LAUSR

We conducted a vertical electrical sounding (VES) survey over the Quaternary sediments covering a transitional region between the eastern part of the Chiang Mai rift basin and the Mae On Depression, northern Thailand and constructed 1D lithologic subsurface models for delineating geological structures of the underlying basement rocks. Thirteen VES stations were collected and divided into northern and southern sub-parallel alignments trending WSW-ENE across the high magnetic contrast zone. We employed a Schlumberger array with the maximum half current electrode separations around 465–500 m, using the ABEM Terrameter SAS4000 resistivity meter. The VES data were inverted using EarthImager™ 1D program. We classified four geoelectric units composed of (1) very-low-resistivity saturated sediments (17–39 Ωm), (2) low-resistivity unsaturated sediments (33–581 Ωm), (3) medium-resistivity sedimentary rocks (57–110 Ωm), and (4) high-resistivity mafic volcanic rocks (176–804 Ωm). Shallow zones of the geoelectric models were interpreted as sediment fill with layered strata with variable resistivity because of different water saturation and clay content. The sediment fill thickens westward to the Chiang Mai rift basin. Deeper zones of higher resistivity are interpreted as the underlying basement rocks. These basement rocks show contrasting resistivities with the western part modeled as clastic sedimentary rocks of sandstone and the eastern part as mafic volcanic rocks. Geological observation evidence of westward-dipping faults and chronostratigraphic results integrated with the 1D lithologic models and drilled well information support a structural model of a reverse fault trending SW-NE between the Permian volcanic and Carboniferous sedimentary basements. This structure is part of the Chiang Mai-Chiang Rai Suture Zone associated with the eastward subduction model during the Permo-Triassic Indosinian Orogeny in northern Thailand.