bluuebunny/crossref_metadata_2025 huggingface dataset:
Nucleation and growth of faults in brittle rocks
Authors: Reches Ze'ev , Lockner David A.
Published: February 2004

We present a model for the nucleation and growth of faults in intact brittle rocks. The model is based on recent experiments that utilize acoustic emission events to monitor faulting processes in Westerly granite. In these experiments a fault initiated at one site without significant preceding damage. The fault propagated in its own plane with a leading zone of intense microcracking. We propose here that faults in granites nucleate and propagate by the interaction of tensile microcracks in the following style. During early loading, tensile microcracking occurs randomly, with no significant crack interaction and with no relation to the location or inclination of the future fault. As the load reaches the ultimate strength, nucleation initiates when a few tensile microcracks interact and enhance the dilation of one another. They create a process zone that is a region with closely spaced microcracks. In highly loaded rock, the stress field associated with microcrack dilation forces crack interaction to spread in an unstable manner and recursive geometry. Thus the process zone propagates unstably into the intact rock. As the process zone lengthens, its central part yields by shear and a fault nucleus forms. The fault nucleus grows in the wake of the propagating process zone. The stress fields associated with shear along the fault further enhance the microcrack dilation in the process zone. The analysis shows that faults should propagate in their own plane, making an angle of 20°–30° with the maximum compression axis. This model provides a physical basis for “internal friction,” the empirical parameter of the Coulomb criterion.
Lateral variations in D″ thickness from long‐period shear wave data
Authors: Kendall J.‐M. , Shearer P. M.
Published: February 2004

We explore global variations in D″ shear wave structure by examining 12 years of long‐period Global Digital Seismograph Network (GDSN) data to identify reflected phases from the D″ layer in the lowermost mantle. We restrict our search to epicentral distances between 63° and 74° where a precritical D″ reflection will lie between the S and ScS arrivals. With GDSN long‐period records a D″ signal at these ranges is generally obscured by the stronger neighboring phases, so to isolate the D″ reflections we develop a technique for stripping away the interfering S and ScS waveforms. We interpret travel time variations in the observed SdS phases in terms of variations in D″ thickness and generate maps of inferred D″ thickness for areas of good seismic coverage. The regions of observed D″ reflections are beneath Australasia, north central Asia, the Arctic, Alaska, and central America. We find considerable variation in D″ structure in these regions, a result consistent with previous studies which have found evidence for lateral heterogeneity in D″. On average, the estimated D″ thickness is 260 km, but there is a fairly uniform distribution of thicknesses between 150 km and 350 km. A possible correlation is observed between D″ thickness and regions of predicted strong horizontal mantle flow.
Present‐day vertical deformation of the Cascadia Margin, Pacific Northwest, United States
Authors: Mitchell Clifton E. , Vincent Paul , Weldon Ray J. , Richards Mark A.
Published: February 2004

We estimate present‐day uplift rates along the Cascadia Subduction Zone in California, Oregon, and Washington in the Pacific Northwest, United States, by utilizing repeated leveling surveys and tide gauge records. These two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast. Uplift rates are extended inland through east‐west leveling lines that connect the north‐south line along the coast to the north‐south line along the inland valleys just west of the Cascades. The results are summarized as a contour map of present‐day uplift rates for the western Pacific Northwest. We find that rates of present‐day uplift vary latitudinally along the coast and inland valleys, as well as longitudinally along transects connecting the coast to the inland valleys. Long‐term tidal records of Neah Bay, Astoria, and Crescent City indicate uplift of land relative to sea level of 1.6±0.2, 0.0±0.2, and 0.9±0.2 mm/yr, respectively (±1 standard error). Unlike previous estimates of relative sea level change at Astoria, we adjust for discharge effects of the Columbia River, including human management influences. After approximating an absolute framework by using 1.8±0.1 mm/yr to compensate for global sea level rise, results indicate that much of the western Pacific Northwest is rising at rates between 0 and 5 mm/yr. The most rapid uplift rates are near the coast, particularly near the Olympic Peninsula, the mouth of the Columbia River, Cape Blanco, and Cape Mendocino. Two axes of uplift are identified: one trends northeast from the southwest Oregon coast, and the other trends south‐southeasterly from the Olympic Peninsula to the Columbia River. The Puget Sound vicinity and a small east‐west region from the north central Oregon coast to the inland Willamette Valley are subsiding at rates up to 1 mm/yr. We interpret the overall pattern of rapid present‐day uplift to be generated by interseismic strain accumulation in the subduction zone. This interseismic elastic strain accumulation implies significant seismic hazard.
The Teton fault, Wyoming: Topographic signature, neotectonics, and mechanisms of deformation
Authors: Byrd John O. D. , Smith Robert B. , Geissman John W.
Published: February 2004

We integrated geophysical and geological methods to evaluate the structural evolution of the active Teton normal fault, Wyoming, and its role in the development of the dramatic topography of Teton Range and Jackson Hole. The Teton fault bounds the precipitous eastern front of the Teton Range and is marked by large, well‐preserved postglacial fault scarps that extend for 55 km along the range front. Comparison of variations in surface offsets with the topographic expression of the Teton range crest and drainage divide, and the overall structure of the range, suggests that the effects of postglacial faulting cannot be discriminated from the influence of pre‐extensional structures and differential erosion on the footwall topography. In contrast, the effects of multiple scarp‐forming normal faulting earthquakes are expressed by the anomalous drainage pattern and westward tilt of the hanging wall, Jackson Hole, toward the Teton fault. Kinematic boundary element fault models suggest that the westward tilt of the valley floor is the product of 110–125 m of displacement on a 45°–75°E dipping Teton fault in the past 25,000–75,000 years. Comparisons with historic normal faulting earthquake displacements imply that this range of displacement corresponds to 10–50, M > 7 scarp‐forming earthquakes. A total throw of 2.5 to 3.5 km across the Teton fault is suggested by inverse ray‐tracing and forward gravity models. These models also suggest that Laramide age structures have been offset across the Teton fault and obscure its geophysical signature but also continue to influence the structural and topographic expression of the footwall and hanging wall blocks. Paleomagnetic analyses of the ∼2.0 Ma Huckleberry Ridge Tuff suggest that the overall westward tilt of the Teton Range is a result of about 10° of west side down tilt across the Teton fault since tuff emplacement. This suggests that much if not all of the throw across the Teton fault has accumulated in the past 2 m.y. Complex demagnetization and rock magnetic behavior and local emplacement of the Huckleberry Ridge Tuff on preexisting topography preclude determination of the amount or variations in throw along strike of the Teton fault from the paleomagnetic data.
The Maras Triple Junction (southern Turkey) based on digital elevation model and satellite imagery interpretation
Authors: Chorowicz Jean , Luxey Pascal , Lyberis Nikos , Carvalho José , Parrot Jean‐François , Yürür Tekin , Gündogdu Niyazi
Published: February 2004

This paper describes major structures in the area of the triple junction of the African, Anatolian and Arabian plates near Maras (southern Turkey). A review of the literature on the paleogeographic evolution since mid‐Cretaceous time leads to the idea of the southern Tethys (Mesogea) persisting between Anatolia and Africa‐Arabia until middle‐late Miocene. Consequently, collision may have started later and northward subducting lithosphere may still exist under Anatolia. The Dead Sea fault may have initiated near the first collision between the Anatolian continent and the western corner of the African/Arabian continent at which point it may have propagated southward. A digital elevation model, SPOT and Landsat‐MSS images, and field work suggest relief js mainly related to finite deformation resulting from on going collision that began at around 13 Ma. The images show that the East Anatolian fault does not crosscut the central Taurus belt. Field data argue that this belt is a left‐lateral oblique‐slip fault zone. The southern contact of Anatolia passes north of the Amanos range (continental par t of the African plate) and connects with the Misis range. To the southwest, the Adana low plain may be regarded as a basin formed at a releasing bend. The Karasu fault zone, which belongs to the Dead Sea fault zone, seems to be transpressive, and in this case the Karasu basin is not a graben. North of the Amanos range, the Giirun arc is thrust to the north and may be considered the result of local collision of the uplifting Amanos range into Anatolia. The Maras triple junction is accompanied by deformations affecting all plates at the regional scale.
Moho geometry beneath the eastern margin of the Andes, northwest Argentina, and its implications to the effective elastic thickness of the Andean foreland
Authors: Whitman Dean
Published: February 2004

The seismically constrained shape of the Moho beneath the eastern margin of the northwest Argentine Andes (24°S) is interpreted in the context of regional isostasy to constrain the flexural rigidity of the lithosphere in this actively deforming orogen. Seismic travel time residuals from regional intermediate depth earthquakes are inverted for three‐dimensional variations in the Moho depth beneath a portable seismic network spanning the eastern margin of the Puna plateau and the Santa Barbara foreland system. Beneath the easternmost Santa Barbara ranges, the Moho is relatively flat and lies at 40–45 km depth. Farther west, beneath the eastern margin of the Puna, the Moho dips 10–15° to the west before flattening out at a depth of 60–65 km beneath the plateau. The Moho inversion model and coincident gravity data constrain the flexural rigidity of the foreland lithosphere to be between 1021 and 1022 m. The corresponding effective elastic thickness of 6–12 km is a factor of 2 to 4 less than that estimated in previous published studies for the Bolivian Altiplano and Subandean ranges farther to the north. These results indicate that in contrast to the Bolivian Andes, the foreland lithosphere in northwest Argentina does not provide significant flexural support to the plateau margin, and the eastern margin of the Andes is nearly in a state of local isostatic compensation. The north to south decrease in the effective elastic thickness of the Andean lithosphere supports published seismic wave attenuation studies which indicate that south of 23°S the lithospheric thickness decreases beneath the plateau and foreland.
Seismic guided waves trapped in the fault zone of the Landers, California, earthquake of 1992
Authors: Li Yong‐Gang , Aki Keiiti , Adams David , Hasemi Akiko , Lee William H. K.
Published: February 2004

A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M7.4 Landers earthquake of June 28, 1992, with a maximum offset of 1 km from the trace. We found a distinct wave train with a relatively long period following the S waves that shows up only when both the stations and the events are close to the fault trace. This wave train is interpreted as a seismic guided wave trapped in a low‐velocity fault zone. To study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone, we eliminated source and recording site effects by the coda normalization method. The normalized amplitudes of guided waves show a spectral peak at 3–4 Hz, which decays sharply with distance from the fault trace. Spectral amplitudes at high frequencies (8–15 Hz) show an opposite trend, increasing with distance from the fault trace. The normalized amplitudes of guided waves at 3–4 Hz also show a systematic decrease with hypocentral distance along the fault zone, from which we infer an apparent Q of 50. In order to confirm the existence of the guided waves, a dense array of 31 stations was deployed at one of the 11 sites. The resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone. By modeling the waveforms as S waves trapped in a low‐velocity waveguide sandwiched between two homogeneous half‐spaces with velocity Vs = 3.0 km/s, we infer a waveguide width of about 180 m, a shear velocity of 2.0–2.2 km/s, and a Q of ∼50. Hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low‐velocity fault zone in three dimensions. We find that the zone extends to a depth of at least 10 km. This zone apparently continues to the south across the Pinto Mountain fault because guided waves are observed at stations north of the Pinto Mountain fault for earthquakes with epicenters south of it. On the other hand, the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter; guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides.
Stratigraphic architecture of extensional basins: Insights from a numerical model of sedimentation in evolving half grabens
Authors: Travis Christopher J. , Nunn Jeffrey A.
Published: February 2004

We present a numerical model for the tectonostratigraphic evolution of half grabens which duplicates many of the stratigraphic relationships observed in extensional terranes of the southwestern United States and elsewhere. The model also generates insights and predictions on the impact of fault geometry, rates of faulting, and rates of erosion and sediment transport on landscape evolution, stratal geometry, and facies distributions. Three processes control basin development in the model: faulting, erosion/deposition, and isostasy. Assumptions are thin plate flexure, normal fault hanging wall deformation by vertical simple shear, linear dependence of sediment flux on slope, and perfect sorting of sediment during deposition. Model limitations reflect oversimplification of the dynamics of sediment erosion and redistribution. During periods of fault motion, half grabens bounded by listric faults accumulate strongly asymmetric sedimentary units. The degree of under‐ or overfilling of the basin is controlled in the short term by basin geometry (fault dip) and in the longer term by extension rates, sediment supply, and out‐of‐plane sediment transport. After fault motion ceases, erosion and sedimentation redistribute the loads on the lithosphere, resulting in progressively more symmetric sedimentary units which overlap the original basin boundaries. Model facies architecture is affected by changing subsidence patterns, stratigraphic geometries, and sediment flux. Under most conditions, syntectonic gravels are trapped near the basin edge on the fault‐bounded side but prograde far out into the basin on the hinge (opposite the fault) side. When fault movement ceases, coarse‐grained sediment from the fault‐bounded side may initially be able to reach farther out into the basin, but overall retrogression is observed on both sides of the basin as topography is eroded and as posttectonic sedimentation buries former sediment sources. Changing fault dips and lithospheric rigidity affect facies patterns, whereas altering the ratio between rates of erosion and sediment transport affects landform development and depositional geometries.
Paleomagnetism of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite, southern Laramie Range, Wyoming and Colorado
Authors: Harlan Steve S. , Snee Lawrence W. , Geissman John W. , Brearley Adrian J.
Published: February 2004

We present the results of a combined paleomagnetic and 40Ar/39Ar geochronologic investigation of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosite complex yield a well‐defined characteristic magnetization of northeast declination (D) and moderate negative inclination (I), although antipodal normal and reverse polarity magnetizations are present at three sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6°, I = −48.7° (k = 77.4, α95 = 3.1°). Alternating field (AF) and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low‐Ti titanomagnetite of single or pseudo‐single domain character that occurs as elongate to rod‐shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization that is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1°, I= −48.1°, k = 46.5, α95 = 7.6°, n = 8/14 sites); rock magnetic data indicate the primary carrier of remanence in Sherman Granite is magnetite. The 40Ar/39Ar geochronologic data from Sherman Granite hornblende, biotite, and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 Ma. Microcline data indicate that the Laramie anorthosite complex and Sherman Granite have probably not been thermally remagnetized. Paleomagnetic poles from the Laramie anorthosite complex and Sherman Granite are indistinguishable at the 95% confidence level, and individual virtual geomagnetic poles (VGPs) from both units are combined to provide a mean pole at 215.0°E, 6.7°S (K = 46.9, A95 = 3.5°, N = 37 VGPs). The location of this pole is similar to paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsewhere in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sibley Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declination and shallow negative inclination (D = 154.9°, I = −16.0°, k = 90.6, α95 = 9.7°, n = 4 sites). This secondary magnetization was probably acquired during late Paleozoic time.
Mapping the lowermost mantle using core‐reflected shear waves
Authors: Wysession Michael E. , Bartkó Lilla , Wilson Joyce B.
Published: February 2004

A map of laterally varying D″ velocities is obtained for the region from 50°S to 50°N in latitude and 70°E to 190°E in longitude. Velocities are found using an analysis of the differential travel time residuals from 481 ScS‐S and 266 sScS‐sS phase pairs. The long‐period data are taken from the Global Digital Seismograph Network digital waveform catalog for the time period of January 1980 to March 1987. Each differential travel time is found by a cross correlation of the S phase ground displacement, corrected to simulate differential attenuation, with all following phases. Travel times are corrected for ellipticity and mantle heterogeneity outside of their D″ paths, and the remaining residuals are interpreted as the result of D″ heterogeneity. Ray‐tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies.The resulting map reveals significant long‐wavelength D″ structure including a 3% low‐velocity region beneath northeastern Indonesia, surrounded by three identified high‐velocity zones beneath northwestern Pacifica (+4%), Southeast Asia (+3%), and Australia (+3–5%). This structure is of continent/ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle. The low‐velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature D″ rock which has been heated by the core has become gravitationally unstable and begun to rise. A chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site, much the way continents are dragged toward subduction zones. The high‐velocity zones possibly result from the downward convection of cold lower mantle plumes, which pond at the core‐mantle boundary. These seismic anomalies may also contain a chemical signature from faster iron‐poor materials brought down through the lower mantle or the additional presence of SiO2 stishovite, perhaps in its higher‐pressure polymorph.
Extension in the northern Mariana inner forearc
Authors: Wessel Jill K. , Fryer Patricia , Wessel Pål , Taylor Brian
Published: February 2004

Seismic reflection, SeaMARC II acoustic imagery, and bathymetric data across the northern Mariana island arc reveal dense, high‐angle, normal faulting of the arc and inner forearc. Faults, with throws up to 780 m, cut a thick sequence of stratified but otherwise undeformed sediments. The major faults display a sinuous or zigzag trace that is commonly associated with rifting. This pattern can be indicative of a single event of triaxial strain that requires two contemporaneous conjugate fault pairs in orthorhombic symmetry. Interpretation and statistical analyses of the data show activity since the last sedimentary deposition and concurrent fault development of two distinct fault populations, one associated with the backarc basin's east boundary fault and the other with extension in the forearc. Extensional faults along the eastern side of the backarc basin, near the active arc at 22°N where active rifting is occurring, have approximately the same trend as the east boundary fault in the survey area. We estimate the direction of extensional (minimum principal) strain for the east boundary fault at 22°N to be 39°±1°, orthogonal to the extensional strain direction measured in the inner forearc. The change in strain is distinct and abrupt, occurring across a 20‐km‐wide region along the arc massif. Within this narrow region of transition, the intermediate and minimum principal strain directions are nearly equal in magnitude with orientations midway between the dominant forearc and backarc basin principal strain directions. Statistical analyses show that the strain from either backarc or forearc regime is not transmitted across this narrow region; the two opposing strain regimes coexist virtually juxtaposed. The coexistence of these two orthogonal, extensional fault regimes within the overriding plate between the Mariana backarc basin and inner forearc may be explained by assuming that a weak, transitional region exists along the east boundary fault zone where stresses can be more easily relieved. This allows active basin‐wide extension in the backarc basin, while the forearc data are consistent with models that require radial forearc deformation resulting from increasing arc‐trench curvature of the northern half of the Mariana island arc.
Phase boundary deflections at 660‐km depth and episodically layered isochemical convection in the mantle
Authors: Solheim Larry P. , Peltier W. R.
Published: February 2004

Numerical simulations have been performed using a multiphase, anelastic, axisymmetric spherical, mantle convection model as part of an ongoing effort to explore the ability of the endothermic phase transition at 660 km depth to cause the circulation to assume a layered style. In particular, model solutions have been constructed for a Rayleigh number of 107, internal heating corresponding to 50% heating from within and 50% heating from below, and Clapeyron slopes for the 410‐km and 660‐km phase boundaries set to +3.0 and −2.8 MPa/K, respectively. In this regime the flow exhibits a substantial degree of radial layering wherein the radial mass flux is reduced significantly at 660‐km depth. This layered regime of flow is episodically disrupted by massive localized avalanches of fluid across the 660‐km boundary Uiat recur at intervals separated by hundreds of millions of years. The degree of layering is related to the magnitude of the 660‐km phase boundary deflection away from its average depth. In these Earth‐like simulations we find that the average magnitude of such phase boundary deflections is similar to the average magnitude of seismically observed deflections of this horizon.
Precise P and S wave velocity structures in the Kitakami Massif, Northern Honshu, Japan, from a seismic refraction experiment
Authors: Iwasaki Takaya , Yoshii Toshikatsu , Moriya Takeo , Kobayashi Akio , Nishiwaki Makoto , Tsutsui Tomoki , Iidaka Takashi , Ikami Akira , Masuda Tetsu
Published: February 2004

The Kitakami massif, which is located in the eastern part of Northern Honshu, Japan, is composed of two geological units. The northern Kitakami terrane is characterized as a Jurassic accretionary complex, while the southern Kitakami terrane consists of pre‐Silurian basement and Silurian‐lower Cretaceous marine sediments. The boundary region of these two units, called the Hayachine tectonic belt (HTB), is composed of mafic to ultramafic rocks. The Kitakami massif experienced intense granitic intrusions in the Cretaceous. We present a detailed crustal structure model for the eastern part of the massif derived from an extensive seismic refraction experiment conducted on a 194‐km N‐S line. The uppermost crust is covered with a very thin (0.5–1 km) surface layer with a velocity of 3.1–5.4 km/s. The velocity structure below this layer shows remarkable lateral variation. In the northern Kitakami terrane the P wave velocity and Vp/Vs at the top of the basement are 5.85–5.95 km/s and 1.68–1.70, respectively. The seismic attenuation in this region is high (Qp = 150–200 and Qs = 70–100). In contrast, the uppermost crust in the southern Kitakami terrane is characterized by a high P wave velocity (6.05–6.15 km/s) and Vp/Vs (1.74–1.77). The Qp and Qs also show high values of 300–400 and 150–200, respectively. Such a structural difference persists to 14‐to 16‐km depth, at which the P wave velocity increases to 6.45 km/s. The low velocity and high attenuation in the northern Kitakami terrane represent a highly deformed structure of the accretionary complex. The high P wave velocity and Vp/Vs in the southern Kitakami terrane indicate the relatively mafic crustal composition, which may result from the fragment of the oceanic crust incorporated by the accretion process or the uplifting in the latest Jurassic‐early Cretaceous. A midcrustal interface determined from wide‐angle reflections shows an abrupt southward depth decrease from 25 to 20 km under the HTB. The P wave velocity and Vp/Vs between 14‐ and 16‐km depth and the midcrustal interface are 6.45–6.55 km/s and 1.74–1.78, respectively. The Moho depth under the northern Kitakami terrane decreases southward from 34 to 32 km. In the southern Kitakami terrane the Moho dips slightly southward. The P wave velocity and the Vp/Vs ratio in the lower crust are 6.9–7.0 km/s and 1.75–1.76, respectively. The P wave velocity in the uppermost mantle is not well resolved but is probably less than 7.7 km/s. The S wave velocity derived from relatively clear Sn is 4.35–4.40 km/s. Our results show that the HTB is a prominent structural boundary extending to the Moho. The crust of Kitakami massif was not homogenized by the Cretaceous granitic intrusions, and the original structural difference remains in the upper crust.
Introduction to Special Section on Tectonics and Topography
Authors: Merritts Dorothy , Ellis Michael
Published: February 2004

This special section on tectonics and topography developed from an American Geophysical Union Chapman Conference convened in the late summer of 1992 in Snowbird, Utah. The intent of the gathering was to assemble a diverse group of Earth scientists working on ultimately the same problem: the interaction between crustal and surficial processes or, euphemistically, tectonics and topography. Through numerous and enthusiastic conversations, it became clear to us prior to the Chapman Conference that many people were working on the interaction between tectonics and topography but about half of these were either unaware of the others or were aghast at the way in which the others simplified their subject (and vice versa for the other half!). To our delight, the conference brought forth a wonderful array of scientists from virtually all subdisciplines of geology, and the desire for mutual help and a sympathetic ear was palpable.
Deformation across the western United States: A local estimate of Pacific‐North America transform deformation
Authors: Humphreys Eugene D. , Weldon Ray J.
Published: February 2004

We obtain a locally based estimate of Pacific‐North America relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern North America from east of the Rio Grande Rift to near the continental escarpment. Data are primarily Quaternary geologic slip rate estimates, and resulting deformation determinations therefore are “instantaneous” in a geologic sense but “long term” with respect to earthquake cycles. We deduce a rate of motion of the Pacific plate relative to North America that is 48±2 mm/yr, a rate indistinguishable from that predicted by the global kinematic models RM2 and NUVEL‐1; however, we obtain an orientation that is 5–9° counterclockwise of these models. A more westerly motion of the Pacific plate, with respect to North America, is calculated from all three paths. The relatively westerly motion of the Pacific plate is accommodated by deformation in the North American continent that includes slip on relatively counterclockwise‐oriented strike‐slip faults (including the San Andreas fault), which is especially relevant in and south of the Transverse Ranges, and a margin‐normal component of net extension across the continent, which is especially relevant north of the Transverse Ranges. Deformation of the SW United States occurs in regionally coherent domains within which the style of deformation is approximately uniform. In the vicinity of the Transverse Ranges, two important shear systems splay from the San Andreas fault: the eastern California shear zone trending NNW from the eastern Transverse Ranges and the trans‐Peninsular faults trending SSE from the western and central Transverse Ranges. Within the Transverse Ranges the right‐lateral San Andreas fault steps left, seemingly requiring large amounts of convergence there. However, most of this convergence is avoided through a “funneling flow” of the crust toward the western Transverse Ranges and into the relatively narrow central California Coast Ranges and the northern motion of the Mojave. The former process involves an alternation of rotation direction from counterclockwise (in and south of the central Transverse Ranges) to clockwise (in the western Transverse Ranges).
Metamorphic evolution of the Kamuikotan high‐pressure and low‐temperature metamorphic rocks in central Hokkaido, Japan
Authors: Sakakibara Masayuki , Ota Tsutomu
Published: February 2004

The Kamuikotan complex in the Sorachi‐Yezo belt, located in central Hokkaido, Japan, has been extensively subjected to subduction‐related high‐pressure and low‐temperature metamorphism. On the basis of geological, petrological, and geochronological data, the coherent metamorphic rocks are divided into six units as follows: Susunai, Horokanai, Harushinai, Biei, Pankehoronai, and Shizunai. Based on their metamorphic mineral sequences, the six units can be classified into the following three baric types: high‐pressure 1 (HP1), high‐pressure 2 (HP2), and high‐pressure intermediate (HI). The Horokanai and Shizunai units belong to the HP1 type, the Harushinai and Biei units belong to the HP2 type, and the Susunai and Pankehoronai units belong to the HI type. Metabasites of the HP1 type are defined by the association of lawsonite + Na amphibole or epidote + Na amphibole (glaucophane or crossite). The HP2 type is characterized by the assemblage of pumpellyite + Na pyroxene + chlorite, and the representative low‐variance asssemblages are lawsonite + pumpellyite + Na pyroxene + chlorite and pumpellyite + Na pyroxene + Na amphibole + chlorite. In the HI type, the most common low‐variance assemblage is pumpellyite + epidote + actinolite + chlorite. These results, together with the previously published K‐Ar and 40Ar‐39Ar ages, suggest that the Kamuikotan metamorphic rocks gradually decreased in pressure during the Cretaceous. This tendency indicates an increase in the Kamuikotan subduction geotherm during Early Cretaceous to early Eocene time. The increase in the paleogeotherm may result from the change of subduction rates and subducting slab ages.
Elastic constants and anelastic properties of an anisotropic periclase sphere as determined by the resonant sphere technique
Authors: Oda Hitoshi , Isoda Satoru , Inouye Yasushi , Suzuki Isao
Published: February 2004

The resonant sphere technique (RST) is applied to measurements of elastic and anelastic properties of single‐crystal periclase (MgO) in order to demonstrate the potential value of this method as an experimental technique. The lowest 22 resonant frequencies and their attenuation properties were measured in the range 1.0 to 2.6 MHz. The oscillation modes were identified by comparing the observed resonant frequencies with computed ones. Using inversion theory, we find perturbations from an assumed set of elastic constants, so that the computed frequencies agree, in a least squares sense, with the measured ones. The final values of the elastic constants were obtained by adding the perturbations to the initial values of elastic constants. Furthermore, the internal friction coefficients were determined from the observed vibrational attenuation data by a least squares method. The values of elastic constants and internal friction coefficients are in good agreement with those determined by other methods. We find better accuracy, by up to 1 order of magnitude, than that found by using the rectangular parallelepiped resonance method. The RST can be an accurate method for measuring elastic and anelastic properties of small anisotropic materials. An applicability of the RST to the Earth's free oscillation is considered. Free oscillation of an elastically anisotropic sphere shows different frequency spectrum, depending on the degree of the elastic anisotropy. The frequency spectrum of an anisotropic sphere can be interpreted as the result of the split of degenerate modes of an isotropic sphere. Therefore the anomalous splitting of the Earth's free oscillation, which cannot be explained by the rotation and ellipticity of the Earth, may be attributed to the elastic anisotropy of the Earth.
Temperature‐induced 4He degassing of specularite and botryoidal hematite: A 4He retentivity study
Authors: Bähr Roland , Lippolt Hans J. , Wernicke Rolf S.
Published: February 2004

Degassing of radiogenic 4He induced by stepwise heating in a vacuum was carried out on various grain size fractions of specular and botryoidal hematite in order to place constraints on their 4He retentivities. Samples from the Vosges, Odenwald, Saarbergland, and Cumbria were used. Below 900°C, specularite yields straight lines in Arrhenius plots. This is consistent with volume diffusion. The diffusion distances correspond with the macroscopic mean grain size. By contrast, botryoidal hematite yields curved lines which are straight only below 250°C. The actual diffusion distances appear to be commensurate with the microscopic size of the constituent crystals (0.1 to 10 μm). The diffusion coefficients of specular (D20) and botryoidal hematite (D20/r2) are 10−28 cm s−1 and 10−22 s−1, and the activation energies range from 110 to 130 and 130 to 210 kJ/mol, respectively. Helium 4 closure temperatures (Tc) of specularite grains with diameters of about 5 mm are above 200°C, and for all grain fractions in botryoidal hematite >10 μm the Tc is above 90°/160°C. On the basis of Tc, both hematite varieties should be useful thermochronometers. Low‐temperature 4He losses of the order of a few percent during a few 100 m.y. are conceivable for botryoidal hematite.
Deep structure of Japan subduction zone as derived from local, regional, and teleseismic events
Authors: Zhao Dapeng , Hasegawa Akira , Kanamori Hiroo
Published: February 2004

We have determined a detailed three‐dimensional P wave velocity structure of the Japan subduction zone to 500‐km depth by inverting local, regional, and teleseismic data simultaneously. We used 45,318 P wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the Japan Islands. The arrival times are recorded by the Japan University Seismic Network which covers the entire Japan Islands densely and uniformly. We also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern Japan. In comparison with the previous results obtained from only local and regional events, the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes. In the crust and uppermost mantle, low‐velocity zones are clearly visible beneath active volcanoes. In the mantle wedge the low‐velocity zones generally parallel with the slab and exist continuously to a depth of about 200‐km, which is consistent with the petrological, geochemical and geodynamic studies. We consider that the existence of volcanism‐related low‐velocity anomalies in the mantle wedge is a general Seismological characteristic of subduction zones, in light of all the available tomographic results for many subduction zones in the world. The Pacific slab beneath Japan is imaged more clearly than in previous studies as a high‐velocity zone with a thickness of 80–90 km and a P wave velocity 4–6% higher than the normal mantle. Lower velocity anomalies are found in the mantle below the slab.
The finite extension of fractal geometry and power law distribution of shallow earthquakes: A geomechanical effect
Authors: Volant Philippe , Grasso Jean‐Robert
Published: February 2004

We study a dome structure (10×10×10 km3) at an intermediate scale between laboratory analyses and tectonic processes. Local gas extraction induced about 1000 events within the dome (1.0 ≤ M1 ≤ 4.2) recorded by a local network during 19 years (1974–1992). Two types of autosimilarity coefficients (b value and correlation dimension) are analyzed in three‐dimensional (3‐D) space. The hypocenter distribution shows a fractal pattern characterized by the noninteger value of the correlation dimension. Moreover the frequency‐magnitude relation of the events obeys a power law. The existence of these two parameters shows that the spatial distribution of the earthquakes induced by the Lacq gas extraction is governed by a nonrandom behavior. We observe no temporal correlation between the temporal behavior of the b value (slope of frequency‐magnitude relation) and D (correlation dimension). Three‐dimensional fractal analysis of induced earthquakes allows us to define two distinct classes of events separated by a critical distance of 500 m. The first class (r > 500 m) shows a diffuse seismicity. This diffuse class of earthquakes (M1 < 3.0) follows the frequency‐magnitude relation. The low number of events prevent us from analyzing whether there is a fractal or random behavior. The second class (r < 500 m) defines nests of seismicity and attests a bifractal character (r < 500 m, D1 ≈ 2.4; r > 500 m, D2 ≈ 1.3). The difference of one unit between the fractal dimension of the seismicity within the nests (D1) and the fractal dimension of the nests distribution (D2) suggests an influence of the geological dome structure on the spatial development of seismic nests. Moreover, a slope break above M1 = 3.0 (G‐R relation) is observed on this second class (1.0 ≤ M1 ≤ 4.2). The slope break of both the b value and the fractal dimension at a common threshold (M1 ≈ 3.0 is equivalent to a 500‐m fracture size) suggests a critical distance for the brittle behavior of the uppermost crust as proposed for tectonic earthquakes by Scholz (1991). Such a critical distance correlates in our study with the maximum thickness of local seismogenic layers (brittle calcareous layer versus ductile marly layer). On this basis we propose that (1) the finite extension of the earthquake power law is driven by the local setting and therefore is also a scale dependent process, (2) the geomechanical link between fractal behavior and fracture size, i.e., a physical mapping of the power law behavior, must also be found in the boundary values of the autosimilarity processes (slope breaks) rather than in the values of the power law exponents.
Influence of kinetics on the smectite to illite transition in the Barbados accretionary prism
Authors: Bekins Barbara , McCaffrey Anne M. , Dreiss Shirley J.
Published: February 2004

Low chloride pore fluids observed along faults in clay‐rich accretionary complexes are commonly attributed to the release of interlayer water during the smectite to illite transformation. However, to date, there has been no thorough analysis of the location and quantity of fluids that may be generated by this mechanism. To address this problem, a temperature and time dependent rate expression describing the dehydration reaction was coupled to a kinematic model of the northern Barbados accretionary complex. Temperatures in the complex were estimated by modeling heat flow through the prism sediments as they thicken arcward. The sediments' temperature‐time histories were computed using a model for the velocities of sediment motion through the complex. In this model the prism sediments follow uniformly diverging paths from the toe, while the underthrust sediments undergo uniaxial strain. The model predictions are validated against clay mineralogy data from Barbados Island mudstones. Our results show that the location of the peak dehydration rate is 20 km farther arcward in the underthrust sediments than in the prism complex. The fresh water produced by the reaction results in pore waters that are 10–30% fresher at a distance of 50–70 km from the prism toe. This constraint indicates a probable fluid migration path of more than 50 km from the reaction zone to the sites where freshened pore fluids have been observed. The peak rate of fluid production when expressed as the volume of fluid per volume of sediment per second is 2×l0−15.
Ultrasonic velocities in cores from the Kola superdeep well and the nature of subhorizontal seismic reflections
Authors: Vernik Lev , Hickman Stephen , Lockner David , Rusanov Michael
Published: February 2004

P wave velocity and orthogonally polarized S wave velocities were measured on 12 cores recovered from the Kola superdeep well at depths of 0 to 12 km. Measurements were made along the core axis at a frequency of 1 MHz, at confining pressures ranging from 2 to 100 MPa, and under dry and water‐saturated conditions. Cores were chosen to sample a variety of lithologies and were used to estimate interval velocities based on a simplified geological column of the well. These interval velocities were then compared with sonic log and vertical seismic profile (VSP) data. High‐pressure lab velocities correlated primarily with rock composition and texture. These laboratory velocities are generally in good agreement with both sonic log and VSP data, suggesting that extremely low velocities, as measured in unconfined laboratory samples or at low confining pressure, are the result of drilling and core‐recovery‐induced damage. The magnitude of this microcrack‐induced damage generally increases with depth in a stepwise manner but with a few notable inversions. These inversions are characterized by a relatively small reduction in dry unconfined velocities compared to the in situ velocities. We interpret these inversions to be due to localized in situ stress relief related to faulting, fracturing, and/or hydrothermal alteration. We also observed pronounced S wave splitting in the cores, the analysis of which suggests that the stress relief microcracks tend to be aligned parallel to the foliation in gneisses and amphibolites (dip angle 28°–45°) rather then being subhorizontal These observations have important implications for the nature of gently dipping seismic reflections detected in the immediate vicinity of the Kola well.
Magnesium grain boundary diffusion in forsterite aggregates at 1000°–1300°C and 0.1 MPa to 10 GPa
Authors: Farver John R. , Yund Richard A. , Rubie David C.
Published: February 2004

Magnesium grain boundary diffusion rates in forsterite aggregates have been experimentally determined from 1000° to 1300°C and 0.1 MPa to 10 GPa. The samples are fine‐grained (mean linear intercept of 4.3 μm) hot‐pressed aggregates. The technique employs a 26Mg‐enriched surface layer and depth profiling using an ion microprobe. Values of the product of the grain boundary diffusion coefficient (D′) and the effective grain boundary width (δ) were calculated using appropriate analytical solutions to the grain boundary diffusion equation of Whipple (1954). The Arrhenius parameters for the 0.1 MPa data for samples annealed in H2 + CO2 and CO + CO2 gas mixtures are D0′δ = 2.1 × 10−10 and 7.7 × 10−10 m3/s and Q = 343 ± 27 and 376 ± 47 kJ/mol, respectively. The reproducibility of D′δ measurements is a factor of 2. A determination of D′ independent of δ yields a calculated effective grain boundary width of ∼3 nm, similar to the physical grain boundary width of 1–3 nm estimated from high‐resolution transmission electron microscopy observations. The data indicate a very low pressure dependence for magnesium grain boundary diffusion in forsterite with an apparent activation volume for grain boundary diffusion of ≤∼1 cm3/mol at 1100°C. The results of this study provide information concerning diffusion creep in solid state deformation of forsterite aggregates over a broad range of pressures and temperatures.