middle to Late Ordovician followed by maximum arch development in the Pennsylvanian in response to the Alleghanian orogeny. Neither arch showed significant migration following initial development as predicted by general models of forebulge development. We attribute this to the effect of preexisting basement anisotropies, such as lithotectonic boundaries, which served to control the initial location of the forebulge and affect its subsequent history. A third major arch, the Kankakee arch, occurs in the western part of the transitional region and is oriented at right angles to the other arches. Unlike the Waverly and Cincinnati arches, this arch owes its development to subsidence in the adjacent Michigan and Illinois basins rather than orogenic activity in the Appalachians.

Measurement of radioactive heat generation in rocks by means of gamma ray spectrometry

Terrestrial heat flow is the sum of the radioactive heat generated in the crust and the heat flow from the mantle. If we can evaluate the radioactive heat generation in the crust more precisely, we can better estimate the heat flow from the mantle and the thermal state in the earth crust. In this study, the authors measured the contents of radioactive elements in rocks of Kohistan volcanic arc, Pakistan and Ichinomegata, northeast Japan. We conclude that low density rocks (ρ_??_2.9) systematically contain much more radioactive elements leading to high heat generation compared with high density rocks (ρ_??_3.0). In the next step, the vertical distribution of radioactive heat generation in the crust was estimated, and the heat generation of the lower crust in the two areas was found to be much lower than previously considered. It means, if the surface heat flow is the same, the estimated temperature at 30km depth is about 80°C higher than previous estimates. This temperature change may strongly affect the seismic activity at the depth.

Controls on extrusion at mid-ocean ridges

A magma lens can erupt to form extrusives only if it is under greater pressure than the static pressure in a column of magma reaching from the lens to the surface. The excess pressure results partly from overburden pressure caused by the presence of high- and low-density rocks (dikes and extrusives, respectively) above the lens. The thicker the pile of low-density extrusives, the lower the average overburden density. Thus, extrusion above a lens should be self-regulating, in that thickening the extrusive layer reduces the driving pressure for subsequent eruptions. Flexural stresses may affect extrusion by altering the pressure on a magma chamber. For ridges lacking an axial valley, we predict that deeper magma lenses should correlate with thicker extrusive layers, consistent with recent observations.

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.

Water‐Holding Characteristics of Metasedimentary Rock in Selected Forest Ecosystems in Southwestern Oregon

AbstractOn sites characterized by shallow soil in semiarid climates, lack of bedrock description may lead to underestimates of total storage capacity of water available for plants, and therefore of potential site productivity. Sample rocks from southwestern Oregon soil‐rock profiles excavated beneath typical forest plant communities were examined for pore volume distribution by means of mercury intrusion porosimetry and oversolution equilibrium. Four ranges of pore size were distinguished: gravitational water, available water content (AWC), slowly available water (SAW), and nonavailable water. Pore volume for AWC and SAW was associated with bulk density and in low density rock, was 20% of volume. Results suggest that metasedimentary rock has the potential to be a major reservoir of plant available water in semiarid climates.

A possible concealed granite beneath part of Anglesey, North Wales

A closed Bouguer gravity low is revealed by land and marine gravity survey data centred on Holyhead Bay, northwest Anglesey. The anomaly extends across areas on land and offshore mainly occupied by Precambrian rocks. A concealed granite offers the best explanation for the anomaly and a late Caledonian age seems most probable for the intrusion, although Precambrian–early Cambrian ages are also possible. The existence of a granite in this area could have influenced the development of mineralization and the low density rock mass also formed a relatively buoyant core to the basement block. It could therefore have been instrumental in the preservation of this Precambrian structural high.

Gravity and Magnetic Maps of the Santa Maria Province, California

In this paper the authors display an isostatic residual gravity map, contour interval 5 mGal, and an aeromagnetic map, which is a mosaic of existing data . Two computer-derived maps, also at a scale of 1:250,000 display the locations of the steepest gravity gradients and of the steepest gradients on the pseudogravity transform of the magnetic map. These gradients indicate boundaries between rock masses with differing physical properties. The gradient boundaries, where steep and linear, are presumably cause by faults. The gravity reduction removes the isostatic effects of the continental margin, a 60 mGal gradient across the Santa Maria basin, leaving a maximum basin low of 40-50 mGal caused by the low-density basin rocks and bounded by various steep gradients at the basin margin faults. Offshore gravity lows indicate local sedimentary basins trending north to north-west. The magnetic anomalies are caused primarily by serpentinite in the basement of Franciscan assemblage and form irregular belts trending northwest across the basin and basin margins, terminating against the offshore Hosgri fault. The magnetic patterns west of the Hosgri are more irregular in character and may reflect a different basement. The Hosgri fault appears to extend south beyond Point Arguello (about 2 km offshore)more » according to the gravity map. The right-lateral slip on this fault is thus presumably accommodated on north-dipping thrust faults on the north side of the Santa Barbara Channel and perhaps along the south side of the Santa Maria basin as well. Along the east side of the Hosgri fault, high gravity values indicate local uplift.« less

Gravity interpretation of the Zaranda Younger Granite ring complex, Nigeria

Gravity and geological data were collected over the Zaranda complex situated near Bauchi, Nigeria for a critical analysis of the deep structure in the area. The result of the study shows that, in addition to the host migmatitic gneiss, five major rock types form the complex. These are leixocratic and melanocratic syenites, granite porphyry, trachytes and basalts, and mafic-rich enclaves in the basement. A negative residual gravity anomaly of about − 10.6 mGal is centred on the complex. This negative residual is bordered on the eastern and southern margins by relatively positive residual anomalies of up to +5.2 mGal. The main negative anomaly displays a prominent NE-SW trend which is similar to those of the major structures in the basement, suggesting that the complex might have intruded the host migmatic gneiss through pre-existing structures. There is also good correlation between the residual anomalies and the geology, with the negative anomalies occurring over areas underlain by low density rocks corresponding to trachyte (2.56 × 10 3 kg m −3), syenite (2.62 × 10 3 kg m −3); while positive anomalies correspond to areas of basic rocks (2.76−2.92 × 10 3 kg m −3). 2-D and 3-D gravity modelling showed that the major pluton of the complex is the syenite body which extends to a depth of about 5.5 km. The mineralogy and chemistry of the rocks in the Zaranda complex suggest that the complex may be underlain by a zone of mafic rocks, as shown by field occurrence of hornblende-rich enclaves in the basement.

Possible Mantle Convection-Current Sink Under United States Cordillera: ABSTRACT

The concept of plate tectonic subduction has helped explain the compressive deformation of the U. S. Cordillera, but continued convergent plate movements have not explained as satisfactorily the end of compression from Alaska to southern Mexico about 40 m.y.B.P., nor the extensional faulting from Canada through Mexico since about 20 m.y.B.P. Accordingly, it may be appropriate to reconsider Hess' original concept of a two-sided mantle convection sink. The axis of a deep Bouguer gravity anomaly (below 125 km or 78 mi) extending from Mexico through western Colorado into Canada may represent the axis of a mantle convection sink. The low-density rocks along the axis of the anomaly may originate from sinking mantle convection currents and/or magmatic differentiation of upper mantle rocks which were carried into the deeper, hotter mantle. If mantle convection currents that transported the Pacific subduction plate eastward and those that transported the North American plate westward both met and sank in the Cordilleran mantle sink, then the two limbs of the sink would be defined. However, a shadow zone between the top of the subduction plate and base of the lithosphere offers the possibility that westward-directed mantle convection currents may have exerted enough stress on the subduction plate to end compression and start extension of the lithosphere, despite continued convergent plate movements and subduction. End_of_Article - Last_Page 468------------

The Carolina slate belt—Evidence of a continental rift zone

Near the South Carolina and Georgia border, the southeastern parts of the Charlotte belt and Carolina slate belt near the Coastal Plain overlap are coincident with the northwestern edge of an isolated negative Bouguer gravity anomaly. This anomaly is interpreted as evidence for a detached fragment of continental crust. The pattern of the magnetic and gravity anomaly contours for the northwestern edge of this fragment is similar to the pattern of magnetic anomaly contours and the Piedmont gravity gradient in central Georgia. This geometric similarity suggests that the fragment may have been separated from a larger continental block by a rift zone nearly 160 km wide. The Carolina slate belt in Georgia and South Carolina may delineate the axis of this continental rift or rift system and may represent remnants of rift-derived volcanic and sedimentary rocks. Gravity profiles across the edges of the hypothesized rift indicate the existence of anomalous high-density rocks in the upper 5 km of the crust in the proposed rift and low-density rocks at the base of the adjacent continental crust. Seismic data in Georgia indicate a typical rift-zone velocity structure that consists of a high-velocity (6.3 km/s) discontinuous surface layer 0.0 to 5.0 km thick over a low-velocity (6.0 km/s) crustal layer which extends to depths of at least 30 km. Magnetic lineations provide evidence for the faults and/or depositional layering of the rift and outline the structures and margins of the rift. The rift developed from late Precambrian through Cambrian time (650 to 520 m.y. B.P.), as determined from radiometric dating of Carolina slate belt rocks. In North Carolina the rift was reactivated during the Triassic, but in Georgia and South Carolina, Triassic extension was restricted to the southeastern edges of the detached crustal fragment.

A model for lower continental crust

This proposed model is based on geological, geophysical and geochemical data. Previous models suggested for the lower continental crust consisted of basalt, gabbro, or charnockitic rocks; however, experimental and field petrological data indicate that the bulk of crustal rocks are metamorphic. A lower crust of heterogeneous metamorphic rocks also agrees with seismic reflection results which show numerous reflections from “layering”. Geothermal conditions favor a “dry” charnockitic or gabbroic lower crust rather than an amphibolitic lower crust because heat production data imply that wet amphibolitic rocks would have a higher heat production than their dry metamorphic equivalents. Relatively high velocities from field and laboratory measurements in such low-density rocks as granite, syenite, anorthosite and granulitic rocks in general imply that the composition of the lower crust is more felsic than gabbro. Variation in seismic velocity and depths from crustal refraction studies and numerous seismic reflections all indicate a highly heterogeneous lower crust. The lower crust, which has traditionally been described as gabbroic or mafic, may consist of such diverse rocks as granite gneiss, syenite gneiss, anorthosite, pyroxene granulite, and amphibolite, interlayered on a small scale, deformed, and intruded by granite and gabbro. Interlayering of these rocks explains the presence and character of seismic reflections. Abrupt changes in dip, tight folding, disruption of layers, intrusion, and changes in layer thickness explain the characteristic discontinuity of deep reflections. Igneous intrusions may be floored by metamorphic rocks. The lower crust consists of a complex series of igneous and metamorphic rock of approximate intermediate composition.

GRANITE BENEATH THE PENNINES IN NORTH YORKSHIRE

( Short communication ) A cored borehole drilled in Raydale, Yorkshire, has proved granite at 495.05 metres depth directly beneath Lower Carboniferous (Visean) beds. The site (SD 9026 8474) lies on the south-east side of Raydale Beck some six kilometres SSW of Bainbridge and 350 metres N 20° E of Raydale House at an altitude of 267.9 metres above O.D. The bore was drilled as part of the research programme of the Institute of Geological Sciences. The Alston Block and the Askrigg Block in the north Pennines have long been recognized by geologists as tectonic units, in which the comparative lack of folding and major faulting affecting the Carboniferous strata was due to some feature of the underlying basement rocks which gave them rigidity. The discovery on the Alston Block of a negative gravity anomaly (Bott and Masson-Smith, 1957), with minima corresponding closely with the postulated emanative centres of mineralization, indicated the existence of low-density rocks and prompted the drilling by the University of Durham Department of Geology of the Rookhope borehole which proved the presence of a granite batholith (Dunham et al. , 1965). This has been named the Weardale Granite, and its age has been determined as 410 ± 10 m.y., by the rubidium-strontium isotope method (Holland and Lambert, 1970), corresponding to an end-Silurian age of emplacement. These results encouraged further studies of the negative gravity anomaly already proved to exist on the Askrigg Block by a gravity traverse extending from Kettlewell in Wharfedale to Aysgarth in Wensleydale (Whetton et al. , …

Tectonic History of the Central Aleutian Arc

The Aleutian arc is a ridge-trench geomorphic system associated with active volcanism, high seismicity, and a hypothetical transition zone from underthrusting to non-underthrusting approximately midway along its length. Two contrasting acoustic signatures can be recognized on seismic reflection profiles in the area, and these are correlated with informally designated early series and late series rock units recognized on the Aleutian Islands. The two series are separated by an unconformity on the summit and flanks of the Aleutian Ridge. The early series consists of altered and deformed sedimentary and volcanic rocks of Eocene to middle Miocene age (45 to 14 m.y.). The late series includes generally unaltered and undeformed sedimentary and volcanic units that range in age from late Miocene to present (10 to 0 m.y.). Reflection profiles show that as much as several kilometers of the late series underlies the Aleutian Trench, the Aleutian Terrace, and summit basins on the Aleutian Ridge; the series reflects sediment draping and basin infilling on an extensionally fragmenting ridge. Magnetic and refraction data and preferred gravity models demonstrate that the southern Aleutian Terrace and inner (northern) Aleutian Trench wall are underlain by low-velocity, low-density rock. A study of the sedimentary fill and inner wall of the trench revealed no structural difference between the postulated underthrust and non-underthrust segments, and it cannot be concluded on this basis that underthrusting has occurred. However, the gravity data (and magnetic and refraction data) do allow the inference that sedimentary offscrapings could form the inner trench wall and underlie the Aleutian Terrace along the postulated zone of underthrusting. Recent JOIDES data from the North Pacific suggest a limited amount (less than 1,000 km) of underthrusting since the Eocene; this interpretation is most compatible with the model of discontinuous plate motion for the North Pacific deduced earlier by Pitman and Hayes. The tectonic, magmatic, and depositional histories of the arc also agree with their model and suggest an episode of active underthrusting in the early Tertiary, a slowing or cessation throughout the middle Tertiary, and a resumption of underthrusting in the late Cenozoic. The early Tertiary underthrusting is indicated by the rapid growth of the ridge during this time. Cessation or slowing in the middle Tertiary is apparently marked by uplift and folding, widespread epizonal plutonism (but no intense volcanic activity), and formation of the unconformity separating the rocks of the early and late series. Resumption of underthrusting in the late Cenozoic is correlated with the outbreak of the present episode of volcanism and fragmentation of the ridge crest to form numerous summit basins.

A gravity survey of Ngorongoro Caldera, Tanzania

A gravity survey of Ngorongoro Caldera is described. The volcano is associated with a negative Bouguer anomaly, which is thought to be due to low-density rocks forming the cone and to sediments deposited on the floor of the caldera. The type of anomaly found at Ngorongoro is compared with those at some other calderas and it is concluded that the mode of formation of a caldera is not necessarily evident from associated gravity anomalies. Finally, some implications for the evolution of the Eastern Rift Valley are discussed.

Seismic refraction surveys, western Queen Elizabeth Islands and Polar continental margin

The Arctic seismic program of 1964 and 1965 comprised two refraction profiles between Prince Patrick Island and Melville Island, and one refraction profile extending 192 km onto the ocean northwest of Brock Island. Numerous intermediate layer (P1) and upper mantle (Pn) events were also recorded on paths not confined to the profiles, thus allowing a "time-term" analysis for these events.High-velocity layers within the sedimentary section cause a velocity inversion problem, whereby the lower velocity and depth of the underlying basement complex is not revealed by the seismic refraction method. The bottom of the basement complex is marked by the intermediate layer having a compressional wave velocity of about 6.25 km/s and depths near 10 km.Velocity of Pn events is 8.18 km/s and these were recorded near the critical distance at one station, providing an estimate of the time-term to depth conversion factor of 9.56 km/s. Pn events traversing Eglinton Island and the west coast of Melville Island show excess times of 0.6 s to 3 s. This region of apparently low mantle velocity may be related to the geomagnetic variation anomaly reported near by.The average time-term of 3.67 s represents an average crustal thickness of 35 km. Some variation in crustal time-terms is caused by differing thicknesses of low velocity sediments rather than undulations on the crust–mantle interface. The time-term of 3.07 s for the most distant shot on the ocean profile does not, by itself, suggest a thinner crust than similar values among the islands; this is shown to be a possible interpretation problem.A negative correlation between crustal time-terms and Bouguer gravity values is noted to be partly due to the effect of varying proportions of low-velocity – low-density rocks and high-velocity – high-density rocks within a few km of depth.

GRAVITY STUDIES OVER THREE EVAPORITE PIERCEMENT DOMES IN THE CANADIAN ARCTIC

Reconnaissance gravity surveys over three evaporite piercement domes in the Canadian Arctic Islands have been interpreted. Each dome was considered as a right‐vertical cylinder divided into two homogeneous regions, a high density anhydrite zone [Formula: see text] overlying a low density gypsum and/or rock salt zone [Formula: see text]. The cylinder is surrounded by a sedimentary sequence which has a uniform density of [Formula: see text]. Interpretation involved a least‐sum‐of‐squares approach to estimate the thickness of the anhydrite and gypsum‐rock salt zones. The three sets of estimates gave a range of 200 to 550 m for the anhydrite thickness and a range of 700 to 5500 m for the vertical extent of domes. In each case the depths were less than expected on the basis of estimates from seismic and geological data. Possible explanations for this are: (a) the cross‐sectional area of each dome decreases with depth; (b) the existence of a transition zone where a gradation occurs between the high and low density zones; and (c) the effective density contrast of the low density zone is less than [Formula: see text].

Hugoniot equation of state of twelve rocks

Shock-wave data have been obtained for the following rocks: one albitite, one anorthosite, two bronzitites, two diabases, two dunites, two eclogites, one granite, and one Jadeite. The experimental pressure range is from slightly over 0.1 to about 1.0 Mb. There is evidence that all rocks investigated exhibit a phase change. It is estimated that the transformations occur at about 0.1 Mb for the low-density rocks and between 0.3 and 0.6 Mb for the high-density rocks. A single-shock Hugoniot consistent with the data has been determined for the high-pressure modification of some of these rocks. These equations of state are compared with similar equations of state derived from geophysical data for the earth. This comparison supports the model of a mantle composed of high-pressure modifications of these basic rocks with an iron content lying somewhere between that of Twin Sisters dunite and Hortonolite dunite. Other thermodynamic quantities have been calculated based on estimates of the specific heat and the Grüneisen parameter.

Gravity, isostasy, and crustal structure in the southern Cascade Range

Gravity measurements taken in northern California reveal an eastward decrease in the Bouguer anomaly of 100 mgal in about 30 km near Mount Shasta. In places the observed gradient approaches 8 mgal/km. Removal of the regional gravity field reveals two major gravity minimums in the southern Cascade Range. Their maximum amplitudes are both close to −50 mgal. These anomalies suggest a striking similarity between the upper crust beneath the Mount Shasta area and the upper crust beneath the Lassen Peak area. Both are regions with large volumes of low-density rocks at relatively shallow depths. The following conclusions are made regarding the anomalies under study: (1) A large volume of low-density material exists at a shallow depth beneath the High Cascades. In places the top surface of this anomalous mass must be within the upper 4 km, and in general it must be within the upper 10 km of the crust. (2) The free-air anomaly in the Interior Platform region averages nearly zero; it follows from Gauss's theorem that this broad region must be in a state of isostatic compensation. (3) The application of Gauss's theorem to the negative gravity anomaly associated with the High Cascades near and including Mount Shasta indicates a mass deficiency of about 4.0×1018 grams. This estimate corresponds well with the mass excess of the range and suggests that the High Cascades are compensated. (4) The application of Gauss's theorem to the negative gravity anomaly at Mount Shasta indicates a mass deficiency beneath Mount Shasta of about 5.4×1017 grams. This estimate corresponds well with the mass excess of the mountain and suggests isostatic compensation, of which a large part must be local.