bluuebunny/crossref_metadata_2025 huggingface dataset:
Constraints on extension of continental lithosphere
Authors: England Philip
Published: February 2008

The extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature‐dependent rheology which is governed primarily by the strength of the upper mantle. The lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time, corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition. The progress of the extension is influenced by the decrease in strength of the lithosphere, owing to its attenuation, and by an increase in strength resulting from cooling. The relative importance of these two effects depends on two parameters, a dimensionless strain rate and the total strain. For a given strain rate the extending lithosphere goes through two phases as the strain increases: in the first, the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to, and perhaps slightly below, its original strength; in the second, the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain, resulting, presumably, in an effective limit on the degree of stretching that the lithosphere experiences.For the case of strain rate decreasing with time, when the initial strain rates are of order 10−15s−1, this maximum extension is between 100% and 150%; for a strain rate of 10−l4 s−1 the maximum lies around 400% to 500%. At higher strain rates than this, very large extension may occur before cooling significantly affects the strength of the lithosphere. This model accords well with the constraints available from the continental margins of the North Atlantic.
Focal depths and fault plane solutions of earthquakes under the Tibetan Plateau
Authors: Molnar Peter , Chen Wang‐Ping
Published: February 2008

We compare synthetic and recorded P wave forms to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts (>4000 m) of the Tibetan plateau. Fault plane solutions for all 16 events show combinations of normal and strike slip faulting with T axes oriented approximately east–west. None of these solutions show thrust faulting. Thus the data corroborate previous inferences that the active tectonics are dominated by east–west extension. Focal depths for all 16 events are less than 15 km and appear to be between 5 and 10 km. This style of deformation and these depths of faulting are similar to those in the Basin and Range province of the western United States. Two intermediate depth events below the crust of southern Tibet also show primarily normal faulting with east–west T axes. The solution for one, discussed by Chen et al. (1981), is unambiguous. The solution for the other, the event of August 1, 1973 (27.59°N, 89.17°E, 85±10 km, mb = 4.9) is less certain. Both apparently occurred in the mantle beneath a thick, aseismic lower crust, and their occurrence suggests that brittle deformation occurs there in response to a stress field similar to that operating at shallow depths beneath Tibet.
An experiment in systematic study of global seismicity: Centroid‐moment tensor solutions for 201 moderate and large earthquakes of 1981
Authors: Dziewonski Adam M. , Woodhouse John H.
Published: February 2008

Data from the Global Digital Seismograph Network were used to obtain ‘centroid‐moment tensor’ solutions using the method of Dziewonski et al. (1981). Results were obtained for 201 earthquakes ranging in seismic moment from 7×1023 to 3×1027 dyne‐cm. The wide dynamic range of the SRO/ASRO stations allows us to investigate, using the same algorithm, series of events among which the smallest and the largest may differ in moment by a factor as large as 1000. Among the events studied is a particularly interesting series, in January 1981, off the island of Honshu. The main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis. There is an indication that the source mechanism changes with the size of the event. Statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1.6×10−8 M01/3. The shifts in origin time for deep‐focus earthquakes are indicative of the general complexity of these events, which depends only weakly on the seismic moment. The extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth. While for shallow earthquakes these departures decrease, from a maximum at about 1025 dyne‐cm, with increasing moment, the reverse appears to be true with respect to deep‐focus earthquakes. Shallow earthquakes north of New Guinea and along the Solomon Islands show systematic and substantial departures from the double couple mechanism.
Wandering terranes in southern Alaska: The Aleutia Microplate and implications for the Bering Sea
Authors: Marlow Michael S. , Cooper Alan K.
Published: February 2008

Paleomagnetic and geological data suggest that much of southern Alaska is a collage of tectonostratigraphic terranes which originated in Mesozoic time at paleolatitudes far south of their present position. The time of ‘docking’ of the terranes against cratonic Alaska is critical to defining their amalgamated size and extent during their northward motion as well as their role in the evolution of the Bering Sea. One of the largest of the tectonostratigraphic terranes, the Peninsular terrane of south central and southwestern Alaska, extends offshore along the outer Bering Sea continental margin (Beringia). Paleomagnetic data suggest that this terrane has moved northward through all of Cenozoic time, but geologic data imply that the terrane had accreted to Alaska by the end of the Mesozoic. In early Cenozoic time the eastern part of the Aleutian arc appears to have been superimposed on the Peninsular terrane, and postulated northward Cenozoic motion of the terrane would therefore have required northward motion of the arc. Two accretion models, based on docking times for terranes in Alaska, are proposed, and they illustrate that large areas of the abyssal Bering Sea, the Alaska Peninsula, the Aleutian arc, and the Beringian continental margin may be part of a superterrane or microplate called Aleutia (microplate as defined by Beck et al. (1980), i.e., a microplate is a displaced segment of lithosphere that has crustal roots, whereas a superterrane is an amalgamation of terranes which may or may not be rootless). Model A implies that the Aleutian arc developed in situ on the southern edge of Aleutia after the microplate had docked. In model B, the final docking time of the Peninsular terrane is late Cenozoic, which implies that the Aleutia microplate encompasses a mammoth area that includes parts of southern Alaska, the Alaska Peninsula, the southern Beringian margin, the abyssal Bering Sea (Kula plate), and the Aleutian arc. If model A is correct, the docking time of the Peninsular terrane is late Mesozoic or earliest Tertiary. The Aleutia microplate in this model is made up solely of the abyssal Bering Sea (Kula plate), which presumably docked at the same time or slightly after the Peninsular terrane accreted against Alaska. If model B is correct, that is, if the Aleutia collided with nuclear Alaska during the Cenozoic, then a late Cenozoic suture zone, the vestige of a large open sea that must have closed between Aleutia and Alaska, must exist in south central and southwest Alaska. Either evidence for Cenozoic closure and suturing has been obliterated in Alaska or the inferences of Cenozoic terrane motion derived from paleomagnetic data are suspect.
Paleomagnetism of a polarity transition in the Lower (?) Triassic Chug Water Formation, Wyoming
Authors: Herrero‐Bervera Emilio , Helsley Charles E.
Published: February 2008

A total of 156 oriented specimens, taken at stratigraphic intervals of 3 to 20 cm, have been collected from the Red Peak Member of the Chugwater Formation at three localities approximately 25 km southeast of Dubois, Wyoming. The stratigraphic interval containing the polarity transition in the remanent magnetic field is about 1.0 m. All specimens were thermally demagnetized at 500°, 550°, 620°, and 660°C and show directions of magnetization of normal, reversed, and transitional character as well as a remarkable decrease of the remanent intensity in the transition zone. The section sampled is divided into five intensity periods: two periods recording the dipolar field, one recording a dipolar field of decreasing intensity, one recording a dipolar field of increasing intensity, and a period of low intensity recording the field present during the 180° directional swing. A virtual geomagnetic pole (VGP) path obtained for a N → R polarity transition makes a fairly smooth transit, with the path confined to a sector of longitude between 85°W and 110°E. The reversal's onset is first indicated by a slow movement of the VGP across northern latitudes. The VGP then lingers to two positions 60°N and 40°N (35°E to 70°E) and 20°N and 40°N (335°E to 5°E), suggesting brief periods of field stability (or nondipolar field dominance). The onset of the transition appears to be more rapid than the recovery. The VGP path observed at a northern hemisphere site is located about 90° from the site. The pole position derived from stable normal and reversed portions of this study (115.5°E, 46.4°N, α95 = 4.1, dp = 2.2, dm = 4.2) lies approximately in the center of the main group of poles reported for other Triassic units.
Local seismicity preceding the March 14, 1979, Petatlan, Mexico Earthquake (Ms= 7.6)
Authors: Hsu Vindell , Gettrust Joseph F. , Helsley Charles E. , Berg Eduard
Published: February 2008

Local seismicity surrounding the epicenter of the March 14, 1979, Petatlan, Mexico earthquake was monitored by a network of portable seismographs of the Hawaii Institute of Geophysics from 6 weeks before to 4 weeks after the main shock. Prior to the main shock, the recorded local seismic activity was shallow and restricted within the continental plate above the Benioff zone. The relocated main shock hypocenter also lay above the Benioff zone, suggesting an initial failure within the continental lithosphere. Four zones can be recognized that showed relatively higher seismic activity than the background. Activity within these zones has followed a number of moderate earthquakes that occurred before or after the initial deployment of the network. Three of these moderate earthquakes were near the Mexican coastline and occurred sequentially from southeast to northwest during the three months before the Petatlan earthquake. The Petatlan event occurred along the northwestern extension of this trend. We infer a possible connection between this observed earthquake migration pattern and the subduction of a fracture zone because the 200‐km segment that includes the aftershock zones of the Petatlan earthquake and the three preceding moderate earthquakes matches the intersection of the southeastern limb of the Orozco Fracture Zone and the Middle America Trench. The Petatlan earthquake source region includes the region of the last of the three near‐coast seismic activities (zone A). Earthquakes of zone A migrated toward the Petatlan main shock epicenter and were separated from it by an aseismic zone about 10 km wide. We designate this group of earthquakes as the foreshocks of the Petatlan earthquake. These foreshocks occurred within the continental lithosphere and their observed characteristics are interpreted as due to the high‐stress environment before the main shock. Pre‐main shock seismicity of the Petatlan earthquake source region shows a good correlation with the aftershocks in their spatial distribution. This suggests that an asperity existing along the Benioff zone may have affected both the pre‐main shock activity in the continental lithosphere and the aftershocks along the Benioff zone. Although major thrust earthquakes at trenches occur along Benioff zones, in the present study we find little activity on this interplate boundary before the Petatlan earthquake. The overlying continental block, on the contrary, is very active seismically. Our data suggest that the activity is probably governed by the stress transmitted from below due to coupling between two plates and the heterogeneity within the continental lithosphere. The continental material is probably the more likely place for precursors.
Generation of arc basalt magmas and thermal structure of the mantle wedge in subduction zones
Authors: Tatsumi Yoshiyuki , Sakuyama Masanori , Fukuyama Hiroyuki , Kushiro Ikuo
Published: February 2008

The chemical compositions of primary magmas of olivine tholeiite (OTB), high‐alumina basalt (HAB), and alkali olivine basalt (AOB) are obtained by the olivine maximum fractionation model for Quaternary magnesian basalts from the Northeastern Japan arc. These basalts are assumed to have fractionated only olivine crystals before eruption. The melting phase relations for three primary basalt compositions have been determined under both anhydrous and water‐undersaturated conditions. The AOB melt coexists with olivine, orthopyroxene, and clinopyroxene at 17 kbar and 1360°C under anhydrous conditions and at 23kbar and 1320°C in the presence of 3wt % water. The HAB melt also coexists with the above three phases at 15 kbar and 1340°C under anhydrous conditions and at 17 kbar and 1325°C in the presence of 1.5 wt % water. The OTB melt, on the other hand, coexists with olivine and orthopyroxene at 11 kbar and 1320°C under anhydrous conditions. The water contents in arc basalt magmas are estimated to be about 3, 1.5, and nearly O wt % for the AOB, HAB, and OTB, respectively, on the basis of the solubility limit of water in silicate melts. Based on these estimates and the experimental results, the AOB, HAB, and OTB magmas are suggested to segregate from the mantle at about 1320°C and at 23, 17, and 11 kbar, respectively. As the temperatures at the segregation of the magmas given above appear to be too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for the magma production in a subduction zone. Considering the heat of formation of melt in the diapir, the region with temperatures higher than 1400°C has to be present in the mantle wedge.
Nonlinear strain buildup and the earthquake cycle on the San Andreas Fault
Authors: Thatcher Wayne
Published: February 2008

Two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault, accompanied by a progressive broadening of the zone of deformation adjacent to it. In the thin lithosphere model, transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere. For an earth model with a thick elastic lithosphere (plate thickness » depth of seismic slip), transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane. Data from the San Andreas fault indicate a long‐term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle. Observations support a cycle‐long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar. If so, the usually lower strain rates and broader deformation zone currently observed on the southern San Andreas represent a later evolutionary stage of the northern locked section, where a great earthquake is a more recent occurrence. Although the data allow some extreme models to be discarded, no sufficiently strong constraints exist to decide between the thin and thick lithosphere models. Regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time‐predictable model.
Oxygen isotopic compositions of minerals from Illinois Deep Drill Hole granites
Authors: Kombrink Mark , Clayton Robert N.
Published: February 2008

The granites sampled by the IDHP have whole‐rock δ18O values of about 7‰, which places them at the low end of the range previously observed for granitic rocks that not undergone extensive modification by hot meteoric waters. Over a depth range of 840 m, no significant variation in bulk initial isotopic composition is observed. All samples have experienced post‐crystallization retrograde isotopic exchange among the major minerals (quartz, feldspars, biotite), as is common in plutonic igneous rocks. In the uppermost sample analyzed (only 3 m below the Paleozoic sediments), feldspar has undergone exchange with an external reservoir, probably water of meteoric origin, and probably at a temperature near 100°C.
Unstable extension of the lithosphere: A mechanical model for basin‐and‐range structure
Authors: Fletcher Raymond C. , Hallet Bernard
Published: February 2008

To investigate the behavior of the lithosphere undergoing extension, we use a simple rheological model broadly consistent with experimental data on rock creep and with the nature of the brittle/ductile transition. A plastic surface layer overlies a substrate that deforms by power law creep with a stress exponent n = 3 and an effective viscosity that decreases with depth. In extension this model shows a strong necking instability, provided that the thermal gradient is sufficiently large; otherwise, stable uniform extension is indicated. The predicted structures display uniformly spaced necks or regions of enhanced extension (basins) alternating with regions of reduced extension (ranges). If the depth to the brittle/ductile transition is roughly 10 km, as suggested by the maximum depth of seismic faulting, the model yields spacings for the incipient Basin and Range structures of about 25–60 km, in excellent agreement with observation.
Travel time analysis of borehole seismic data
Authors: Stephen R. A. , Harding A. J.
Published: February 2008

A method is presented for reducing travel time data from multiple offset borehole seismic experiments to velocity‐depth structure. The technique, which treats simultaneously data from any number of depths in the borehole in addition to surface data, is based on the linear inversion scheme of Dorman and Jacobson (1981). Given the parameters ζ(p) = T(p) + pX(p) and τ(p) = T(p) − pX(p), the method solves for depth to specified slownesses, assuming linear gradients (in velocity) between slowness values. A practical limitation on the use of surface travel time measurements to resolve velocity‐depth structure is the necessity of assuming a surface velocity. This is not necessary for the case of borehole data. For a borehole receiver the velocity at the depth of the receiver can be obtained from the slope of the inflection point of the travel time curve. Thus a direct measure of the uppermost velocity of a section can be obtained from the travel time data of a shallow borehole receiver. Estimation of velocities from the inflection points of deeper receivers improves the resolution of the velocity‐depth function, which would be obtained from surface data alone. The technique is applied to data from three borehole seismic experiments in oceanic crust. The experiments were carried out in the western Atlantic (crustal age ∼110 m.y.), the Gulf of California (crustal age ∼1 m.y.) and the Costa Rica Rift Area (crustal age ∼6 m.y.). All three experiments show relatively high upper crustal velocities ( >4.0 km/s), suggesting that layer 2A is not present even in the very young crust. All sites had over 100 m of sediment thickness, and it is postulated that sediment thickness and sediment permeability, not merely age, govern the velocity of the upper crust by accelerating the cementation of fractures and cracks.
Tree ring indications of uplift at Icy Cape, Alaska, related to 1899 earthquakes
Authors: Jacoby Gordon C. , Ulan Linda D.
Published: February 2008

Previous studies show that tree ring analysis can provide information about past major earthquakes. If occurrence of large, prehistoric seismic events and extent of areas affected by such events can be determined by analysis of old‐aged trees, this information will be an important addition to the paleoseismological record. Two great earthquakes occurred in September 1899 on the southeastern coast of Alaska near Icy Cape, an area now included within the Yakataga seismic gap. Due to glacial coverage of Icy Bay and the scarcity of observers, any possible uplift at Icy Cape related to these earthquakes was unreported. In 1902 the presumed former shoreline trees at two sites, 2.4 km apart, show both an absolute increase in growth and an increase in growth relative to inland trees. The ages of the trees on the lowland seaward of the sample sites are all post‐1899. If uplift occurred, a seaward shifting of the shoreline would improve the growth environment of trees growing on the former wave‐beaten shoreline and former beach areas might be protected enough for initial afforestation. Other possible causes for tree growth and shoreline changes are considered; however, from the tree ring evidence, geomorphology, and reports of uplift in other parts of the region, it is the authors' interpretation that uplift occurred at Icy Cape related to the 1899 earthquakes. Subsequent accumulation of fluvial and littoral sediments has built out over and around the uplifted area, resulting in the present‐day shoreline configuration. The methodologies used in this study have paleoseismological applications to other uplifted, forested, shoreline areas.
Earthquake frequency distribution and the mechanics of faulting
Authors: Wesnousky S. G. , Scholz C. H. , Shimazaki K. , Matsuda T.
Published: February 2008

The level of intraplate seismicity in Japan generally shows a positive correlation with the density of Quaternary faulting. In southwest Japan, where intraplate seismicity is concentrated on land, rates of seismic moment release (
) are similar when calculated from either the 400‐year historical record of seismicity or geologically determined slip rates of Quaternary faults. A data set of 18 earthquakes with seismic moments (M0) ranging from ∼0.01 to 3×1027 dyn cm shows a relationship between rupture lengthl and M0(log M0 = 23.5+1.94 · log l). When seismic moment on each Quaternary fault is assumed to occur in discrete events every T = M0/
years (where M0 is estimated for a rupture extended over the entire fault length, and
is proportional to the slip rate of each Quaternary fault), the moment frequency distribution of earthquakes (log N =A − B · log M0) predicted from the geologic record is virtually identical to that seen with the 400‐year record of seismicity. In contrast, if it is assumed that earthquakes on each fault occur according to the Gutenberg‐Richter relation, we obtain poor agreement with the observed seismicity. Thus, while regional seismicity satisfies the relation log N =A − B · log M0 (or equivalently, log N = a − b · log M, where M is magnitude), it appears that seismicity on individual faults does not. This further implies that the primary factor that leads to the magnitude frequency distribution in regional seismicity studies is the relative distribution of the slip rates and lengths of preexisting faults.
Overlapping rift zones at the 5.5°S offset of the East Pacific Rise
Authors: Lonsdale Peter
Published: February 2008

A Seabeam and magnetometer survey of the Pacific‐Nazca plate boundary around 5.5°S mapped a “nontransform offset” whose geology and kinematics seem typical of a whole class of structures formed where fast‐spreading rises are laterally offset for distances less than the width of a subaxial magma chamber. Though the spreading center is abruptly displaced by 15 km right laterally, there is no trace of strike‐slip faulting. Instead the ends of two 100 km‐long axial volcanic ridges veer 15° toward each other and overlap for more than 20 km. In this overlapping region, between 5°22′S and 5°32′S, accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one‐sided spreading. A continuous but dog‐legged magma chamber is inferred to underlie both rift zones and an intervening 9‐km wide volcano‐studded basin. A strong magnetic signature from the crust, which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank; in the past 0.5×106 years the southern axial ridge has propagated north at 40 mm/yr, 55% of the spreading half‐rate.
Ophiolite emplacement by collision between the Sula Platform and the Sulawesi Island Arc, Indonesia
Authors: Silver Eli A. , McCaffrey Robert , Joyodiwiryo Yoko , Stevens Scott
Published: February 2008

Much of the tectonic complexity displayed in eastern Indonesia results from a series of Neogene collision events between island arcs, continental fragments, and the Australian continent. Here we examine the emplacement of a large ophiolite belt, resulting from the Miocene collision between the Sulawesi island arc and a continental fragment, the Sula platform. We present the results of several marine geophysical expeditions to the SW Molucca Sea and the NW Banda Sea, plus gravity and geology on the east arms of Sulawesi. The Batui thrust separates the ophiolite from sedimentary rocks deformed along the leading edge of the Sula platform. We mapped this thrust eastward from Sulawesi along the southern margin of the Gorontalo basin. The latter is floored by oceanic crust, and its south edge is uplifted against the thrust. Thus the Sulawesi ophiolite can be traced offshore to its origin as basement of the Gorontalo basin. The ophiolite is composed of harzburgite in the Southeast Arm and passes upward through a complex of gabbros and diabase dikes in the East Arm. Ophiolite melange underlies the harzburgites on the Southeast Arm beneath low‐angle thrust contacts where seen. Our local observations show the melange to be composed of thrust packets of both serpentine and red shale matrix varieties. The packets are several hundred meters thick, and the melange, where studied, has a moderate north to northeast dipping foliation. This orientation, if regionally representative of the melange fabric, is consistent with a significant northward component of movement of the lower plate, probably the Sula platform or its margin. Where the ophiolite is in contact with rocks of the central schist belt, it dips under the schist, but where it encounters melange, or Mesozoic or Paleogene sedimentary rocks, the ophiolite is thrust over them. The tectonic overlap sequence, from west to east, is schist over ophiolite over older sediments or melange. The ophiolite appears to have been emplaced by oblique convergence of the Sula platform along the southern edge of the Gorontalo basin. We suggest that the Gorontalo basin represents a forearc basin and the ophiolite is its basement, analogous to a number of other forearc settings. Deformation of the ophiolite may have occurred in part on the seafloor prior to emplacement, but we feel that much of the deformation occurred during emplacement. The Sulawesi ophiolite is only one of a number of ophiolites in the Indonesian region, each of which has a very different origin and tectonic history.
An upper bound on the radius of a highly electrically conducting lunar core
Authors: Hobbs B. A. , Hood L. L. , Herbert F. , Sonett C. P.
Published: February 2008

Parker's [1980] nonlinear inverse theory for the electromagnetic sounding problem is converted to a form suitable for analysis of lunar day‐side transfer function data by (1) transforming the solution in plane geometry to that in spherical geometry; and (2) transforming the theoretical lunar transfer function in the dipole limit to an apparent resistivity function. The theory is applied to the revised lunar transfer function data set of Hood et al. [1982a] which extends in frequency from 10−5 to 10−3 Hz. On the assumption that an iron‐rich lunar core, whether molten or solid, can be represented by a perfect conductor at the minimum sampled frequency, an upper bound of 435 km on the maximum radius of such a core is calculated. This bound is somewhat larger than values of 360–375 km previously estimated from the same data set via forward model calculations because the prior work did not consider all possible mantle conductivity functions.
Pieces of the ancient lunar crust: Ages and composition of clasts in consortium breccia 67915
Authors: Marti K. , Aeschlimann U. , Eberhardt P. , Geiss J. , Grögler N. , Jost D. T. , Laul J. C. , Ma M.‐S. , Schmitt R. A. , Taylor G. J.
Published: February 2008

We discuss the composition and chronology of clasts representing three minor lithologies in consortium breccia 67915. The lithologies studied are: sodic ferrogabbro, pristine troctolitic anorthosite (67915,26), and granulitic troctolitic anorthosite (67915,67). These lithologies were presumed to represent samples of the ancient lunar crust. The pristine troctolitic anorthosite, a typical member of the pristine anorthosite suite, contains little or no trapped liquid consistent with its two‐phase mineralogy. The plagioclase separates have 39Ar‐40Ar plateau ages of 4.10±0.06 b.y., and a final rise in the age pattern may represent a ‘memory’ of the earlier evolution. The granulitic troctolitic anorthosite, which has 13 times larger REE abundances, displays a continuous rise of the 40Ar/39Ar ratio. The last 25% of the 39Ar release data establish a minimum age of >3.5 b.y., but the 244Pu/Nd chronology suggests an age of ∼4.3 b.y. The sodic ferrogabbro samples, which appear to be members of the Mg‐gabbronorite group of pristine rocks, show large Ar losses but indicate plateaus in the low‐temperature data, suggesting a metamorphic event more recently than 260 m.y. ago, a time that is consistent with the North Ray impact event 50 m.y. ago.
Chondrules and other components in C, O, and E chondrites: Similarities in their properties and origins
Authors: Scott Edward R. D. , Taylor G. Jeffrey
Published: February 2008

We describe three types of chondrules that are common to H3, LL3, CM2, CO3, and CV3 chondrites. Low and high‐FeO, porphyritic olivine chondrules contain olivine with Fa0.3–8 and Fa5–50 respectively, and can easily be distinguished petrographically. Poikilitic pyroxene chondrules have 1–20 vol % olivine (Fa0.2–8) enclosed by low‐Ca pyroxene (Fs0.5–7), and also occur in E chondrites. These three types formed in separate nebular regions which had dimensions and spacings such that a few percent of the chondrules that collided to form compound chondrules were of different types. Sorting of chondrule precursor material and mixing of chondrule types probably account for most variations in bulk and mineral chemistry among chondrite groups. Metallic Fe,Ni grains containing 0.1–1% Cr, Si, and P are present in low‐FeO olivine chondrules from all type 2 and least metamorphosed type 3 chondrites. Metal compositions reflect reduction during chondrule formation in the nebula, not nebular condensation. Opaque matrices in type 3 ordinary and carbonaceous chondrites are somewhat similar in composition and mineralogy, and probably have related origins. We conclude that chondrules in all known chondrite groups share similar nebular origins.
A partially molten magma ocean model
Authors: Shirley David N.
Published: February 2008

The properties of the lunar crust and upper mantle can be explained if the outer 300–400 km of the moon was initially only partially molten rather than fully molten. The top of the partially molten region contained about 20% melt and decreased to 0% at 300–400 km depth. Nuclei of anorthositic crust formed over localized bodies of magma segregated from the partial melt, then grew peripherally until they covered the moon. Throughout most of its growth period the anorthosite crust floated on a layer of magma a few km thick. The thickness of this layer is regulated by the opposing forces of loss of material by fractional crystallization and addition of magma from the partial melt below. Concentrations of Sr, Eu, and Sm in pristine ferroan anorthosites are found to be consistent with this model, as are trends for the ferroan anorthosites and Mg‐rich suites on a diagram of An in plagioclase vs. mg in mafics. Clustering of Eu, Sr, and mg values found among pristine ferroan anorthosites are predicted by this model.
The geology of Tethys
Authors: Moore Jeffrey M. , Ahern Judson L.
Published: February 2008

The surface of the Saturnian satellite Tethys has been recognized to consist of four major landforms or physiographic provinces: hilly cratered terrain, the Odysseus impact structure, rifts and lineaments (including Ithaca Chasma), and plains terrain. Two alternate scenarios for the geological evolution of the satellite are developed. The first model assumes that Ithaca Chasma and the other rifts and lineaments were created by the Odysseus impact event. The second model assumes that rifting was a result of the freeze‐expansion of Tethys' liquid water interior. In both models, the hilly cratered terrain is postulated to be the oldest unit and the plains terrain, the youngest.
Nuclear track and compositional studies of olivines in CI and CM chondrites
Authors: Goswami J. N. , Macdougall J. D.
Published: February 2008

A small fraction of the olivines in CI and CM chondrites contain solar flare tracks. There are no major compositional differences between the irradiated and nonirradiated olivines, suggesting that they form a single population. Solar flare irradiated grain aggregates and chondrules also occur in CM chondrites. The grain aggregates were irradiated as entities, indicating that they formed prior to solar flare irradiation. Angular distribution data for tracks in irradiated olivine grains suggest a single stage exposure to solar flare radiation in most cases. The combined solar flare track data for olivine grains, grain aggregates, and chondrules require that their irradiation took place with variable shielding and are compatible with the model of Goswami and Lal [1979] in which irradiation took place in the near‐surface regions of bodies up to a meter in size. However, they also require partial disaggregation and mixing of these small bodies before or during agglomeration to form larger (≳km) objects. Irradiation, fragmentation, and redistribution of CI and CM components in their parent body regoliths was probably minimal.
The Manicouagan Impact Structure: An analysis of its original dimensions and form
Authors: Grieve Richard A. F. , Head James W.
Published: February 2008

Manicouagan, Quebec (51°23'N; 68°42'W) is the most intensively studied large complex terrestrial impact structure in a predominantly crystalline target. The ground truth data available from Manicouagan have considerable potential for interpreting the subsurface characteristics of lunar impact structures of comparable morphology and size. Two contrasting hypotheses, however, exist for the preerosional form of Manicouagan: (1) a multi‐ring basin with a final diameter of 75 km and a transient cavity diameter of 30–45 km [Floran and Dence, 1976], and (2) an endogenically modified peak‐ring basin with a final diameter of 100 km and a transient cavity diameter of 80 km [Orphal and Schultz, 1978]. Both hypotheses are based largely on topographic data and interpret the prominent 65 km diameter annular moat as a graben‐like feature formed either (1) by collapse of the outer slope of the transient cavity rim or (2) by uplift of the final crater floor through post‐impact intrusive activity.

The present analysis suggests that this annular moat is primarily an erosional feature and that the original form of the impact structure cannot be determined unequivocally on the basis of present topography. If all the available topographic, geologic, and geophysical data are considered, however, then an internally consistent interpretation of original dimensions is possible. The distribution of shock deformation effects in the basement rocks of the present crater floor suggests an original transient cavity diameter of 60 km. The 55 km‐diameter, impact melt‐covered inner plateau provides a minimum estimate for the diameter of the final floor of the crater and the annular moat, with its outliers of downdropped Ordovician limestone, is interpreted as marking the contact between the floor and the innermost slump blocks of the final rim. There is no compelling evidence to regard the annular moat as a graben. By analogy with fresh lunar and mercurian structures, a floor width of 55 km suggests a final rim diameter of 86–95 km. This is consistent with the topographic data from outside the annular moat and with the residual peripheral gravity low, which suggests a final rim diameter of 85–95 km. Due to erosion, it is difficult to assign Manicouagan to a particular morphological class of impact structure, but on the basis of the available data it is most likely that the preerosional form was that of a central peak crater or possibly a peak‐ring basin. At the time of its formation, Manicouagan may have been dimensionally and possibly morphologically similar to the 96 km diameter lunar crater Copernicus.
Primordial retention of nitrogen by terrestrial planets and meteorites
Authors: Fegley Bruce
Published: February 2008

Thermodynamic calculations of the amount of dissolved nitrogen in Fe‐Ni alloy in a solar composition gas were done over a wide range of temperatures and pressures. The stabilities of the nitride minerals found in meteorites, a large number of other nitrides, and ammonium aluminosilicates were also calculated in a solar gas. This thorough study indicates that equilibrium mechanisms cannot account for the nitrogen contents of the terrestrial planets and meteorites. The best available data indicate that the observed nitrogen contents of planets and meteorites are several orders of magnitude greater than the predicted nitrogen contents of condensed material in a solar gas. It is suggested that the nitrogen in meteorites was originally retained as organic compounds produced by disequilibrium mechanisms. Nitrogen retention by the terrestrial planets could be due to homogeneous accretion or to accretion of a volatile‐rich veneer. However the actual mode of nitrogen accretion cannot yet be determined.
Induced seismicity in The Geysers Geothermal Area, California
Authors: Eberhart‐Phillips Donna , Oppenheimer David H.
Published: February 2008

A simultaneous inversion for hypocenters, velocities, and station delays was performed from the P arrival times of 55 microearthquakes and four explosions distributed throughout a 90‐ by 100‐km region centered at The Geysers geothermal area in northern California. The resulting one‐dimensional layered model shows velocity increasing with depth from 4.4 km/s at the surface to 5.9 km/s at 8 km and deeper. We relocated 7215 earthquakes which occurred between May 1975 and February 1982, using the new velocity model. These relocations show a significant spatial clustering near production wells in the steam field that was not apparent in earlier studies. We infer from the spatial and temporal pattern of seismicity that the seismicity at The Geysers is induced by geothermal production activities. With the expansion of power production by approximately 70% in 1979–1980, seismic activity has developed near the new production areas. Seismicity has also expanded northwestward of the present production area, although there are no active steam wells in this area. Statistical cross correlations between the number of earthquakes per month and both steam withdrawal per month and injection volume per month were calculated. For the wells studied, we find no consistent pattern of correlation between injection and seismicity. Likewise, we find little or no correlation between steam withdrawal and seismicity for steam wells in production longer than 7 years. Two mechanisms remain plausible for inducing seismicity at The Geysers. Volumetric contraction due to mass withdrawal may perturb the stress field and cause faulting in reservoir rock already near failure due to the regional stress field. Alternatively, aseismic deformation due to regional tectonic stress may be converted to stick‐slip(seismic) deformation due to an increase of coefficient of friction along fault surfaces, such as could occur from exsolution of dissolved silica into fractures. Neither mechanism would be likely to show strong correlation between production and seismicity rates. However, we expect seismicity to continue in The Geysers and for seismicity to appear in areas where new production occurs.
Attenuation and envelope formation of three‐component seismograms of small local earthquakes in randomly inhomogeneous lithosphere
Authors: Sato Haruo
Published: February 2008

A unified model is proposed for explaining the frequency dependence of Q−1 and the formation of velocity seismograms by a single scattering process. Adopting Birch's law and a linear correlation between P and S wave velocities, we statistically describe the inhomogeneous medium by one random fluctuation, which is spatially characterized by the exponential autocorrelation function. We calculate Q−1 for P and S waves on the basis of the Born approximation, supposing seismic waves attenuate due to scattering largely by rapidly fluctuating random structure. Resulting Qs−1 explains the observed frequency dependence well, when the mean square fractional fluctuation and the correlation distance are chosen to be 0.01 and 2 km, respectively. Nonspherical radiation due to a point shear dislocation and frequency dependent nonisotropic scattering including conversion between P and S waves complicate the seismograms. Taking those complexities into account, we synthesize the “envelopes” of three‐component velocity amplitudes of P and S coda waves by summing up energy singly scattered by distributed inhomogeneities. From numerical calculations, we found that the radial component of the P coda wave is excited even in the nodal direction of the P wave radiation mostly due to SP scattering occurring near the hypocenter. The P coda waves appearing on the transverse components are a complex mixture of PP, PS, and SP scattering. The S coda waves are excited mostly due to SS scattering in all directions, and appear in all three components; their polarizations are controlled by the radiation pattern even in the later part of S coda waves.
Local gravity anomalies produced by dislocation sources
Authors: Savage J. C.
Published: February 2008

Rundle (1978) and Walsh and Rice (1979) have shown that the change in the vertical component of gravity is proportional to uplift for a spherical source of dilatation and for slip on an infinitely long dip‐slip fault. In the first case, no free air gravity anomaly is produced and in the second case no Bouguer gravity anomaly. Gravity anomalies due to other dislocation sources in three dimensions are as follows: For strike‐slip faulting the ratio of the gravity change to uplift depends upon position; however, the gravity change contours are roughly similar to those corresponding to a zero free air gravity anomaly. Nor is the ratio constant for dip‐slip faulting except for the two special cases of dip slip on a vertical fault and horizontal slip on a horizontal fault, neither of which produce a Bouguer anomaly. The Bouguer anomaly produced by an open horizontal crack is the same as would be produced had the material within the crack been mined out without deforming the solid. If the horizontal crack were filled with material of density equal to the host rock (a good approximation to sill formation), no Bouguer anomaly is produced. For cracks of other inclinations the ratio of gravity change to uplift is not constant. Thus, dilatancy, in general, does not correspond to the absence of a free air anomaly, as might be suggested by the special case of a spherical source of dilatation. For two‐dimensional models a cylindrical source of dilatation produces no free air gravity anomaly, dip‐slip faulting produces no Bouguer anomaly, and open cracks produce a Bouguer anomaly equal to that which would be produced had the material within the crack been mined out without deforming the solid. A two‐dimensional crack filled with material of density equal to that of the host rock would produce no Bouguer anomaly. Jachens et al. (1983) have reported temporal changes in gravity, elevation, and area strain along the San Andreas fault in southern California such that the Bouguer anomaly apparently remains unchanged and the uplift‐to‐strain ratio is about −100 km. Several dislocation mechanisms are proposed that fulfill these constraints, but these mechanisms appear to be rather contrived and are not regarded as satisfactory explanations.
Patterns of earthquakes and the effect of solid earth and ocean load tides at Mount St. Helens prior to the May 18, 1980, eruption
Authors: McNutt S. R. , Beavan R. J.
Published: February 2008

Seismographs near Mount St. Helens Volcano recorded an earthquake swarm lasting nearly 2 months prior to the catastrophic May 18, 1980, eruption. The earthquakes are divided into four classes based on station CPW (Δ = 116 km) seismogram characteristics: (1) events with Sv:P amplitude ratio >3 and dominant frequency >3 Hz; (2) events with Sv:P ratio between 1 and 3 and dominant frequency >2 Hz; (3) events similar to characteristic 2 but with a strong (probably surface wave) phase just after the S phase; and (4) events with frequencies between 1 and 2 Hz lacking a clear S phase. The seismicity pattern for each of the four classes is unique, and we assume each group of events with similar wave‐forms represents a common physical process, depth, source area, or fault orientation. Maximum likelihood b values range between 0.97±0.05 and 1.81±0.09 for the four groups. We calculate solid earth stress and strain tides at the average hypocentral depth of 4 km. We also calculate stress and strain tides induced by ocean loading; their amplitudes are typically 20–40% those of the solid earth tides at the location of Mount St. Helens. A weak but significant correlation exists between the latter two classes of events and the tides for a time interval of about 5 days preceding the first onset of volcanic tremor and about 5 days thereafter. The polarity of the correlation is opposite for the two classes of events. In each case, the phase of the correlation changes systematically with time, the changes coinciding with the onset of tremor on March 31 and with a pronounced decrease in earthquake energy release rate on April 3. There are no significant correlations between the tides and the number of events or energy release of these two classes of earthquakes during any other interval between March 20 and May 18, 1980. The first two classes of events show no evidence of significant tidal correlation at any time during the study period.