bluuebunny/crossref_metadata_2025 huggingface dataset:
A heat pipe mechanism for volcanism and tectonics on Venus
Authors: Turcotte D. L.
Published: February 2008

A heat pipe mechanism is proposed for the transport of heat through the lithosphere on Venus. This mechanism allows the crust and lithosphere on Venus to be greater than 150 km thick. A thick crust and thick lithosphere can explain the high observed topography and large associated gravity anomalies. For a 150‐km‐thick lithosphere the required volcanic flux on Venus is 200 km3/yr; this is compared with a flux of 17 km3/yr associated with the formation of the oceanic crust on Earth. A thick basaltic crust on Venus is expected to transform to eclogite at a depth of 60 to 80 km; the dense eclogite would contribute to lithospheric delamination that returns the crust to the interior of the planet completing the heat pipe cycle. Topography and the associated gravity anomalies can be explained by Airy compensation of the thick crust. The principal observation that is contrary to this hypothesis is the mean age of the surface that is inferred from crater statistics; the minimum mean age is about 130 Ma, and this implies an upper limit of 2 km3/yr for the surface volcanic flux. If the heat pipe mechanism was applicable on Earth in the Archean, it would provide the thick lithosphere implied by isotopic data from diamonds.
Late Cenozoic right‐lateral strike‐slip faulting in southern Tibet
Authors: Armijo Rolando , Tapponnier Paul , Han Tonglin
Published: February 2008

Field observations collected during the Sino‐French Himalaya‐Tibet cooperation project (1980–1982) show that right‐lateral faulting between southern and northern Tibet, roughly along the “chord” joining the eastern and western syntaxes of the Himalayan arc, is a major component of the active tectonics north of the Himalayas. Just north of the well developed rifts of southern Tibet, we studied three right‐lateral, 100–200 km long, strike‐slip faults forming an en échelon array along that chord. Evidence for earthquake surface breaks was found on each of these faults. For the M = 8, Beng Co (or Damxung) strike‐slip earthquake of November 18, 1951, rupture parameters could be constrained to be ū = 8 ± 2 m and L = 90 km. Offset glacial and postglacial morphological markers (105 years and 104 years B.P.) imply displacement rates of the order of 1–2 cm/yr on the three faults. The en échelon fault zone appears to extend westward ≈2000 km across the plateau to the Karakorum fault and is thus referred to as the Karakorum‐Jiali fault zone (KJFZ). Fast rates of slip on it and on faults along the northern rim of Tibet appear to contrast with the slower rates of deformations in the interior of the plateau. Our estimates of slip rates on the strike‐slip and normal faults of southeastern Tibet suggest that eastward extrusion of North Tibet between the right‐lateral KJFZ and the left‐lateral Altyn Tagh and Kun Lun faults absorbs at least 30% of the present convergence between India and Asia. We propose an instantaneous velocity model of the active tectonics of Asia compatible with such faulting rates and geometries. Evidence for Tertiary right‐lateral strike‐slip movements in southern Tibet and around the eastern syntaxis of the Himalayas is also found in the structural record. Offsets of various large‐scale geological features imply that these movements have been large.
Effective elastic plate thickness beneath the East African and Afar plateaus and dynamic compensation of the uplifts
Authors: Ebinger C. J. , Bechtel T. D. , Forsyth D. W. , Bowin C. O.
Published: February 2008

The broad topographic domes, or plateaus, of East Africa and Afar are characterized by long‐wavelength negative Bouguer gravity anomalies and volcanically active rift valleys. Gravity and topography data from the East African and Afar plateaus and data from the stable cratonic regions to the west were subdivided into 17 smaller regions to study the variation of elastic plate thickness within part of the African continent, its relation to rifting processes within these intracontinental plateau regions, and compensation mechanisms for the broad uplifts. Assuming that loads at the surface, within, and beneath the base of a thin elastic plate contribute to the observed Bouguer gravity anomalies, the wavelength dependence of the coherence between gravity and topography was used to determine the effective elastic plate thickness in each of the subregions. Estimates of elastic plate thickness were found to be 64–90+ km in the stable cratonic areas, provided that surface and subsurface loads are uncorrelated. Lower estimates (43–49 km) were obtained in the largely unfaulted regions encompassing the broad uplifted plateaus and the narrower Darfur dome to the west of the Afar plateau. Estimates of 21–36 km correspond to regions that include the severely faulted and commonly volcanically active Kenya, Western, and Ethiopian rift valleys as well as unfaulted regions adjacent to the rift valleys. We attribute the smallest estimates of elastic plate thickness (21–36 km) to averaging unfaulted topography with mechanically weakened topography within the severely faulted Kenya, Western, and Ethiopian rifts. The linear transfer function between gravity and topography within the uplifted East African plateau region at wavelengths longer than 1000 km can be explained by a dynamic uplift mechanism and associated heating of the thermal lithosphere above a convecting region within the asthenosphere. These isostatic and dynamical compensation mechanisms are consistent with existing geological and geophysical data and with constraints on the timing of volcanism and uplift within the East African plateau region.
Tectonic history of the north portion of the San Andreas Fault System, California, inferred from gravity and magnetic anomalies
Authors: Griscom Andrew , Jachens Robert C.
Published: February 2008

Geologic and geophysical data for the San Andreas fault system north of San Francisco suggest that the eastern boundary of the Pacific plate migrated eastward from its presumed original position at the base of the continental slope to its present position along the San Andreas transform fault by means of a series of eastward jumps of the Mendocino triple junction. These eastward jumps total a distance of about 150 km since 29 Ma. Correlation of right‐laterally displaced gravity and magnetic anomalies that now have components at San Francisco and on the shelf north of Point Arena indicates that the presently active strand of the San Andreas fault north of the San Francisco peninsula formed recently at about 5 Ma when the triple junction jumped eastward a minimum of 100 km to its present location at the north end of the San Andreas fault. Prior to 5 Ma the triple junction was located at the north end of a proposed northwesterly extension of the Pilarcitos fault. This jump took place at what is now about latitude 38°20′N on the North American plate and may have occurred during part of the time when the relative motion between the North American and Pacific plates rotated 20° clockwise, resulting in transpression along the earlier transcurrent San Andreas fault system. The proposed 150‐km eastward movement of the triple junction explains the submarine topography near Cape Mendocino where the continental shelf south of the Mendocino fault extends about 130 km farther west than does the shelf directly north of the fault.
Deformation of the Long Valley Caldera, eastern California from mid‐1983 to mid‐1988: Measurements using a two‐color geodimeter
Authors: Langbein John
Published: February 2008

Deformation of Long Valley caldera in eastern California has been closely monitored since June 1983 with a two‐color geodimeter, which has a precision between 0.12 and 0.20 ppm of the baseline length. Initial trilateration measurements were concentrated in the south moat of the caldera, the source of the January 1983 earthquake swarm. Since then, the network has been expanded from 13 to 42 baselines. The additional baselines span the resurgent dome and the western half of the caldera. With a combination of measurements made several times per week on 7–11 baselines and monthly or yearly measurements on the remaining baselines, we have measured both the temporal and spatial deformation of the caldera. Although the M 6.4 Chalfant Valley earthquake was located 35 km from the nearest baseline in the network, we observed coseismic strain changes of up to 1.5 ppm on several baselines. Using fault planes selected to coincide with the hypo‐centers of the main shock and the M 5.9 foreshock, we have inferred a combined moment of 3.0×1025 dyn cm from the two‐color data. This cumulative moment of both earthquakes is equivalent to a magnitude 6.3 shock. Over the period from mid‐1983 to mid‐1988, the long‐term trends determined from the length changes of the baselines within the caldera show a general decrease in the rate of extension. During 1983, measurements show a maximum extension rate of 5.0 ppm/yr. After mid‐ to late‐1984, the extension rates decreased by a factor of 2–4 relative to those in the first year. With the exception of the coseismic step, the extension rates have been fairly constant during the interval between late 1984 and mid‐1988. The long‐term changes in rate can be modeled by using two Mogi point sources representing inflation beneath the resurgent dome of the caldera and a dislocation surface representing slip in the south moat. Modeling indicates that both the rates of slip and inflation have decreased since mid‐1983, but these two processes are still occurring at a measureable and constant rate.
Thermal evolution of a rift basin: The Tyrrhenian Sea
Authors: Wang Chi‐Yuen , Hwang Win‐Tsuang , Shi Yaolin
Published: February 2008

Existing geological and geophysical data support the view that the opening of the Tyrrhenian Sea was by rifting of a formerly continuous continental lithosphere, which began in upper Miocene on the upper Sardinian margin and migrated southeastward to the Calabria margin. Heat flow across the Tyrrhenian Sea shows a pronounced asymmetry, from about 50–70 mW m−2 over the Sardinian margin, to more than 150 mW m−2 over its southeastern margin off southern Italy and Calabria. In this study we use the newly acquired data from Ocean Drilling Program Leg 107, heat flow, seismic velocity, and bathymetric data to constrain a group of models that supposes the Tyrrhenian Sea opened by detachment of a formerly continuous lithosphere along a southeastern facing, low‐angle normal fault. Specific models tested include (1) the Wernicke model, in which thinning and extension of the continental lithosphere are supposed to be accomplished by “simple shear” along a large‐scale, gently dipping detachment fault zone which cut through the entire lithosphere, and (2) the delamination model, in which the low‐angle detachment fault is supposed to cut only the upper and middle crust but to merge subhorizontally into the lower crust, below which concurrent pure shear may occur. A versatile, two‐dimensional finite element procedure is used to evaluate the tectonic and thermal evolution of these models, and the results are compared with observations to test the validity of the models. Delamination models with large pure shear components in the lower lithosphere and the Wernicke model predict heat flow within ±1σ of the measurements. The Wernicke model, however, predicts a basin configuration much too deep in comparison with bathymetry and a subbasinal lithospheric thickness substantially different from Rayleigh wave dispersion results. The delamination model with a large component of pure shear in the subbasinal lower lithosphere, on the other hand, predicts satisfactorily both the bathymetry and the lithospheric thickness beneath the basin.
Improved oxygen‐buffering techniques for C‐O‐H fluid‐saturated experiments at high pressure
Authors: Taylor W. R. , Foley S. F.
Published: February 2008

A new oxygen‐buffering technique has been developed for experiments in high‐pressure, solid media apparatus where saturation with a reduced C‐O‐H fluid is required. The buffer (abbreviated “WCWO”) consists of a mixture of WC, WO2 and graphite. Stearic acid (C18H36O2) is mixed with the buffer components to provide the C‐O‐H fluid source. The silicate sample is enclosed and separated from the buffer by an inner graphite capsule thereby eliminating iron loss problems. Analysis of quenched fluids by capsule piercing/mass spectrometry demonstrates that WCWO controls oxygen fugacity (ƒO2) at ∼1 log unit above the iron‐wustite (IW) buffer with a reproducibility better than 0.2 logƒO2 units. Fluid compositions buffered by WCWO can be maintained in sealed noble metal capsules for run times exceeding 50 hours. Fluids are CH4‐H2O dominated with a wider compositional range (CH4/(CH4 + H2O) ∼0.8 to 0.1 depending on temperature) than is accessible with the more reduced iron‐wustite‐graphite (IWG) buffer. In liquidus experiments on olivine lamproite, we found the IWG buffer, in contrast to WCWO, to be unreliable in controlling fluid compositions. IWG did, however, perform satisfactorily in the absence of a silicate sample. We show that the IWG technique can be modified so that fluids of H2O ≫ CH4 > CO2 character can be reliably reproduced (termed the “CWI” technique). The WCWO buffer has been employed in melting studies on the system peridotite‐C‐O‐H (Taylor and Green, 1988b) and should have wide application in experiments above 1000°C where control of ƒO2 and water activity (aH2O) is required. The CWI technique is based on fluid monitoring and may be used in experiments where high water activity but low ƒO2 (∼IW+1.5 log units) is desired.
Introduction to Special Section on the Northern Chugach Mountains‐Southern Copper River Basin Segment of the Alaskan Transect, Part 1
Authors: Page Robert A.
Published: February 2008

This special section of the Journal of Geophysical Research is the first of a two‐part collection of papers addressing the nature and evolution of the lithosphere in southern Alaska. Collectively, these papers document the important role of subduction‐related accretion in continental growth, a process which continues today in southern Alaska.

The studies reported in the collection were undertaken as part of the Trans‐Alaska Lithosphere Investigation (TALI), a geological and geophysical transect of the Alaskan lithosphere along a north‐south corridor. The transect follows the route of the trans‐Alaska oil pipeline and extends offshore across the Pacific and Arctic continental margins (Figure 1). Unique among transect studies of the North American lithosphere, TALI features coordinated geological and geophysical investigations along a common corridor crossing the entire continent, from the active convergent Pacific margin to the rifted Arctic margin of Mesozoic age.
Petrologic and age constraints on the origin of a low‐pressure/high‐temperature metamorphic complex, southern Alaska
Authors: Sisson V. B. , Hollister L. S. , Onstott T. C.
Published: February 2008

The interrelationships between metamorphism, deformation, magma intrusion, and40Ar/39Ar geochronology were determined for a low‐pressure/high‐temperature metamorphic complex which formed from an accretionary prism in the Chugach Mountains, southern Alaska. Compressional deformation, which first produced south verging folds and associated thrusts, was followed by magma intrusion and development of north verging folds. Synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall, as documented by the distribution of mineral assemblages and by pressure‐temperature modeling of garnet growth. The initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust. Regionally developed amphibolite facies metamorphism followed intrusion of felsic sills. The peak metamorphic conditions derived from geothermobarometry, mineral assemblages, and fluid inclusions ranged from 400° to 600°C at a depth of ∼10 km. The increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite‐bearing migmatites. Subsequent cooling was initially rapid (≈55°C/Ma) to ∼350°C based on40Ar/39Ar dates of 53 Ma for hornblende and 50 Ma for biotite and may have slowed to ∼11°C/Ma until 200°C based on an40Ar/39Ar date of 35 Ma for plagioclase. Intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds. This suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism.
Coexisting calcalkaline and high‐niobium basalts from Turrialba Volcano, Costa Rica: Implications for residual titanates in arc magma sources
Authors: Reagan Mark K. , Gill James B.
Published: February 2008

Turrialba volcano, the southeasternmost volcano in the Central American arc, is constructed of medium to high‐K calcalkaline basalts, andesites, and dacites, plus rare basalts with unusually high Nb concentrations. The compositions of these high‐Nb basalts are more similar to those of intraplate basalts than they are to typical calcalkaline or arc‐tholeiitic basalts. The association of calcalkaline and high‐Nb basalts is rare in arc front volcanoes, seemingly being restricted to volcanoes that overlie Oligocene or younger subducting crust or that overlie the edges of subducting plates. The calcalkaline and high‐Nb basalts at Turrialba have generally similar major element, trace element, and isotopic compositions but differ significantly in their Ba/La and La/Nb ratios. The geochemical similarities imply that they were derived from similar ocean island basalt sources. Their geochemical differences suggest that residual rutile stabilized by a large ion lithophile element bearing slab‐derived fluid was present during calcalkaline basalt genesis but not during high‐Nb basalt genesis. To explain the stability of rutile in a calcalkaline melt with a relatively low TiO2 concentration, we use a model that involves two stages of melting for both basalt types. Silica saturated high degree melts with mid‐ocean ridge basalt like incompatible element concentrations generated by upwelling mantle are used as mixing end‐members for both the calcalkaline and the high‐Nb basalts. The calcalkaline basalts represent mixtures of the high‐degree melts and oxidized small‐degree melts generated by amphibole breakdown in mantle overlying the subducting slab. This small‐degree melt has high incompatible element concentrations and is saturated in rutile. Arc‐related lamprophyric rocks have compositions that are appropriate for these small‐degree melts. High‐Nb basalts are mixtures of the high‐degree melts and more reduced small‐degree melts that are undersaturated in rutile. These reduced melts may migrate around or through the subducting slab into the wedge to become involved in arc magma genesis.
Triassic shoshonites from the dolomites, northern Italy: Alkaline arc rocks in a strike‐slip setting
Authors: Sloman L. E.
Published: February 2008

Triassic igneous rocks of the Dolomites are typical shoshonites, with high K2O/SiO2 and K2O/Na2O ratios, and low Fe contents. They have mid‐ocean ridge basalt‐like abundances of high field strength elements but are highly enriched in large ion lithophile (LIL) elements (Rb, Sr, Ba, K). The effects of crustal contamination and fractional crystallization have been superimposed on initial LIL element enrichment. Lavas of similar composition occur in Papua New Guinea, Fiji, the Absaroka Mountains, and the Eolian Islands. These suites are all associated with destructive plate margins but were not erupted until after subduction had ceased. Examination of Mediterranean paleogeography indicates that no subduction event which could have given rise to the shoshonites of the Dolomites occurred during the Triassic. In fact, the tectonic setting at the time of eruption of the lavas was probably one of strike‐slip faulting. The suite may have inherited its “arc signature” from Permo‐Carboniferous subduction of the ocean between Laurasia and Gondawanaland prior to the Hercynian orogeny.
Archean lamprophyre dikes of the Superior Province, Canada: Distribution, petrology, and geochemical characteristics
Authors: Wyman D. A. , Kerrich R.
Published: February 2008

Shoshonitic lamprophyre dikes of late Archean age are a volumetrically minor, yet widespread component of magmatism in the Superior Province, and preferentially occur along major structures that delineate subprovince boundaries. Lamprophyre swarms are most prominently developed in fault graben, where they are associated with alkali gabbro and trachytes, and molasse sediments. Mineralogically, the dikes are characterized by zoned phlogopite or hornblende phenocrysts, restriction of feldspar to the groundmass, globular segregations of K‐feldspar and calcite, panidiomorphic texture, and the sporadic presence of crustal xenoliths. Compositionally, fresh dikes have contents of SiO2 (47–51 wt %), TiO2 (0.55–0.60), and P2O5 (0.36–1.58), and K2O/Na2O ratios (1.5–2.3) and Mg numbers (78–80), comparable to Phanerozoic shoshonitic lamprophyres. Unaltered dikes are characterized by extreme enrichments of the lithophile elements K, Rb, Ba, Cs, Th, U, and light rare earth elements (LREE), and enhanced abundances of Cr (650–920 ppm), Co (37–42), Ni (264–355) and Sc (22–28). Mid‐ocean ridge basalt normalized plots display (1) consistent enrichments of K, Rb, and Ba relative to Sr and LREE, (2) variable enrichments of Rb and Ba relative to K, and (3) troughs at Ta‐Nb and Ti. Compared with fresh rocks, altered samples have systematically lower K, Rb, Ba, and Sr contents, but similar REE distributions and profiles from Th‐Yb and Sc‐Ni on normalized diagrams. Suites of post‐Archean ultramafic lamprophyres in the Superior Province are enriched in large ion lithophile elements (LILE), LREE, and compatible elements, relative to their Archean counterparts and lack troughs at Ta‐Nb and Ti. Emplacement of the lamprophyres was diachronous from north (2705 Ma) to south (2674 Ma), probably being related to localized extension during the successive accretion of individual subprovinces. Archean lamprophyre magmas were derived from a depleted mantle source, variably enriched in LILE and LREE by sediments and altered oceanic crust, with small variable degrees of crustal contamination, whereas the post‐Archean lamprophyres lacked a plate margin setting.
Petrology and age of alkalic lava from the Ratak Chain of the Marshall Islands
Authors: Davis Alicé S. , Pringle Malcolm S. , Pickthorn Leda‐Beth G. , Clague David A. , Schwab William C.
Published: February 2008

Volcanic rock dredged from the flanks of four volcanic edifices in the Ratak chain of the Marshall Islands consist of alkalic lava that erupted above sea level or in shallow water. Compositions of recovered samples are predominantly differentiated alkalic basalt and hawaiite but include strongly alkalic melilitite. Whole rock 40Ar/39Ar total fusion and incremental heating ages of 87.3±0.6 Ma and 82.2±1.6 Ma determined for samples from Erikub Seamount and Ratak Guyot, respectively, are within the range predicted by plate rotation models but show no age progression consistent with a simple hot spot model. Whole rock major element, and mineral chemistry are similar to those of alkalic lavas from other oceanic islands, but isotopic and certain trace element ratios reveal distinctive mantle source characteristics. Zr/Nb ratios of ∼3.0±0.2 for Bikar and Ratak Guyot, compared to 5.2±0.3 for Majuro Atoll and Erikub Seamount, at comparable degree of differentiation, suggest a more alkalic trend for the northern sites. The 143Nd/144Nd isotopic ratios range from 0.51283 to 0.51289 and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios range from 20.55 to 21.11, 15.71 to 15.77, and 39.93 to 40.75, for samples from Erikub and Ratak. Variations in isotopic and some incompatible element ratios suggest interisland heterogeneity. Similar highly radiogenic lead isotopes, coupled with distinct incompatible element ratios, especially with low Ba/Nb ratios (∼6), are observed for St. Helena in the South Atlantic and for Tubuai and Mangaia in the South Pacific. The similar mantle signature shown by lavas from Tubuai and Mangaia in the Austral‐Cook chain, which are located at approximately the position where the Ratak edifices formed during the Late Cretaceous, indicates that this mantle anomaly is not of recent origin but has erupted distinctive lava compositions for at least 87 m.y.
Evolution of linear volcanic ranges in Marie Byrd Land, West Antarctica
Authors: LeMasurier W. E. , Rex D. C.
Published: February 2008

The Marie Byrd Land volcanic province consists of a regionally extensive foundation of subhorizontal alkaline basaltic rocks, surmounted by 18 shield volcanoes of predominantly trachytic and other felsic compositions. Eleven of the felsic volcanoes occur in three N‐S and E‐W oriented chains. In each of these, chronologic data and morphology suggest systematic migrations of felsic activity along linear paths 90–154 km long. Basaltic rocks show no migration patterns, although they are the most abundant and wide‐spread rocks in the province. Plate motion is a very unlikely cause for the migrations of felsic activity because of the opposing and orthogonal directions of contemporaneous migration and because geophysical evidence indicates that the Antarctic plate has remained stationary in late Cenozoic time. Fracture propagation is a more likely mechanism, but wedging by forcible injection of magma is difficult to reconcile with an apparent lack of mixing between closely adjacent magma reservoirs. The pronounced space‐time patterns of felsic volcanism coupled with the lack of pattern in mafic activity suggest a more complex process. We propose a twostage mechanism in which the rise of mafic magma from the mantle is random in space and time, but passage through the crust has been controlled by a relict system of N‐S and E‐W fractures that have been systematically reactivated by crustal doming.
Isotopic variations in continental basaltic lavas as indicators of mantle heterogeneity: Examples from the western U.S. Cordillera
Authors: Lum Clinton C. L. , Leeman William P. , Foland Ken A. , Kargel Jeff A. , Fitton J. Godfrey
Published: February 2008

The nature and significance of Sr and Nd isotopic variations in late Cenozoic basalts from the western U.S. Cordillera region are examined in the light of major and trace element characteristics of two end member suites from the Lunar Crater (LCVF) and the Snake River Plain (SRP) volcanic fields. Like many other late Cenozoic Basin and Range basalts, those from LCVF are mildly alkalic and have isotopic (Sr, Nd) compositions similar to those of many oceanic island basalts (OIB). SRP basaltic lavas are tholeiitic and have lower incompatible trace element contents and systematically higher 87Sr/86Sr and lower 143Nd/144Nd isotopic ratios compared to LCVF lavas. These two suites are unlikely to have any genetic relation to one another but are representative of the range of isotopic variations observed in the region. We specifically address the question of how such isotopic diversity arises. For example, (1) could all of these magmas be generated from isotopically similar source materials and then be variably contaminated by crustal material during ascent, or (2) do their isotopic compositions reflect actual heterogeneities within the underlying mantle? To answer these questions, processes of contamination were evaluated using combined major and trace element and isotopic data, and it is concluded that isotopic distinctions between SRP and LCVF basalts cannot be entirely due to contamination. Indeed, they seem to reflect compositional differences in the respective magma source regions. We interpret the LCVF magmas to represent partial melts of dominantly “asthenospheric” mantle (i.e., similar to OIB sources) which, in the case of the Basin and Range province, has presumably upwelled beneath a region of major lithospheric extension. In contrast, isotopically distinct SRP basalts could be derived from ancient or enriched mantle that most likely resides in the continental lithosphere. Thus lateral and/or vertical heterogeneities inferred to exist in the mantle beneath the western United States can be reconciled with geodynamic models that are consistent with regional tectonic evolution.
Flexural uplift of the Transantarctic Mountains
Authors: Stern Tim A. , ten Brink Uri S.
Published: February 2008

The Transantarctic Mountains have formed at the continent‐continent boundary between East and West Antarctica. High heat flow, thin crust, normal faulting, and past and present volcanism indicate that this approximately 3000‐km‐long boundary is divergent in character. Three principal structures have developed at and adjacent to the boundary: the Transantarctic Mountains, the Wilkes Basin, and the Victoria Land Basin. The Transantarctic Mountains form the east edge of East Antarctica and consist of a block‐tilted mountain range up to 4500 m high. Running parallel but 400–500 km behind, or to the west of, the Transantarctic Mountains is the Wilkes Basin. This is a broad subglacial basin where the bedrock surface is now as much as 1 km below sea level. East of and immediately adjacent to the Transantarctic Mountain front is an area of extension called the Victoria Land Basin where at least 4–5 km of Cenozoic sediments have been interpreted from seismic reflection data. The wavelengths and amplitudes of these three structures can be accounted for by the elastic flexure of two cantilevered lithospheric plates if the boundary between East and West Antarctica is taken as a stress‐free edge. Specifically, the Wilkes Basin is modeled as a flexural “outer low” coupled to uplift of the Transantarctic Mountains. Similarly, subsidence within the Victoria Land Basin is also linked to uplift of the Transantarctic Mountains via the Vening Meinesz uplift‐subsidence mechanism and sediment loading. The maximum flexural rigidity for East Antarctica is estimated to be about 1025 N m (or effective elastic thickness, Te, of 115±10 km), one of the highest values for continental rigidity from long‐term loads. Flexural rigidity for the Ross Embayment in West Antarctica is, on the other hand, found to be more than 2 orders of magnitude less at 4×1022 N m (Te = 19 ± 5 km). This rigidity variation suggests a marked contrast in effective thermal age, and hence geotherms, between East Antarctica and the western Ross Embayment. Accordingly, one of the principal uplift mechanisms for the Transantarctic Mountains is considered to be a thermal uplift associated with lateral heat conduction from the extended and thinned West Antarctic lithosphere into the thicker lithosphere of East Antarctica. Augmenting thermal uplift of the Transantarctic Mountains are the effects of erosion and the Vening Meinesz uplift effect.
Paleomagnetic evidence for poleward transport of Upper Jurassic Rocks in the Decatur terrane, San Juan Islands, Washington
Authors: Bogue Scott W. , Cowan Darrel S. , Garver John I.
Published: February 2008

The San Juan Islands are underlain by a west‐vergent Late Cretaceous thrust and nappe system, part of a larger thrust belt exposed in the northwest Cascades of Washington and the southwestern Coast Mountains of British Columbia. Published paleomagnetic data from Cretaceous plutons (for example, the Coast Plutonic Complex) intruding the thrust belt suggest that it has been displaced northward by as much as 2400 km since 90–100 Ma. This study focuses on the preintrusive travel history of the Decatur terrane, one of the units involved in the San Juan thrusting. The Decatur terrane is of special interest because it may be correlative with the Coast Range ophiolite (CRO) and Great Valley sequence of the southern California Coast Ranges. We report new paleomagnetic data from the James Island Formation, an Upper Jurassic, preintrusive turbidite unit, and from the Obstruction Formation, a terrigenous sandstone unit deposited on the Decatur terrane in the Late Cretaceous. Nine sites in the Obstruction Formation yield a single remanence direction (grand mean D = 90.7°, I = 67.9°) that is clearly a postfolding overprint, possibly of Late Cretaceous age. The six site mean remanence directions from the James Island Formation cluster well only after restoring the beds to horizontal by a scheme that takes account of their complex structure. This positive fold test and the presence of two polarities provide evidence that the remanence is primary. The mean paleolatitude corresponding to the corrected remanence direction from the James Island Formation is 0°±9.2°. The data thus corroborate equatorial paleolatitudes derived from parts of the CRO by others and suggest that the Decatur terrane and CRO represent fragments of a larger terrane originally located near the Jurassic equator. Unlike the CRO, however, the Decatur terrane was still near the equator at 145 Ma. If the plutons intruding the San Juan‐north Cascades thrust belt formed 2400 km to the south, then the paleomagnetism of the Decatur terrane suggests it moved 20° farther poleward in the interval 145 Ma to 97 Ma than did an equivalent site on North America. A much smaller amount of pre‐Late Cretaceous motion is allowed by error limits on the paleolatitudes, but more is required if tilting is partly responsible for the shallow paleomagnetism of the Cretaceous plutons.
Crustal structure, flexure, and subsidence history of the Hawaiian Islands
Authors: Watts A. B. , ten Brink U. S.
Published: February 2008

Seismic reflection profile data have been used to estimate the long‐term (>106 years) mechanical properties of the oceanic lithosphere underlying the Hawaiian Islands. The data show prominent reflectors associated with the top of the oceanic crust (reflector 2), the M discontinuity, and the base of the crust (reflector 3). The two‐way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three‐dimensional elastic plate models. A good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere Te of 40 km. The main discrepancies with the refraction data occur beneath Oahu and Molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of Hawaii where it is too deep. We attribute the thickened crust beneath Oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of Hawaii to crustal melting beneath the youngest part of the chain. The effective Te of 40 km is higher than expected for 80 m.y. oceanic lithosphere, possibly due to the effects of subsurface loads that act at or near the base of the flexed crust. According to free‐air gravity anomaly and bathymetry data, Te does not change significantly along the island chain, retaining a similar value for Hawaii as it does for Oahu. We have used this result to estimate the pattern of subsidence and uplift that would be expected along the Hawaiian Islands because of flexural loading of Hawaii. The observed depth of the Haleakala wave‐cut terrace northeast of Maui can be generally explained by the flexure model. The main discrepancies between observed and the predicted depths occur northwest of Hawaii where they suggest a somewhat higher value of Te, possibly due to incomplete adjustment of the lithosphere. We suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle “hotspot” is accommodated by progressive flexing “downstream,” causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge. The progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the Hawaiian Islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat. A progressive flexure model may also apply to the case of a continental plate moving over a mantle “hotspot” and could explain the observations of (1) a “clinoform” pattern of stratigraphy in some flood basalt provinces and (2) a seaward dipping reflector sequence at some passive continental margins.
Flux of the vertical negative muons stopping at depths 0.35–1000 hg/cm2
Authors: Bilokon H. , Castagnoli G. Cini , Castellina A. , Piazzoli B. D'Ettorre , Mannocchi G. , Meroni E. , Picchi P. , Vernetto S.
Published: February 2008

The vertical negative muon stopping rate at depths 0.35–1000 hg/cm2 of standard rock has been evaluated by means of a Monte Carlo simulation of the muon transport underground. The muon energy losses take into account both continuous (ionization and pair production) and stochastic (bremmstrahlung and nuclear interaction) processes. The dependence of the charge ratio μ+/μ− on the muon momentum is included. The effect of muon multiple scattering has been estimated. Results are given both for high‐latitude and equatorial sites, and they are compared to previous calculations and to experimental data.
Segmentation of the Pacific‐Nazca Spreading Center, 1°N–20°S
Authors: Lonsdale Peter
Published: February 2008

A continuous Sea Beam swath along 2500 km of the crest of the fast‐spreading East Pacific Rise (EPR) defines the location and morphology of seven systems of spreading centers. They are bounded by six transform fault systems, where reconnaissance swaths mapped additional short axes of plate accretion. The spreading center systems are subdivided by nontransform offsets, most of which step right on the central part of the plate boundary (12°S–4.5°S) and left on the northern and southern parts. These staircases of small lateral offsets were probably created by changes in spreading direction, the right steps by a counterclockwise rotation as the 12°S–4.5°S EPR matured after 6 Ma from the propagating western boundary of the Bauer microplate. Because the evolution of this microplate also eliminated transform offsets inherited from the Pacific‐Farallon boundary (the pre‐24‐Ma ancestor of the Pacific‐Nazca EPR), led to segmentation of the (fossil) Galapagos Rise, and created the present right‐offset transform fault systems on the EPR, it is identified as the principal event in the development of the mapped plan pattern. A recent small clockwise rotation in Pacific‐Nazca relative motion is inferred to be responsible for most of the small left steps along the boundary and for segmentation of the transforms into systems of en echelon fault zones linked by short intratransform spreading centers. The plan segmentation that results from the plate and microplate histories obstructs the shallow horizontal distribution of magma along the rise crest, allowing spatial variations in the rate of vertical magma delivery to be topographically expressed by the variable cross section of the axial ridge and by the along‐strike relief of the neovolcanic zone. The pattern of variations is much more complex than that predicted by models of transform‐bounded asthenosphere upwelling and regularly spaced melt diapirs. For example, the shallowest spreading segments, as well as the deepest, are short links between long fault zones in transform fault systems, and some long straight parts of the rise crest have humped long profiles consistent with a diapir‐fed model, whereas others have a remarkably flat crest line with no topographic evidence for spreading cells.
Modeling periodic megaplume emission by black smoker systems
Authors: Cann J. R. , Strens M. R.
Published: February 2008

Two huge bodies of anomalously warm water, megaplumes, have been discovered above the normal black smoker plumes on the southern Juan de Fuca Ridge. Their chemistry indicates that they contain high‐temperature mature hydrothermal fluid mixed with entrained seawater. Their excess heat content (estimated at about 1017 J) is equivalent to that in about 0.1 km3 of black smoker fluid. Their buoyancy flux, estimated from the height of rise of the plume, indicates flow rates of up to about 20,000 kg s−1. The size, buoyancy flux, and hydrothermal characteristics of megaplumes strongly suggest that they are the result of a massive, catastrophic emission from a black smoker system. Such flow rates require a high recharge permeability (10−12 m2 or higher). An abrupt increase in discharge permeability is essential to allow black smoker flow rates to increase by orders of magnitude. This can be achieved in two ways. The most obvious way is a tectonic stretching event, such as those observed in Iceland, where some spreading segments undergo periodic extension with episodes of activity lasting a few years. This would result in a fundamental change in the hydraulic properties of the system. Megaplume emission would be associated with periods of tectonic activity, and it is estimated that several tectonically induced megaplumes might occur each year globally. We investigate hydrofracturing as an alternative mechanism for spontaneous megaplume discharge. This relies on the presence of a clogged cap to the discharge and a clogged cylindrical shell around the discharge pipe, both the result of subseafloor precipitation of sulfides and quartz through mixing of hydrothermal fluid with cool recharge water. With this structure, decrease in fluid density can lead to pressures sufficient to fracture the clogged cap and release a megaplume. Fluid density can be decreased either by injection of magmatic CO2, in which case megaplumes would be related to magmatic evolution or, if heat transport in a system is less than the rate of heat supply, by increase of fluid temperature to about 400°C, where there is rapid, nonlinear expansion of seawater at subseafloor pressures. We model hydrofracturing by increasing temperature and show that it can occur under geologically realistic conditions. We calculate the change in temperature, buoyancy pressure, and flow rate before and after initial fracture and show that megaplume flow rates can be generated by hydrofracturing. Periodic emission of megaplumes from a normal black smoker system is possible if the hydrofractures are clogged during the latter stages of megaplume activity. Sites of hydrofracture‐induced megaplumes should be distinguished by mounds of fractured and recemented sulfides surrounding hydrothermal vent areas.
Detailed tectonics near the tip of the Galapagos 95.5°W propagator: How the lithosphere tears and a spreading axis develops
Authors: Kleinrock Martin C. , Hey R. N.
Published: February 2008

A model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using SeaMARC II, Sea Beam, Deep‐Tow, and Alvin near the tip of the propagator growing westward at ∼52 km/m.y. along the east‐west trending Galapagos spreading axis near 95.5°W. Initial lithospheric rifting appears to be accommodated along reactivated roughly east‐west abyssal hill faults at the “tectonic tip.” Extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures. The initial volcanism (“initial volcanic tip”) occurs about 6.5 km behind the tectonic tip as basalt erupts through these fissures, covering most of the keystone block. The “neovolcanic axial tip” occurs another 4.5 km back at the western termination of a well‐defined axial pillow ridge along which volcanism is localized. Spreading along the propagator axis accelerates to the full rate (29 mm/yr half rate) at the “full rate tip,” interpreted to be ∼10 km behind the neovolcanic axial tip, or ∼21 km behind the tectonic tip. A complex pattern of strain in the tip area results from the interaction of plate spreading, transform shear stress, crack propagation, dynamic depression of the western end of the axial valley and the transform zone, and variations in the strength, thickness, heterogeneity, and anisotropy of the lithosphere. Southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young, thin, weak, relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction, relative to the spreading direction, due to the influence of shear stress from the transform zone. Similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older, thick, strong, heterogeneous, and anisotropic lithosphere by oblique slip along a set of left‐stepping, reactivated, preexisting east‐west abyssal hill structures. Although on a time scale of ∼106 years the overall process of propagation is mostly continuous, rifting at the tectonic tip appears discontinuous at a scale of ∼1500 m or 30,000 years, based on the east‐west spacing of grabens formed as rifting begins. The scale of discontinuity of propagation of the neovolcanic axis is ∼3–5 km or ∼50,000–100,000 years, based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes. The extension involved in rifting is ∼10–20%, based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting. These values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting.
Tectonics of the failing spreading system associated with the 95.5°W Galapagos propagator
Authors: Kleinrock Martin C. , Searle Roger C. , Hey R. N.
Published: February 2008

The failing spreading system associated with the Galapagos 95.5°W propagator system comprises a complex set of volcanic and structural provinces. Data from detailed surveys using GLORIA, SeaMARC II, Sea Beam, Deep‐Tow, the submersible Alvin, magnetics, and bottom cameras constrain the plate boundary geometry and evolution of this area. Although the failing spreading system may involve diffuse spreading, the patterns of sedimentation, volcanism, and faulting suggest localization. The failed spreading axis, composed of a set of right‐stepping en echelon basins, extends and ages southeastward from offset and overlapping doomed/failing axes. The failing axis is offset ∼13 km left‐laterally from the doomed axis. Although there may have been a preexisting offset in the doomed axis, lack of structural evidence for this and lack of an offset of this magnitude in the Brunhes/Matuyama magnetic reversal argue against this possibility. A model whereby the offset is transient does not explain the pattern of failed spreading basins which leads directly away from the currently failing axis. Detailed structural and volcanic patterns suggest that this offset may be caused by a westward progression of northward jumps in the location of the failing axis (like a secondary propagator), probably in association with the migrating strain/stress field of the migrating transform zone of the propagator system. This is modelled as a “migrating extensional relay zone” (MERZ) similar to the relay zones observed at some fast‐slipping transform faults except that it propagates westward with the overall system. Bathymetric and magnetic evidence for tectonic elements associated with the MERZ supports this model, although the expected extinct trace of the doomed axis is not obvious in the present, incomplete data set. Unlike other models for the plate boundary geometry, the MERZ model does not require serendipity for temporal and spatial juxtaposition of the primary propagator and the offset within the failing spreading system. Observation of a similar geometry in the Lau Basin supports this preferred model. Present data from the Galapagos failing spreading system are inconclusive, however, and the MERZ model remains speculative. If valid, this model implies that in at least some cases, paired propagators act together. The MERZ is too short and deep to drive its own propagation according to standard cracking models.