Stéphane Brusset

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.

Miocene semidiurnal tidal rhythmites in Madre de Dios, Peru

New data from upper Miocene deposits in the Madre de Dios region, southern Peru, allow the delineation of tidal regime for the first time in western Amazonia and provide strong evidence of elevated tidal range and brackish-water influence. The results point out the insufficiency of the current depositional models and support the earlier hypothesis that western Amazonia was also connected to the Paranan Sea during the late Miocene. In this paper we present sedimentological, ichnological, and statistical (Fourier transformation) data from two selected outcrops containing rhythmite successions from an area that is traditionally considered as continental. The sediments are interpreted to represent tide-dominated, inner-middle estuarine deposits. The cyclic rhythmites display semidiurnal cyclicity. The results are significant because (1) they contradict recent interpretations of the areas paleoenvironmental history; (2) the evidence for tidal processes is persuasive; and (3) the delineated tidal regime and range provide a unique insight into the depositional dynamics of a system having many important paleogeographic implications.

Three-dimensional geologic imaging and tectonic control on stratigraphic architecture: Upper Cretaceous of the Tremp Basin (south-central Pyrenees, Spain)

In the Tremp Basin area (south Pyrenean foreland, Spain), the Campanian–Maastrichtian Orcau-Vell and Santa Engracia depositional sequences onlap the western termination of the Sant Corneli anticline. The precise mapping of the different systems tracts belonging to these depositional sequences, their spatial arrangement, and the structural control of the anticline on the sedimentation still remained unclear. To accurately interpret the geometry of the depositional sequences and to determine the factors influencing the sedimentation, we have developed a method that aims to build a three-dimensional (3-D) geological picture of this area. The originality of our approach is that the 3-D map, which consists of the volume and shape of all the systems tracts, has been produced mainly from the interpretation and combination of surface data, including a mosaic of aerial photographs at 50-cm (20-in.) pixel size and a digital elevation model at 10-m (33-ft) resolution. We have additionally constrained the model by integrating bedding dip and strike data and balanced cross sections. With respect to the 2-D, the 3-D visualization and field observations reveal the structural control at different scales of the lateral propagation of a fault-propagation fold (Sant Corneli anticline) on the stratigraphic architecture. The Orcau-Vell depositional sequence was controlled by the rise of the base level and was characterized by differences in the sedimentation rates. The emplacement of a north-south–trending gravitational normal fault, located at the western tip of the Sant Corneli anticline, was coeval with the emplacement of the Santa Engracia depositional sequence. This fault resulted from the westward propagation of the Sant Corneli anticline, generating a local slope and a depression that channeled the turbidites and the Gilbert-type delta deposits of the Santa Engracia depositional sequence. Uplift of the Sant Corneli anticline may have subsequently stopped, and the area subsided, inducing a rapid rise of the base level.

Time, place and mode of propagation of foreland basin systems as recorded by the sedimentary fill: examples of the Late Cretaceous and Eocene retro-foreland basins of the north-eastern Pyrenees

Abstract The relationship between tectonics and sedimentary fill has been studied in two syncontractional basins of the western Corbières (eastern North Pyrenean retro-foreland basin). The Late Cretaceous basin formed during c. 10–12 Ma as a result of left-lateral transpressional deformation, and is composed of forward-younging sub-basins characterized by reworking of the forelimbs of growing fold-propagation folds. Thrust-wedge advance and cratonward migration of the platform are recorded by a deepening-upward stacking pattern indicating increased regional subsidence with a limited contribution of the submarine orogen. Tectonic quiescence and erosional unloading lasting 29–30 Ma are recorded by a shallowing-upward stacking pattern, and fluvial sedimentation issued from widespread sources in the emerging inner orogen. The Early to Middle Eocene basin formed as a result of pure shortening normal to the range. The marine Early Eocene basin developed during c.2 Ma by widening of a single basin provoked by the two-step propagation of a basement duplex. This is recorded by growth-stratal patterns and coarsening-upward depositional sequences indicating the increasing contribution of the emerged orogen. The Middle Eocene continental deposits infilled two sub-basins working synchronously and were transported by alluvial fans with a provenance in the inner orogen, during decreasing thrust-wedge advance and increasing erosional unloading.

Partition between collision and subduction accretionary prisms along an inherited transcurrent fault zone: New insights on the Taiwan fold and thrust belt

A new geotectonic framework of the Taiwan orogen is presented in accordance with the hypothesis of an oblique arc- arc collision. The colliding Luzon arc is physically connected to the eastern Coastal Range in which a subduction complex remnant is preserved and backthrust with intra-arc sediments in a small retroforeland basin. A southern and extinct extension of the Ryukyu arc is characterized in western Taiwan. It displays a duplex structure (Tananao and Backbone horses and Lishan triangle zone) between a buried floor thrust located in the arc crust and a roof thrust developed in the arc cover (Hsfiehshan Range and South Backbone Range). Westward the basal thrust climbs in the sedimentary series of the western proforeland (Foothills and Hengchun Peninsula) and dies out in a buried tip line. The northern part of the orogen, including all the Tananao arc core, is shown as an intra-oceanic-continental arc-arc collision belt characterized by an unroofed duplex culmination above a leading floor thrust and both proforeland and retroforeland basins. The southern part, which displays a roof thrust sequence above a buried duplex, is shown as an accretionary prism built in a transition zone between continent and oceanic subduction (transition from the Asian continental crust, including the former Ryukyu arc, to the oceanic Old Philippine Sea crust). The partition is believed to be induced by a deep intracontinental transcurrent fault zone able to influence the difference in shortening, duplex pattern, and leading thrust depth. The evolution was controlled by the Ryukyu subduction (backarc extension, arc magmatism extinction, and cooling and intra-arc collapse) until the early middle Miocene (around 15 Ma) and then it was controlled by the Luzon arc progression (continental subduction, collision, indentation, and hinterland uplift and frontal thrust propagation).

Seismic evidence of gas hydrates, multiple BSRs and fluid flow offshore Tumbes Basin, Peru

Identification of a previously undocumented hydrate system in the Tumbes Basin, localized off the north Peruvian margin at latitude of 3°20′—4°10′S, allows us to better understand gas hydrates of convergent margins, and complement the 36 hydrate sites already identified around the Pacific Ocean. Using a combined 2D–3D seismic dataset, we present a detailed analysis of seismic amplitude anomalies related to the presence of gas hydrates and/or free gas in sediments. Our observations identify the occurrence of a widespread bottom simulating reflector (BSR), under which we observed, at several sites, the succession of one or two BSR-type reflections of variable amplitude, and vertical acoustic discontinuities associated with fluid flow and gas chimneys. We conclude that the uppermost BSR marks the current base of the hydrate stability field, for a gas composition comprised between 96% methane and 4% of ethane, propane and pure methane. Three hypotheses are developed to explain the nature of the multiple BSRs. They may refer to the base of hydrates of different gas composition, a remnant of an older BSR in the process of dispersion/dissociation or a diagenetically induced permeability barrier formed when the active BSR existed stably at that level for an extended period. The multiple BSRs have been interpreted as three events of steady state in the pressure and temperature conditions. They might be produced by climatic episodes since the last glaciation associated with tectonic activity, essentially tectonic subsidence, one of the main parameters that control the evolution of the Tumbes Basin.

Petroleum Systems Restoration of the Huallaga—Marañon Andean Retroforeland Basin, Peru

Abstract The Huallaga–Maranon retroforeland basin system of northern Peru is deformed by both thick- and thin-skinned tectonics. The thrust system is complex and resulted from the reactivation of a west-verging Permian fold and thrust belt capped by an important salt detachment. This chapter presents 2-D petroleum modeling from an updated balanced cross section and sequential restoration through the Huallaga–Maranon wedge-top basin. The sequential restoration has been calibrated by thermochronological dating and thickness variations in Cenozoic synorogenic sediments. It shows two important stages of the deformation (Middle Eocene and Late Early Miocene). Late Triassic/Early Jurassic Pucara Group and Late Cretaceous (Raya and Chonta formations) classic source rocks are present in the Huallaga–Maranon foreland basin, but the revision of the stratigraphy replaced in its updated structural context allowed us to highlight a new Late Permian source rock (Shinai Formation). 2-D modeling of kerogens maturity evolution and hydrocarbon (HC) accumulations in the sequential restoration shows that first Andean structures (Middle Eocene and Late Early Miocene) could preserve HC accumulations in the Chazuta thrust sheet footwall. In the eastern Maranon basin, more recent structures (Late Miocene–Pliocene) such as Santa Lucia could also have been charged. Deep subthrust structures stay unexplored in the Peruvian fold and thrust belts. The Huallaga–Maranon foreland system is probably the best example of subtrap attractiveness in Peru.

Deciphering the Late Cretaceous‐Cenozoic Structural Evolution of the North Peruvian Forearc System

The link between plate tectonics and the evolution of active margins is still an ongoing task to challenge since the acceptance of plate tectonic paradigm. This paper aims at deciphering the structural architecture and uplift history of the North Peruvian forearc system to better understand the evolution and the mechanics that govern the Late Cretaceous-Cenozoic building of this active margin. In this study, we report surface structural geology data, interpretation of seismic reflection profiles, apatite fission-track data, and the construction of two offshore-onshore crustal-scale balanced cross sections. The structure of the North Peruvian forearc system is dominated by an accretionary style with northwestward propagation of thrust-related structural highs involving continental/oceanic basement rocks, and off-scrapped sediments. The thrust systems bound thick thrust-top forearc depocenters mainly deformed by crustal normal to strike-slip faults and thin-skinned gravitational instabilities. The sequential restoration of the margin calibrated with apatite fission-track data suggests a correlation between uplift, shortening and plate convergence velocity during Late Cretaceous and Miocene. Pliocene-Quaternary shortening and uplift of the coastal zone is rather related to the subduction of asperities during convergence rate decrease. The development of crustal normal to strike-slip faulting and subsidence zones might be the consequence of slab flexure, local basal erosion along subduction fault, and/or oblique subduction associated with sediment loading control. We conclude that the evolution of the North Peruvian forearc system was controlled by subduction dynamics, strong sediment accumulation and recent ridge subduction, and it recorded the orogenic loading evolution of the Andes over the Cenozoic.

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)