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

238 1 2 T H I N T E R N AT I O N A L I N Q U A M E E T I N G O N PA L E O S E I S M O L O G Y , A C T I V E T E C T O N I C S A N D A R C H A E O S E I S M O L O G Y ( PATA ) , O C T O B E R 6 T H - 1 1 T H , 2 0 2 4 , L O S A N D E S , C H I L E PATA Days 2024 K E Y W O R D S San Ramón Fault, Quaternary thrust fault, Santiago, Chile (1) Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile. *Email: Ignacio.gutierrez.p@ug.uchile.cl / geaston@uchile.cl Ignacio Gutierrez P. (1) Gabriel Easton (1) Sofía Rebolledo (1) CHARACTERIZATION OF THE STRUCTURAL GEOLOGY AND DAMAGE ZONE OF THE SAN RAMÓN FAULT A B S T R A C T The San Ramón Fault is a reverse, west-vergent active fault, which overthrusts Cenozoic volcanic rocks associated to the Abanico Formation over Quaternary alluvium. We studied an outcrop of the eastern branch of the fault, located in the north- eastern area of Santiago, capital city of Chile. The main fault plane exhibits an orientation of N32W/43E (strike/dip). The fault core is around 50-75 cm thick, and it’s constituted by an angular, poorly sorted gravel- to-block-size clasts with a sand-to-clay-sized matrix breccia. In the hanging wall, shales interbedded with tuffs are intensively affected by joints, which decrease in number per cubic meters at distance greater than 6 m, and abruptly diminish beyond 10 m, defining a wide damage zone from the fault core. The joints are characterized by four main sets, which would indicate both reverse and strike-slip kinematics in this area. At the surface, the outcrop is characterized by a ca. 1 m thick soil, suggesting some development of the surface during Quaternary times. I N T R O D U C T I O N The Andes Cordillera is a chain of mountains formed by tectonic plate’s interaction, that, from a wide perspective, accumulates a shortening of about 150- 300 kilometres since the Gondwanan period, resulting in a crustal thickness of up to 70 kilometers (Allmendinger et al., 1997), including contributions of magmatism (Horton, 2018). Consequently, along the mountain margin we can find evidence of abundant tectonic activity until the Cenozoic times (e.g. Armijo et al., 2010). In this context, the reverse San Ramón Fault is located at 33°S (Figure 1). This is an active, north-south trending thrust fault dipping to the east, situated at the foothills of the Andean mountain range to the east of the city of Santiago (Armijo et al., 2010). It crosses the