ALBERT

Beschreibung: We discuss an innovation in traveltime tomography that combines wavelet-based, multiscale parameterization and finite-frequency theory to solve two outstanding issues that inevitably arise from uneven source station distributions and from the three-dimensional (3-D) nature of wavefront healing: how to objectively address the intrinsically multiscale nature of data coverage while simultaneously maintain model resolution at each scale level. We apply the new, integrated methodology to investigate 3-D variations of P and S wave speeds (δlnVP and δlnVS) beneath the Himalayan-Tibetan orogen. In particular, we are able to constrain variations in the Poisson's ratio via δln(VP/VS). The formulation is naturally data adaptive, resolving features at each scale only if the required data converge is available. The very first, long-wavelength feature that emerges is a clear anomaly of high δlnV that extends over more than 500 km beyond the northern edge of the Lhasa terrane at places. Farther northward, a strong negative anomaly underlies the region where recent volcanism occurs in northern Tibet. Regions of negative δln(VP/VS) delineate a slab-like, subhorizontal feature concentrated between depths of ∼100–250 km. Such characteristics are consistent with the notion that chemically refractory, and therefore buoyant, mantle lithosphere of the Indian shield (“Greater India”) has advanced subhorizontally northward far beyond the surficial Bangong-Nujiang suture. In the crust, two isolated regions of low δlnV, each extending to depths near 100 km, occur along the Lunggar and the Yadong-Gulu active rifts in southern Tibet. Deep penetrating rifts imply that only a limited amount of horizontal displacement is being accommodated on subvertical structures.

Beschreibung: We present results of detailed paleomagnetic investigations on deep-sea cores from sediment drifts located along the Pacific continental margin of the Antarctic Peninsula. High-resolution magnetic measurements on u channel samples provide detailed age models for three cores collected from drift 7, which document an age of 122 ka for the oldest sediments recovered near the drift crest at site SED-07 and a high sedimentation rate (11 cm/kyr) at site SED-12 located close to the Alexander Channel system. Low- and high-temperature magnetic measurements in conjunction with microscopic and mineralogic observations from drifts 4, 5 and 7 indicate that pseudosingle-domain detrital titanomagnetite (partially oxidized and with limited Ti substitution) is the dominant magnetic mineral in the drift sediments. The titanomagnetite occurs in two magnetic forms: (1) a low-coercivity form similar to laboratory-synthesized titanomagnetite and (2) a high-coercivity form (Bcr 〉 60 mT). These two forms vary in amount and stratigraphic distribution across the drifts. We did not find evidence for diagenetic magnetic iron sulfides as has been previously suggested for these drift deposits. The observed change of magnetic mineralogy in sediments deposited during Heinrich events on drift 7 appears to be related to warming periods, which temporarily modified the normal glacial transport pathways of glaciogenic detritus to and along the continental rise and thus resulted in deposition of sediments with a different provenance. Understanding this sediment provenance delivery signature at a wider spatial scale should provide information about ice sheet dynamics in West Antarctica over the last ∼100 kyr.

Beschreibung: The origin of pulverized rocks (PR) in surface outcrops adjacent to the fault cores of the San Andreas and other major faults in Southern California is not clear, but their structural context indicates that they are clearly associated with faulting. An understanding of their origin might allow inferences to be drawn about the nature of dynamic slip on faults, including rupture mechanisms and their speed during earthquakes. In the present study, we use split Hopkinson bar recovery experiments to investigate whether PR can be produced under dynamic stress wave loading conditions in the laboratory and whether PR is diagnostic of any particular process of formation. The results of the study indicate that in Westerly granite for transition from sparse fracture to pervasive pulverization requires high strain rates in excess of 250/s and that the formation of PR may be inhibited at the larger burial depths. The constraint imposed by field observations of the relatively low strains (1–3%) in PR recovered from the field and the laboratory derived threshold for the critical strain rate (∼250/s and higher) together indicate that a dynamic supershear-type rupture may be necessary for the origin of pulverized rocks at distances of tens of meters away from the fault plane as observed in the field for both large strike-slip-type and the relatively small dip-slip-type fault ruptures in nature.

Beschreibung: This study investigates the spatial and temporal distribution of energy release of large, intermediate-depth earthquakes using a modified back projection technique first used to study the 2004 Sumatra-Andaman megathrust event. Multiple seismic phases are included in the back projection analysis, which provides the capability to determine the energy distribution with respect to depth and time. A total of 22 intermediate-depth earthquakes with moment magnitudes greater than or equal to 6.5 are investigated with hypocentral depths between 100 and 300 km. For most of these events, the vertical extent of energy release is either below the resolution of this study (≤5 km) or slightly above (≤15 km). This observation agrees with previous studies that find large, intermediate-depth earthquakes have subhorizontal rupture planes. The results also show a significant portion of the events have multiple rupture planes that are well separated in depth. The closeness in time of the ruptures on separate planes and the distance between the planes suggest dynamic triggering where the P waves from the first rupture initiate rupture on the second plane. We propose that a dehydration embrittlement mechanism combined with preferentially hydrated subhorizontal faults can explain the observations of dominant subhorizontal rupture planes and the frequent occurrence of rupture complexity involving multiple subevents.

Beschreibung: We have synthesized phase D at 24 GPa and at temperatures of 1250–1100°C in a multianvil press under conditions of high silica activity. The compressibility of this high-silica-activity phase D (Mg1.0Si1.7H3.0O6) has been measured up to 55.8 GPa at ambient temperature by powder X-ray diffraction. The volume (V) decreases smoothly with increasing pressure up to 40 GPa, consistent with the results reported in earlier studies. However, a kink is observed in the trend of V versus pressure above ∼40 GPa, reflecting a change in the compression behavior. The data to 30 GPa fit well to a third-order Birch-Murnaghan equation of state (EoS), yielding Vo = 85.1 ± 0.2 Å3; Ko = 167.9 ± 8.6 GPa; and K′o = 4.3 ± 0.5, similar to results for Fe-Al-free phase D reported by Frost and Fei (1999). However, these parameters are larger than those reported for Fe-Al-bearing phase D and for Fe-Al-free phase D. The abnormal volume change in this study may be attributed to the reported hydrogen bond symmetrization in phase D. Fitting a third-order Birch-Murnaghan EoS to the data below 30 GPa yields a bulk modulus Ko = 173 (2) GPa for the hydrogen-off-centered (HOC) phase and Ko = 212 (15) GPa for the data above 40 GPa for the hydrogen-centered (HC) phase, assuming K′o is 4. The calculated bulk modulus Ko of the HC phase is 18% larger than the bulk modulus Ko of the HOC phase.

Beschreibung: Knowledge of the mechanical properties of gas-hydrate-bearing sediments is essential for simulating the geomechanical response to gas extraction from a gas-hydrate reservoir. In this study, drained triaxial compression tests were conducted on artificial methane-hydrate-bearing sediment samples under hydrate-stable temperature-pressure conditions. Toyoura sand (average particle size: D50 = 0.230 mm), number 7 silica sand (D50 = 0.205 mm), and number 8 silica sand (D50 = 0.130 mm) were used as the skeleton of each specimen. Axial loading was conducted at an axial strain rate of 0.1% min−1 at a constant temperature of 278 K. The cell and pore pressures were kept constant during axial loading. We found that the strength and stiffness of the hydrate-sand specimens increased with methane hydrate saturation and with the effective confining pressure, and the secant Poisson's ratio decreased with the effective confining pressure. The stiffness depends on the type of sand forming the skeleton of the specimens, although the strength has little dependence on the type of sand. According to an earlier work, hydrate-sand specimens are thought to contract in the early stage of axial loading before starting to expand owing to the dilatancy effect, as is the case for many other geological materials. The test results in this study are discussed in relation to the deformation mechanism proposed in an earlier work.

Beschreibung: We have conducted a paleointensity study of the Matuyama-Brunhes (M-B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40Ar/39Ar ages yielding a weighted mean of 771 ± 8 (1σ) ka indicate that 30 lava flows recorded the geomagnetic field across the M-B transition. The 215 specimens from 32 flows were subjected to the double-heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation-like variation between 3 and 38 μT corresponding to virtual dipole moments (VDMs) between 0.9 × 1022 and 9.6 × 1022 Am2. For the major directional changes, a weak paleointensity of 5 μT is obtained, which gives a VDM of 1.0 × 1022 Am2. For the normal polarity period, paleointensities are 14–21 μT, giving VDMs of 3.5–5.2 × 1022 Am2. For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M-B transition.

Beschreibung: Recent exploration drilling of the Manicouagan impact structure has revealed local developments of impact melt that are substantially thicker than the previously accepted average of ∼300 m and a more complex melt sheet–basement interface showing considerable “topography.” The thickest section (1045 m of clast-free to clast-poor impact melt overlying 425 m of clast-laden impact melt) is associated with a centrally located, fault-bounded graben that was in place prior to melt crystallization. Here we report the results of a geochemical investigation of 115 samples obtained from nine drill holes and a set of field samples. The results reveal that most of the drill core comprises an undifferentiated unit, showing minimal geochemical variation. This is typically ∼300 m thick and equates with previous studies performed on samples from exposed rock. In contrast, the deeper section exhibits differentiation and is divided into three layers based on chemical, mineralogical, and textural variations: a quartz monzonite to quartz monzodiorite upper zone (276 m), a quartz monzodioritic middle zone (244 m), and a mainly monzodioritic lower zone (525 m). The mineralogy is defined by plagioclase 〉 orthoclase 〉 clinopyroxene 〉 orthopyroxene, with ubiquitous amphibole and biotite and olivine locally developed in the lower zone and below. Our results reveal that the thicker section of the impact melt sheet has undergone fractional crystallization. Differentiation of parts of the Manicouagan impact melt indicates that lunar impact melt samples possessing different chemistries, previously considered to be derived from distinct impact basins, may be related via the fractionation of common, much smaller diameter impact structures.

Beschreibung: Peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation. The precise lithology and kinematics of peak ring formation, however, remains unclear. Previous work has revealed a suite of bright inward dipping reflectors beneath the peak ring at the Chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity. New two-dimensional, full waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin (∼100–200 m thick) layer of low-velocity (∼3000–3200 m/s) rocks. This low-velocity layer is most likely composed of highly porous, allogenic impact breccias. Our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors. Across the peak ring, velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation, where the rocks that form the peak ring originate from an uplifted basement that has been subjected to high shock pressures (10–50 GPa) and lie above downthrown sedimentary rocks that have been subjected to shock pressures of

Beschreibung: Oceanic crust is accreted through the emplacement of dikes at spreading ridges, but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood. Between 2005 and 2009 the ∼70 km long Dabbahu-Manda Hararo rift segment in Ethiopia has experienced 14 large volume dike intrusions, 9 of which were recorded on temporary seismic arrays. A detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems. All of the migrating swarms of earthquakes started from a 80% of energy is released during the propagation phase, with minimal seismic energy release after the dike propagation ceases. We interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip, coincident with the onset of low-frequency earthquakes. Dike lengths show no systematic reduction in length with time, suggesting that topographic loading and stress barriers influence dike length, as well as changes in tectonic stress. The propagation velocities of all the dikes follow a decaying exponential. Northward propagating dikes had faster average velocities than those that propagated southward, suggesting preconditioning by the 2005 megadike, or ongoing heating from a subcrustal magma source north of the midsegment.