Geology of the Western Sierra Madre and Copper Canyons of Mexico, a brief summary.

Gary R Ziegler

During the middle Tertiary period around 25 million years ago, the Sierra Madre region of Northwestern Mexico was subject to intensive volcanic and pyroclastic activity. Massive intrusions of molten material, magma, reached the surface in batholith sized domes and shields. Much of this material exited through zones of weakness in the Earth's crust flowing out through vents, fissure and blown out through volcanism. Vast amounts cooled and solidified below the surface as well.

A recent study suggests that much molten material may have surged out along fault zones created by basin and range alignment resulting from ongoing plate tectonics. Too few surface volcanic sources, calderas and vents have been identified to explain the vast amount of rock of extrusive origin (1). This material cooled and solidified, formed the various igneous rock types that now make up the rugged topography and deep cliff-sided canyons of the the Sierra Madre. Later, secondary enrichment, and infusion of less extensive intrusive material, probable created the several mineral rich deposits that became the historic mining centers at Batopilas, Tejaban and elsewhere.

Unlike the Grand Canyon of the Colorado presenting a slice through geological history down to the basement, two billion plus years old Vishnu Schist , the Sierra Madre canyons cut down through massive, assorted Tertiary period, igneous rocks of relatively uniform age. Uplift, erosion and alluvial deposition followed forming the great canyons that we see today.

As one travels through the region, the predominant rock types seen are rhyolites, basalts, tuft, breccia and andesite. A closer look in the canyons will produce an interesting array of minerals, agates, geodes, quartz crystals, malachite, galena and exceptional quality, clear calcite crystals from near Divisadero.


Geological survey on horseback. The Oteros Canyon near Divisadero, Mexico.

References:

1) Fissure ignimbrites: Fissure-source origin for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with Basin and Range faulting
Gerardo J. Aguirre-Díaz*,1 and Guillermo Labarthe-Hernández
http://geology.geoscienceworld.org/cgi/content/abstract/31/9/773?ck=nck

2) Major ignimbrites and volcanic centers of the Copper Canyon area: A view into the core of Mexico's Sierra Madre Occidental.
Eric R. Swanson*,1, Kirt A. Kempter2, Fred W. McDowell3 and William C. McIntosh
http://geosphere.geoscienceworld.org/cgi/content/abstract/2/3/125

3) Province, Mexico, Revealed by U–Pb Geochronology.
Scott E. Bryan1,*, Luca Ferrari2, Peter W. Reiners1,pastedGraphic.pdf ¨, Charlotte M. Allen3, Chiara M. Petrone4, Aldo Ramos-Rosique2,5 and Ian H. Campbe
http://petrology.oxfordjournals.org/cgi/content/abstract/49/1/47

		Gary Ziegler is an archaeologist, mountaineer and explorer who has spend a lifetime studying remote regions of Peru and Mexico. He is co-owner of Adventure Specialists, a Colorado ranch based adventure tour operation that runs educational treks, horse trips and research expeditions in Colorado, Peru and Mexico's Copper Canyon. His expeditions have located the important Inca sites of Corihuayrachina, Cota Cota and Llactapata. He is a Fellow of The Royal Geographical Society, The Explorers Club and a sometime lecturer at Colorado College. He has appeared on BBC specials about the Inca and as guest expert on the Discovery and History Channel He can be contacted at: info@adventurespecialists.org US phone: 719-783-2076