Rocky Mountain Geology 34.1

Introduction to special issues, Part II: Nature of tectonic boundaries in lithosphere of the Rocky Mountains

K. E. Karlstrom

Keywords: Introduction to special issues, Part II: Nature of tectonic boundaries in lithosphere of the Rocky Mountains

Insights into the Proterozoic geology of the Park Range, Colorado

C. T. Foster, M. K. Reagan, S. G. Kennedy, G. A. Smith, C. A. White, J. E. Eiler, and J. R. Rougvie

Keywords: Proterozoic stratigraphy, Proterozoic volcanism, Proterozoic metamorphism, Proterozoic shear zones, Proterozoic sutures, Park Range, Colorado.

Proterozoic rocks in the Park Range comprise an assemblage of highly deformed metavolcanic and metasedimentary supracrustal rocks intruded by numerous plutons. The oldest rocks in the region are a suite of bimodal metavolcanic and associated metasedimentary rocks on Farwell Mountain. These rocks are similar to the Green Mountain Formation to the north, and are geochemically similar to modern continental volcanic arcs related to subduction. The most widely exposed Proterozoic supracrustal unit in the Park Range consists of a highly metamorphosed and deformed, bimodal volcanic sequence with trace-element patterns similar to those of continental interior bimodal suites instead of subduction-related volcanism. The metavolcanic rocks are overlain by a suite of metasedimentary rocks. Protoliths for the basal metasedimentary rocks are graded sequences of feldspar-rich lithic sandstones and conglomerates interpreted as turbidites. These grade stratigraphically upward into a sequence of interbedded shales and finegrained quartz arenites, similar to those found on passive-margin shelves. The top of the sedimentary sequence consists of a medium- to coarse-grained, cross-bedded quartzite. The supracrustal rocks are intruded by plutonic rocks with compositions from gabbro to granite and ages of 1.78 to 1.735 Ga. The youngest Proterozoic unit in the area is the 1.4-Ga Mount Ethel pluton.

Upper amphibolite-facies regional metamorphism produced abundant sillimanite in aluminous rocks throughout the Park Range and overprints structural fabrics associated with the older intrusions. Garnet-biotite/GASP thermobarometry indicates temperatures for this event ranging from 550–700°C and pressures of 4 to 6 kbar for most of the region south and east of greenschist-facies rocks at Farwell Mountain. Evidence of an earlier period of higher pressure metamorphism is present near Lester Mountain where kyanite that contains small blebs of staurolite has partly broken down to sillimanite. Evidence for a late, lower-pressure rehydration event is also recorded at Lester Mountain.

The dominant fabric in the supracrustal rocks throughout the range consists of a steeply dipping northeast-southwest-striking foliation with a steeply plunging stretching lineation. This fabric is also recorded in the oldest of the felsic plutonic rocks, although the amount of strain in the dated plutonic rocks decreases with age. The intensely deformed Soda Creek-Fish Creek shear zone might be a suture between major terranes in the Park Range. There are also changes in metamorphic grade and ages of plutonic rocks that may be related to the shear zone. The main deformation along this shear zone predated the Mount Ethel pluton and was northside-down. However, some left-lateral displacements along late mylonite zones offset rocks attributed to the Mount Ethel magmatic system. A second candidate for a suture is the area between Farwell Mountain and Lester Mountain, where metamorphic grades and compositions of metavolcanic rocks change abruptly.

Metamorphism and deformation near the ~1.4-Ga Mount Ethel pluton, Park Range, Colorado

M. F. Barinek, C. T. Foster, and P. P. Chaplinsky

Keywords: Soda Creek–Fish Creek shear zone, Mount Ethel pluton, 1.4-Ga plutonism, Proterozoic deformation, Proterozoic metamorphism.

The ~1.4-Ga Mount Ethel pluton is a northeast-trending elliptical body of granodiorite to quartz monzonite surrounded by Lower Proterozoic (1.8–1.7 Ga) rocks in the Park Range, northeast of Steamboat Springs, Colorado. The contact along the northern and southern margins of the pluton is concordant with the northeast-striking regional foliation, whereas the western margin is discordant to the regional foliation. The southern margin of the pluton is adjacent to the subvertical, northeast-striking, northwest-dipping Soda Creek–Fish Creek shear zone, which is located entirely within metasedimentary and metavolcanic country rocks. Kinematic indicators within the shear zone yield a north-side-down sense of shear with a left-lateral component. In contrast to several other well-studied ~1.4-Ga plutons associated with shear zones in the western U.S., penetrative strain related to the shear zone does not appear to extend into the pluton. Late dikes, which appear to be genetically related to the Mount Ethel pluton, cut the main penetrative fabric of the shear zone but are also offset by narrow, discontinuous mylonites of the shear zone. Scarce discontinuous mylonites are also present along the southern margin of the pluton. The pluton contains magmatic fabrics that parallel the strike of the shear zone along the north and south margins of the pluton, but are at a high angle to it along the western margin. These data suggest that early deformation associated with the Soda Creek–Fish Creek shear zone occurred prior to pluton emplacement, that the Mount Ethel pluton was emplaced along a northeast-striking zone of anisotropy, and that some shearing also took place after crystallization and cooling of the pluton.

Country rocks in the vicinity of the Mount Ethel pluton with garnet–biotite–plagioclase–muscovite ±sillimanite have two generations of garnet: one that is syntectonic and another that is post-tectonic relative to the northeast-striking foliation. Core compositions of pre/syntectonic garnet paired with matrix biotite compositions yield temperatures of >600°C at ~5 kbar that are associated with early (pre-Mount Ethel) deformation within the Soda Creek–Fish Creek shear zone. Core compositions of post-tectonic garnet paired with matrix biotite compositions reveal a temperature gradient on the south side of the pluton that increases from ~540°C 5 km away to >~630°C within 1 km of the Mount Ethel pluton. This may represent the thermal aureole around the intrusion, or it may represent a gradient that pre-dated the pluton. Garnet-rim compositions from both syntectonic and post-tectonic garnet, paired with compositions of biotite near garnet, yield consistent temperatures of ~550 ±50°C that are spatially unrelated to the Mount Ethel pluton. These rim temperatures may be related to a third thermal event; or alternatively, they may represent partial re-equilibration of garnet rims with nearby biotite during cooling following the last metamorphism.

The Yavapai–Mazatzal crustal boundary in the Southern Rocky Mountains

C. A. Shaw and K. E. Karlstrom

Keywords: province boundary, continental accretion, Proterozoic, island arcs, isotopic provinces, Colorado, Southern Rocky Mountains, sutures, shear zones.

A major geologic boundary has been proposed in the Southern Rocky Mountains separating Proterozoic crustal provinces with different ages and tectonic histories. These provinces probably correlate with the Yavapai (1.8–1.7 Ga) and Mazatzal (1.7–1.6 Ga) provinces of Arizona. Geologic, geochemical, geochronologic, and xenolith data suggest that the boundary lies within a ~300 km-wide zone that trends northeastward through southern Colorado and northern New Mexico. This zone also seems to have focused later tectonic and thermal effects. However, no major shear zone that might represent a discrete tectonic suture has been identified in the area, and there is no agreement on precisely where the boundary is or what tectonic significance it may have.

We present a review of evidence supporting extrapolation of the Yavapai–Mazatzal boundary through the Southern Rocky Mountains. Limitations in the precision, quantity, and interpretation of available data probably contribute to disagreement over the location of the boundary. However, the disparity in boundaries defined by different data sets may partly reflect a complex or gradational transition between crustal domains. We propose a speculative model for the boundary based on a preliminary structural analysis. Tectonic fabrics appear to be consistent with the initial juxtaposition of arc terranes of the Yavapai and Mazatzal provinces on a low-angle thrust system with later modification and steepening of the boundary during continued crustal shortening. This model explains the diffuse isotopic boundary as a manifestation of a vertically heterogeneous crustal column that might promote isotopic mixing. The cryptic structural expression of the suture may result from a layer-parallel style of suturing and complex post-accretionary tectonic overprinting.

New Mexico middle-crustal cross sections: 1.65-Ga macroscopic geometry, 1.4-Ga thermal structure, and continued problems in understanding crustal evolution

M. L. Williams, K. E. Karlstrom, A. Lanzirotti, A. S. Read, J. L. Bishop, C. E. Lombardi, J. N. Pedrick, and M. B. Wingsted

Keywords: Proterozoic geology, New Mexico, Mazatzal, Yavapai, tectonic evolution.

Recent detailed work in key regions along two north–south transects in northern New Mexico highlights continued controversy about Proterozoic tectonic evolution. Ductile deformation features (folds, ductile thrusts, and associated foliations and lineations) are grouped into three deformation generations. D1 includes cryptic bedding-parallel foliation and fold nappes. D2 involves north-verging, km-scale inclined folds, the main shortening foliation, and D2’ structures that further attenuate or reactivate F2 folds. D3 involves east–west-trending open folds and domes and associated crenulation cleavage. Although others can dominate locally, S2 is the dominant regional foliation that could possibly be imaged seismically. Map relationships around ca. 1.65- and ca. 1.42-Ga plutons and porphyroblast-matrix studies of dated minerals show that D3 occurred at ca. 1.42. The age of D2 is more uncertain and could be 1.65 or 1.42 Ga. Metamorphic studies also indicate multiple metamorphic events, M1–M3, that may relate to the deformational events. New geochronology indicates that most metamorphic minerals grew (or were reset) at ca. 1.47–1.35 Ga. U-Pb dates on metamorphic zircon, monazite, titanite, staurolite, garnet, and tourmaline suggest regional metamorphism to 550–700°C at 1.47–1.42 Ga. Metamorphic aureoles are present around plutons, but the highest grades of metamorphism are in areas with no exposed 1.42-Ga plutons. Metamorphism is interpreted to record a regional mantle-driven thermal event, the latter parts of which correspond to a time of pluton emplacement. 40Ar/39Ar dates record post–1.42-Ga cooling: the highest grade rocks yield the youngest cooling ages, indicating slow cooling and gradual unroofing of the 1.42-Ga thermal profile following 1.42-Ga metamorphism. Our preferred model is that macroscopic geometries (D1–D2) were established by 1.65 Ga, and that regional amphibolite-grade metamorphism and associated D3 deformation at 1.47–1.42 Ga produced localized high-strain domains and fabric reactivation at exposed levels. At deeper levels, structures and assemblages may increasingly record 1.42-Ga reactivation.

A middle-crustal cross section from the Rincon Range, northern New Mexico: Evidence for 1.68-Ga pluton-influenced tectonism and 1.4-Ga regional metamorphism

A. S. Read, K. E. Karlstrom, J. A. Grambling, S. A. Bowring, M. Heizler, and C. Daniel

Keywords: Rincon Range, Mora, Guadalupita, rheology, granite, Paleoproterozoic, Mesoproterozoic, metamorphism, Mazatzal orogeny, middle crust.

In the Rincon Range, north of Mora, New Mexico, a relatively abrupt regional change in dominant fabric orientation occurs within Paleoproterozoic rocks which are nearly continuously exposed for ~70 km in adjacent Laramide uplifts of the southern Sangre de Cristo Mountains. Near the village of Guadalupita, these rocks display a smooth but abrupt south-to-north change from subhorizontal to subvertical dominant foliation (S2) over a distance of ~2 km. This change in dominant fabric orientation coincides with a regional change in metamorphic grade from near-granulite grade (~650°C, 4–6 kbar) in rocks with a subhorizontal fabric to amphibolite grade (~500°C, 4–6 kbar) in rocks with a subvertical fabric. The shallowly dipping S2 fabric and highest temperature assemblages are both centered around an ~1682-Ma granitic orthogneiss, the Guadalupita pluton, which engulfs the overturned lower limb of an ~15 km-scale, north-facing F1 fold. Porphyroblast-matrix microstructural studies suggest that S1 and S2 formed during a progressive event that was synchronous with pluton emplacement and regional metamorphism at ~1682 Ma. Granite emplacement and its incorporation into the core of a fold-nappe at ~1.68 Ga appears to have facilitated subhorizontal S2 fabric development late during the progressive S1/S2 event and heat from the granite enhanced regional metamorphic conditions to create the ~150°C temperature gradient. However, metamorphic monazites aligned in S2 yield U-Pb dates of ~1421 Ma, suggesting that monazite grew during renewed tectonism that reactivated the older subhorizontal fabric during ~1.42-Ga regional metamorphism. Present geometries therefore reflect a superposition of major tectonometamorphic events at 1.68 and 1.42 Ga. This study suggests that: (1) large temperature gradients around plutons can cause regionally heterogeneous middle-crustal pressure–temperature–time–deformation (P–T–t–D) paths; (2) plutons may both localize and be localized by subhorizontal shear zones; and (3) middle-crustal rheologies are strongly influenced by thermal weakening near plutons.

Thermal, structural, and petrological evidence for 1400-Ma metamorphism and deformation in central New Mexico

J. R. Marcoline, M. T. Heizler, L. B. Goodwin, S. Ralser, and J. Clark

Keywords: 40Ar/39Ar thermochronology, Manzano Mountains, metamorphism, deformation.

Amphibolites from the Manzano Mountains, New Mexico, include two chemically and microstructurally distinct amphibole populations. Petrographic and electron-microprobe studies show that early actinolite is overgrown and crosscut by younger foliation-forming hornblende. An older foliation defined by actinolite porphyroclasts (S1) is discordant to the regionally extensive hornblende foliation (S2). Microstructural relationships both in amphiboles and muscovite-chlorite schists indicate that S1 and S2 record two distinct episodes of metamorphism and deformation, rather than a single progressive event. 40Ar/39Ar geochronologic analyses on hornblende, actinolite, muscovite, and biotite constrain timing of the youngest metamorphic/deformational event. Most amphiboles yield complex 40Ar/39Ar age spectra, but two hornblende concentrates give preferred ages of 1410 ±12 Ma and 1399.1 ±5.4 Ma, and one actinolite has a preferred age of 1391.6 ±5.0 Ma, suggesting cooling below ~450°C at this time. These ages are interpreted to record the timing of near-peak metamorphism. Muscovite sampled over a 1.5-km vertical section of muscovite-chlorite schists shows an age discordance, with the structurally highest sample being ~50 m.y. older than the structurally lowest sample. This age discordance is interpreted to suggest cooling from ~300°C at 0.5°C/m.y. following the peak of ca. 1400-Ma metamorphism. Biotites from similar structural levels yield variable preferred ages, which range from 1402.6 ±5.1 to 1267.8 ±5.7 Ma and corroborate the slow cooling suggested by the muscovite results.

Together, the thermochronologic, structural, and petrologic data support a model of regional deformation, metamorphism, and mineral growth at ca. 1450–1400 Ma. These data add to a growing body of evidence from the southwestern United States that ca. 1400-Ma plutonism was not anorogenic, but rather was contemporaneous both with metamorphism and deformation.

The Rio Grande rift: A geological and geophysical overview

G. R. Keller and W. S. Baldridge

Keywords: Rio Grande rift, tectonics, geophysics.

The Rio Grande rift is a major structural element of the Southern Rocky Mountain region. During the last 20 years this feature has become widely recognized as a major Cenozoic continental rift zone. During this time we have learned much about the structure and evolution of the rift. However, many gaps in our knowledge remain that prevent us from fully understanding its evolution and the processes that formed it. The rift should not be studied in isolation, because the Laramide orogeny, uplift, complex magmatism, and extension that have occurred since the Late Cretaceous are all to some extent related.

Seismic expression of Late Cretaceous to Recent structure in southwestern New Mexico

J. Y. Chang, K. C. Miller, and G. R. Keller

Keywords: New Mexico, Basin and Range, Laramide, structure, seismic reflection, gravity.

We present an interpretation of seismic-reflection data from the Playas and Hachita valleys of southwestern New Mexico which places new constraints on the geometry of structures responsible for Laramide shortening, and subsequent Basin and Range extensional structures. With seismic interpretation tied to well data and gravity modeling, we address aspects of the debate that still surrounds the question of whether Laramide shortening was accommodated by “thin-skinned” thrusting along a regional décollement or by basement-involved block uplifts. Our interpretation suggests that shortening was at least in part accommodated with low-angle, basement-involved thrusts. Subsequent extension occurred on range-bounding normal faults that become listric near 10-km depth. The relative lack of recent basin fill in the Playas Valley, and particularly the Hachita Valley, suggests either that uplift of the mountains along normal faults has been relatively recent or that extension has only just compensated for earlier shortening, and has yet to create deeply subsided basins.