Nicolas Espurt

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

Horizontal subduction zones, convergence velocity and the building of the Andes.

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene-Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene.

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.

Amber from western Amazonia reveals Neotropical diversity during the middle Miocene

Tertiary insects and arachnids have been virtually unknown from the vast western Amazonian basin. We report here the discovery of amber from this region containing a diverse fossil arthropod fauna (13 hexapod families and 3 arachnid species) and abundant microfossil inclusions (pollen, spores, algae, and cyanophyceae). This unique fossil assemblage, recovered from middle Miocene deposits of northeastern Peru, greatly increases the known diversity of Cenozoic tropical–equatorial arthropods and microorganisms and provides insights into the biogeography and evolutionary history of modern Neotropical biota. It also strengthens evidence for the presence of more modern, high-diversity tropical rainforest ecosystems during the middle Miocene in western Amazonia.

Lithospheric structural control on inversion of the southern margin of the Black Sea Basin, Central Pontides, Turkey

To illustrate the structural evolution of the Black Sea Basin in the context of Neotethyan subduction and subsequent continental collisions, we present the first lithosphere-scale, ∼250-km-long, balanced and restored cross section across its southern continental margin, the Central Pontides. Cross-section construction and restoration are based on field, seismic-reflection, geophysical, and apatite fission-track data. The structure of the onshore Pontides belt is predominantly controlled by inverted normal faults, whereas the offshore areas are devoid of large structural inversion. The restored section indicates that Cretaceous crustal thinning occurred synchronously with (probably buoyancy-driven) exhumation of a forearc high-pressure blueschist wedge likely during Neotethyan slab retreat. Apatite fission-track data show that structural inversion of the forearc zone, which formed the Central Pontides fold-and-thrust belt, started at ca. 55 Ma. This Eocene structural inversion followed upon collision of the Kirs¸ehir continental block and the arrest of Neotethyan oceanic subduction below the Central Pontides. Compared to the Central Pontides belt, which underwent significant shortening (∼28 km, i.e., ∼33%), the relatively colder and stronger Black Sea lithosphere prevented the northern offshore areas from undergoing inversion. We propose that the location of Cenozoic contractional deformation is related to the absence of lithospheric mantle below the southern Pontides (forearc) zone as a consequence of the Cretaceous high-pressure wedge exhumation.

A scenario for late Neogene Andean shortening transfer in the Camisea Subandean zone (Peru, 12°S): Implications for growth of the northern Andean Plateau

Precise knowledge of the timing of deformation in the Subandean zone of the Andean Plateau is a prerequisite for deciphering the late Neogene growth of the Andean Plateau. In this paper, we report new apatite fission-track (AFT) and vitrinite reflectance (Ro) data for a regional balanced cross section of the Camisea Basin in the central Peruvian Subandean zone, adjacent to the northern Andean Plateau. The balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust- related anticlines. The balanced cross-section restoration shows 53 km (39%) of total horizontal shortening. We sampled Paleozoic to Cenozoic sedimentary strata for AFT and Ro analyses along the ∼4-km-thick vertical profile of the Mainique back thrust (passive roof thrust), the innermost preserved Subandean structure. Young components of AFT ages are spread between ca. 6 Ma and ca. 24 Ma. A break in the slope in the AFT ages determines the geometry of the Miocene partial annealing zone and the exhumation of the Mainique back thrust at ca. 6 Ma. Sequential restoration calibrated by AFT and Ro data indicates that the last ∼23 km horizontal shortening were accommodated by the Camisea thrust system over the past ∼6 m.y., giving a mean shortening rate of 3.8 mm/yr. Using this shortening rate for the first ∼30 km horizontal shortening, we calculate that the Andean shortening transfer into the Peruvian Subandean zone initially started at ca. 14 Ma. This result suggests that the transfer of shortening from the northern Andean Plateau to the Subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca. 10 Ma and ca. 7 Ma. We rather propose that the late Neogene growth of the northern Andean Plateau mostly resulted from a continuous crustal shortening combined with lower-crustal flow.

Reconstruction of the Provence Chain evolution, southeastern France

The Provence fold-and-thrust belt forms the eastern limit of the Pyrenean orogenic system in southeastern France. This belt developed during the Late Cretaceous-Eocene Pyrenean-Provence compression and was then deformed by Oligocene-Miocene Ligurian rifting events and Neogene to present-day Alpine compression. In this study, surface structural data, seismic profiles, and crustal-to-lithospheric-scale sequentially balanced cross sections contribute to the understanding of the dynamics of the Provence Chain and its long-term history of deformation. Balanced cross sections show that the thrust system is characterized by various structural styles, including deep-seated basement faults that affect the entire crust, tectonic inversions of Paleozoic-Mesozoic basins, shallower decollements within the sedimentary cover, accommodation zones, and salt tectonics. This study shows the prime control of the structural inheritance over a long period of time on the tectonic evolution of a geological system. This includes mechanical heterogeneities, such as Variscan shear zones, reactivated during Middle Cretaceous Pyrenean rifting between Eurasia and Sardinia. In domains where Mesozoic rifting is well marked, inherited basement normal faults and the thermally weak crust favored the formation of an inner thick-skinned thrust belt during Late Cretaceous-Eocene contraction. Here 155 km (similar to 35%) of shortening was accommodated by inversion of north verging crustal faults, north directed subduction of the Sardinia mantle lithosphere, and ductile thickening of the Provence mantle lithosphere. During the Oligocene, these domains were still predisposed for the localized faulting of the Ligurian basin rifting and the seafloor spreading.

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)