Active sinking at the bottom of the Rincón de Parangueo Maar, Guanajuato, Mexico 169 Abstract Rincon de Parangueo is a Quaternary maar that had a perennial lake until the 1980s. The lake was gradually desiccated as a con- sequence of drawdown in the Salamanca-Valle de Santiago regional aquifer and now functions as a playa-lake. In contrast with the features observed in other crater-lakes in the region (La Alberca, Cíntora, and San Nicolás), which also dried up at the same time, the bottom of the Rincón crater displays clear evidence of active deformation associated with mass movement of lake sediments towards the depocenter inside the crater. The most conspicuous topographic feature is a 10 – 12 m high scarp parallel to the former lake coast. The scarp is produced by an annular shaped normal fault system, down towards the depocenter. Evidence of active mass movement is observed along the topographic scarp. Rotational slides associated with rollover anticlines and local grabens produced by antithetic faults are common on the eastern and northern parts of the scarp. Planar slides with open folds at their base occur at western part of the lake basin. Evaporites (trona, thermonatrite, eitelite, halite, and silvite) are abundant in the playa-lake sediments. Their presence makes us believe that a mass removal process is acting as a consequence of salt dissolution and iniltration of the brine towards the aquifer. This process, probably in conjunction with lake sediment compaction and/or diatreme subsidence, may explain the signiicantly higher fault displacement rate observed inside the crater ( ≈ 50 cm/year ) in comparison with active faults elsewhere in the Salamanca-Valle de Santiago aquifer ( ≈ 6 cm/year ). Keywords: drawdown, maar, evaporite, stromatolite, landslide. Resumen Rincón de Parangueo es un maar cuaternario que hasta los años ochenta contenía un lago perenne en su interior. El lago fue gradualmente desecado como consecuencia de sobre-explotación del acuífero regional Salamanca-Valle de Santiago y ahora funciona como un lago-playa. A diferencia de lo que se observa en otros lagos-cráter de la región (La Alberca, Cíntora y San Nicolás) que también fueron desecados en la misma época, en el fondo del cráter de Rincón se pueden apreciar evidencias claras de deformación Boletín de la Sociedad GeolóGica Mexicana VoluMen 65, núM. 1, 2013, p. 169-188 Active sinking at the bottom of the Rincón de Parangueo Maar (Guanajuato, México) and its probable relation with subsidence faults at Salamanca and Celaya José Jorge Aranda-Gómez 1,* , Gilles Levresse 1 , Jesús Pacheco Martínez 2 , José Alfredo Ramos-Leal 3 , Gerardo Carrasco-Núñez 1 , Elizabeth Chacón-Baca 4 , Gildardo González-Naranjo 1 , Gabriel Chávez-Cabello 4 , Marina Vega-González 1 , Gabriel Origel 5 , Cristina Noyola-Medrano 3 1 Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. México. 2 Centro del Diseño y la Construcción, Universidad Autónoma de Aguascalientes, Av. Universidad 940, Aguascalientes, Ags., México. 3 División de Geociencias Aplicadas, Instituto Potosino de Investigación Cientíica y Tecnológica, San Luis Potosí, SLP, México. 4 Facultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León, Carretera a Cerro Prieto Km. 8, Linares, Nuevo León, México. 5 AGROASEMEX S.A. * jjag@servidor.unam.mx M E X I C A N A A . C . S O C I E D A D G E O L Ó G I C A 1904 2004 C i e n A ñ o s