Welded-tuff dikes were the vents for major ignimbrites of the Sierra Madre Occidental, Mexico
Abstract
It has been a major unsolved problem to identify the sources of the voluminous ignimbrites of the Sierra Madre Occidental (SMO), NW Mexico, which is considered the largest continuous ignimbrite province in the world. A conservative estimate of the physical volume of the SMO ignimbrites is about 360,000 km3. At least 250 calderas the size of those at San Juan Volcanic Field in Colorado would be needed to have produced these ignimbrites, but there are less than 10 calderas identified so far in the SMO. Field evidence indicates that much of the ignimbrite's volume was related to fissure-type eruptions, most of them coinciding with Basin and Range faults. These fissures are several kilometers long and are marked by welded-tuff dikes. In some cases co-ignimbrite lithic-lag breccias occur next to these dikes confirming that the tuff dike was the source of the adjacent ignimbrite. Tuff dikes reach up to 50 m wide and can be followed for tens of km within, or at, the shoulders of large grabens of the SMO. Fiamme and other ignimbrite's planar features are aligned vertically or semi-vertically in these dikes. Shearing along margins and wall-cooking are also evident. Some dikes are composite and include up to three distinct ignimbrites with differences in welding, pumice contents, and general aspect. Each ignimbrite shows variations in welding degree, from poorly- to highly-welded. In some cases, the dikes are lithics-poor and in others are lithics-rich. In a regional scheme, ignimbrite flare-up and Basin and Range faulting overlap in time and space in many places of the SMO. On this basis, we propose a model in which batholith-sized magma chambers reached shallow crustal levels, and were evacuated when Basin and Range normal faults cut the roof of these chambers; this caused fast decompression when the system was opened, and voluminous silicic magmas were erupted explosively through these faults. The fissures were filled by the pyroclastic material and closed the vent after cooling, producing the welded-tuff dikes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.V11B1398A
- Keywords:
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- 8010 Fractures and faults;
- 8109 Continental tectonics: extensional (0905);
- 8404 Ash deposits;
- 8414 Eruption mechanisms