Nicola De Paola

Fault lubrication and earthquake propagation in thermally unstable rocks

Experiments performed on dolomite or Mg-calcite gouges at seismic slip rates ( v > 1 m/s) and displacements (d > 1 m) show that the frictional coefficient μ decays exponentially from peak values (m p ≈ 0.8, in the Byerlee9s range), to extremely low steady-state values (μ ss ≈ 0.1), attained over a weakening distance D w . Microstructural observations show that discontinuous patches of nanoparticles of dolomite and its decomposition products (periclase and lime or portlandite) were produced in the slip zone during the transient stage (d w ). These observations, integrated with CO 2 emissions data recorded during the experiments, suggest that particle interaction in the slip zone produces flash temperatures that are large enough to activate chemical and physical processes, e.g., decarbonation reactions ( T = 550 °C). During steady state (d ≥ D w ), shear strength is very low and not dependent upon normal stresses, suggesting that pressurized fluids (CO 2 ) may have been temporarily trapped within the slip zone. At this stage a continuous layer of nanoparticles is developed in the slip zone. For d >> D w , a slight but abrupt increase in shear strength is observed and interpreted as due to fluids escaping the slip zone. At this stage, dynamic weakening appears to be controlled by velocity dependent properties of nanoparticles developed in the slip zone. Experimentally derived seismic source parameter W b (i.e., breakdown work, the energy that controls the dynamics of a propagating fracture) (1) matches W b values obtained from seismological data of the A.D. 1997 M6 Colfiorito (Italy) earthquakes, which nucleated in the same type of rocks tested in this study, and (2) suggests similar earthquake-scaling relationships, as inferred from existing seismological data sets. We conclude that dynamic weakening of experimental faults is controlled by multiple slip weakening mechanisms, which are activated or inhibited by physicochemical reactions in the slip zone.

Calibration and validation of reservoir models: the importance of high resolution, quantitative outcrop analogues

Abstract Rapidly developing methods of digital acquisition, visualization and analysis allow highly detailed outcrop models to be constructed, and used as analogues to provide quantitative information about sedimentological and structural architectures from reservoir to subseismic scales of observation. Terrestrial laser-scanning (lidar) and high precision Real-Time Kinematic GPS are key survey technologies for data acquisition. 3D visualization facilities are used when analysing the outcrop data. Analysis of laser-scan data involves picking of the point-cloud to derive interpolated stratigraphic and structural surfaces. The resultant data can be used as input for object-based models, or can be cellularized and upscaled for use in grid-based reservoir modelling. Outcrop data can also be used to calibrate numerical models of geological processes such as the development and growth of folds, and the initiation and propagation of fractures.

The signature and mechanics of earthquake ruptures along shallow creeping faults in poorly lithified sediments

Seismic slip episodically occurring along shallow creeping faults in poorly lithified sediments represents an unsolved paradox, largely due to our poor understanding of the mechanics governing creeping faults and the lack of documented geological evidence showing how coseismic rupturing overprints creep in near-surface conditions. Here we describe the signature of seismic ruptures propagating along shallow creeping faults affecting unconsolidated forearc sediments. Field observations of deformation band–dominated fault zones show widespread foliated cataclasites in fault cores, locally overprinted by sharp slip surfaces decorated by thin (0.5–1.5 cm) black gouge layers (herein, black gouge). Compared to foliated cataclasites, black gouges have much lower grain size, porosity, and permeability. Moreover, they are characterized by distinct mineralogical assemblages compatible with high temperatures (180–200 °C) due to frictional heating during seismic slip. Foliated cataclasites were also produced by laboratory experiments performed on host sediments at subseismic slip rates (≤0.1 m/s), displaying high residual friction (µf = 0.65) and strain-hardening behavior. Black gouges were produced during experiments performed at seismic (1 m/s) slip rates, displaying low residual friction (µf = 0.3) due to dynamic weakening. Our results show that black gouges represent a potential diagnostic marker for seismic faulting in shallow creeping faults. These findings can help understanding the time-space partitioning between aseismic and seismic behavior of faults at shallow crustal levels.

The Internal Structure of Dilational Stepovers in Regional Transtension Zones

We present a theoretical model and discuss field-based observations from the Carboniferous Northumberland Basin (UK), describing the structures developed at stepovers associated with regionally oblique divergence (i.e., transtension zones). We show that these structures have significantly different geometries and evolution compared to those found in stepovers along strike-slip faults. The development of complex and heterogeneous patterns of structures, accommodating both brittle (fault/fracture mesh) and ductile deformation (folds), is observed at these sites. The dilational mesh structures in the stepover region experience a complex evolution due to finite strain-controlled switches from wrench- to extension-dominated transtension. This disrupts the development of a smoothly evolving structural fabric and may inhibit/perturb the development of a throughgoing fault linking adjacent fault segments. Markedly curvilinear and locally curviplanar folds, compartmentalized by strike-slip faults, are also developed. Significant amounts of hinge-parallel extension are accommodated by calcite-filled tensile veins and conjugate tension gash arrays. The fold-fracture associations described here contrast strongly with the more widely recognized patterns of strike-slip conjugate shear planes and extension fractures associated with folds developed in contractional and wrench tectonic settings. This represents a diagnostic feature that allows transtensional folds in both surface and subsurface environments to be distinguished from structures formed by later episodes of compressional or strike-slip inversion.

Putting the geology back into Earth models

New digital methods for data capture can now provide photorealistic, spatially precise, and geometrically accurate three-dimensional (3-D) models of rocks exposed at the Earths surface [Xu et al., 2000; Pringle et al., 2001; Clegg et al., 2005]. These “virtual outcrops” have the potential to create a new form of laboratory-based teaching aids for geoscience students, to help address accessibility issues in fieldwork, and generally to improve public awareness of the spectacular nature of geologic exposures from remote locations worldwide. This article addresses how virtual outcrops can provide calibration, or a quantitative “reality check,” for a new generation of high-resolution predictive models for the Earths subsurface.

Partitioning of oblique convergence coupled to the fault locking behavior of fold-and-thrust belts : evidence from the Qilian Shan, northeastern Tibetan Plateau.

Oblique plate convergence is common, but it is not clear how the obliquity is achieved by continental fold-and-thrust belts. We address this problem in the Qilian Shan, northeastern Tibetan Plateau, using fieldwork observations, geomorphic analysis and elastic dislocation modeling of published geodetic data. A thrust dips SSW from the northern range front, and underlies steeper thrusts in the interior. Cenozoic thrust-related shortening across the Qilian Shan is ~155-175 km, based on two transects. Elastic dislocation modeling indicates that horizontal strain in the interseismic period is consistent with oblique slip on a single low angle detachment thrust below ~26 km depth, dipping SSW at ~17o. We suggest this detachment is located above North China Block crust, originally underthrust during Paleozoic orogeny. Horizontal shear strain is localized directly above the up-dip limit of creep on the detachment, and is coincident with the left-lateral Haiyuan Fault. This configuration implies oblique slip on the detachment below seismogenic depths is partitioned in the shallow crust onto separate strike-slip and thrust faults. This is consistent with strain partitioning in oceanic subduction zones, but has not previously been found by dislocation models of continental interiors. The marginal, strike-slip, Altyn Tagh Fault influences thrusting within the Qilian Shan for 100-200 km from the fault, but does not control the regional structure, where Paleozoic basement faults have been reactivated. The Qilian Shan resembles the main Tibetan Plateau in nascent form: active thrusts are marginal to an interior that is developing plateau characteristics, involving low relief, and low seismicity.

Earthquake nucleation on rough faults

Earthquake nucleation is currently explained using rate and state stability analysis, which successfully models the behavior of laboratory simulated faults with constant thickness gouge layers. However, roughness is widely observed on natural faults and its influence on earthquake nucleation is little explored. Here we conduct frictional sliding experiments with different roughness on granite samples at upper crustal conditions (30–200 MPa). We observe a wide range of behaviors, from stable sliding to stick slip, depending on the combination of roughness parameters and normal stress. Stick slip is repeatedly observed in velocity-strengthening regimes, and increases in normal stress stabilize slip; these features are not fully predicted by current stability analysis. We derive a new instability criterion that matches our observations, based on fracture energy considerations and the size of weak patches created by fault roughness.

FAULT LUBRICATION AND EARTHQUAKE PROPAGATION IN CARBONATE ROCKS

Friction experiments performed on dolomite/calcite at conditions typical during the propagation of large earthquakes (slip velocities v > 1 m/s and displacements d > 1 m) show that the frictional strength of experimental faults decays exponentially from peak values, in the Byerlees’ range (μ p ≈ 0.8), to extremely low steady-state values (μ ss ≈ 0.1), attained over a weakening distance Dw. The integration of CO2-emission data, recorded during the experiments, with microstructural observations shows that nanoparticles were produced in the slip zone due to cataclastic and thermally activated chemical/thermal processes (e.g. decarbonation reactions). Steady-state shear strength, soon after the onset of CO2 emissions during the transient stage, is reduced below best fit exponential law. During the transient stage of dynamic weakening, flash heating temperature rises, greater than the value necessary to activate the thermal decomposition of dolomite (T=550°C), have been locally reached at the highly stressed frictional microcontacts. Pressurized fluids (CO2), temporarily trapped within the slip zone, may also have contributed to dynamic weakening in accordance with the effective normal stress principle. Flash heating and thermal pressurization processes will be inhibited at the end of the transient stage as nanoparticles are produced and fluids can escape the slip zone. Steady-state dynamic weakening may be controlled by velocity-dependent frictional properties of nanoparticles. Seismic source parameters (e.g. slip weakening distance Dw and breakdown work Wb), calculated from experimental data, match those obtained by modelling of seismological data from earthquakes nucleated in the same carbonate rocks in Italy during the 1997 M=6 Colfiorito earthquakes. Wb scales with Dw according to a best fit power law, in a similar fashion to that inferred from existing seismological data sets.