Active sinking at the bottom of the Rincón de Parangueo Maar (Guanajuato, México) and its probable relation with subsidence faults at Salamanca and Celaya

Resumen

Rincon de Parangueo is
a Quaternary maar that had a perennial lake until the 1980s. The
lake was gradually desiccated as a consequence 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 infiltration of the brine towards the aquifer.
This process, probably in conjunction with lake sediment compaction
and/or diatreme subsidence, may explain the significantly 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 ).