Paleomagnetic evidence for the timing of collapse and resurgence of the Lake City Caldera, San Juan Mountains, Colorado

Journal of Geophysical Research B: Solid Earth
By: , and 



Rocks of the 23.1‐m.y.‐old Lake City caldera consist of the compositionally zoned Sunshine Peak Tuff, postcollapse intracaldera lava flows, and resurgent quartz syenite intrusions. Declinations of reversely magnetized (I = −45° to −75°) Sunshine Peak Tuff change from easterly (D = 93°–130°) throughout most of the tuff to southerly (D = 195°–207°) within the late eruptive phases. The postcollapse lava flows are also reversely magnetized, and their emplacement appears to have spanned resurgence of the caldera. In contrast, the uppermost levels of the resurgent intrusion record two distinct components of magnetization: a reversed component isolated by thermal demagnetization typically at temperatures greater than 400°C and a normal component defined by thermal demagnetization at lower temperatures. A baked‐contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity. Assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data, the time for caldera development (from eruption of the ash flow tuffs to emplacement of the resurgent intrusion) was less than about 300,000 years on the basis of best estimates of durations of reversed polarities in the early Miocene. The estimated 300,000‐year time span for the development of the Lake City caldera is very close in duration to the radiometrically determined time spans of much younger (Pleistocene) calderas having similar dimensions and similar volumes of erupted material.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Paleomagnetic evidence for the timing of collapse and resurgence of the Lake City Caldera, San Juan Mountains, Colorado
Series title Journal of Geophysical Research B: Solid Earth
DOI 10.1029/JB091iB09p09599
Volume 91
Issue B9
Year Published 1986
Language English
Publisher American Geophysical Union
Contributing office(s) Geosciences and Environmental Change Science Center
Description 15 p.
First page 9599
Last page 9613
Country United States
State Colorado
Other Geospatial Lake City Caldera, San Juan Mountains
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