Alluvial fans are important sedimentary environments. They trap sediment delivered from mountain source areas, and exert an important control on the delivery of sediment to downstream environments, to axial drainages and to sedimentary basins. They preserve a sensitive record of environmental change within the mountain source areas. Alluvial fan geomorphology and sedimentology reflect not only drainage basin size and geology, but change in response to tectonic, climatic and base-level controls. One of the challenges facing alluvial fan research is to resolve how these gross controls are reflected in alluvial fan dynamics and to apply the results of studies of modern fan processes and Quaternary fans to the understanding of sedimentary sequences in the rock record. This volume includes papers based on up-to-date research, and focuses on three themes: alluvial fan processes, dynamics of Quaternary alluvial fans and fan sedimentary sequences. Linking the papers is an emphasis on the controls of fan geomorphology, sedimentology and dynamics. This provides a basis for integration between geomorphological and sedimentological approaches, and an understanding how fluvial systems respond to tectonic, climatic and base-level changes.

Alluvial Fans: Geomorphology, Sedimentology, Dynamics

[1] We document a new model of crustal structure of the Andean front in Argentina where numerous historic earthquakes destroyed the cities of Mendoza in 1861 (Ms = � 7) and San Juan in 1944 (Mw = 7.0). The Cerro Salinas anticline is formed above the west directed Cerro Salinas thrust. An east facing monocline with an amplitude of about 2 km folds the Cerro Salinas thrust and overlying Neogene succession. This monocline is formed above a blind crustal thrust in the basement. Its dip of 14 west is inferred from fold geometry. This thick-skinned east directed blind thrust and the thin-skinned west directed Cerro Salinas thrust define a tectonic wedge; the wedge tip occurs at a depth of 5.4 km. Growth of the monocline after � 8.5 Ma is revealed on multichannel seismic (MSC) profile 31017 (Repsol-YPF). Rates of Cerro Salinas thrust displacement are of the order of 1 mm/yr, whereas vertical uplift of � 0.45 mm/yr results from the combined displacement on the Cerro Salinas thrust and growth of east facing monocline. The lateral extent of the east directed crustal blind ramp corresponds with the along-strike extent of the Eastern Precordillera. When combined with the low displacement rate, a long earthquake recurrence interval is implied. Smaller magnitude earthquakes, however, indicate that segments of the blind thrust ramps ruptured in historic events. If all the segments of the blind thrust ruptured together the fault area is � 7000 km 2 and could produce a Mw � 7.7 earthquake. The crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region.

/pdf/crustal-wedging-triggering-recent-deformation-in-the-andean-4ashp7eeux.pdf

Crustal wedging triggering recent deformation in the Andean thrust front between 31°S and 33°S : Sierras Pampeanas-Precordillera interaction

Because earthquakes on large active thrust or reverse faults are not always accompanied with surface rupture, paleoseismological estimation of their associated seismic hazard is a difficult task. To improve the seismic hazard assessments in the Andean foreland of western Argentina (San Juan Province), this paper proposes a novel approach that combines structural geology, geomorphology and exposure age dating. The Eastern Precordillera of San Juan is probably one of the most active zones of thrust tectonics in the world. We concentrated on one major regional active reverse structure, the 145 km long Villic´um-Pedernal thrust, where this methodology allows one to: (1) constrain the Quaternary stress regime by inversion of geologically determined slip vectors on minor or major fault planes; (2) analyse the geometry and the geomorphic characteristics of the Villic´um-Pedernal thrust; and (3) estimate uplift and shortening rates through determination of in situ-produced 1 0 Be cosmic ray exposure (CRE) ages of abandoned and uplifted alluvial terraces. From a structural point of view, the Villic´um-Pedernal thrust can be subdivided into three thrust portions constituting major structural segments separated by oblique N40°E-trending fault branches. Along the three segments, inversion of fault slip data shows that the development of the Eastern Precordillera between 31°S and 32°S latitude is dominated by a pure compressive reverse faulting stress regime characterized by a N110° ′ 10°E-trending compressional stress axis (σ 1 ). A geomorphic study realized along the 18 km long Las Tapias fault segment combined with CRE ages shows that the minimum shortening rate calculated over the previous ∼20 kyr is at least of the order of 1 mm yr - 1 . An earthquake moment tensor sum has also been used to calculate a regional shortening rate caused by seismic deformation. This analysis of the focal solutions available for the last 23 yr shows that the seismic contribution may be three times greater than the shortening rate we determined for the Las Tapias fault (i.e. ∼3 mm yr - 1 ), suggesting that the San Juan region may have experienced a seismic crisis during the 20th century. Moreover, the ramp that controls the development of the Eastern Precordillera appears to be one of the main seismic sources in the San Juan area, particularly the 65 km long Villic´um-Las Tapias segment. A first-order evaluation of the seismic hazard parameters shows that this thrust segment can produce a maximum earthquake characterized by a moment magnitude of ∼7.3 (′0.1) and a recurrence interval of 2.4 (′1.5) kyr. This part of the Villic´um-Pedernal ramp may have ruptured during the Ms =7.4, 1944 San Juan earthquake producing very few surface ruptures and only distributed flexural slip deformation on to the Neogene foreland bedding planes between the Eastern Precordillera and Pie de Palo.

https://hal-amu.archives-ouvertes.fr/hal-01458847/document

Seismic hazard reappraisal from combined structural geology, geomorphology and cosmic ray exposure dating analyses: the Eastern Precordillera thrust system (NW Argentina)

The Precordillera thrust belt of western Argentina is anomalously close, both horizontally and vertically, to the coeval subduction zone of the Nazca plate. The thin-skinned part of the belt has an unusually deep decollement that is well defined by industry seismic reflection and recent broadband experiments. New area and line-length balanced cross sections show that the central Precordillera has accrued ∼90 ± 21 km of shortening since 13 Ma; much of that shortening occurred between 12 and 9 Ma. Fault-slip data generally show shortening approximately west-northwest–east-southeast, orthogonal to the traces of the thrust and folds in the Precordillera and oblique to the mean vector of local global positioning system (GPS) data. The GPS strain rate is –63 ± 9 × 10 –9 /yr, whereas strain rate in the thrust belt, averaged over 13 m.y., is –56 ± 4 × 10 –9 /yr. Although the decollement of the Precordillera cannot cut into Paleozoic Cuyania(?) terrane basement east of the crest of the high Andes, broadband receiver function data show that significant crustal thickening must occur beneath and even east of the thrust belt. We suggest that top-to-the-west shear and thickening of the lower crust due to flat subduction explains the distribution of crustal thickening.

/pdf/the-argentine-precordillera-a-foreland-thrust-belt-proximal-456gmp1uum.pdf

The Argentine Precordillera: A foreland thrust belt proximal to the subducted plate

Late Cenozoic fluvial dynamics of the River Tana, Kenya, an uplift dominated record

Quaternary alluvial terraces in an active tectonic region: the San Juan River Valley, Andean Ranges, San Juan Province, Argentina

The structural character of the Andean Precordillera represents a high-level of fold and thrust belt formed during the alpine orogeny. The studied area is characterised by a set of eastwards imbricated overthrusts and folded rocks which are in movement, as evidenced by a number of earthquakes in recent times. The main rivers flow approximately perpendicular to the main structural lines and show severa/ terrace levels with different morphologies. The generation ot the terraces has often been attributed to the main climatic changes. In our opinion this development is directly related to the repeated base level changes controlled by neotectonics.

Aterrazamientos en el río Albarracín, cuenca del río San Juan: génesis y significado geodinàmico. (Provincia de San Juan, Argentina)

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Quaternary alluvial terraces in an active tectonic region: the San Juan River Valley, Andean Ranges, San Juan Province, Argentina

Aterrazamientos en el río Albarracín, cuenca del río San Juan: génesis y significado geodinàmico. (Provincia de San Juan, Argentina)