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Richard Chamberlin

The Magdalena radial dike swarm (MRDS) is a large semi-continuous radial array of basaltic-andesite dikes of Oligocene age broadly focused on the westward younging Socorro-Magdalena caldera cluster (SMCC) of Oligocene age. The MRDS fans through 220° of arc from Pie Town clockwise to Acoma, Chupadera, Bingham and Elephant Butte. Preliminary observations of cross cutting dikes near Riley and La Joya imply a westerly migrating source for the mafic magmatism that is coincident with the westerly migrating core of the caldera cluster. Rhyolitic ignimbrites erupted from the SMCC are intercalated with a thick pile of basaltic andesite lavas along the north margin of the caldera cluster. The total volume of this Oligocene bimodal volcanic pile is 7000 km3. Our working hypothesis (Chamberlin, Chapin and McIntosh, 2002) suggests that the Socorro-Magdalena magmatic system is the surface expression of a sheared upper mantle diapir or diapir chain that formed during westward retreat and disintegration of the Farallon slab. Mapping of dikes and sampling for 40Ar/ 39Ar dating are now underway; preliminary age data are favorable.

For more information, see the page-sized PDF version of my 2002 GSA poster: [ABSTRACT] or [PDF 3.29 Mb file].

Photo: Looking southwest along zig-zagging basaltic-andesite dike in the northern Joyita Hills. Dike cuts west-tilted sandstone beds of the Spears Formation and is buried by horizontal conglomerate beds of the upper Santa Fe Group that cap the mesa. Field relationships indicate this dike fed a 32-33 Ma basaltic-andesite flow and tephra beds that are well exposed behind the mesa.

References:

• Chamberlin, Richard M., Chapin, Charles E., and McIntosh, William C., 2002, Westward Migrating Ignimbrite Calderas and a Large Radiating Mafic Dike Swarm of Oligocene Age, Central Rio Grande Rift, New Mexico: Surface Expression of an Upper Mantle Diapir?: Geological Society of America, Abstracts with Programs, Vol. 34, No.6, p.438.
• Chamberlin, R. M., Cather, S. M., McIntosh, W. C., Anderson, O. J., and Rattè, J. C., 1994, First-day road log from Socorro to Magdalena, Datil, western Crosby Mountains, Sawtooth Mountains, Pie Town, Quemado and Quemado Lake: New Mexico Geological Society, Guidebook 45, p. 1–45. (See p.35-36).
  

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