Vincent Regard

Flat subduction dynamics and deformation of the South American plate: Insights from analog modeling

Received 14 June 2007; revised 13 January 2008; accepted 12 March 2008; published 21 June 2008. [1] We present lithospheric-scale analog models, investigating how the absolute plates’ motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction, possibly resulting in the appearance of flat slab segments, and how it changes the overriding plate tectonic regime. Experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions, part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab. Slab flattening is a long-term process (� 10 Ma), which requires the subduction of hundreds of kilometers of buoyant plateau. The overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect. Slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate. The increase of the overriding plate topography close to the trench results from (1) the buoyancy of the plate subducting at trench and (2) the overriding plate shortening. Experiments are compared to the South American active margin, where two major horizontal slab segments had formed since the Pliocene. Along the South American subduction zone, flat slab segments below Peru and central Chile/NW Argentina appeared at � 7 Ma following the beginning of buoyant slab segments’ subduction. In northern Ecuador and northern Chile, the process of slab flattening resulting from the Carnegie and Iquique ridges’ subductions, respectively, seems to be active but not completed. The formation of flat slab segments below South America from the Pliocene may explain the deceleration of the Nazca plate trenchward velocity. Citation: Espurt, N., F. Funiciello, J. Martinod, B. Guillaume, V. Regard, C. Faccenna, and S. Brusset (2008), Flat subduction dynamics and deformation of the South American plate: Insights

How does the Nazca Ridge subduction influence the modern Amazonian foreland basin

The subduction of an aseismic ridge has important consequences on the dynamics of the overriding upper plate. In the central Andes, the Nazca Ridge subduction imprint can be tracked on the eastern side of the Andes. The Fitzcarrald arch is the long-wavelength topography response of the Nazca Ridge flat subduction, 750 km inboard of the trench. This uplift is responsible for the atypical three-dimensional shape of the Amazonian forelland basin. The Fitzearrald arch uplift is no older than Pliocene as constrained by the study of Neogene sediments and geomorphic markers, according to the kinematics of the Nazca Ridge subduction.

Cumulative right-lateral fault slip rate across the Zagros-Makran transfer zone: role of the Minab-Zendan fault system in accommodating Arabia-Eurasia convergence in southeast Iran

The Zendan-Minab zone is the transition zone between the Zagros collision to the west and Makran subduction to the east. It is also linked to the north with the Nayband-Gowk fault system that bounds the Lut Block to the east. The total convergence rate between Arabia and Eurasia is estimated to range between 23 and 35 mm yr-1 in a NNE-trending direction. The deformation through the Minab-Zendan system is accommodated within two fault systems, the western N160°E-trending Minab-Zendan fault system and the eastern north-south Sabzevaran-Jiroft fault system. The study area is characterized by a well-defined succession of Quaternary deposit levels. The age of these deposits was estimated by archaeological data, regional palaeoclimate correlations and constrained by additional in situ10Be dating in another paper in this study. These deposits exhibit offsets, both lateral and vertical, that are evaluated by satellite image analysis and GPS profiles. Thanks to offsets and ages the strike-slip rates associated with the Minab-Zendan and the Sabzevaran-Jiroft fault systems are calculated to be 5.1 +/- 1.3 or 6.6 +/- 1.5, and 6.2 +/- 0.7 mm yr-1, respectively. These results allow an evaluation of the velocity vector of the Musandam Peninsula (Oman) with respect to the Lut Block of 11.4 +/- 2.0 or 12.9 +/- 2.2 mm yr-1 in a N10 +/- 15°E direction, close to the GPS estimates. This study also constrains the in-plane slip rates for each fault. Previous works indicate that the Zagros accommodates only 10 mm yr-1 of shortening, while 10 mm yr-1 should be accommodated by the Alborz mountains in northern Iran. This last 10 mm yr-1 may be accommodated through the Nayband-Gowk system and the East Iranian ranges, implying that the two fault systems constituting the Zagros-Makran transfer zone have different geodynamic roles. The western Minab-Zendan fault system links the Makran and Zagros deforming zones, whereas the northwestern Jiroft-Sabzevaran fault system is transmitting the deformation to the Nayband-Gowk system and then to the Alborz ranges. The presence of another such strike-slip zone within the Makran seems to indicate that the accommodation zone between the Zagros and Makran is wide, of the order of 400 km. We interpret this deformation pattern that accompanies the genesis of the immature transform zone by a flexure of the slab under the Zagros-Makran transfer zone instead of a tear in the slab that may be expected to induce a sharper transition zone.

Dynamical effects of subducting ridges: insights from 3-D laboratory models

SUMMARY We model using analogue experiments the subduction of buoyant ridges and plateaus to study their effect on slab dynamics. Experiments show that simple local (1-D) isostatic considerations are not appropriate to predict slab behaviour during the subduction of a buoyant ridge perpendicular to the trench, because the rigidity of the plate forces the ridge to subduct with the dense oceanic lithosphere. Oceanic ridges parallel to the trench have a stronger effect on the process of subduction because they simultaneously affect a longer trench segment. Large buoyant slab segments sink more slowly into the asthenosphere, and their subduction result in a diminution of the velocity of subduction of the plate. We observe a steeping of the slab below those buoyant anomalies, resulting in smaller radius of curvature of the slab that augments the energy dissipated in folding the plate and further diminishes the velocity of subduction. When the 3-D geometry of a buoyant plateau is modelled, the dip of the slab above the plateau decreases, as a result of the larger velocity of subduction of the dense ‘normal’ oceanic plate on both sides of the plateau. Such a perturbation of the dip of the slab maintains long time after the plateau has been entirely incorporated into the subduction zone. We compare experiments with the present-day subduction zone below South America. Experiments suggest that a modest ridge perpendicular to the trench such as the present-day Juan Fernandez ridge is not buoyant enough to modify the slab geometry. Already subducted buoyant anomalies within the oceanic plate, in contrast, may be responsible for some aspects of the present-day geometry of the Nazca slab at depth.

Slope and climate variability control of erosion in the Andes of central Chile

Climate and topography control millennial-scale mountain erosion, but their relative impacts remain matters of debate. Confl icting results may be explained by the infl uence of the erosion threshold and daily variability of runoff on long-term erosion. However, there is a lack of data documenting these erosion factors. Here we report suspended-load measurements, derived decennial erosion rates, and 10 Be-derived millennial erosion rates along an exceptional climatic gradient in the Andes of central Chile. Both erosion rates (decennial and millenial) follow the same latitudinal trend, and peak where the climate is temperate (mean runoff ~500 mm yr ‐1 ). Both decennial and millennial erosion rates increase nonlinearly with slope toward a threshold of ~0.55 m/m. The comparison of these erosion rates shows that the contribution of rare and strong erosive events to millennial erosion increases from 0% in the humid zone to more than 90% in the arid zone. Our data confi rm the primary role of slope as erosion control even under contrasting climates and support the view that the infl uence of runoff variability on millennial erosion rates increases with aridity.

Probabilistic coastal cliff collapse hazard from repeated terrestrial laser surveys: case study from Mesnil Val (Normandy, northern France)

ABSTRACT Dewez, T.J.B., Rohmer, J., Regard, V., and Cnudde, C., 2013, Probabilistic coastal cliff collapse hazard from repeated terrestrial laser surveys: case study from Mesnil Val (Normandy, northern France) Along cliff edges, coastal managers dealing with life-safety matters often wonder “how much time have I got left before I need to expropriate this house?” Here, we present a case study where repeated terrestrial laser scanner surveys (TLS) were performed to monitor a chalk cliff section in Normandy. The Mesnil Val cliff, cut in Upper Cretaceous chalk known as the Upper Lewes Nodular Chalk, is a ca. 750-m-long, 20–80m-high cliff section. It was surveyed 6 times between December 2005 and March 2008 at a 1 –point-per-5cm resolution. Successive Digital Surface Models (DSM) of the cliff surface were subtracted to reveal the location and shape of erosion scars. Scars detection relies on a robust indicator of observed noise distribution. The resulting scar inventory contains more than 8500 objects with volumes spanning 8 orders of magnitude (10−4 to 104) cubic meters. Probability Distribution Functions (PDF) of erosion scar thickness, area and volume scale as power laws. The scaling relationships between area (0.77) and volume (0.53) power law exponents demonstrate that rockfall are simply scaled (area/volume exponent ratio = 3/2). This finding legitimates the inference of rockfall volume distribution from easy-to-measure area PDF. PDF can also be turned into complementary cumulative distribution function (CCDF) which then gives the annual rate of exceeding a given scar dimension. Our statistical estimates have been empirically validated with five complete cliff collapse events occurring on the same cliff section in 13 years. Repeated TLS surveys provide invaluable information to help coastal managers make informed decision.

Erosion in the Chilean Andes between 27°S and 39°S: tectonic, climatic and geomorphic control

Abstract The effect of mean precipitation rate on erosion is debated. Three hypotheses may explain why the current erosion rate and runoff may be spatially uncorrelated: (1) the topography has reached a steady state for which the erosion rate pattern is determined by the uplift rate pattern; (2) the erosion rate only depends weakly on runoff; or (3) the studied catchments are experiencing different transient adjustments to uplift or to climate variations. In the Chilean Andes, between 27°S and 39°S, the mean annual runoff rates increase southwards from 0.01 to 2.6 m a−1 but the catchment averaged rates of decadal erosion (suspended sediment) and millennial erosion (10Be in river sand) peak at c. 0.25 mm a−1 for runoff c. 0.5 m a−1 and then decrease while runoff keeps increasing. Erosion rates increase non-linearly with the slope and weakly with the square root of the runoff. However, sediments trapped in the subduction trench suggest a correlation between the current runoff pattern and erosion over millions of years. The third hypothesis above may explain these different erosion rate patterns; the patterns seem consistent with, although not limited to, a model where the relief and erosion rate have first increased and then decreased in response to a period of uplift, at rates controlled by the mean precipitation rate.

Coastal chalk platform erosion modulated by step erosion and debris shielding: example from Normandy and Picardy (northern France)

ABSTRACT Regard, V., Dewez, T., Cnudde, C. and Hourizadeh, N., 2013. Coastal chalk platform erosion modulated by step erosion and debris shielding: example from Normandy and Picardy (northern France) Coastal platforms and cliffs compose an inter-dependant dynamic system. To understand how cliffed coast evolve, it is therefore crucial to quantify the magnitude and dependence of their processes. Here we question the role of erosion by waves at platform steps, often linked to lithological discontinuities. Along the French chalky coast of the English Channel, we determine, by comparison of aerial photographs at 23 yr interval that the mesoscale steps are retreating at a rate of ~2 cm/yr, ca. 10% cliff retreat rate. This corresponds to an average erosion rate of 0.32 ± 0.08 mm/a, quite uniform across the study area, suggesting that the geology has more of a control on erosion processes than the position in the tidal frame. In reality, erosion occurs only where platform surface is not shielded by debris fallen from the cliff. We study their size distribution (16% of the platform) and location and draw the conclusion that small falls are quickly erased by wave action while far reaching fans may cover the entire platform width. The peak shielding by fan debris occurs at a distance of ~40 m from the cliff. Step erosion by block quarrying is the process likely responsible for 20% to 100% of platform erosion.

Comment on "Tectonic record of strain buildup and abrupt coseismic stress release across the northwestern Peru coastal plain, shelf, and continental slope during the past 200 kyr" by Jacques Bourgois et al.

Citation: Pedoja, K., L. Ortlieb, T. J. Devries, J. Machare, L. Audin, and V. Regard (2011), Comment on “Tectonic record ofstrain buildup and abrupt coseismic stress release across the northwestern Peru coastal plain, shelf, and continental slope duringthe past 200 kyr” by Jacques Bourgois et al., J. Geophys. Res., 116, B09401, doi:10.1029/2011JB008321.

How does the Nazca Ridge subduction influence the modern Amazonian foreland basin?: COMMENT and REPLY REPLY

In their Comment of our Geology paper ([Espurt et al., 2007][1]), [Clift and Ruiz (2008)][2] argue that: 1) the flat-slab subduction of the Nazca Ridge is unlikely to have produced uplift of the Fitzcarrald Arch in the Amazonian retroforeland basin (using geologic data from the forearc area); 2)