Abstract

Analysis of repeated leveling surveys in the Rio Grande rift shows a pronounced zone of relative subsidence north of Espanola, New Mexico, which appears to be associated spatially with anomalous seismic activity. Maximum subsidence relative to nearby bench marks was 4.9 cm between September 1934 and March 1939. Observed subsidence occurs over 19 km and extends from 4 km north to 23 km north-northwest of Espanola. The leveling anomaly does not appear to be associated with either ground-water effects or leveling errors and thus most likely represents crustal deformation. The zone of relative subsidence occurs near some of the few late Tertiary dikes mapped in the rift and lies within the zone of high heat flow (> 2.5 HFU) along the western part of the rift. The crustal movement anomaly appears qualitatively similar but opposite in sign to movements associated with a crustal magma body that have been observed in the Socorro, New Mexico, area of the Rio Grande rift. The movements reported here appear consistent with either deflation of a shallow (< 10 km) magma body or normal faulting within the crust, possibly associated with an intrarift graben.

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