Bernard Delcaillau

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

Abstract 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.

Geomorphic response to growing fault-related folds: example from the foothills of central Taiwan

Abstract In this paper we concentrate particularly on the geomorphological indicators left by active tectonics. In the central foothills of Taiwan, we used topography, drainage pattern and structural data to perform quantitative morphometric analysis and to determine relative age of fault-related anticlines. The Tiehchen, Tatu and Pakua ridge belt is a fault-related anticline system located in the hanging wall of the Changhua fault along the western thrust front of the foothills. Geomorphic systems are analysed with intent to detect the various responses of landforms and drainage pattern to late Quaternary deformation. Topography and drainage basin register uplift and are valuable tools to discriminate lateral propagation of an active frontal fold. Geomorphic field evidence and quantitative morphometric parameters are used to define the evolution of the rising anticline ridges and to infer tectonism style along an active front. Geometry of alluvial fans, formed along the frontal side of the anticlines, and weathered terrace deposits provide relevant information on neotectonics. Knowledge concerning these younger anticline ridges, makes this area a good example of an actively forming mountain front. We discuss in detail the origin of N045°, N095 and N120° trending oblique fault scarps which delimite numerous fault blocks. The fault scarps morphology is characterized by imbricate talus facets. Steeper topography accompanied by breaks in the slope along some transverse profiles, seems to correspond to the traces of successive uplifts.

Active tectonics, seismicity and geomorphology with special reference to Normandy (France)

Integrated studies based on tectonic, seismotectonic and geomorphological analyses indicate that Normandy (northwest France) has been an active area during the Quaternary. Topography and landform discontinuities reflect the dislocation and differential uplift of a late Cenozoic platform. The tectonic activity is represented by (i) active faults, indicated by linear scarps and seismic activity, (ii) offsetting of pre-existing surfaces, (iii) Plio-Pleistocene sedimen- tation restricted within narrow subsiding zones, and (iv) morphometric properties of drainage basins that indicate zones of differential uplift. The inferred strain pattern involves (i) a shortening direction that strikes NW-SE as expected in the European context of Alpine compression, and (ii) a NE-SW trending extension accommodated by NW-SE normal faults. The geomorphological systems encountered in Normandy preferentially record differential vertical displacements. Copy- right

New geomorphic criteria on lateral propagation of blind thrust-related fold growth accommodating oblique convergence

The aim of this contribution is to show the styles of growing anticlines and the ways in which they accommodate oblique convergence through proximal foreland basins and intramontane basins. The four natural examples of individual fault-related-folds are: the Chandigarh anticline in the Siwalik foothills of Himalaya (NW India), the Pakuashan anticline in the western foothills of Central Range (Taiwan), the J. Bou Dhar-Tamdafelt fold belt (Morocco) and the Marand anticline in the North-Tabriz Fault-Zone (NW Iran). The evolution of these geomorphic criteria and drainage patterns shows evidence of lateral propagation of folds accommodating oblique convergence. This paper focuses on the geomorphic indicators that could be used to provide information on the timing of fault-related folding and direction of lateral propagation. Several tectonic scenarios has been analysed and an attempt has been made to delineate the sequential evolution of anticlines accommodating oblique convergence.

Landscape response to progressive tectonic and climatic forcing in NW Borneo: Implications for geological and geomorphic controls on flood hazard

Empirical models have simulated the consequences of uplift and orographic-precipitation on the evolution of orogens whereas the effects of these forcings on ridgelines and consequent topography of natural landscapes remain equivocal. Here we demonstrate the feedback of a terrestrial landscape in NW Borneo subject to uplift and precipitation gradient owing to orographic effect, and leading to less-predictable flooding and irreversible damages to life and property. Disequilibrium in a large catchment recording the lowest rainfall rates in Borneo, and adjacent drainage basins as determined through χ, a proxy for steady–state channel elevation, is shown to result in dynamic migration of water divide from the windward-side of the orogen towards the leeward-side to attain equilibrium. Loss of drainage area in the leeward-side reduces erosion rates with progressive shortening resulting in an unstable landscape with tectonic uplift, gravity faults and debris flows. 14C dating of exhumed cut-and-fill terraces reveal a Mid–Pleistocene age, suggesting tectonic events in the trend of exhumation rates (>7 mm a−1) estimated by thermochronology, and confirmed by morphotectonic and sedimentological analyses. Our study suggests that divide migration leads to lowered erosion rates, channel narrowing, and sediment accretion in intermontane basins on the leeward-side ultimately resulting in enhanced flooding.

Unraveling the roles of asymmetric uplift, normal faulting and groundwater flow to drainage rearrangement in an emerging karstic landscape

Landscape adjustment to tectonic, lithologic and climatic forcing leads to drainage reorganization and migration of divides. The respective contribution of these forcings, especially on carbonate landscapes is not well defined. Here, we have addressed this issue by combining field observations, satellite image interpretation and DEM quantitative analysis to assess drainage response to spatially heterogeneous rainfall, asymmetric uplift, and normal faulting on an emerging carbonated platform (Sumba Island, Indonesia). We map geomorphic markers of fluvial dynamics and drainage rearrangement and compute a χ parameter that incorporates the contributions of unevenly distributed precipitation and asymmetric uplift to estimate erosional disequilibrium across drainage divides. We find that asymmetric emergence of Sumba Island created an initial parallel drainage, asymmetric across a divide that propagates landwards. Soon after establishing itself on the emerging slopes this drainage was disturbed by normal faulting, which has become the main force driving drainage rearrangement. Vertical offsets across normal fault scarps first triggered aggradation within valleys over the hanging walls, and then disconnected upstream reaches from downstream reaches, leading to the formation of wind gaps atop the fault scarps and upstream perched sedimentary basins. The defeat of rivers by growing fault scarps was catalysed by the possibility for surface water to be rerouted near the fault scarps into underground water networks inside the underlying carbonates. At the end of the process, the opposite drainage across the main water divide captured the struggling drainage. Capture mechanisms include initial groundwater capture of the perched alluvial aquifers, followed by ground sapping at the head of the opposite drainage and surface stream diversion by avulsion. Finally, normal faulting is the main driving force of drainage rearrangement allowing avulsion and karstic rerouting whereas asymmetric uplift and climate forcings have shown a low efficiency. The role of karstification is more ambiguous, catalyzing or inhibiting drainage rearrangement.

Conjectures et réfutations [Réponse à la note de Monsieur le Professeur Calvet]

Carozza Jean-Michel, Delcaillau Bernard. Conjectures et refutations [Reponse a la note de Monsieur le Professeur Calvet]. In: Geomorphologie : relief, processus, environnement, Octobre-decembre, vol. 6, n°4. pp. 272-274.

L'enregistrement géomorphologique de la tectonique quaternaire par les nappes alluviales: l'exemple du bassin de la Têt (Roussillon, France)

Abstract In a low sismo-tectonic activity zone, the geometry of a Quaternary alluvial body can be used as a neotectonic tracer tool. Geometry characteristics of individualized terrace levels are extracted from a Digital Elevation Model by superposing elevation and geological data. The longitudinal profile of each terrace is compared to the best fitting exponential theoretical profile, deduced from hydrodynamic parameters. This analysis, applied to the Tet river terraces (Mediterranean southern France) shows the splitting of a sedimentary body, corresponding to hydrodynamic break interpreted as morphostructural segmentation of the Roussillon basin.

Évolution géomorphologique d'un bassin versant côtier: l'exemple de la Touques (Basse-Normandie)

Abstract In the Touques drainage basin, five alluvial terraces have been distinguished. A multiscale approach in a continental basin was undertaken, combining different stratigraphic and geomorphic analyses. Stratigraphic and sedimentologic analyses play an essential role, because of the interaction between climate and sea-level variations and tectonics. In the Touques valley, the alluvial sheets cover the Middle and Upper Quaternary. Each fluvial sequence is correlated with a glacialinterglacial climatic cycle. The numerous episodes of channel entrenchment and terrace formation arise from a Quaternary regional uplift. Valuable information is obtained through detailed analysis of the morphology (morphometric and drainage pattern analyses). The longitudinal profiles of the rivers and the hypsometric integrals are related to the immaturity of the drainage basins subjected to recurrent sea-level changes.