bluuebunny/crossref_metadata_2025 huggingface dataset:
Total tectonic subsidence: A parameter for distinguishing crust type at the U.S. Atlantic Continental Margin
Authors: Sawyer Dale S.
Published: April 2008

Total tectonic subsidence (TTS) is the difference between the prerifting continental crust elevation and the present, sediment‐unloaded, basement depth in a sedimentary basin. TTS is calculated using observations of bathymetry and basement depth and assumptions about sediment density and the loading response of the lithosphere. I use the extensional model for sedimentary basin formation to interpret the TTS of a passive continental margin. That model predicts that the TTS on an old margin should define the major crust‐type boundaries and indicate the lateral distribution of extension within the extended continental crust. TTS analysis of the U.S. Atlantic continental margin indicates that the boundary between extended continental and oceanic crusts lies along the East Coast Magnetic Anomaly (ECMA) to the north of the Blake Spur Fracture Zone (BSFZ), and along the Blake Escarpment to the south. There is no TTS anomaly associated with the Blake Spur Magnetic Anomaly which seems to be underlain by normal oceanic crust. The ECMA appears to be associated with a TTS minimum, but I attribute that to artificially shallow estimates of depth to magnetic basement under the ECMA. TTS estimates of extended continental crust thickness agree well with gravity models except where the problem with magnetic basement depth estimates interferes. The amount of crust extension prior to seafloor spreading varies along the strike of the margin. The most abrupt change is at the BSFZ. Across the BSFZ the amount of extension varies by 160 km, but the volume of continental material does not vary. The extension variation may be caused by lateral changes in the gross rheology of the continental lithosphere. In the case of the BSFZ it would be the suture of more “ductile” pre‐late Paleozoic African craton with more “brittle” North American craton that could cause the abrupt change.
Folding of oceanic lithosphere
Authors: McAdoo David C. , Sandwell David T.
Published: April 2008

Folding of the lithosphere just south of the Bay of Bengal appears as (1) undulations in acoustic basement topography and (2) as linear geoid undulations in the Seasat altimeter data. From the Seasat data we find that the east‐west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km. The horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mGal. Elastic models of oceanic lithosphere have, in the past, been used to demonstrate the implausibility of lithosphere buckling, or folding, in response to compression. These elastic models typically predict that compressive stresses of about 5 GPa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern Indian Ocean (40–70 Ma). These stresses exceed the strength of lithospheric rock. We use an elastic‐plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12% of the elastic buckling stress. We further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km, i.e., the wavelength observed from Seasat. Our results reinforce earlier speculation that this folding may be related to the Himalayan orogeny.
Block rotation and deformation by strike‐slip faults: 2. The properties of a type of macroscopic discontinuous deformation
Authors: Garfunkel Z. , Ron H.
Published: April 2008

Study of the general features of deformation by systems of subparallel strike‐slip faults shows that the governing constraints are kinematic: the fault blocks must remain in contact with each other and the deformed area must fit with its surroundings. As a result, fault blocks that move laterally without significant internal deformation also rotate about vertical axes relative to boundaries of the fault domain by an amount that is quantitatively related to fault slip, spacing, and orientation. Areas where left‐and right‐lateral faults move simultaneously are usually divided into domains that contain faults of one kind only. Deformation of such multidomain areas is nonhomogeneous, but the differences between domains can compensate for each other, so that the regional deformation is quite simple. Strike‐slip faulting combined with block rotation is an efficient mechanism of deformation because modest offsets and rotations change the linear dimensions of faulted areas by several tens of percents, which may often greatly exceed the results of other types of coeval deformation. Paleomagnetic data can provide an independent measure of block rotations and should be used to supplement structural data. Block rotation changes the initial angles between right‐ and left‐lateral faults and also their orientation relative to the stress field; the faults probably tend to rotate away from the axis of maximum compression. Rotated faults continue to move because they are weak surfaces. Since the deformation depends on geometric factors such as fault orientation and domain geometry, it is not simply related to the stress field. These properties, including rotation of material units, characterize discontinuous deformation in general.
Causes and rate‐limiting mechanisms of ridge propagation: A fracture mechanics model
Authors: Morgan Jason Phipps , Parmentier E. M.
Published: April 2008

All known propagating rifts (PR's) are growing away from shallow ridge axis seafloor. We examine a model for rift propagation in which the ridge axis conduit, like a crack in the lithosphere, propagates in response to a stress concentration at its tip. The stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity‐spreading stresses associated with anomalously shallow ridge axis along the growing ridge. This stress concentration near the tip is characterized by a stress intensity factor K, which must exceed a threshold value to cause failure of the plate. A remotely applied tensile stress would make a positive contribution to K but would act equally on both ridge segments and not favor the propagation of either. The pressure in the crack may, depending on its distribution, make either a positive or negative contribution to K. For a high enough propagation rate, viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the PR tip would make K < 0. This simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well‐studied 95.5°W PR of the Galapagos spreading center. Analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the PR axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 Pa s (1020 P). We also examine a more general model of a ridge‐transform‐ridge system on the basis of energy conservation. In terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis. However, the more general model also includes mechanisms that may stop ridge propagation. Propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis, i.e., if the stress intensity factors for both ridge axes are similar. Both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85°W in the Galapagos. Finally, in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation.
Hydraulic fracturing stress measurements at Yucca Mountain, Nevada, and relationship to the regional stress field
Authors: Stock J. M. , Healy J. H. , Hickman S. H. , Zoback M. D.
Published: April 2008

Hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes USW G‐1 and USW G‐2 at Yucca Mountain as part of the Nevada Nuclear Waste Storage Investigations for the U.S. Department of Energy. Eight tests in the saturated zone, at depths from 646 to 1288 m, yielded values of the least horizontal stress Sh that are considerably lower than the vertical principal stress Sv. In tests for which the greatest horizontal principal stress SH could be determined, it was found to be less than Sv, indicating a normal faulting stress regime. The borehole televiewer logs showed the presence of long (in excess of 10 m), vertical, drilling‐induced fractures in the first 300 m below the water table. These are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures (up to 5.2 MPa) applied during drilling. The presence of these drilling‐induced hydrofractures provides further confirmation of the low value of the least horizontal stresses. A least horizontal principal stress direction of N60°W–N65°W is indicated by the orientation of the drilling‐induced hydrofractures (N25°E–N30°E), and the orientation of stress‐induced well bore breakouts in the lower part of USW G‐2 (N65°W). This direction is in good agreement with indicators of stress direction from elsewhere at the Nevada Test Site. The observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults. Using these data, the Coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0.6, movement on favorably oriented faults could be expected. For coefficients of friction of 1.0, preexisting faults of all orientations should be stable. Laboratory studies on the Yucca Mountain tuffs, reported elsewhere, yield coefficients of friction ranging from 0.6 to 0.9.
Secular variation, crustal contributions, and tectonic activity in California, 1976–1984
Authors: Johnston M. J. S. , Silverman S. A. , Mueller R. J. , Breckenridge K. S.
Published: April 2008

Five to ten years of data from an array of 34 total field magnetometers are used to define the temporal and spatial characteristics of secular variation throughout central and southern California. For this period, well‐determined rates of secular variation are obtained at each site. These rates are temporally linear but spatially variable, ranging from −45 nT/a near San Francisco to −54 nT/a near the Mexican border. Least squares analysis of all data indicates secular variation decreases in a general southeasterly direction according to
, where
is in nanoteslas per year, θ and ϕ are the geographic latitude and longitude, and k1, k2, and K are 1.66±0.13 nT/a deg, −0.13±0.10 nT/a deg, and −123.2±0.2 nT/a, respectively. Deviations of as much as 1 nT/a occur on scales of a few tens of kilometers. These apparent small‐scale secular variation anomalies result, in part, from differences in local induction and remanent magnetization and may be reduced by determination of a site transfer function. A planar surface fit to the corrected data has the form
, where k1 k2, and K are now 1.50±0.08 nT/a deg, −0.23±0.06 nT/a deg, and −129.2±0.1 nT/a, respectively. Residual field variations obtained after correction of all data for secular variation are most apparent on the San Andreas fault in southern California between Palm Springs and the Salton Sea and marginally so along the recent Coyote (ML = 5.9 of August 6, 1979) and Morgan Hill (ML = 5.8 of April 24, 1984) aftershock zones. These residuals could be explained by stress localization in these regions and, particularly in the case of the southern San Andreas anomaly, may indicate the location of a future damaging earthquake. Incomplete correction for complex site effects may be an alternative explanation. In the first large‐scale test of global secular variation models we find that the models for this region do not predict either the amplitudes or the mean isogram directions of these data to better than several tens of nanoteslas per year and several tens of degrees, respectively. This may result from a failure to correct for site response effects at some observatories before using the data in global spherical harmonic expansions. Local magnetization response can therefore bias estimates of secular variation and yield apparent impulsive behavior when external fields are perturbed.
Geographical distribution of 3He/4He ratios in Japan: Implications for arc tectonics and incipient magmatism
Authors: Sano Yuji , Wakita Hiroshi
Published: April 2008

The 3He/4He and 4He/20Ne ratios of 115 natural gas samples, including various types of gases from volcanoes, hot springs, mineral springs, water wells, petroleum fields, and natural gas fields, were measured using a mass spectrometer. The observed 3He/4He and 4He/20Ne ratios range from 7.47×10−8 to 9.65×10 −6 and from 0.26 to 1100, respectively. The 3He/4He ratios reflect well the geotectonic structure of the Japanese Islands. In northeastern (NE) Japan there is a clear geographical difference in the 3He/4He ratios between the frontal arc (forearc) and volcanic arc (back arc) regions. Lower ratios were found in the trench side region and higher ratios in the back arc side. This result suggests that the mantle‐derived helium in the volcanic arc region is associated with the diapiric uprise of a magma. Lower 3He/4He ratios in the frontal arc region may be due to radiogenic He produced by radioactive decay of U and Th in the crustal and sedimentary rocks. In southwestern (SW) Japan there is no clear geographical contrast in the 3He/4He ratios. Some samples in the frontal arc region show quite high 3He/4He ratios. The tendency of the 3He/4He ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from NE Japan. The high 3He/4He ratios observed in the frontal arc region in SW Japan may be indicative of renewed or incipient magmatism due to a descending young and warm slab.
Trace element and isotopic geochemistry of lavas from Haleakala Volcano, east Maui, Hawaii: Implications for the origin of Hawaiian basalts
Authors: Chen Chu‐Yung , Frey Frederick A.
Published: April 2008

Haleakala volcano on East Maui, Hawaii, consists of a tholeiitic basalt shield which grades into a younger alkalic series that was followed by a posterosional alkalic series. Tholeiitic, transitional, and alkalic basalts range widely in Sr and Nd isotopic ratios (from mid‐ocean ridge basalt to bulk earth ratios) and incompatible element (P, K, Rb, Sr, Zr, Nb, Ba, REE, Hf, Ta, and Th) abundances, but isotopic ratios and incompatible element abundance ratios (e.g., Ba/La, Nb/La, La/Ce, La/Sm) vary systematically with age. The youngest series (posterosional alkalic lavas) has the highest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the lowest 87sr/86sr ratios, whereas the oldest series (dominantly tholeiitic basalts) has the lowest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the highest 87sr/86sr ratios. The most striking features of the trace element and isotopic data are the inverse correlations between isotopic ratios and parent/daughter abundance ratios in the Sr and Nd systems. Although some of the geochemical variations can be explained by shallow level fractional crystallization (e.g., alkali basalt to mugearite [Chen et al., 1984, and manuscript in preparation, 1985]), the temporal geochemical trends require a major role for mixing. We propose a model in which melts from a diaper interact with incipient melts of its wall rocks, presumed to be oceanic lithosphere. Because of motion between the lithosphere and mantle hot spot the relative contribution of melts from the diapir (mantle plume) material to the lavas decreases with time; consequently, with decreasing age the basalts become more enriched in incompatible trace elements and acquire Sr and Nd isotopic ratios which overlap with mid‐ocean ridge basalts. This model quantitatively explains the isotopic ratios and incompatible trace element abundances in representative samples from the three Haleakala volcanic series. On the basis of the degrees of melting inferred for the mixing components we conclude that the lower lithosphere and much of the asthenosphere beneath Hawaiian volcanoes are involved in creating these volcanoes.
Correlation between Rayleigh loops and frequency‐dependent and quadrature susceptibility: Application to magnetic granulometry of rocks
Authors: Bloemendal J. , Barton C. E. , Radhakrishnamurthy C.
Published: April 2008

We report the results of measurements of frequency‐dependent and quadrature low‐field magnetic susceptibility of basalt specimens showing different types of low‐field hysteresis (Rayleigh) loop. A correlation among the three techniques is demonstrated, with specimens showing broad and constricted loops exhibiting relatively high‐frequency dependence of susceptibility and a high ratio of quadrature to in‐phase susceptibility. This result has implications for magnetic granulometry studies of rocks. Broad and constricted Rayleigh loops indicate the significant contribution of hyperfine grains to the magnetic properties of specimens; however, with the current instrumentation, low‐field hysteresis loop measurements are restricted to relatively strongly magnetic specimens. For weakly magnetic rocks and sediments, measurement of the frequency dependence of susceptibility or the quadrature susceptibility permits rapid identification of specimens in which hyperfine grains significantly influence the magnetic properties.
Effect of clay and salinity on the dielectric properties of rock
Authors: Rankin D. , Singh R. P.
Published: April 2008

The effect of clay and saline solutions on the complex dielectric properties of geologic formations is not theoretically understood, nor are there satisfactory empirical relations for such complex media. In this work, measurements of the dielectric constant are reported for consolidated sandstone cores containing a fixed percentage of clay with varying salinity and water content. In order to study the effect of varying clay content, the dielectric constants of different sand‐clay mixtures were also determined. The measurements were carried out at higher frequencies than previously reported, i.e., in the VHF and microwave range (20 MHz to 1 GHz). The effect of clay increases the real part and decreases the imaginary part of the complex dielectric constant compared to the “clean” sandstone samples. The pronounced frequency dependence on clay, water content, and salinity disappears at our higher frequencies.
A comparison between LAGEOS laser ranging and very long baseline interferometry determined baseline lengths
Authors: Kolenkiewicz R. , Ryan J. , Torrence M. H.
Published: April 2008

The NASA Crustal Dynamics Project uses satellite laser ranging (SLR) and very long baseline interferometry (VLBI) data collected and analyzed by the Goddard Space Flight Center and the Jet Propulsion Laboratory for the measurement of geodynamic and geodetic parameters. An important parameter is the determination of the baseline length or intersite distance. Since October 1979, baselines determined from SLR data and VLBI data have been compared to help assess the accuracy of the baseline measurements from these independent techniques. Eight locations in the continental United States participated in the comparison: Westford, Massachusetts; Fort Davis, Texas; Platteville, Colorado; and Quincy, Owens Valley, Goldstone, Pasadena, and Monument Peak, California. Twenty‐two baseline lengths between these locations, as determined by SLR, were differenced from the corresponding VLBI determinations. The rms scatter about zero for the length differences is 5.2 cm with a mean of 1.0±1.1 cm, and there is no apparent scale factor between these two distance determinations. For this result, the Monument Peak‐Quincy baseline has been adjusted for tectonic plate motion by using the SLR‐determined baseline rate. For the eight other baselines crossing a plate boundary, tectonic motion was not modeled. If a model for this motion based upon baseline changes derived from strain rates is applied, the rms scatter about zero for the 22 differences becomes 7.2 cm, with a mean of 1.0±1.1 cm. The above results are within the combined error budgets of the SLR and VLBI baseline length determinations.
Starlette geodynamics: The Earth's tidal response
Authors: Williamson Ronald G. , Marsh James G.
Published: April 2008

The Starlette satellite was launched by the French Centre National d'Etudes Spatiales in 1975. The primary objective of this project was to further knowledge of the “solid” earth and ocean tides and other long‐period gravitational perturbations. In the present study, Starlette laser ranging data for 1980 have been analyzed to map the tidal response of the earth to about 0.01 in the effective solid earth Love number k2. These results are based upon the solution of normal equations by processing the laser ranging data in 5‐day orbital arc segments. The resonant response near the K1 tidal frequency is clearly shown. This is mostly due to the nonrigid body character of the earth's internal structure and is the expected type of response considering modern earth models. If we assume the Wahr free space potential for the solid earth tidal response, the recovered values for the K1, O1, M2, N2, S2, and Mƒ ocean tidal coefficients are in good agreement with those of the conventional nonsatellite techniques, Schwiderski and Parke, and with those derived from long‐period orbital evolution studies.
Excitation study of the LAGEOS‐derived Chandler wobble
Authors: Gross Richard S. , Chao B. Fong
Published: April 2008

We have used the University of Texas LAGEOS series of pole positions to obtain a Chandler wobble data set that has then been deconvolved. The deconvolution filter used here can be derived from the ideas of Backus‐Gilbert inverse theory. By trading off resolution against accuracy a model for the Chandler wobble's excitation function can be obtained that is able to reproduce all of the features of the input Chandler wobble data set except for the noise contained therein. Two observed features of this excitation function are discussed. First, a change of amplitude 0.04″ in the direction −54°E longitude is observed to have taken place from August 14.0, 1977, to September 3.0, 1977. The only notable geophysical event known to us to have occurred in this time interval is the great Sumba, Indonesia, earthquake of August 19, 1977. However, elastic dislocation theory applied to a double‐couple model of this event is unable to explain the observed pole shift. We speculate that if the observed shift is causally related to the Sumba event it could be due to motions of the subducted slab occurring within 20 days of the main shock. By deconvolving Chandler wobble data sets obtained by other independent methods we find that this shift is not evident in the excitation function derived from Bureau International de l'Heure data. Therefore we cannot conclusively determine whether or not the shift seen in the LAGEOS‐derived excitation function is geophysically significant. Second, a study is conducted into the possible influence on the Chandler wobble of the extraordinarily strong 1982–1983 Southern Oscillation/El Niño event which has been observed to have had a great influence on the length of day. We find in the Chandler wobble's excitation function a very encouraging signature that can be explained by the 1982–1983 El Niño event. It is encouraging in the sense that the y component ψy(t) of the excitation function appears to be temporally correlated with the Southern Oscillation index and that the polarity and overall magnitude of the change in ψy as well as the quiescence of the excitation function's x component during this period are all consistent with observations of the 1982–1983 El Niño.
Reassessment of the charge and neutral drag of LAGEOS and its geophysical implications
Authors: Afonso G. , Barlier F. , Berger C. , Mignard F. , Walch J. J.
Published: April 2008

The influence of an external drag to account for the secular decrease of the semi‐major axis of LAGEOS is reexamined. We develop a theory for the neutral and electric drag more refined than those previously available. The charge drag appears to be the most likely cause of the secular decrease of LAGEOS of about 1.1 mm day−1. Likewise we show the existence of annual and semiannual variations connected with the orientation of the satellite's orbital plane with respect to the sun. Finally we express the electric drag as n5/6 T2/3 in terms of ion number density and absolute temperature. The rate of fall of LAGEOS constrains the range of possible physical states of the plasmasphere at altitudes as high as 6000 km.
LAGEOS and UT measurements of long‐period Earth tides and mantle Q
Authors: Merriam J. B.
Published: April 2008

The zonal response coefficient κ and the tidal effective potential Love number
are two parameters which measure the amplitude of long‐period earth tides. It is shown that reasonable models for the frequency dependence of Q in the mantle can result in an increase in the amplitude of these parameters of several percent. UT measurements of κ at Mf, Mm, and LAGEOS measurement of
at Ssa have uncertainties which are small enough to place valuable constraints on acceptable anelastic models. The LAGEOS measurement of the amplitude of the 18.6‐year tide is not yet accurate enough to constrain further these models, but this situation will improve markedly as a larger fraction of an 18.6‐year cycle is observed.
On the geodetic applications of simultaneous range differences to LAGEOS
Authors: Pavlis Erricos C.
Published: April 2008

The possibility of obtaining accurate geodetic results by use of simultaneously (from two stations) observed range differences to LAGEOS is investigated. Using simplified and computationaly cheaper orbital models, one can still achieve accuracies comparable to those of laser measurements for such parameters as baselines and earth orientation. Simulation tests show that model errors can be effectively minimized by the proposed “simultaneous range differencing”. Elimination of radial, along‐, and across‐track biases is examined. Sensitivity analyses show that data collected from orbits parallel to the baseline direction are best suited for its determination. Cross ‐ or L‐shaped baselines near the meridians of definition are best suited for polar motion determination. Individual solutions for each baseline with respect to an a‐priori fixed orbit decreases orbit biases and reduces the cost tremendously. The method is also free from reference frame orientation errors, biasing baseline determinations.
Crustal accretion in and around Iceland
Authors: Steinthorsson S. , Jacoby W.
Published: April 2008

Since the advent of the new global tectonics, geological problems and processes are being redefined in terms of plate tectonics. This is particularly so in Iceland, which is traversed by the divergent boundary between the Eurasian and North‐American plates and at the same time is a prominent hotspot. Many of the peculiarities of Icelandic geology and geochemistry appear now to be understandable only in these terms, and as a result, investigations in Iceland should shed light on various processes taking place under water along the more inaccessible oceanic ridges.

Each period of human history has its own way of looking at the earth. In the Middle Ages, Mount Hekla, Iceland's most famous volcano, was reputed to be the gateway to Hell. Science oftentimes grows more by replacement than by addition, and the Medieval scheme was superseded by a new one which, surprisingly, at times even appeared to hold more questions than answers: Why is Iceland there at all, in the middle of the Atlantic? It looked like an accident of nature. Wegener [1915] interpreted Iceland to be a relic of continental scum left from the separation of Greenland from Europe, although earlier [Wegener, 1912] he had contemplated something similar to seafloor spreading [Jacoby et al., 198O]. H. Hess (personal communication, 1966), one of the pioneers of the new geology, also felt that Iceland did not fit into the idea of seafloor spreading and Belousov [1970] actually used Iceland as a strong argument against that hypothesis.
Plate tectonic evolution of the ICelandic Transverse Ridge and adjacent regions
Authors: Bott Martin H. P.
Published: April 2008

The northeastern North Atlantic evolved in the following three stages: (1) 54–44 Ma, initial separation, (2) 44–26 Ma, separation of Jan Mayen microcontinent from Greenland with complementary fan‐shaped spreading on both sides, and (3) 26 Ma to present, spreading about a single axis passing west of the microcontinent. Poles of rotation for each stage have been refined, and the plate tectonic evolution of the Icelandic transverse ridge in relation to adjacent regions has been reconstructed. During the early stage the Iceland‐Faeroe Ridge and the complementary east Greenland shelf were formed. The Faeroe transform fault separated the region from the Norwegian Sea and the spreading axis jumped westward prior to anomaly 22. During the intermediate stage, the line of the Faeroe transform fault enabled the Jan Mayen microcontinent to slide out, with the development of a ridge‐ridge‐transform triple function which migrated westward, and the formation of a wedge‐shaped region of crustal accretion joining the triple junction to the Aegir Ridge. The Greenland‐Iceland Ridge eastward of the shelf break and most of the Vestfirdir region of Iceland were formed during this stage. At the start of the last stage, the Reykjanes and Kolbeinsey spreading axes became joined by a new split which formed an eastward bulging embayment across the Icelandic transverse ridge approximately coincident with the eastern edge of the insular shelf of Iceland. The asymmetrical shape of Iceland itself, and of the whole transverse ridge, is explained by this reconstruction.
Stratigraphy and K/AR ages across the west flank of the northeast Iceland Axial Rift Zone, in relation to the 7 MA volcano‐tectonic reorganization of Iceland
Authors: Jancin Mark , Young Kirby D. , Voight Barry , Aronson James L. , Saemundsson Kristján
Published: April 2008

Lithostratigraphy, magnetostratigraphy, and new K/Ar ages from the region between Eyjafjördur and Skjálfandi, north central Iceland, reflect tectonic reorganization of crustal accretion zone activity ∼7 Ma. Two flood basalt piles are in unconformable contact along this western flank of the Northeast Iceland Axial Rift Zone (NEIARZ). The older ranges in age from ∼9.5 to 13 Ma and is largely composed of tholeiite flows. In eastern Dalsmynni the upper to middle portions of this older basaltic pile define a 15°–35° SE dipping, monoclinal flexure developed ∼6–7 Ma during early development of the present NEIARZ. Lavas of a flood basalt group younger than about 6.5 Ma were deposited unconformably on the older, flexured basalt pile. These postflexure age flows mostly consist of compound doleritic basalts and olivine tholeiites; possible tillites are found in the upper portion of this sequence. The unconformity between the older and younger basalt piles represents a major structural, temporal, and lithological boundary in north central Iceland. Zeolite zones appear subhorizontal through the Dalsmynni flexure zone and have been largely reset, subsequent to flexuring, by westward onlap of the younger basalt sequence onto the older subsided and flexured flows. Radiometrie ages, regional isochron patterns and timing constraints for Tjornes Fracture Zone‐related shear deformation indicate the model of a singular, ∼130 km eastward “jump” of the axial rift zone in north Iceland, ∼6–7 Ma, is too simple. Crustal accretion older than 12 Ma along a “proto‐NEIARZ” is apparently required, indicating that northern Iceland has had multiple‐branched crustal accretion zones up until about 7 Ma, at which time a single spreading zone along the present NEIARZ developed. An eastward shift of axial rift zone activity ∼6–7 Ma in southwestern Iceland is time correlative with reorganization of spreading activity in northern Iceland. Our proposed 6–7 Ma reorganization of axial rift zone activity on land is time correlative with an independently proposed increase in discharge from the Iceland mantle plume.
Transform deformation of tertiary rocks along the Tjornes Fracture Zone, north central Iceland
Authors: Young Kirby D. , Jancin Mark , Voight Barry , Orkan Nebil I.
Published: April 2008

Significant fracture zone deformation has been imparted to Tertiary lavas on the peninsula (Flateyjarskagi) located between Eyjafjördur and Skjálfandi in north central Iceland. The region lies immediately south of the Flatey fault, one of a series of WNW trending, left‐stepping faults comprising the right‐lateral, oblique slip Húsavík‐Flatey fault system (HFFS). The HFFS defines the present southern margin of the 70 to 80‐km‐wide Tjörnes Fracture Zone (TFZ). Lava bedding, dikes, and faults in the western half of Flateyjarskagi display a progressive 0°–110° clockwise change of trend and steepening of lava dip angles to 15°–45° over an 11‐km‐wide zone nearest the Flatey fault. Accompanying this structural curvature is an increase in fault and dike frequency and amygdule/vein mineralization. Northeast striking faults develop predominant downthrows to the east approaching the TFZ, accommodating the steep lava dips via repetition of stratigraphic section and normal fault‐bounded block rotations. Although the possibility cannot yet be conclusively ruled out that structural trends formed in curved orientations, the simplest interpretation suggests that the lava pile was tectonically rotated by heterogeneous simple shear. Maximum on‐land shear strain has been crudely estimated at γ = 3.5; estimates of shear displacement suggest that the TFZ was active for at least 2 m.y. sometime prior to 6–7 Ma, at a strain rate of approximately 5.5×10−14 s−1. Antithetic faulting and associated flexural bending may have been the means by which a great part of the proposed simple shear rotations were accomplished. WNW extension produced by rotational normal faulting is interpreted to have accommodated systematic variations in shear strain parallel to the direction of shear. Paleomagnetic and structural evidence suggest that rocks of Tertiary age on Tjörnes peninsula immediately to the east also have been tectonically rotated, implying an original shear zone width of at least 25 km.
Petrological and geochemical variations along Iceland's Neovolcanic Zones
Authors: Meyer Peter S. , Sigurdsson Haraldur , Schilling Jean‐guy
Published: April 2008

Petrological, geochemical, and geophysical gradients along the SE volcanic zone in Iceland imply systematic variations in melting and crystallization conditions and in magma supply and eruption rates. At the southern tip of the zone, in Vestmannaeyjar, alkali basalt magmas are generated by small degrees of melting under a thick lithosphere. Farther north, in the Hekla‐Katla region, greater degrees of melting result in the generation of transitional basalt magmas. Magma supply rates exceed eruption rates, and melts begin to accumulate at the base of the crust, as indicated by magnetotelluric evidence. Uniform rare earth element patterns in the Hekla‐Katla basalts may be explained by homogenization in the melt accumulation zone or by uniform melting conditions. Infrequent replenishment of magma reservoirs in this region leads to mixing of compositionally diverse magmas and, consequently, to basalts with diverse phenocryst compositions and textures. Even farther north, in central Iceland, the melting anomaly associated with the SE zone has developed to the same degree as it has beneath the SW axial rift zone, leading to similar magmatic conditions. High magma supply rates and low cooling rates inhibit fractionation and lead to the eruption of voluminous olivine tholeiites. In these areas a broad spectrum of melt compositions is generated by variable degrees of melting over a wide depth range. The compositional diversity, e.g., in large ion lithophile element enrichment, is masked somewhat by reequilibration and mixing of melts on ascent and in the melt accummulation zone. Compositional diversity may be preserved, however, in the melt accummulation zone in a lateral direction away from the rift axis since distal parts of the melt zone are fed only by melts segregating at greater depths. The variations in magmatic conditions along the SE zone, which are analogous to those inferred along propagating rifts, may be related to a mantle blob that ascended beneath central Iceland 2–3 m.y. ago, spread out laterally and triggered a southward propagating rift.
Shifts of the plate boundary in Iceland: Some aspects of Tertiary volcanism
Authors: Helgason Jóhann
Published: April 2008

This paper points out five anomalies of the Icelandic upper crust that are not compatible with currently accepted construction models. These anomalies are (1) assumption of a single time‐averaged plate boundary, (2) irregular depth distribution to seismic layer 3, (3) the assumption of a uniform vertical compositional zonation, (4) the “excess width” of Iceland, and (5) abnormal variations in the duration and stratigraphic thickness of chron 5 time lavas in western, northern, and eastern Iceland. A model involving frequent shifts of the volcanic zones in Iceland is outlined that suggests a way to account for the above structural anomalies. The model also suggests that detailed studies of basalt lava facies types may be used effectively to trace small‐scale shifts of the Tertiary plate boundary.
Magnetotelluric measurements across the eastern neovolcanic zone in south Iceland
Authors: Eysteinsson Hjálmar , Hermance John F.
Published: April 2008

A magnetotelluric survey consisting of 18 separate soundings was performed in 1982 along a 125‐km profile crossing the eastern neovolcanic zone in south Iceland. Although the three‐dimensional ocean effect may be significant at the longest periods which previous workers have analyzed to study features in the upper mantle beneath Iceland, our studies suggest that this effect is minimal for periods less than 1000 s at distances greater than 50 km from the coast. Therefore we are confident that two‐dimensional interpretations are valid at periods suitable for interpreting crustal resistivities. Such a two‐dimensional interpretation presented here indicates the presence of a layer (∼5 km thick) of low resistivity (5 ohm m) at the crust‐mantle interface (10–20 km depth). The depth to this layer increases from the center of the neovolcanic zone. This feature has been seen elsewhere in Iceland by other workers. However, the present study suggests that east of the neovolcanic zone the low‐resistivity layer is not present, which we argue may be associated with a recent eastward shift of the axial rift zone to a new location within older crust. A striking feature of our two‐dimensional crustal interpretation is that pockets of low‐resistivity material appear to be present at approximately 8 km depth at the base of the crust beneath the two volcanic centers that we have studied, Torfajokull and Hekla. These may be manifestations of local magma chambers.
Partial melt below Iceland: A combined interpretation of seismic and conductivity data
Authors: Schmeling Harro
Published: April 2008

A combined interpretation of published seismic and electric data is carried out to impose constraints on the state of melting below Central Iceland. These data indicate high electrical conductivity and low vp velocity (regionally below 6 km/s) for the crust‐mantle transition region, while the mantle below shows lower conductivity but still anomalous low velocity. The velocity data are treated by utilizing gravity data to quantify the elastic modulus decrease due to partial melt. A comprehensive set of theoretical models on elasticity, anelasticity, and electrical conductivity of partially molten rock is applied to this data set. The geometry, the fraction, and the interconnection of the melt phase are varied systematically. The conductivity models are based on published laboratory data on conductivity of solid and melt phases of rocks. Anisotropy and possible unexplored conductivity mechanisms near the melting temperature are not considered here. By means of the melt models the data can be explained most easily as follows: in a depth range roughly between 20 and 50 km only a small percentage of melt (≦2%, at the most 5% if melt pockets are also present) occurs in thin films, possibly with a distribution of aspect ratios and a low degree of interconnection. If, in the crust‐mantle transition region, zones of low p wave velocity (≦6 km/s) are correlated with those of high conductivity (≧0.08 S/m), melt fractions between 10 and 20% are likely and the degree of interconnection should be high. Thin films can be excluded as a dominant melt geometry in that region. Apart from improved constraints on melt geometry the results are in good agreement with published models based only on conductivity data. However, they disagree with published models based entirely on seismic data. The discrepancy is now resolved by the combined interpretation.
On the lithospheric seismic structure of Reykjanes Ridge at 62.5°N
Authors: Ritzert Michael , Jacoby Wolfgang R.
Published: April 2008

A long reversed refraction line on the southeast flank of Reykjanes Ridge subparallel to the axis is modeled with synthetic seismograms. These are calculated using the reflectivity method. The data have been recorded by two ocean bottom seismometers during the Reykjanes Ridge Iceland Seismic Program 1977 (RRISP77 experiment). The observed amplitudes are characterized by a low at around 80‐km range and a pronounced maximum at around 100‐km distance. Here the apparent velocity increases from about 8 km/s to at least 8.6 km/s in both directions. The modeling demonstrates that most of the observed features can be described by a laterally homogeneous structure except for layer 2 which thickens markedly toward Iceland. The structure of the lower crust seems to be similar to that of the Bay‐of‐Islands ophiolite, Newfoundland. There is an indication of a low‐velocity zone at the base of the crust which may be caused by partial serpentinization of the olivine component of the gabbros at that depth. The Moho seems to be modeled best by a discontinuity about 10 km below the water surface. In the uppermost mantle there is a very slight velocity increase from 8 to 8.25 km/s. It is underlain by a transition zone to velocities as high as 8.6–8.9 km/s at about 20 km depth. This could possibly be explained by velocity anisotropy, perhaps together with a petrological boundary. The direction of maximum velocity of the deep anisotropic layer should be parallel to the ridge axis. This could be caused by a pipe flow from the Iceland mantle plume which is possibly indicated by the shear wave structure at greater depth.
Resistivity structure of the upper crust in Iceland
Authors: Flóvenz Ólafur G. , Georgsson Lúdvík S. , Árnason Knútur
Published: April 2008

The results of numerous resistivity soundings from all parts of Iceland are analyzed. The resistivity and the P wave velocity of the uppermost kilometer in the crust show a strong correlation with the geological structure of Iceland. By estimating porosity from P wave velocity, Archie's law is found not to be valid in general. The temperature dependence of nine water‐saturated basaltic rock samples was investigated in a laboratory showing stronger temperature dependence than expected from pore fluid conduction. These two facts suggest that interface conduction is the most important conduction mechanism. A relation between bulk resistivity, fracture porosity, pore fluid resistivity, and temperature is established. Comparison of regional bulk resistivity with different zeolite zones shows that the resistivity is mainly a function of the maximum depth of burial of the lava pile, emphasizing the importance of clay minerals and zeolites for the measured bulk resistivity.
Geodetic measurements of surface deformations during the present rifting episode in NE Iceland
Authors: Wendt K. , Möller D. , Ritter B.
Published: April 2008

Iceland is part of the accreting plate boundary between the American and European plates. Since 1965 a number of repeated triangulation‐trilateration surveys spanning the neovolcanic zone in NE Iceland were made in order to detect crustal movements. All the measurements for a single observation epoch are separately adjusted by the method of least squares. Statistical tests are carried out to check the observations and adjustment models for outliers and model errors. To detect significant movements in repeatedly measured geodetic networks with multivariate design, a testing rule is described. After unexpected horizontal contractions of the surface of approximately 0.5 m from 1965 to 1971, enormous deformations have been found since the beginning of a major rifting episode in December 1975. During the period from 1971 to 1980 a horizontal expansion of about 7.5 m and vertical displacements of 3 m were observed across the approximately 3‐km‐wide active rifting zone of the Krafla fissure swarm. In adjacent zones, compressional strains of up to 2×10−4 occurred, rapidly decreasing to a value of 1×10−5 at a distance of 40 km from the rifting axis. After 1980 the pattern of ground deformation has altered. The east‐west shifts have decreased and are superposed by a radial component of motion around the Krafla caldera. West of the Krafla fissure swarm a shear zone is found.
Possible influences of thermal stresses on basin and range faulting
Authors: Reiter Marshall , Minier Jeffrie
Published: April 2008

Reasonable if not conclusive arguments may be made for the heating and subsequent cooling of the Basin and Range Province during middle and late Cenozoic time. It is proposed that thermal stresses developed during this time and played a role in the formation of Basin and Range structure. The analysis depends upon physical models which are surely a simplification of the actual heating and/or cooling occurring in geological environments where rock failure and faulting are influenced by regional tectonic stresses, the rock fabric, and previously induced zones of weakness. Nevertheless, the one‐dimensional models which represent crustal heating by means of an increase of temperature at depth and crustal cooling in concert with overall lithospheric cooling allow an appreciation that thermal stresses developing more or less contemporaneously with heating or cooling events are of sufficient magnitude to be considered important contributions to rock failure. The thermal stresses are oriented in a manner generally consistent with observed faulting across the Basin and Range Province. It is therefore suggested that thermal stresses may have combined with regional tectonic stresses to help promote the structures observed in the Basin and Range Province.