Morphotectonic evidence from lateral propagation of an active frontal fold; Pakuashan anticline, foothills of Taiwan

The Pakuashan anticline is uniquely suited for study of the forward and lateral growth of fault-related folds. The Pakuashan ridge development arises from the late Quaternary uplift of the most external thrust zone of the western foothills of Taiwan. From Kaoshiung to Taichung, recent and active westward thrusting occurs at the front of the foothills. The Pakuashan anticline, trending N 150°E in the northern part to N 000° in the southern part, has been active throughout the Quaternary period. This activity is marked by geological structures, tectonic geomorphology and seismicity. A multisource and multiscale approach to study of the continental collision setting has been undertaken to combine tectonics, sedimentology and geomorphology. Studies of fracture patterns allow identification of two main features of stress orientations: a WNW/ESE compression direction, and E–W and N–S extension directions. Quantitative geomorphic parameters have been used to define the morphotectonic evolution and to infer tectonic style along the mountain front. Geomorphic evidence provides significant information on the processes that govern lateral propagation of an active anticline. Quaternary terraces are uplifted, tilted and folded over the Pakuashan ridge. Drainage systems in areas of active compression give information on the thrust zone structures and their development. Steep drainage and high local relief indicate that the Pakuashan anticline forms a well-defined zone of high uplift, especially in the southern part. The two main controls on drainage in that area are rock strength in the hanging wall and propagation of the deformation towards the south.     

Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps-Ligurian basin junction during the Cenozoic

We focus on the northern Ligurian margin, at the geological junction of the subalpine domain and the Ligurian oceanic basin, in order (1) to identify the location of the southern limit of the Alpine compressive domain during the Cenozoic, and (2) to study the influence of a compressive environment on the tectonic and sedimentary evolution of a passive margin.Based on published onshore and offshore data, we first propose a chronology of the main extensional and compressional regional tectonic events.High-resolution seismic data image the margin structure down to ∼3 km below seafloor. These data support that past rifting processes control the present-day margin structure, and that 2800-4000 m of synrift sediment was deposited on this segment of the margin in two steps. First, sub-parallel reflectors indicate sediment deposition within a subsident basin showing a low amount of extension. Then, a fan-shaped sequence indicates block tilting and a higher amount of extension. We do not show any influence of the Miocene Alpine compression on the present-day margin structure at our scale of investigation, despite the southern subalpine relief formed in the close hinterland at that time. The southern front of the Miocene Alps was thus located upslope from the continental margin.Finally, a comparison with the Gulf of Lions margin suggests that the tectonic influence of the Alpine compression on the rifting processes is restrited to an increase of the subsidence related to flexure ahead of the Alpine front, explaining abnormally high synrift thicknesses in the study area. The Alpine environment, however, has probably controlled the sedimentary evolution of the margin since the rifting. Indeed, sediment supply and distribution would be mainly controlled by the permanent building of relief in the hinterland and by the steep basin morphology, rather than by sea-level fluctuations, even during the Messinian sea-level low-stand.     

Tectonique superposée de la zone de faille d’Arakapas (Massif de Troodos,Chypre)

Résumén

Une analyse tectonique du couloir faillé d’Arakapas est menée à partir de la mesure de plus de 3 000 plans striés relevés dans les formations ophiolitiques du Troodos et du Kellaki. Des mesures ont également été effectuées dans les formations du Crétacé terminal — Paléogène et du Néogène, afin de préciser l’âge des différents stades de la déformation. Elles ont été traitées par les méthodes des dièdres droits et du calcul du tenseur des contraintes. Il est montré que le couloir d’Arakapas a été le siège de plusieurs déformations successives. Au Néogène règne un régime en extension qui succède à un jeu décrochant dextre contemporain des compressions d’âge Crétacé terminal à Miocène. Un jeu décrochant antérieur peut-être mis en évidence. Il serait compatible avec le fonctionnement en faille transformante océanique proposé par divers auteurs.     

Analyses of fault mechanisms and expansion of southwestern Anatolia since the late Miocene

Neotectonic field studies and detailed analyses of Neogene and Quaternary fault mechanisms in southwestern Anatolia enable us to recognize a succession of compressional and extensional events, and to characterize the direction of corresponding regional stresses. The three most important compressive phases occurred during the Miocene, and a much smaller one near the Plio-Quaternary boundary. The last one or two interrupted a widespread extension of much greater duration and amplitude. The whole tectonic evolution resembles that of the Aegean. The large extension by normal faulting is consistent with a minimum stress along a NNE-SSW average direction. It appears that this direction was N-S during the Pliocene and changed to NE-SW sometime during the Quaternary. This dominant NNE-SSW extension, which began during late Miocene or earliest Pliocene, was related to the development of the southwestern Anatolian graben system.     

Tectonic analysis of an oceanic transform fault zone based on fault-slip data and earthquake focal mechanisms: the Húsavík–Flatey Fault zone, Iceland

The Húsavík–Flatey Fault (HFF) is an oblique dextral transform fault, part of the Tjörnes Fracture Zone (TFZ), that connects the North Volcanic Zone of Iceland and the Kolbeinsey Ridge. We carry out stress inversion to reconstruct the paleostress fields and present-day stress fields along the Húsavík–Flatey Fault, analysing 2700 brittle tectonic data measured on the field and about 700 earthquake focal mechanisms calculated by the Icelandic Meteorological Office. This allows us to discuss the Latest Cenozoic finite deformations (from the tectonic data) as well as the present-day deformations (from the earthquake mechanisms). In both these cases, different tectonic groups are reconstructed and each of them includes several distinct stress states characterised by normal or strike-slip faulting. The stress states of a same tectonic group are related through stress permutations (σ₁−σ₂ and σ₂−σ₃ permutations as well as σ₁−σ₃ reversals). They do not reflect separate tectonic episodes. The tectonic groups derived from the geological data and the earthquake data have striking similarity and are considered to be related. The obliquity of the Húsavík–Flatey Fault implies geometric accommodation in the transform zone, resulting mainly from a dextral transtension along an ENE–WSW trend. This overall mechanism is subject to slip partitioning into two stress states: a Húsavík–Flatey Fault-perpendicular, NE–SW trending extension and a Húsavík–Flatey Fault-parallel, NW–SE trending extension. These three regimes occur in various local tectonic successions and not as a regional definite succession of tectonic events. The largest magnitude earthquakes reveal a regional stress field tightly related to the transform motion, whereas the lowest magnitude earthquakes depend on the local stress fields. The field data also reveal an early extension trending similar to the spreading vector. The focal mechanism data do not reflect this extension, which occurred earlier in the evolution of the HFF and is interpreted as a stage of structural development dominated by the rifting process.     

Large earthquake-triggered landslides and mountain belt erosion: The Tsaoling case,Taiwan

The 1999 Chi-Chi earthquake triggered the catastrophic Tsaoling landslide in central Taiwan. We mapped the landslide area and estimated the landslide volume, using high-resolution digital elevation model from airborne LiDAR (Light Detection and Ranging), satellite images, aerial photographs and topographic maps. The comparison between cut and fill volumes, about 0.126 and 0.150 km³, respectively, suggests a volume increase of 19% due to decompaction during landsliding. In April 2002, the cut and fill volumes were about 0.137 and 0.116 km³, respectively. These estimates suggest that 2.5 years after the event, the volume of landslide debris removed by river erosion was nearly 0.045 km³. Such a large value highlights the importance of landslide processes for erosion and long-term denudation in the Taiwan mountain belt. To cite this article: R.-F. Chen et al., C. R. Geoscience 337 (2005).     

Topographical changes revealed by high-resolution airborne LiDAR data: The 1999 Tsaoling landslide induced by the Chi–Chi earthquake

The 1999 Chi–Chi earthquake triggered the catastrophic Tsaoling landslide in central Taiwan. We mapped the landslide area and estimated the landslide volume, using a high-resolution digital elevation model from airborne LiDAR (Light Detection And Ranging), aerial photographs and topographic maps. The comparison between scar and deposit volumes, about 0.126 km³ and 0.150 km³ respectively, suggests a coseismic volume increase of 19% due to decompaction during landsliding. In July 2003, the scar and deposit volumes were about 0.125 km³ and 0.110 km³ respectively. These estimates suggest that 4 years after the event, the volume of landslide debris removed by river erosion was nearly 0.040 km³. These determinations are confirmed by direct comparison between the most accurate topographic models of the post-landslide period, indicating a very high erosion rate at the local scale (0.01 km³/year) for the deposit area of the landslide. Such a large value highlights the importance of landslide processes for erosion and long-term denudation in the Taiwan mountain belt.     

Coseismic deformation revealed by inversion of strong motion and GPS data: the 2003 Chengkung earthquake in eastern Taiwan

A moderate earthquake of   M _w= 6.8  occurred on 2003 December 10. It ruptured the Chihshang Fault in eastern Taiwan which is the most active segment of the Longitudinal fault as a plate suture fault between the Luzon arc of the Philippine Sea plate and the Eurasian plate. The largest coseismic displacements were 13 cm (horizontal) and 26 cm (vertical). We analyse 40 strong motion and 91 GPS data to model the fault geometry and coseismic dislocations. The most realistic shape of the Chihshang fault surface is listric in type. The dipping angle of the seismic zone is steep (about 60°–70°) at depths shallower than 10 km and then gradually decreases to 40°–50° at depths of 20–30 km. Thus the polygonal elements in Poly3D are well suited for modelling complex surfaces with curving boundaries. Using the strong motion data, the displacement reaches 1.2 m dip-slip on the Chihshang Fault and decreases to 0.1 m near surface. The slip averages 0.34 m, releasing a scalar moment of 1.6E26 dyne-cm. For GPS data, our model reveals that the maximal dislocation is 1.8 m dip-slip. The dislocations decrease to 0.1 m near the surface. The average slip is 0.48 m, giving a scalar moment of 2.2E26 dyne-cm. Regarding post-seismic deformation, a displacements of 0.5 m were observed near the Chihshang Fault, indicating the strain had not been totally released, as a probable result of near-surface locking of the fault zone.     

Tectonic stress regimes,rift extension and transform motion: the South Iceland Seismic Zone

Abstract

The South Iceland Seismic Zone (SISZ) is located at the junction of three rift segments in southwestern Iceland. The presence of different types of faulting and of differently orientated subgroups in Upper Pliocene to Holocene formations indicate polyphase tectonism. We measured 736 minor faults at 25 sites. Two types of relationships between stress regimes are represented. The first type, named IDS (inhomogeneous data set), is characterized by the presence of two types of fault mechanisms, normal and strike-slip, consistent with a single direction of extension. The second type, named OSR (opposite stress regimes), is characterized by the presence of perpendicular directions of extensions for a single type (normal or strike-slip) of faulting. Because of contradictory chronological criteria, we infer that the OSR alternated during the brittle tectonic activity of the SISZ. Two stress regimes, primary and secondary, are characterized by directions of extension NW-SE and NE-SW, respectively. The general fracture pattern characterized for the primary stress regime in the SISZ includes NNE-SSW trending right-lateral strike-slip faults, conjugate ENE-WSW trending left-lateral faults and NE-SW normal faults. This distribution is quite consistent with a Riedel- type model of fault pattern in a left-lateral shear zone. The stress states characterized based on analysis of both the earthquake focal mechanisms and the recent faulting sow great similarity in terms of stress directions. The main difference is the larger ratio of strike-slip motions representing 71% of the total population in the case of earthquake focal mechanisms, whereas for the whole set of faults the proportion of strike-slip faulting was 50 %. We explain that a témpora evolution of the tectonic regime in the SISZ region, accompanied by a gradual change in stress field, starts with rift-type pure extension and progressively leads to development of preferentially strike-slip structures in the kinematic context of left- lateral transform motion. © Elsevier, Paris     

De la distribution des diaclases dans des flexures à la chronologie tectonique : exemples dans le Plateau du Colorado (Utah-Arizona-Nouveau Mexique,U.S.A.)

Résumé

L’établissement de la chronologie relative de systèmes de diaclases ou joints est difficile à établir, surtout si les conditions d’observation ne permettent pas d’analyser de très nombreux recoupements de joints. On peut toutefois situer dans le temps, grâce à une étude principalement géométrique, la formation de familles de joints affectant des structures flexurées d’âge connu.

Les flexures monoclinales, d’âge laramien, sont les structures les plus caractéristiques du Plateau du Colorado. Les multiples systèmes de joints qui les affectent ont pour orientations : N000° à N020°, NO40°, N080°-N090°, Ν110^ο-Ν120°. N140°-N150° et Ν170°. L’étude des relations géométriques entre ces joints et les flexures, complétée par des analyses qualitatives d’intersection des joints et de rugosité de surface, permettent la classification suivante : 1) des joints anté-laramiens N080°-N090° puis N000° et Ν150°, 2) des joints N110° et N020° à N040 contemporains de la compression laramienne, 3) des joints post-laramiens Ν170° dus aux mécanismes extensifs Basin & Range.

A partir de cette étude, plusieurs idées plus générales sur la structuration des zones peu déformées sont proposées. La chronologie de la facturation ainsi définie peut également être appliquée aux zones tabulaires du Plateau du Colorado.