Charles E. Chapin

Time-stratigraphic framework for the Eocene-Oligocene Mogollon-Datil volcanic field, southwest New Mexico

A time-stratigraphic framework for discontinuously exposed regional ignimbrites in the Eocene-Oligocene Mogollon-Datil volcanic field has been established using correlations aided by 40Ar/39Ar age determinations and paleomagnetic analyses. 40Ar/39Ar age spectra from sanidine separates (25 regional ignimbrites, 85 samples, 97 spectra) yield well-defined plateau ages that are precise (within-sample and within-unit 1σ < ± 0.5%) and agree closely with independently established stratigraphic order. Paleomagnetic remanence directions (404 sites) from individual ignimbrite outflow sheets are generally vertically and horizontally uniform throughout facies ranging from thick (100-500 m), densely welded, proximal ignimbrites to thin (1.5-30 m), unwelded, distal fringes. Between-unit differences in paleomagnetic directions provide useful correlation criteria, particularly for units having ages too close to be resolved using 40Ar/39Ar dating. The Mogollon-Datil time-stratigraphic framework clarifies ignimbrite history and provides improved age control for intercalated lavas and sedimentary rocks. Ignimbrite activity was strongly episodic; outflow sheets were primarily erupted in four discrete pulses representing synchronized activity of two separate cauldron complexes. Activity in the southern complex began at 36.2 Ma near Las Cruces, New Mexico, and subsequently migrated 220 km northwest, culminating in the 28.0 Ma Bursum cauldron. Activity in the northern complex, located west of Socorro, New Mexico, underwent a less defined and more modest 40-km westward migration over its 32.0 to 24.3 Ma life span. The four pulses of ignimbrite activity were (1) 36.2-24.3 Ma, 12 major units, >1,500 km3 total volume; (2) 32.0-31.4 Ma, three major units, >1,500 km3 volume; (3) 29.1-27.4 Ma, nine major units, >6,000 km3; and (4) 24.3 Ma, one major unit. The third and largest ignimbrite pulse was accompanied by extensive rhyolitic dome and flow eruptions in the area between the two main cauldron complexes.

Interplay of oceanographic and paleoclimate events with tectonism during middle to late Miocene sedimentation across the southwestern USA

Continental sedimentation reflects a complex interplay of tectonics and climate. A 2000-km transect from coastal California to the western Great Plains documents a major increase in sedimentation (ca. 16–6 Ma) coeval with deposition of the hemipelagic Monterey Formation along the California coast. Basin and Range-style regional extension following elongation of the Pacific–North American transform boundary at ca. 17.5 Ma provided fault-bounded basins for accommodation space, but sedimentation also occurred on unextended erosional surfaces of the Great Plains and Colorado Plateau. Two global climate transitions bracket this sedimentary interval. The middle Miocene transition (ca. 17–12 Ma) records the global change from equatorial to meridional circulation caused by: (1) closing of the eastern Tethys Seaway (ca. 18 Ma); (2) opening of the Arctic–North Atlantic connection (ca. 17.5 Ma); (3) growth of the East Antarctic Ice Sheet (ca. 14 Ma); and (4) closing of the Indonesian Seaway (ca. 12 Ma). Upwelling of cold waters along the California coast, abetted by domination of La Nina phases of El Nino–Southern Oscillation (ENSO), progressively aridified the Southwest as reflected in sedimentary and biologic records. The second climate transition occurred as opening of the Gulf of California (ca. 6 Ma) intensified the North American monsoon, resulting in integration of drainages, incision of uplifts, and exhumation of basin fills. The Miocene ended with the driest climate of the Tertiary (both regional and global) accompanied by conversion of savanna to steppe or scrub desert, spread of C4 grasses, and the greatest mammal extinction of the Neogene.

Diachronous episodes of Cenozoic erosion in southwestern North America and their relationship to surface uplift, paleoclimate, paleodrainage, and paleoaltimetry

The history of erosion of southwestern North America and its relationship to surface uplift is a long-standing topic of debate. We use geologic and thermochronometric data to reconstruct the erosion history of southwestern North America. We infer that erosion events occurred mostly in response to surface uplift by contemporaneous tectonism, and were not long-delayed responses to surface uplift caused by later climate change or drainage reorganization. Rock uplift in response to isostatic compensation of exhumation occurred during each erosion event, but has been quantified only for parts of the late Miocene–Holocene erosion episode. We recognize four episodes of erosion and associated tectonic uplift: (1) the Laramide orogeny (ca. 75–45 Ma), during which individual uplifts were deeply eroded as a result of uplift by thrust faults, but Laramide basins and the Great Plains region remained near sea level, as shown by the lack of significant Laramide exhumation in these areas; (2) late middle Eocene erosion (ca. 42–37 Ma) in Wyoming, Montana, and Colorado, which probably occurred in response to epeirogenic uplift from lithospheric rebound that followed the cessation of Laramide dynamic subsidence; (3) late Oligocene–early Miocene deep erosion (ca. 27–15 Ma) in a broad region of the southern Cordillera (including the southern Colorado Plateau, southern Great Plains, trans-Pecos Texas, and northeastern Mexico), which was uplifted in response to increased mantle buoyancy associated with major concurrent volcanism in the Sierra Madre Occidental of Mexico and in the Southern Rocky Mountains; (4) Late Miocene–Holocene erosion (ca. 6–0 Ma) in a broad area of southwestern North America, with loci of deep erosion in the western Colorado–eastern Utah region and in the western Sierra Madre Occidental. Erosion in western Colorado–eastern Utah reflects mantle-related rock uplift as well as an important isostatic component caused by compensation of deep fluvial erosion in the upper Colorado River drainage following its integration to the Gulf of California. Erosion in the western Sierra Madre Occidental occurred in response to rift-shoulder uplift and the proximity of oceanic base level following the late Miocene opening of the Gulf of California. We cannot estimate the amount of rock or surface uplift associated with each erosion episode, but the maximum depths of exhumation for each were broadly similar (typically ∼1–3 km). Only the most recent erosion episode is temporally correlated with climate change. Paleoaltimetric studies, except for those based on leaf physiognomy, are generally compatible with the uplift chronology we propose here. Physiognomy-based paleo elevation data suggest that near-modern elevations were attained during the Paleogene, but are the only data that uniquely support such interpretations. High Paleogene elevations require a complex late Paleogene–Neogene uplift and subsidence history for the Front Range and western Great Plains of Colorado that is not compatible with the regional sedimentation and erosion events we describe here. Our results suggest that near-modern surface elevations in southwestern North America were generally not attained until the Neogene, and that these high elevations are the cumulative result of four major episodes of Cenozoic rock uplift of diverse origin, geographic distribution, and timing.

Origin of the Colorado Mineral Belt

The Colorado Mineral Belt (CMB) is a northeast-trending, ∼500-km-long, 25–50-km-wide belt of plutons and mining districts (Colorado, United States) that developed within an ∼1200-km-wide Late Cretaceous–Paleogene magma gap overlying subhorizontally subducted segments of the Farallon plate. Of the known volcanic gaps overlying flat slabs in subduction zones around the Pacific Basin, none contains zones of magmatism analogous to the CMB. I suggest that the primary control of the CMB was a northeast-trending segment boundary within the underlying Farallon flat slab. The boundary was dilated during warping of slab segments by the overriding thick (∼200 km) lithospheres of the Wyoming Archean craton and the continental interior craton during acceleration of Farallon–North American convergence beginning in mid-Campanian time (ca. 75 Ma). Because the primary control was not in the North American plate, the CMB cut indiscriminately across the geologic grain of Colorado, seemingly independent of the tectonic elements it crossed. A series of discontinuous shear zones of Proterozoic ancestry provided some local control at the district level but were not the primary control. Geologic contrasts north and south of the CMB reflect its relationship to a segment boundary in the Farallon plate. The dominant trends of Laramide basement-cored uplifts are northwestward north of the CMB but northward south of the CMB. Laramide sedimentary deposits of Late Cretaceous and Paleogene age (exclusive of the Sevier foredeep) are as much as 6 km thick north of the CMB versus only ≤3 km south of the CMB. The Farallon segment south of the CMB rolled back to the southwest and sank into the mantle beginning ca. 37 Ma with resultant major ignimbrite volcanism and generation of the large San Juan and Mogollon-Datil volcanic fields. Volcanism in the Rocky Mountains north of the CMB was sparse. Laramide plutons (ca. 75–43 Ma) are mainly alkaline monzonites and quartz monzonites in the northeastern CMB, but dominantly calc-alkaline granodiorites in the central CMB. Geochemical and isotopic studies indicate that CMB magmas were generated mainly in metasomatized Proterozoic intermediate to felsic lower crustal granulites and mafic rocks (± mantle). Late Eocene–Oligocene rollback magmatism superimposed on the CMB during waning of Laramide compression (ca. 43–37 Ma) resulted in world-class sulfide replacement ores in the Leadville area. Overprinting of the CMB by Rio Grande Rift extension beginning ca. 33 Ma resulted in intrusion of evolved alkali-feldspar granites and generation of major porphyry molybdenum deposits at Climax and Red Mountain.

High-precision 40Ar/39Ar sanidine geochronology of ignimbrites in the Mogollon-Datil volcanic field, southwestern New Mexico

Abstract40Ar/39Ar age spectra have been obtained from 85 sanidine separates from 36 ignimbrites and one rhyolitic lava in the latest Eocene-Oligocene Mogollon-Datil volcanic field of southwestern New Mexico. Of the 97 measured age spectra, 94 yield weighted-mean plateau ages each giving single-spectrum 1σ precision of±0.25%–0.4% (±0.07–0.14 Ma). Replicate plateau age determinations for eight different samples show within-sample 1σ precisions averaging ±0.25%. Plateau ages from multiple (n=3–8) samples of individual ignimbrites show 1σ within-unit precision of ±0.1%–0.4% (±0.04–0.13 Ma). This within-unit precision represents a several-fold improvement over published K-Ar data for the same ignimbrites, and is similar to the range of precisions reported from single-crystal laser fusion studies. A further indication of the high precision of unit-mean 40Ar/30Ar ages is their close agreement with independently established stratigraphic order. Two samples failed to meet plateau criteria, apparently due to geologic contamination by older feldspars. Effects of minor contamination are shown by six other samples, which yielded slightly anomalous plateau ages. 40Ar/39Ar plateau ages permit resolution of units differing in age by 0.5% (0.15 Ma) or less. This high resolution, combined with paleomagnetic studies, has helped to correlate ignimbrites among isolated ranges and has allowed development of an integrated timestratigraphic framework for the volcanic field. Mogollon-Datil ignimbrites range in age from 36.2 to 24.3 Ma. Ignimbrite activity was strongly episodic, being confined to four brief (<2.6 m.y.) eruptive episodes separated by 1–3 m.y. gaps. Ignimbrite activity generally tended to migrate from the southeast toward the north and west.

The Rocky Mountain Front, southwestern USA

The Rocky Mountain Front (RMF) trends north-south near long 105°W for ∼1500 km from near the U.S.-Mexico border to southern Wyoming. This long, straight, persistent structural boundary originated between 1.4 and 1.1 Ga in the Mesoproterozoic. It cuts the 1.4 Ga Granite-Rhyolite Province and was intruded by the shallow-level alkaline granitic batholith of Pikes Peak (1.09 Ga) in central Colorado. The RMF began as a boundary between thick cratonic lithosphere to the east (modern coordinates) and an orogenic plateau to the west and remains so today. It was reactivated during the 1.1 to 0.6 Ga breakup of the supercontinent Rodinia and during deformation associated with formation of both the Ancestral and Laramide Rocky Mountains. Its persistence as a cratonic boundary is also indicated by emplacement of alkalic igneous rocks, gold-telluride deposits, and other features that point to thick lithosphere, low heat flow, and episodic mantle magmatism from 1.1 Ga to the Neogene. Both rollback of the Farallon flat slab ca. 37 Ma and initiation of the Rio Grande Rift shortly thereafter began near the RMF. Geomorphic expression of the RMF was enhanced during the late Miocene to Holocene (ca. 6–0 Ma) by tectonic uplift and increased monsoonal precipitation that caused differential erosion along the mountain front, exhuming an imposing 0.5–1.2 km escarpment, bordered by hogbacks of Phanerozoic strata and incised by major river canyons. Here we investigate four right-stepping deflections of the RMF that developed during the Laramide orogeny and may reveal timing and structural style. The Sangre de Cristo Range to Wet Mountains and Wet Mountains to Front Range steps are related to reactivation of the eroded stumps of Ancestral Rocky Mountain uplifts. In northern Colorado, the Colorado Mineral Belt (CMB) ends at the RMF; no significant northeast-trending faults cross the Front Range–Denver Basin boundary. However, several features changed from south to north across the CMB. (1) The axis of the Denver Basin was deflected ∼60 km to the northeast. (2) The trend of the RMF changed from north–northwest to north. (3) Structural style of the Front Range–Denver Basin margin changed from northeast-vergent thrusts to northeast-dipping, high-angle reverse faults. (4) Early Laramide uplift north of the CMB was accompanied by southeastward slumping and decollement faulting of upper Cretaceous sedimentary units. (5) The Boulder-Weld coal field developed within the zone of decollement faulting. (6) The huge Wattenberg gas field formed over a paleogeothermal anomaly. (7) Apatite fission track (AFT) cooling ages in the Front Range north of the CMB are almost all associated with Laramide deformation (ca. 80–40 Ma), whereas south of the CMB, AFT ages in the Front Range and Wet Mountains vary widely (ca. 449–30 Ma). Proterozoic rocks still retain pre-Laramide AFT ages in a zone as much 1200 m thick south of the CMB, revealing comparatively modest uplift and erosion. A fourth step is a ∼250 km deflection of the RMF from the Laramie Range to the Black Hills of South Dakota along the southeastern boundary of the Wyoming Archean province. Laramide synorogenic sedimentation occurred mainly in Paleocene and early Eocene time on both sides of the Front Range in Colorado, but the timing and style of basin-margin thrusting differed markedly. Moderate- to high-angle thrusts and reverse faults characterized the east side beginning in the Maastrichtian (ca. 68 Ma). On the west side, low-angle thrusts overrode the Middle Park and South Park basins by 10–15 km beginning in the latest Paleocene–early Eocene. This later contraction correlates temporally with the third major episode of shortening in the Sevier fold and thrust belt, when the Hogsback thrust added ∼21 km of shortening to become the easternmost major thrust in southwest Wyoming and northern Utah. A remarkable attribute of the RMF is that it maintained its position through multiple orogenies and changes in orientation and strength of tectonic stresses. During the Laramide orogeny, the RMF marked a tectonic boundary beyond which major contractional partitioning of the Cordilleran foreland was unable to penetrate. However, the nature of the lithospheric flaw that underlies the RMF is an unanswered question.

Two-Stage Laramide Orogeny in Southwestern United States: Tectonics and Sedimentation: ABSTRACT

The Laramide orogeny (80-40 Ma) in the southwestern United States is usually thought of as a single tectonic event with attention concentrated on its early stage because of its dramatic expression in the sedimentary record. The sharpest pulse of deformation, however, occurred in the latest Paleocene-early Eocene and was separated from the early stage by a tectonic lull and development of widespread lateritic weathering profiles, remnants of which are preserved in some early Tertiary basins. The first stage correlates plate tectonically with opening of the North Atlantic and Labrador Sea, the second stage with opening of the Norwegian Sea and Eurasian basin in the eastern Arctic. Rapid convergence between the North American and Farallon plates decreased the dip of the subd cting Farallon plate, until by early Eocene (55 Ma), strong viscous coupling was occurring between the Farallon plate and the overlying lithosphere. Change in direction of convergence from west-southwest-east-northeast during the early stage to southwest-northeast during the late stage brought the first-order shear direction into near parallelism with the north-northeast-trending southern Rocky Mountain deformed belt in New Mexico. This parallelism allowed the Colorado Plateau to decouple from the craton along right-lateral wrench faults. The Colorado Plateau was translated 100-130 km north-northeast with a comparable shortening across the Wyoming province. Both sedimentation rates and coarseness of synorogenic sediments increased dramatically from Wyoming to the Gulf Coast of Texas beginning in the early Eocene. Simultaneously, an en echelon series of strike-slip basins formed along the zone of decoupling from southern New Mexico to southern Wyoming. Deformation in the Wyoming province also increased sharply, resulting in as much as 21 km of overhang on range-front thrusts and up to 15 km of structural relief between adjacent uplifts and basins. As much as 2,500 m of Eocene orogenic sediments were deposited in rapidly subsiding basins. Deformation was so rapid that surface drainage was disrupted and runoff was impounded in huge lakes in which as much as 1,000 m of lacustrine sediments accumulated to form the oil shale province. Wrench faulting along the eastern margin of the Colorado Plateau was distributed across a 100-km wide belt that followed zones of weakness inherited from Precambrian and late Paleozoic deformation. Wrenching changed from nearly parallel in New Mexico to strongly convergent in Colorado and southern Wyoming because of a change in structural grain from north-northeast in New Mexico to north-northwest in Colorado. At least 25 km of shortening across the zone of decoupling during convergent wrenching caused conspicuous low-angle thrusting, which has tended to mask the lateral component of movement. Major north-northeast shortening across the Wyoming province and a lack of such shortening on the High Plains demand right-lateral wrench faulting. Offset of transverse aeromagnetic anomalies ac oss the zone of decoupling in Colorado and New Mexico, offset of Precambrian metavolcanic belts and age province boundaries, and offset of northeast-trending lineaments and distinctive rock types provide independent measures of the magnitude of right slip. Northward translation of the Colorado Plateau was a relatively minor part of regional right-lateral shear that extended through much of the North American cordillera in the early Tertiary. Recognition of the wrench-faulted nature of the eastern margin of the Colorado Plateau and Wyoming province creates new opportunities for petroleum exploration. These can be divided into: (1) subthrust plays beneath low-angle faults caused by convergent wrenching; (2) structural, stratigraphic, and fracture-controlled traps along wrench faults; and (3) fracture reservoirs along northeast-trending lineaments dilated during wrenching. Recognition of wrench faults paralleling the structural grain of the southern Rocky Mountains also means that isopach and facies maps drawn smoothly across the zone of decoupling should be reevaluated with careful attention to control points. End_of_Article - Last_Page 2044------------

Eocene Paleotectonics and Sedimentation in the Rocky Mountain-Colorado Plateau Region: ABSTRACT

End_Page 1331------------------------------The Laramide orogeny (c. 80 to 40 m.y.B.P.), which culminated during early Eocene time, resulted in the development of numerous uplifts and basins in the foreland of the western United States. Uplifts are assignable to three general classes: (1) Cordilleran thrust belt uplifts, (2) basement-cored, fault-bounded uplifts of the classic Laramide Rocky Mountains, and (3) monocline-bounded uplifts of the Colorado Plateau. Basins were also of three types: (1) Green River type--large equidimensional to elliptical basins bounded on three or more sides by uplifts and commonly containing lake deposits, (2) Denver type--asymmetrical, synclinal downwarps with a related uplift along one side, and (3) Echo Park type--narrow, highly elongate basins with through drainage and of strike-slip origin. Gr en River-type basins exhibit quasiconcentric zonation of facies, in contrast to the unidirectional, proximal-to-distal facies tract of Denver-type basins. Facies distribution in Echo Park-type basins is complex and often difficult to reconstruct due to faulting, erosional truncation, and cover. The prevalence of en echelon structures in the deformed zone east of the Colorado Plateau, and evidence for significant crustal shortening north of the plateau, suggest that the major structural features of the Laramide foreland were produced by large-scale, north-northeastward translation of the relatively rigid Colorado Plateau block. The magnitude of this motion, as indicated by dextral offset of lineaments which cross the eastern margin of the plateau and by the amount of crustal shortening in the Wyoming province, may be as great as 65 to 120 km (40 to 75 mi). This translation probably resulted from the interaction of relatively competent Colorado Plateau lithosphere with the underlying, gently dipping Farallon plate, which was being overridden by the western United States in Lar mide time. Evidence for increased strain rates in early Eocene time includes: (1) markedly higher rates of deposition and sand/shale ratios in the Gulf Coast geosyncline (Wilcox Group), (2) formation of several new basins in the southern Rocky Mountains in which Eocene deposits rest unconformably on pre-Cenozoic rocks, and (3) the generally coarser and more arkosic nature of Eocene sediments, as compared to older Laramide deposits, in many areas throughout the foreland. The early Eocene culmination of Laramide tectonism appears to result from two factors. First, the subducted Farallon plate achieved its shallowest dip at about 55 m.y.B.P., resulting in increased viscous coupling with the overriding continental lithosphere. Second, changing spreading-center geometries in the Labrador Sea, Norwegi n Sea, and Arctic Ocean caused the maximum horizontal stress direction to shift to a northeasterly orientation, causing the Colorado Plateau block to increasingly decouple from the craton along north-trending wrench faults in the southern Rocky Mountains. Translation of the Colorado Plateau to the north-northeast during Laramide time resulted in a series of transpressive uplifts and basins along its eastern margin and large-scale crustal shortening in the Wyoming province to the north. End_of_Article - Last_Page 1332------------