A neotectonic–geomorphologic investigation of the prehistoric rock avalanche damming Laguna de Metztitlán (Hidalgo State, east–central Mexico)

Abstract

Laguna de Metztitlán (Hidalgo State, east-central Mexico) is a natural lake dammed by an unbreached, large-scale rock avalanche (sturzstrom) deposit (area: 2.5 km²; volume: ~0.6 km³; up to~400 m thick in the valley axis; horizontal runout distance: 2,600 m; vertical fall height: 860 m) that impounds the Metztitlán River. The natural outflow of the lake is by seepage, the difference between the lake level and the level of resurgence being ~250 m; this value also corresponds approximately to the maximum thickness of the lake deposits. Based on the earlier extent of the lake beyond the town of Metztitlán, indicated in the «Relación Geográfica de Metztitlán» (A.D. 1579), the delta progradation rate of the Metztitlán River is estimated at ~30 m/yr. Based on this rate and the maximum extent of the paleolake, the rock avalanche damming Laguna de Metztitlán must have occurred ~500–1,100 yr B.P. There are general observations that support a seismic origin of the rock avalanche. Two regional-scale, late Cenozoic, east-west striking, and south-dipping normal faults at the northeastern margin of this rock avalanche feature pronounced scarps, up to ~250 m high. These faults have the same orientation as other normal faults with Quaternary activity farther west in the stress and deformation province of the Trans-Mexican Volcanic Belt, which is characterized by north-south oriented extension. Thick colluvium layers have been cut off and rotated along the northern fault. The southern fault forms a scarp with a height of 275–305 cm where it crosses the debris mound of the rock avalanche, which suggests the occurrence of a surface-rupturing earthquake on this fault in the past 1,100 yr.