Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

R E S U LT S The ENE trending Las Coloradas Mountain range in the Eastern Precordillera is bounded to the west of the Zonda Fault, and characterized by steeper slopes (Fig. 1, 2). The eastern slopes are gentle and drained by several rivers (Fig. 2). Four generation of Alluvial Fan surfaces (AFS1-AFS4) has been identified based on the elevation variation. The AFS1 shows the highest elevation of 802±2 m, the AFS2 is located at 793±2 m, AFS4 at 788±2 m and AFS4 at 784±2 m (Fig. 2). The alluvial fan surfaces are deformed and uplifted by the Marquesado Fault giving rise to west-facing fault scarps (Fig. 2c, d). The Marquesado Fault scarp runs parallel to Las Coloradas, perpendicular to the current stream channel, and cuts the fan approximately half a kilometer from its source at our site. Within the AFS1, we have identified a fluvial terrace (T0) deformed by the fault and vertically displaced approximately by 3.29 m (Fig. 2c, P3).To maintain the safety of a hand-excavated trench, we selected to grid only the south-wall with a fluorescent grid and logged at a 1:10 m scale. Important features such as imbrications or clear unit boundaries were measured from gridlines and marked on scaled mylar paper for accurate logging. The trench exposed four fault splays (F1-F4), with varying dip angles. Fault F1 displays a dip of around 40°, F2 displays an average dip of 22°, and F4 displays a gentle dip of around 14° between grid 2-3 and gets steeper to around 35° as it reaches the surface (Fig. 3). Fig. 2: Mapped alluvial surfaces near the Marquesado trench site. Four alluvial surfaces are seen and labeled AFS1-4. Floodplains are labeled as Fp and given a light brown color. T0 is the westfacing scarp along which the Marquesado Trench was excavated (seen in 2b). (c) shows a scarp height of 3.29 m derived from the ArcGIS exploratory 3D analysis of a hillshade generated from the 5 m MDE DEM. (d) uses the same analysis and data to produce a larger scale profile showing the location of the scarp along the alluvial surface.