Science Center Objects

Volcanic rocks and their plutonic underpinnings are temporally and spatially coincident with, or superimposed on, prominent tectonic structures from the late Cretaceous in the Southern Rocky Mountains. By virtue of their mineralogy, these dominantly post-Laramide deposits provide the principal lithologic control for quantitative age determinations on Neogene faulting, estimates of long-term fault slip rates, basin sedimentation rates, and basin sediment provenance. Additionally, the petrology, geochemistry, eruptive style and volumes and, in places, spatial distribution of eruptive centers yield information on:

  1. Lithospheric structure
  2. Magma chamber and plumbing system architecture
  3. Upper crustal residence times
  4. Eruption duration

All of these subsurface concerns are either directly influenced by or are a result of regional tectonism and associated heat flow, and local expression of stress orientation and strain magnitude. Consequently, the characterization of stratigraphy, geochronology, lithology, and geophysical rock properties in volcanic terranes is fundamental to the study of the Neogene tectonic evolution and basin formation of the Southern Rocky Mountains.

Cenozoic magmatism in the Southern Rocky Mountains is manifest primarily as

  1. Exhumed plutons (75-43 Ma) along the 500-km-long, SW-NE-trending Colorado Mineral Belt (CMB).
  2. Intermediate to silicic composition volcanic and subvolcanic deposits (37-25.5 Ma) of the Southern Rocky Mountain volcanic field.
  3. Late Oligocene to Pleistocene mafic and intermediate composition volcanic rocks (26-1 Ma) usually ascribed to Rio Grande continental rifting.

 

Volcanic fields of Rio Grande Rift
Schematic regional map showing the locations of major pre- and syn-rift volcanic fields in Colorado and New Mexico and their spatial association with conventionally defined borders of the Rio Grande rift and major uplifts of the Southern Rocky Mountains. Modified from Thompson and others (1991) after Tweto (1979)

Categorically, the first two likely reflect lithospheric melting dominated by fluid fluxing of mantle overriding a shallowly subducting slab, ostensibly the Farallon Plate; the third may reflect combined effects of slab rollback and melting of upwelling depleted mantle. The deposits preserve both the temporal and spatial evolution of Paleogene and Neogene magmatism of the Southern Rocky Mountains and expose multiple levels of magmatic plumbing systems and crustal structure. These include the roots of volcanoes, ranging from large composite plutons to subvolcanic, hypabyssal intrusions and feeder dikes. Superimposed on the volcanic terrane is tectonic deformation spanning the range from latest Laramide events to regional extensional deformation associated with active basin development of the Rio Grande rift.

Previous work has established that volcanic rocks are universally associated with major tectonic deformation in the Southern Rocky Mountains and provide, in places, a detailed stratigraphic framework for the eruptive sequences. Additionally, geochemical characteristics of major eruptive centers and subvolcanic plutons are moderately well known, especially in regions of significant economic mineral potential, and models of petrogenesis at many scales have been proposed and in most cases are reasonably well accepted. However, few studies enable establishment of:

  1. Quantitative age constraints on basin-fill stratigraphic sequences
  2. Temporal correlation of basin deposits in spatially detached basins or sub-basins
  3. Provenance studies of Neogene sediments linking deposits to source areas
  4. Correlating provenance, stratigraphic studies and fault history to palinspastic reconstruction of drainage patterns and landscape evolution

These factors are critical to the delineation of the tectonic history of the Southern Rocky Mountains and are proposed elements of this study.

Bridge over Rio Grande gorge
View looking south at the high bridge crossing of the Rio Grande gorge.

Objectives

The objective is to establish the volcano-tectonic history of the Southern Rocky Mountains in areas where additional study provides temporal and spatial constraints on Neogene extensional tectonics, fault deformation and basin formation. This requires developing a regional stratigraphic and geochronologic framework in areas underlain by remnants of the Southern Rocky Mountain volcanic field and volcanic fields of the Rio Grande rift.

The aerially and volumetrically dominant deposits of the Southern Rocky Mountain volcanic field occupied an area >125,000 km2 centered about the San Juan Mountains of southern Colorado, one of the world's largest contiguous volcanic fields. Deposits were largely emplaced south of the CMB and north of areas subsequently underlain by large Pliocene volcanic fields of northern New Mexico and southern Colorado, and extension-related basin-fill sediments of the Española and San Luis basins. North of the CMB scattered but volumetrically significant remnants of late Oligocene to Pliocene eruptive centers are preserved at moderate to high elevations and are associated with basins of the North Park area in northern Colorado, Flat Tops area of northwest Colorado, the Rabbit Ears Range and Elkhead Mountains of the Park Range in northern Colorado, and Leucite Hills of southern Wyoming. Given the wide geographic extent of volcanic terrane in the Southern Rocky Mountains, focus areas include:

  • Taos Plateau area and adjacent basin bounding highlands of the southern San Luis Basin, New Mexico and Colorado
  • Southeastern San Juan Mountains of southern Colorado
  • Rabbit Ears Range and Elkhead Mountains, Colorado
  • Flat Tops area of west-central Colorado
Mapping area of Ute Mountain 7.5' quadrangle
Recently published geologic map of Ute Mountain. The Ute Mountain 7.5' quadrangle is located in the south-central part of the San Luis Basin of northern New Mexico, in the Rio Grande del Norte National Monument, and contains deposits that record volcanic, tectonic, and associated alluvial and colluvial processes over the past four million years.