Michel Sébrier

Changes in the tectonic regime above a subduction zone of Andean Type: The Andes of Peru and Bolivia during the Pliocene‐Pleistocene

This paper adresses the changes in the tectonic regime in the Peruvian and Bolivian Andes that have occurred since the upper Miocene when the present-day elevation of the Cordillera above sea level has been almost reached. The stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes. The Cuzco fault system in southern Peru is chosen as an example to illustrate the methodology used. In this region, striations on both active and Holocene faults are in agreement with a N-S extension. But faults affecting early Pleistocene deposits exhibit two families of striations. The younger results from the previous N-S extension: the older, involving reverse motions, results from either an E-W or a N-S compression. Faults affecting Pliocene formations often show an oldest family of striations resulting from a NE-SW or an E-W trending extension. Thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data: (1) a Pliocene extensional regime, (2) a lower Pleistocene compressional regime, and (3) a mid-Pleistocene-present-day extensional regime. Similar analyses conducted in the Pacific and sub-Andean lowlands allow sketching of the successive Pliocene-Pleistocene stress patterns in the Central Andes. The Quaternary and present-day stress pattern is characterized by a N-S extension in the High Andes and in the Pacific lowlands and by an E-W compression in the sub-Andean lowlands and at the contact between the Nazca and South American plates. This stress pattern is interpreted at a large wavelength (>100 km) as an effect of compensated topography. This model supposes that the vertical lithospheric stress, σzz, increases with the topography, the crustal thickness, and the low-density mantle beneath and that the lithospheric maximum (compressional) horizontal stress σHmax, trending E-W roughly parallel to the convergence, is fairly constant. On both edges of the Andes, the tectonics being compressional, σzz is σ3 and σHmax is σ1. In the High Andes, σzz becomes σ1, then the E-W trending σHmax is σ2 and σHmin trending N-S is σ3, allowing extension to occur in this direction. The Pliocene stress pattern was characterized by a NE-SW or an E-W trending extension in the High Andes, in the Pacific lowlands, and possibly in the sub-Andean lowlands. This stress pattern was clearly different from the present-day one because the E-W trending stress was σHmin. This required a weak push or, eventually, tractional boundary forces acting on the South American lithosphere. It is suggested that this might result from a strong slab pull due to a long, steeply dipping slab which decreased the value of the σxx stress transmitted to the overriding plate. The early Pleistocene state of stress was compressional. Since the elevation of the Andes had not markedly decreased during this period, this required an increase of the E-W trending stress value. This resulted from a strong coupling between the two lithospheric plates, possibly due to a rupture of a long slab under its own weight. Other spatial changes in the stress pattern are related to the particular situation of the forearc, to the subduction of the buoyant Nazca ridge, and to the different dips of the slab. Extension in the High Andes is of small magnitude, of the order of 1% during the last 1–2 m.y.; in a few basins, it may have attained 40% during the Pliocene (≈5–3 m.y.).

Quaternary state of stress in the Northern Andes and the restraining bend model for the Ecuadorian Andes

Inversion of shallow focal mechanisms in the Northern Andes, together with a neotectonic analysis of the Ecuadorian Andes, shows that the state of stress is homogeneous in most of the Northern Andes (E-W-trending σ1). However, north of 5°N, σ1 is roughly NW-SE trending. This difference in state of stress is due to the force balance between the Nazca, Caribbean and South American plates. South of 5°N, the state of stress appears to be mainly controlled by the Nazca-South American plate interaction, while, north of 5°N it appears to be mainly controlled by the Caribbean-South American plate interaction. This state of stress difference from south to north is consistent with dextral motions along faults trending parallel to the chain south of 5°N and sinistral motions along faults trending parallel to the chain north of 5°N. This inversion of shallow focal mechanisms also illustrates the incomplete strain partitioning of the oblique convergence between the Nazca and South American plates along the Ecuadorian-southern Colombian trench. The convergence obliquity that increases northward controls the active transcurrent fault slip rate in the upper plate. Finally, this convergence obliquity is responsible for the location of the seismicity in the upper plate. In addition, the Andean Block is considered in this paper as a large triple zone under constriction, with one of its borders (Caribbean) behaving partly as a free border.

Relationship between tectonism and volcanism along the Great Sumatran Fault Zone deduced by spot image analyses

Abstract Satellite images provide evidence for numerous stepovers, pull-apart grabens and volcanic structures along the NW-trending right-lateral Great Sumatran Fault Zone. High-resolution SPOT images permit us to analyze the relationship between volcanic and tectonic structures. This analysis reveals that releasing stepovers and pull-aparts are ephemeral structures along the Great Sumatran Fault Zone and that the geometry of the strike-slip fault evolves permanently through time. The interpretative structural maps obtained from SPOT image analysis reveal that the formation of huge, peculiarly shaped, volcanic calderas has occurred in large releasing stepover fault zones and that the bounding faults of rectangular pull-apart basins are analogous to the circular ring faults of calderas. In particular, detailed study of the segments at the southernmost, 150-km-long termination of the Great Sumatran Fault Zone, from northwest of Ranau Lake to Semangka Bay, allows us to describe the development of the Ranau collapse caldera. This caldera is located within a pull-apart basin bounded by a stepover. Its peculiar rectangular shape and its relatively large size (about 200 km 2 ) are controlled by the evolution of the extinct Ranau releasing stepover. Similarly, the Toba elliptical caldera, one of the largest volcanic caldera in the world, is elongated parallel to the present trace of the Great Sumatran Fault and appears to be related to a releasing stepover associated with a wide pull-apart basin that is not active at present.

Local erosion rates versus active tectonics: cosmic ray exposure modelling in Provence (south-east France)

Over the past decade, in situ-produced cosmogenic nuclides have revolutionised the study of landscape evolution. In particular, numerous studies have demonstrated that, in active tectonic settings, cosmic ray exposure dating of deformed or displaced geomorphic features makes it possible to quantify long-term deformation rates. In western European countries, erosion due to climatically driven processes and human activities is probably the factor that most limits the accuracy of exposure ages and landscape modification rates. In this study, we present the results of a depth-profiling technique applied to alluvial terraces located along the Rhone and the Moyenne Durance rivers. The expected decrease with depth of the measured 10Be concentrations has been modelled using a χ2 inversion method in order to constrain the exposure history of the alluvial sediments. The results suggest that: (1) over the Quaternary, the local surface erosion rates including both regional uplift and climatically driven processes acting on landforms are on the order of 30 m/Myr in southeastern France, and (2) providing a fairly good bracketing of the exposure age, the modelled abandonment age of alluvial terraces affected by the Moyenne Durance Fault allows estimating incision rates, comparing the alluvial terrace elevations with topographic river profiles, and a minimum vertical slip rate value of roughly 0.02 mm/yr for the southern segment of the Moyenne Durance Fault.

Cosmogenic dating ranging from 20 to 700 ka of a series of alluvial fan surfaces affected by the El Tigre fault, Argentina

It is crucial to date continental landforms to quantify processes involved in terrestrial surface evolution, especially in regions affected by active tectonics. Andean quaternary alluvial fan surfaces affected by the El Tigre strike-slip fault have been studied using combined geomorphic and 10 Be exposure age approaches. Field observations and SPOT (French acronym for “Satellite for Observation of the Earth”) image analysis enable the identification of six alluvial fan units. Measurements of in situ–produced cosmogenic 10 Be concentrations in quartzite boulders exposed on the top of fan surfaces show that the depositional periods ended during successive major interglacial stages. The calculated minimum exposure ages date the abandonments of the alluvial fan surface from 41 000 ± 8500 yr for the youngest to 670 000 ± 140 000 yr for the oldest unit. When linked to the measured maximum cumulative right-lateral displacement of stream channels, the exposure ages yield a horizontal slip rate of about 1 mm/yr on the El Tigre fault. This study shows that for arid regions, where fan surface erosion is minimal, in situ–produced 10 Be can be used to constrain the age of stratigraphically separate alluvial fan surfaces. These fan surface exposure ages can be further used to calculate slip rates on active faults and infer depositional periods correlative with climatic events.

Paleoseismicity in France: Fault trench studies in a region of moderate seismicity

Abstract France is a country of moderate to low seismicity with a ten-century record of historical seismicity. Paleoseismicity appears to be the only available tool which is able to extend the record of seismic activity beyond this short time-window. However, moderate to low seismicity should be associated with low slip rates and, thus, geomorphic evidence for detecting active fault traces should be weak, impeding the development of paleoseismic fault studies. To test the paleoseismic method in France, we have developed a programme on paleoseismology during the past few years. First, we performed a critical reappraisal of existing data on French active tectonics, then we selected several suitable sites, and finally we trenched on some sites. Much of the revised evidence from recent and active tectonics was not selected because their origins were either dubious or non-tectonic. In particular, many reported sites from the Alps, seen in moraines, actually correspond to glacial processes (‘glacitectonics’). In addition, some clearly observed scarps could result from gravitational processes such as post-glacial rebound. Among the selected sites, three were studied in detail; they correspond to the Nimes, Moyenne Durance, and Argentera fault zones. The most reliable information was provided by trenching observations on the Moyenne Durance Fault. The trench site exhibited a N35 °E-striking knee fold, parallel to the Moyenne Durance Fault, covered unconformably by colluvial deposits. Radiocarbon dates indicate that folding occurred during the latest Pleistocene or earliest Holocene. The observed stratigraphical relations strongly suggest that folding resulted from a single coseismic slip event associated with a strong earthquake of magnitude Mw = 6.4–6.9. Our data also indicate long recurrence intervals for such earthquakes, at least of the order of 25 ka. Paleoseismic data collected on the other studied fault zones confirm that nearly Mw = 7 events may be generated by French active faults and that the recurrence intervals should be over 10 ka. Thus, the paleoseismic study of active faults is a robust tool to reconstruct seismic history where slip rates are low to moderate; however, specific exploration methods should be developed to localize fault traces more accurately and study hidden faults.

Paleoseismicity and seismic hazard along the Great Sumatran Fault (Indonesia)

Abstract The Great Sumatran Fault (GSF) is a 1650-km-long dextral strike-slip fault zone which accommodates part of the oblique convergence of the subduction between the Indo-Australian and Eurasian plates. To define the seismic hazard along this fault, we used paleoseismology and neotectonics. To characterise the seismic history of the southern GSF we excavated four trenches. Within these trenches, the occurrence of only one paleosol related to a seismic event indicates that in a wet, tropical region, the degradation rate of organic matter could be faster than seismic recurrence. The trenching method permitted us to identify only one recent earthquake, reactivating the southern GSF. As the trenching method does not seem efficient to constrain knowledge of seismicity in this region, we have developed an active tectonic study to characterise the seismic hazard along the GSF. We created a large-scale segmentation map which allows 18 major fault segments with lengths ranging between 45 and 200 km to be recognised. We complemented the segmentation map reporting major earthquake ruptures on the basis of the historical seismicity which recorded 17 earthquakes since 1835. The segmentation map indicates a northward increase of segment lengths which parallels the GSF slip-rate increase. This observation suggests a northward increase of seismic hazard along the GSF. Segmentation and historical seismicity provide evidence of a 300-km-long seismic gap (between 3 °N and 5 °N) around a locked restraining bend which can be considered as having high potential for seismic hazard in Sumatra. The magnitude of the maximum expected earthquake for each segment was estimated through two empirical methods. These estimates give higher maximum magnitude and shorter seismic recurrence intervals for segments in northern Sumatra, confirming a northward increase of seismic hazard.

Holocene liquefaction and soft-sediment deformation in Quito (Ecuador): A paleoseismic history recorded in lacustrine sediments

Abstract We studied the Holocene fluvial-lacustrine sediments of the northern Quito Basin to determine a paleoseismic history for Quito, the capital of Ecuador. Fault-controlled sedimentation and coseismic deformation demonstrate the Holocene activity of the Quito reverse fault system. In the 450 years of historical seismicity, just a few events recorded in Quito could be related to local seismic sources. Numerous liquefaction horizons are possible evidence of paleoseismic activity in Quito. Potential paleoseismic horizons have a maximum thickness of 1.5 m and were produced in a lacustrine environment. Several horizons have been successfully correlated throughout the basin thanks to the presence of six volcanic marker beds. We found no evidence of widespread liquefaction processes originating in an aerial environment at water-table depth, clearly suggesting a present-day low liquefaction potential for the city. Assuming a seismic origin for contorted bedding features, we compared thickness distributions of these horizons and intensity distributions from the historical seismicity record, and proposed a methodology for the determination of seismic paleointensities. We constructed a paleoseismic history which suggests the occurrence of 28 earthquakes of intensity > V for a roughly 1500-year time span, prior to the historical record. From these 28 earthquakes, we determined the probable occurrence of a major event of intensity X (MSK) between the 10th and the 16th centuries. The correlation between its paleoseismic horizon and buried coseismic faulting in the Quito Basin suggests the occurrence of a local earthquake. The occurrence of a MM 6.5–7.0 event due to the rupture of the entire Quito fault appears to be the most probable origin for this high seismic intensity which exceeds the maximum recorded historical intensity by almost one degree.

K–Ar age of the Ranau Tuffs: implications for the Ranau caldera emplacement and slip-partitioning in Sumatra (Indonesia)

The Sumatran subduction is an example of oblique convergence which is partitioned into a component normal to the plate boundary and a wrench component taken up by strike-slip deformation within the overriding plate. Indeed, off Sumatra, the approximately NNE-trending convergence is mechanically accommodated both by subduction processes and strike-slip deformation partly localised on the Great Sumatran dextral Fault (GSF). The GSF parallels the trench and follows approximately the magmatic arc, where major calderas are installed. The Ranau caldera is one of those located along the GSF in south Sumatra. A Ranau Tuff sample yielded 40K–40Ar ages of 0.55±0.15 Ma for its separated feldspars, which places the major Ranau caldera collapse between 0.7 and 0.4 Ma, a period of paroxysmal calderic activity along the Sumatran Arc. Geomorphic features affecting the Ranau Tuff and offset by the GSF yield a long-term dextral slip rate of 5.5±1.9 mm/yr at 5°S. Consequently, south Sumatra represents an intermediate case between complete slip-partitioning and purely oblique thrusting, where the leading edge is characterised by a low convergence obliquity (<20°) accommodated by strike-slip deformation in the overriding plate. This demonstrates that even for low obliquity, slip-partitioning can exist.

Is the Cauca-Patia and Romeral Fault System left or rightlateral?

Extrusion driven collision is controlled by strike-slip faults of opposite senses whereas subduction related oblique convergence is accommodated by margin parallel faults of identical senses. Northern Andes geodynamics has been interpreted either as resulting from the obliquity of convergence between Nazca and South American plates or from the collision of the Carnegie Ridge with Ecuadorian margin. The sense of the Cauca-Patia and Romeral Fault System (CPRFS) has been interpreted in two opposite manners: right or left-lateral agreeing with oblique or collisional patterns, respectively. We analyze arguments which favor, South to 4°N, a right-lateral motion along the CPRFS that is consistent with obliquity driven geodynamics. However, North of 5°N, the CPRFS is actually left-lateral. These opposite motions between the northern and southern part of the CPRFS have to be accommodated around 5°N by N-S stretching and/or shortening. Existing data support N-S shortening around 5°N, this buttressing could be due to the collision between the Panama and Andean Blocks.