Rio Grande Rift

• the San Luis and Espanola Basins of the northern rift
• numerous volcanic cones in the rift
• adjacent Sangre de Cristo Mts. (east of rift)
• adjacent Jemez volcanic field (west of and over western rift boundary
• nearbu Colorado Plateau (far west)

I. Location and Extent:

• A well defined series of asymmetrical grabens from Leadville, CO to Presidio, TX and Chihuahue, MX
• >1,000 km in length
• main rift grabens with up to 6 km vertical offset
• shallow astenosphere beneath rift

II. Extension and faulting:

• The Rio Grande Rift (RGR) consists of a number of asymmetrical grabens between Leadville, CO., and Chihuahua, Mexico (see class overhead).
• The RGR is comprised of a series of right-stepping basins which were nothing more than broad shallow depression during the Oligocene/early Miocene ~30 to 20 Ma. Low angle faulting.
• The rift developed during two major phases: the first, occurring during the late Oligocene through early Miocene (~30-18 Ma) and involving low-angle faults. the second, occurring during the late Miocene-Pliocene (~9-3 Ma) and involving high-angle and low-angle normal faults.
• similar extensional history to Basin and Range

III. Northern Rift Volcanism Taos field:

• In northern New Mexico, the RGR consists of three basins which from south to north are the Albuquerque-Belen Basin, Española Basin, and San Luis Basin .
• The RGF coincides with a north-trending tectonic zone previously deformed during the late Paleozoic (Ancestral Rocky Mts.) and Laramide orogeny. The central graben of the Española Basin formed between 5 and 3 Ma. The Taos graben of the San Luis Basin formed entirely in the last 4.5 Ma.
• location of a major thermal source.
• The Taos Plateau field consists dominantly of low-alkali tholeiitic magma (Servilleta Basalt).
• Minor quantities of intermediate to silicic melts.
• Jemez field w/volcanism starting at 13 Ma with large quantities of intermediate to silicic magmas.
• Jemez Mts. calderas formed 1.4-1.1 Ma.
• BandeLier tuff-1.1 Ma.
• Cerros del Rio-2.6 Ma (once damned Rio Grande)

• Magmatism of very small volume of volcanic rocks of a specific age (exception Jemez lineament).
• Diverse composition of volcanics. Not sourced from a single large heat source. No single composition is dominant along the rift or evolution of composition with time or unique tectonic setting.

The following is a more detailed history of the northern Rio Grande Rift. You may want to review this prior to attending Field Camp.

OLIGOCENE-MIOCENE

• Some volcanism appears to have begun contemporaneously with Laramide uplift.
• During the late Oligocene to early Miocene (27-18 Ma) more extensive volcanism occurred throughout the developing western rift region.
• Extensive flows, tuffs, and other volcanic debris buried the slopes of volcanic peaks and local volcanic fans and fanglomerates with interbedded thin flows coalesced to form debris sheets which buried topographic irregularities (Abiquiu tuff and Los Pinos Formations).
• Pre-rift volcanism began in the Jemez region between 15 and 14 Ma with eruption of the Lobato Basalt. Here basaltic volcanism became widespread between 10 and 9 Ma and then largely shut off.
• An incipient Rio Grande rift began in the Abiquiu embayment between 10 and 7 Ma.
• High angle faults with less than 500m of offset separated the embayment from the Chama basin but downthrowing Paleozoic to Tertiary sediments into the embayment.
• After 7 Ma this aborted rift boundary was aborted, leaving the embayment as a shallow platform between the Chama Basin and active rift.
• The western Rio Grande rift boundary jumped east to the Embudo fault zone, decoupling the embayment from the rift.
• Deepening of the Rio Grande rift occurred primarily during the late Miocene and Pliocene in the Espanola and San Luis Basins.
• Vertical offset between the basin and the surrounding structural highs is up to 7-8 Km!. Earlier more silica reach volcanics were largely buried by a main phase of basaltic and intermediate volcanism between 4.5 and 3.5 Ma.
• Early Pliocene flood basalts (highly fluid olivine tholeiites (1500 sq. km.) filled the San Luis Basin forming the TPVF.

• Subsidence of the rift basins continued.
• Major explosive volcanism occurred in the Jemez region forming the Jemez Mts. by the accumulation of volcanic debris (not structural uplift).
• These eruptions buried portions of the Albuquerque, Espanola, and Chama Basins with an estimated 2000 cubic km of debris.
• Rhyolitic ignimbrite eruptions (1.61 and 1.22 Ma) formed the Valles caldera.
• Further uplift occurred in the Sangre de Cristos and Picuris Mts. Glaciation sculpted higher elevations of the Sangre de Cristos and Tusas-Brazos.