Alfredo Taboada

Geodynamics of the northern Andes: Subductions and intracontinental deformation (Colombia)

New regional seismological data acquired in Colombia during 1993 to 1996 and tectonic field data from the Eastern Cordillera (EC) permit a reexamination of the complex geodynamics of northwestern South America. The effect of the accretion of the Baudo-Panama oceanic arc, which began 12 Myr ago, is highlighted in connection with mountain building in the EC. The Istmina and Ibague faults in the south and the Santa Marta-Bucaramanga fault to the northeast limit an E-SE moving continental wedge. Progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the Cordilleras (northward of 5°N) and transpressive deformation in the Santander Massif. Crustal seismicity in Colombia is accurately correlated with active faults showing neotectonic morphological evidences. Intermediate seismicity allows to identify a N-NE trending subduction segment beneath the EC, which plunges toward the E-SE. This subduction is interpreted as a remnant of the paleo-Caribbean plateau (PCP) as suggested by geological and tomographic profiles. The PCP shows a low-angle subduction northward of 5.2°N and is limited southward by a major E-W transpressive shear zone. Normal oceanic subduction of the Nazca plate (NP) ends abruptly at the southern limit of the Baudo Range. Northward, the NP subducts beneath the Choco block, overlapping the southern part of the PCP. Cenozoic shortening in the EC estimated from a balanced section is ∼120 km. Stress analysis of fault slip data in the EC (northward of 4°N), indicates an ∼E-SE orientation of σ1 in agreement with the PCP subduction direction. Northward, near Bucaramanga, two stress solutions were observed: (1) a late Andean N80°E compression and (2) an early Andean NW-SE compression.

Identification of rolling resistance as a shape parameter in sheared granular media.

Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

Coseismic elastic models of folds above blind thrusts in the Betic Cordilleras (Spain) and evaluation of seismic hazard

Abstract Although it is generally considered that near-surface earthquakes result from movements along faults that cut through the surface, several recent large earthquakes have been partly attributed to blind thrusts. Movements along blind thrusts lead to the formation of surface folds, which are highly dependent upon fault geometry at depth and often not considered in seismic hazard evaluation. Several authors have studied the relationship between surface folding and thrusting for geological situations in which fault geometries are quite simple. However, active fault geometries can be quite complex e.g., segmented thrust faults associated with strike-slip faults. The aim of this contribution is to reconstruct the fault kinematics at depth for a relatively complex geological structure located in the Eastern Betic Cordilleras (Orihuela-Guardamar-Torrevieja region) using the patterns of kilometre-scale folds observed in the field. In order to model surface deformation, the assumption is made that surface km-scale folds have been created by coseismic deformation associated with movement along blind thrusts. By means of a coseismic deformation model, movements at depth have been calculated for three possible hypotheses. Hypothesis 1 assumes that each superficial fold is created by an independent fault. Hypotheses 2 and 3 assume that a sequence of two superficial folds can be created by movement along a single fault displaying a flat and ramp geometry. In Hypothesis 2, the flat is a superficial decollement level between the sedimentary cover and the Betic basement; in Hypothesis 3, it is a deeper decollement level within the Betic basement. Knowing the approximate age of surface deformation, rough estimates of fault slip-rates and recurrence periods for two possible earthquake magnitudes (7 Ms and 6.7 Ms) have been made, from calculated dislocations at depth. Slip-rates and recurrence periods for flat and ramp fault geometries are in the range of 0.75–1 mm/yr and 1000–2000 yr, respectively. These values are close to those calculated by direct methods in similar seismotectonic contexts.

Discrete element simulation of the Jiufengershan rock‐and‐soil avalanche triggered by the 1999 Chi‐Chi earthquake, Taiwan

We present Contact Dynamics discrete element simulations of the earthquake-triggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping similar to 22 degrees SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple slip-weakening"" criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure at the toe of the dip slope. As the avalanche propagates, the pop-up deforms into an overturned fold, which overrides the surface of separation along a decollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (mu(SS) = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake- and rain-triggered avalanches are similar.

Kinematic model for postorogenic Basin and Range extension

The Raft River extensional shear zone is exposed in the Albion-Raft River-Grouse Creek metamorphic core complex. Several studies of ductile deformation have shown that it accommodated crustal stretching in Tertiary time during late orogenic collapse of the thickened Cordilleran crust. Progressive deformation that results from mixed pure and simple shear produces a complex strain pattern along the shear zone. The authors propose a numerical kinematic model that relates strain variations in the shear zone to the different amounts of extension between the brittlely extended upper plate rocks and the undeformed lower plate rocks. A possible hypothesis for postorogenic crustal extension involves diffuse heterogeneous extension and flow in the ductile lower crust and localized extensional shear zones in the middle and upper crust.

The 19 January 1995 Tauramena (Colombia) earthquake: geometry and stress regime

Abstract The Tauramena (Colombia) earthquake, M w =6.5, occurred on January 19, 1995, in the Andean Eastern Cordillera foothill region, the so-called Piedemonte Llanero. The Harvard CMT focal mechanism indicates an almost pure reverse fault rupture. There was no surface faulting associated with this earthquake. This event was located at the northern tip of a zone, about 90 km in length, with relatively low microseismic activity along the central segment of the Piedemonte Llanero in Colombia. A field expedition to the epicentral area was organized and a temporary portable network was installed for 1 month to register aftershock activity. More than 800 events were recorded during this period. A subset of the best located aftershocks (319 events) shows epicenters extended over an area of 800 km 2 , and suggests two antithetic planes on which most of the activity was concentrated. The main event and a subset of 41 aftershocks occurred before the installation of the portable network but were recorded by the Colombian National Seismic Network and were relocated. They show an epicentral distribution similar to that of the subsequent events. Based on geologic information, aftershock locations, and focal mechanisms, we built a model for the Tauramena earthquake. Our results indicate that the Tauramena earthquake was produced by reverse faulting along a steep-dipping plane (dip ∼50°NW) associated with the Guaicaramo System. The fault plane cuts through basement rocks and folded Mesozoic and Cenozoic sedimentary rocks. Distribution of aftershocks in depth, suggesting the presence of splay reverse faults and a backthrust, is consistent with the hypothesis that the Guaicaramo Fault System was an old Mesozoic normal fault, reactivated during the Andean compression as a reverse fault. Stress tensor inversion of P-wave first motion polarities was performed, and focal mechanisms for the best recorded aftershocks were calculated. We found a well-defined sub-horizontal principal compression axis oriented in the NW–SE direction. The predominant reverse faulting for the Tauramena earthquake and the stability of σ 1 suggest that the tectonics in the central segment of the Eastern Cordillera Frontal Fault System is dominated by a compressive regime orthogonal to the cordillera, the controlling processes being probably the Caribbean subduction and the Choco Block collision.

Stress and strain from striated pebbles. Theoretical analysis of striations on a rigid spherical body linked to a symmetrical tensor

Striations on a pebble are interpreted as resulting from slip due to either a homogeneous stress state or a small homogeneous coaxial deformation in the matrix. In terms of stress, striations are assumed to be parallel to the applied shear stress. In terms of strain, striations are considered to be parallel to the relative tangential displacement between the pebble and adjacent matrix particles. Slip on the surface of a spherical rigid body enclosed in a deformable matrix (brittle or ductile) is theoretically analysed for different stress and strain regimes. The analysis predicts the topology of the resulting striations and singularity distribution on the sphere. Both in terms of stress and strain, the tangential vector field on the spheres surface derives from a potential function proportional to the magnitude of the normal vector field. Tangential and normal vectors represent either shear and normal stresses, or displacement components (in terms of strain). The plot of continuous curves parallel to striations (integral curves) and of equipotential curves on the sphere, allows simultaneously the magnitude and orientation of the tangential and normal vector fields to be visualized. Close to singular points, the integral curves correspond to power laws and the equipotentials correspond to conic sections. This theoretical analysis allows graphical method for estimating the stress ratio (σ2 − σ3)(σ1 − σ3) from striated faults to be proposed, once the orientations of the principal stress directions are known (i.e. by means of other graphical methods).

Impact of erosion and décollements on large-scale faulting and folding in orogenic wedges: analogue models and case studies

Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small- and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. Recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold–nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy). Supplementary material: Raw data of the experiments are available at www.geolsoc.org.uk/SUP18658.

Comparison of the effects of rolling resistance and angularity in sheared granular media

In this paper, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. For this purpose, we use contact dynamics simulations. By means of a simple shear numerical device, we investigate the mechanical behavior of these materials in the steady state in terms of shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. We find that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

Recent Morphological Changes of the Nice Continental Slope

Time-series bathymetric data acquired between 1991 and 2011 have been used to evaluate the recent morphological evolution of the Nice upper continental slope (SE France, Ligurian Sea). Small-scale landslides lead to a retrogressive evolution of the continental shelf/upper slope transition but their frequency, size and impact are not well known. Mapping was undertaken to identify the morphology of landslide scarps and the location of the shelfbreak. Map comparisons were performed using ArcGIS “raster calculator”. Sediment remobilization on the upper slope (up to depths of 200 m) is fast and significant; landslide scars with volumes greater than 25,000 m3 can appear with a frequency less than 8 years. Shelfbreak migration toward the coastline can reach rates of 60 m over 7–8 years where the continental shelf is over 200 m wide. Furthermore, this quantitative analysis highlights alternations between periods of strongly erosive events and sedimentation periods. On the upper slope, eroded volumes can be multiplied by 10 during periods of enhanced landslide patterns (1999–2006). Such cycle-like landslide activity raises the issue of the triggering processes. On the Nice continental slope thick poorly consolidated beds rapidly deposited on a steep slope, earthquakes and rainfall leading to fresh water circulation below the shelf were identified as potential triggers. Our 4D bathymetric study suggests that over the last 20 years the greatest impacting factor may be freshwater outflows.