Abstract

In intraplate areas, low tectonic deformation frequently resembles denudational processes. Fault scarps are eroded and buried by Quaternary alluvial deposits. The eastern termination of the Pyrenean orogenic belt displays a moderate tectonic and seismic deformation. In the northern segment of the Tet fault (F.T.N.), geomorphic systems are analysed with intent to detect the various response of landforms and drainage pattern to recent tectonics. We used topography, structural data, drainage pattern and sedimentological syntectonic deposits which determine the evolution of the fault scarp since the Pliocene.

The F.T.N., trending N080 to 050oE, is about 30 km long. The fault, separating the Pliocene sequences in the hanging wall and granite in the footwall, is emergent along the scarp. The change in fault orientation along strike gives the fault zone a “dog-leg” pattern in plan view (fig. 1 and 2). Structural data indicate that the faults are steeply dipping (> 60o). The stratigraphy of the study area can be divided up into three main depositional units, bounded above and below by sequence boundaries (fig. 6 and 7). The lithofacies of the Pliocene deposits is composed of coarser conglomerate, sandstones and silts. Most of the sediments exposed in the sections were deposited by braided rivers. Palaeocurrent data indicate a dominant SW-NE directed palaeoflow, although there are also a range of lateral flow directions. Proximal breccias provide evidence for the degradation of the fault scarp (fig. 7b). In westward direction, the offset of middle Pliocene level shows a differential uplift from 150 to 300 m. In several places, the fault is covered by middle Pleistocene terrace deposits that are not apparently affected. The activity of the Têt fault and transversal faults is thus probably lower to upper Pliocene age.

The geomorphological indices are useful tools in the evaluation of neotectonics. One of the major features of the relief is the fresh and immature aspect of the fault scarp. They are characterized by a sharp contact between sediments and basement, a low sinuosity index and two triangular spurs. Some statistical properties of landscape (relief, hydrographic network geometry, hypsometric integral ...) may be dominated by the influence of tectonic processes [Merrits and Vincent, 1989 ; Lucazeau and Hurtrez, 1997 ; Font, 2002]. These geomorphic parameters can provide quantitative estimates of surface displacements. But the factors other than tectonics (climate, glacioeustatism and lithology) control morphometric properties. In our study, we used topography, drainage pattern and structural data to perform quantitative morphometric analysis on the basis of field investigation and digital elevation model processed with Arcview V.3.1. The geomorphic indices of drainage basins (area, length, basin relief, slope...), physiographic of the drainage network and degree of topographic dissection of the fault scarp provide information to analyse the timing of the landscape development (tables I & II, fig. 12). Hypsometry has been determined both for basins and selected lithologies within these basins. Hypsometric analysis is performed to summarize the form and evolution of drainage pattern (fig. 10). This F.T.N. is the surface expression of vertical movement located along the strike-slip fault with a normal component. The fault scarp is characterized by three segments which have different morphogenic comportements : the western segmented scarp is poorly exhumed and steeped, the central and eastern scarps are highly eroded and present a poorly geomorphic expression. In the western area, steeper topography accompanied by breaks in the slope along transverse profiles seem to correspond to the trace of recent uplift. Triangular shaped analysis, procured by degradation of the fault scarp, is a geomorphic indicator of neotectonics [Delcaillau, 2001]. Lateral propagation of the deformation and rapid valley incision in the Pliocene formation and the granite of the Millas block are the main factors of the geomorphological evolution of the fault scarp.

Divergent-oblique stress leads to the development of strike slip main faults, normal faults and transtensional subbasins. The Conflent basin is in a pull-apart that was initiated during the Pliocene. The elements of the basin fill are underlain by local syndepositional relief, such as fault-flank conglomeratic wedges and evidence of syndepositional tectonism including intraformational fault and roll-overs (fig. 6c).

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