Probing shallow subsurface structures in the arc-continent collision suture zone near Hualien in Eastern Taiwan with magnetotelluric methods

Abstract

We used the magnetotelluric method (MT) to investigate near-surface structures at the northern end of the Longitudinal Valley in Hualien City, eastern Taiwan. This valley is considered the suture zone of the arc-continental collision between the Eurasian plate and the Philippine Sea plate, making it crucial to understand the geological settings at its northern end. We conducted MT measurements along the L2 and L3 lines on the north and south sides of the city, respectively. On the L2 survey line to the north of Hualien City, our inverted image indicates that the Tananao metamorphic complex, with a resistivity of several hundred ohm-m, is in vertical contact with sedimentary rock with a resistivity higher than 1000 Ω-m. This vertical contact zone is consistent with the distribution of the Beipu fault. However, the vast area between L2A and L2E is occupied by airports and military facilities, making it challenging to lay out survey stations to clearly analyze critical regional structures such as the Beipu and Milun faults. The 2D inversion results of the L3 are consistent with past reflection seismic interpretation results. The resistivity of the metamorphic Yuli Formation, which belongs to the continental basement rock in the west, is less than 200 Ω-m. Its top dips eastward, reaching depths greater than 2.5 km at the eastern end of the L3. The rocks above the metamorphic bedrock, composed of material with a resistivity greater than 1000 Ω-meters, are likely to be sedimentary rocks of the foreland basin. The difference in geological structure between the two survey lines, L2 and L3, may suggest the possible existence of an E-W orienting fault structure in the urban area between the two survey lines. Additionally, the rapid thickening of sedimentary rocks north of Hualien City may provide indirect evidence for the detachment model proposed by Shyu et al. (Tectonophysics 692:295-308, 2016). Further studies are required to resolve the resistivity structures and fault orientations in the urban region between the two survey lines.