ALBERT

Description: Using an up-to-date global plate rotation model, applied to the endpoints of preserved major spreading ridge isochrons, we have calculated the explicitly reconstructable length-weighted mean global half-spreading rate (HSR), ridge length and area production as a function of time since the end of the Cretaceous Normal Superchron at 83.0 Ma. Our calculations integrate uncertainties in rotation parameters and chron boundary ages with the partial sampling uncertainties arising from progressive subduction of older oceanic lithosphere and its preserved spreading record. This record of directly reconstructable oceanic ridge production provides a well-constrained baseline that can be compared to reconstructions that include the largely unconstrained extrapolated histories of entirely subducted oceanic plates. The directly reconstructable global mean HSR has not varied by more than ±15 per cent about an average rate of 28.4 ± 4.6 mm a –1 since 83 Ma. No long-term secular trend is evident: a maximum global mean half-rate of 32 ± 6 mm a –1 occurred from 33.1 Ma to about 25.8 Ma, with minima of 26 ± 5 mm a –1 between about 56 and 40.2 Ma, and 24 ± 1 mm a –1 since 3.2 Ma. Only this most recent interval has a rate that differs significantly (at ±2) from the long-term mean. The global, reconstructable ridge length at 56 Ma decreases by less than 15 per cent relative to the modern ridge system; by 83 Ma it has decreased by 38 per cent. These relatively high preserved ridge fractions mean that the estimated uncertainty due to partial sampling stays roughly equivalent to the estimated rotation model uncertainties, allowing long-term spreading rate variations of 〉20 per cent since the Late Cretaceous to be ruled out. In contrast, prior to 83 Ma too little oceanic lithosphere is preserved to reliably reconstruct global spreading rates.

Description: In this study, we present a new synthesis of GPS velocities for tectonic deformation within the Tibetan Plateau and its surrounding areas, a combined data set of ~1854 GPS-derived horizontal velocity vectors. Assuming that crustal deformation is localized along major faults, a block modelling approach is employed to interpret the GPS velocity field. We construct a 30-element block model to describe present-day deformation in western China, with half of them located within the Tibetan Plateau, and the remainder located in its surrounding areas. We model the GPS velocities simultaneously for the effects of block rotations and elastic strain induced by the bounding faults. Our model yields a good fit to the GPS data with a mean residual of 1.08 mm a –1 compared to the mean uncertainty of 1.36 mm a –1 for each velocity component, indicating a good agreement between the predicted and observed velocities. The major strike-slip faults such as the Altyn Tagh, Xianshuihe, Kunlun and Haiyuan faults have relatively uniform slip rates in a range of 5–12 mm a –1 along most of their segments, and the estimated fault slip rates agree well with previous geologic and geodetic results. Blocks having significant residuals are located at the southern and southeastern Tibetan Plateau, suggesting complex tectonic settings and further refinement of accurate definition of block geometry in these regions.

Description: We performed complex conductivity measurements on 28 core samples from the hole drilled for the Humu'ula Groundwater Research Project (Hawai'i Island, HI, USA). The complex conductivity measurements were performed at 4 different pore water conductivities (0.07, 0.5, 1.0 or 2.0, and 10 S m –1 prepared with NaCl) over the frequency range 1 mHz to 45 kHz at 22 ± 1 °C. The in-phase conductivity data are plotted against the pore water conductivity to determine, sample by sample, the intrinsic formation factor and the surface conductivity. The intrinsic formation factor is related to porosity by Archie's law with an average value of the cementation exponent m of 2.45, indicating that only a small fraction of the connected pore space controls the transport properties. Both the surface and quadrature conductivities are found to be linearly related to the cation exchange capacity of the material, which was measured with the cobalt hexamine chloride method. Surface and quadrature conductivities are found to be proportional to each other like for sedimentary siliclastic rocks. A Stern layer polarization model is used to explain these experimental results. Despite the fact that the samples contain some magnetite (up to 5 per cent wt.), we were not able to identify the effect of this mineral on the complex conductivity spectra. These results are very encouraging in showing that galvanometric induced polarization measurements can be used in volcanic areas to separate the bulk from the surface conductivity and therefore to define some alteration attributes. Such a goal cannot be achieved with resistivity alone.

Description: We investigate the relationship between complex conductivity spectra and both permeability and pore mean size and distribution of 22 core samples (21 volcanic rocks and 1 clayey sandstone). The volcanic core samples were extracted from a wellbore drilled for the Humu‘ula Groundwater Research Project in the Humu‘ula saddle region between Mauna Kea and Mauna Loa volcanoes (Hawaii). The quadrature conductivity spectra of volcanic rocks exhibit a subtle, but generally detectable, relaxation frequency in the range 0.3 Hz to 45 kHz similar to the relaxation frequency observed for clayey sandstones. We find a fair relationship between this relaxation frequency and the pore size determined by mercury porosimetry. Combined with the intrinsic formation factor of the core samples, the relaxation frequency can be used as an indicator of the permeability of the material. The predicted values of the permeability are grossly consistent with the permeability values to air (in the range 0.001–100 mD) within two orders of magnitude. The measured permeability values are highly correlated to the peak of the pore size distribution determined from mercury porosimetry divided by the intrinsic formation factor. By fitting the complex conductivity spectra with the pore size distribution, we obtain the normalized chargeability of the core samples, which is, in turn, highly correlated to the measured cation exchange capacity.

Description: Kinematics of divergent boundaries and Rift-Rift-Rift junctions are classically studied using long-term geodetic observations. Since significant magma-related displacements are expected, short-term deformation provides important constraints on the crustal mechanisms involved both in active rifting and in transfer of extensional deformation between spreading axes. Using InSAR and GPS data, we analyse the surface deformation in the whole Central Afar region in detail, focusing on both the extensional deformation across the Quaternary magmato-tectonic rift segments, and on the zones of deformation transfer between active segments and spreading axes. The largest deformation occurs across the two recently activated Asal-Ghoubbet (AG) and Manda Hararo-Dabbahu (MH-D) magmato-tectonic segments with very high strain rates, whereas the other Quaternary active segments do not concentrate any large strain, suggesting that these rifts are either sealed during interdyking periods or not mature enough to remain a plate boundary. Outside of these segments, the GPS horizontal velocity field shows a regular gradient following a clockwise rotation of the displacements from the Southeast to the East of Afar, with respect to Nubia. Very few shallow creeping structures can be identified as well in the InSAR data. However, using these data together with the strain rate tensor and the rotations rates deduced from GPS baselines, the present-day strain field over Central Afar is consistent with the main tectonic structures, and therefore with the long-term deformation. We investigate the current kinematics of the triple junction included in our GPS data set by building simple block models. The deformation in Central Afar can be described by adding a central microblock evolving separately from the three surrounding plates. In this model, the northern block boundary corresponds to a deep EW-trending trans-tensional dislocation, locked from the surface to 10–13 km and joining at depth the active spreading axes of the Red Sea and the Aden Ridge, from AG to MH-D rift segments. Over the long-term, this plate configuration could explain the presence of the en-échelon magmatic basins and subrifts. However, the transient behaviour of the spreading axes implies that the deformation in Central Afar evolves depending on the availability of magma supply within the well-established segments.

Description: Volcanic crises are often preceded and accompanied by volcano deformation caused by magmatic and hydrothermal processes. Fast and efficient model identification and parameter estimation techniques for various sources of deformation are crucial for process understanding, volcano hazard assessment and early warning purposes. As a simple model that can be a basis for rapid inversion techniques, we present a compound dislocation model (CDM) that is composed of three mutually orthogonal rectangular dislocations (RDs). We present new RD solutions, which are free of artefact singularities and that also possess full rotational degrees of freedom. The CDM can represent both planar intrusions in the near field and volumetric sources of inflation and deflation in the far field. Therefore, this source model can be applied to shallow dikes and sills, as well as to deep planar and equidimensional sources of any geometry, including oblate, prolate and other triaxial ellipsoidal shapes. In either case the sources may possess any arbitrary orientation in space. After systematically evaluating the CDM, we apply it to the co-eruptive displacements of the 2015 Calbuco eruption observed by the Sentinel-1A satellite in both ascending and descending orbits. The results show that the deformation source is a deflating vertical lens-shaped source at an approximate depth of 8 km centred beneath Calbuco volcano. The parameters of the optimal source model clearly show that it is significantly different from an isotropic point source or a single dislocation model. The Calbuco example reflects the convenience of using the CDM for a rapid interpretation of deformation data.

Description: Pulverized rocks (PR) are extremely incohesive and highly fractured rocks found within the damage zones of several large strike-slip faults around the world. They maintain their crystal structure, show little evidence of shearing or chemical alteration, and are believed to be produced by strong tensile forces. Several mechanisms for pulverization have been proposed based on simple qualitative analyses or laboratory experiments under simplified loading conditions. Numerical modelling, however, can offer new insights into what is needed to produce PR and likely conditions of formation. We perform dynamic rupture simulations of different earthquakes, varying the magnitude, the slip distribution, and the rupture speed (supershear and subshear), while measuring the stresses produced away from the fault. To contextualize our results, a basic threshold of 10 MPa is set as the tensile strength of the rock mass and recordings are made of where, when, and by how much this threshold is exceeded for each earthquake type. Guided by field observations, we discern that a large (〉 M w 7.1) subshear earthquake along a bimaterial fault produces a pulverized rock distribution most consistent with observations. The damage is asymmetric with the majority on the stiffer side of the fault extending out for several hundred metres. Within this zone there is a large and sudden volumetric expansion in all directions as the rupture passes. We propose that such an extreme tensile stress state, repeated for every earthquake, eventually produces the PR seen in the field.

Description: Described are results of laboratory experiments which revealed regularities of gradual transition from stick-slip mode to aseismic creep. The behaviour of model gouge-filled fault was investigated with experimental setup of the spring-bock model. It was experimentally proven that small variations of a percentage of materials with velocity strengthening and velocity weakening properties in the fault principal slip zone may result in significant variation of the portion of seismic energy radiated during a fault slip event. The tests simulated different modes of interblock sliding whose characteristic values of scaled kinetic energy varied by several orders of magnitude, while differences in contact strength and shear stress drop remained relatively small. The obtained results led to the conclusion that the earthquake radiation efficiency and the fault slip mode are governed by the ratio of two parameters—maximum fault slip-weakening rate and shear stiffness of the enclosing massif. The ratio can be essentially changed by small variations of the material composition of the fault principal slip zone.

Description: Among the outstanding tectonic questions regarding the convergence between the Tien Shan and Tarim basin in northwestern China are the manner in which deformation is accommodated within their lithospheres, and the extent that the Tarim lithosphere underthrusts the Tien Shan. In particular, the amount and type of deformation within the Tarim basin is poorly understood. It is also uncertain if the convergence between the Tarim and the Tien Shan takes place mainly along a discrete boundary, or if the Tarim lithosphere simply indents into the Kazach shield, forming the Tien Shan through crustal thickening accommodated by a distributed series of thrust faults. In this study we use hypocentres from published earthquake catalogues and waveforms recorded by regional seismic networks to determine earthquake source parameters through regional centroid moment tensor inversion. The entire dataset consists of 160 earthquakes that occurred between 1969 and 2009 and with moment magnitudes between 3.5 and 7 distributed throughout the central Tien Shan and northwestern Tarim Basin. The estimated focal depths of these earthquakes range from the near-surface to about 44 km. Focal mechanisms throughout much of the Tien Shan indicate active deformation accommodated by thrust faults from at least the upper crust to 30 km depth. South of the Tien Shan, the Jia-shi earthquake sequence within the Tarim basin suggests that both crustal shortening and localized flexure are part of a complicated process involving rotational convergence. Inside the Tarim basin, two earthquakes with thrust faulting mechanisms near the crust–mantle boundary beneath the Bachu uplift imply a brittle rheology of the lower crust. High-angle thrust events occur broadly across the Tien Shan, suggesting that the Tarim lithosphere as a whole is strong and indents into the Kazach shield to create the mountain range.

Description: The subduction of lithospheric plates is partitioned between subducting plate motion and lateral slab migration (i.e. trench retreat and advance). We use 3-D, dynamic models of subduction to address the role of a power-law mantle viscosity on subduction dynamics and, in particular, rates of trench retreat. For all numerical models tested, we find that a power-law rheology results in reduced rates of trench retreat, and elevated slab dip angles, relative to the equivalent isoviscous mantle model. We analyse the asthenospheric pressure distribution and the style of mantle flow, which exhibits only limited variability as a function of mantle rheology, in order to compute estimates of the mantle forces associated with subduction. The inclusion of a power-law rheology reduces the mantle shear force (which resists subducting plate motion) to a greater degree than it reduces the dynamic pressure gradient across the slab (which resists trench retreat). Therefore, the inclusion of a power-law mantle rheology favours a shift towards a subduction mode with a reduced trench retreat component, typically a relative reduction of order 25 per cent in our 3-D models. We suggest that this mechanism may be of importance for reducing the high trench retreat rates observed in many previous models to levels more in line with the average subduction partitioning observed on Earth at present (i.e. trench velocity ≤ plate velocity), for most absolute plate motion reference frames.