Muhammad Shafiqullah

New K-Ar dates from basalts and the evolution of the southern Rio Grande rift

In the southern Rio Grande rift, two extensional regimes of different origin (but transitional with each other through the Miocene) can be interpreted from structures and rocks formed within the past 28 to 29 m.y. The earlier regime, which began about 28 to 29 m.y. B.P., is characterized by emplacement of “basaltic andesite” flows with relatively high strontium isotope ratios; formation of broad, relatively deep, northwest-trending basins; and incipient uplift of some of the region9s fault-block mountains. This regime appears to have developed in a back-arc setting, perhaps behind a rapidly steepening slab and a westward-sweeping arc system. The younger episode seemingly represents a renewal or acceleration of block faulting and volcanism during the latest Miocene and Pliocene, 9 to 3 m.y. B.P., after a long transitional period during the early and mid-Miocene when volcanism was absent and tectonism was less vigorous. The latest Miocene-Pliocene episode produced the modern northerly-trending rift basins and uplifts, regional uplift of the rift 1 to 2 km above sea level, and renewal of volcanism, this time dominated by relatively primitive alkali-olivine basalt. New basalt dates reveal that in the southern rift, modern ranges and basins were almost fully developed and that near-modern drainage ways were established across uplifts into bolsons by about 5.0 m.y. B.P. An ancestral Rio Grande had extended itself southward into the southern rift by 3 to 4 m.y. B.P., and the river entrenched itself into its modern valley between 0.7 and 0.5 m.y. B.P. Horst-graben development of the southern Basin and Range province, as well as associated basaltic volcanism, swept progressively eastward from southeastern California in the past 20 m.y., culminating in formation of the Rio Grande rift and other fault-block terrane in west Texas, New Mexico, and northern Chihuahua in the latest Miocene and Pliocene. Late Quaternary Basin and Range fault scarps increase in density eastward, which also suggests that more easterly parts of the province are youngest. These relationships support a previous model of an eastward-expanding, slab-free triangle (related to growth of the San Andreas transform), through which mantle upwelling triggers eastward-younging patterns of tectonism, volcanism, and uplift and promotes lithospheric thinning and increased heat flow. Across most of the southern Basin and Range and Rio Grande rift, the horst-graben structures related to growth of this triangle are superimposed on somewhat older (late Oligocene-middle Miocene) extensional terrane that appears to have formed in a back-arc or arc setting.

Isotopic provenance of sandstones from the Eocene Tyee Formation, Oregon Coast Range

The Tyee Formation of Eocene age in the Oregon Coast Range has been studied by a variety of isotopic techniques in order to determine its provenance. Traditional basin analyses including paleocurrent measurements, lithofacies mapping, and study of sandstone compositions made previously suggest derivation from the Klamath Mountains, which lie to the south. In contrast, the isotopic compositions of whole-rock sandstone samples, white mica, and potassium feldspar separates preclude derivation solely from this local source area. Nd-Sm, Rb-Sr, K-Ar, 18 O/ 16 O, and D/H analyses of sandstones from the Tyee and related formations yield the following information about their source areas. (1) Whole-rock ϵ Nd values between −7.1 and −7.3 at the time of deposition indicate that an old crustal component (∼700 Ma) was incorporated in the source rocks. (2) Whole-rock Rb-Sr systematics implies an older age than those of sandstones clearly derived from the Klamath Mountains. These Rb-Sr values are similar to those of modern sands of the Columbia River that were derived from eastern source areas. (3) Either apparent Rb-Sr ages of potassium feldspars are too old or their initial 87 Sr/ 86 Sr ratios are too high to have been derived from plutonic rocks of the Klamath Mountains or Sierra Nevada. (4) White micas have a fairly consistent Late Jurassic Rb-Sr isochron but have K-Ar ages of 68 Ma, an overprint not recognized in the Klamath terranes. (5) White micas have δ 18 O values of ∼9.5, a value typical of S-type granges such as are found in the Idaho batholith, but too high for normal I-type granites such as in the northern Sierra Nevada and too low for metamorphic rocks in the Klamath Mountains. (6) White micas have δD values consistent with those observed for plutonic white mica in the Idaho batholith, but markedly lower than those of white mica from schist in the Klamath Mountains. (7) Potassium feldspars have δ 18 O values that vary widely and that mainly are not in oxygen-isotope equilibrium with coexisting white mica, suggesting that these minerals were not derived from the same source area. These results indicate that the provenance of these sandstones included S-type (two-mica) granites that formed in Late Jurassic time from sources that included an old crustal component. Minerals in the granites underwent subsequent thermotectonic age resetting in Late Cretaceous time. Rocks in the Klamath Mountains and northern Sierra Nevada do not possess these features and consequently are precluded from being major source areas for the Tyee Formation. The sandstones most likely were derived from the Idaho batholith. Abundant detritus from that source area is consistent with a model in which the Oregon Coast Range basin lay much farther east, closer to Idaho, during deposition and subsequently moved westward to its present position. Such major displacement is compatible with the tectonic-rotation history documented for the Oregon Coast Range that began during the time of deposition of the Tyee Formation.

Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre-Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large-magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.

Cenozoic sedimentation and paleotectonics of north-central New Mexico: Implications for initiation and evolution of the Rio Grande rift

The Rio Grande rift is one of the major late Cenozoic continental rifts of the world, sharing most geophysical, geochemical, and geological characteristics with other rifts. Cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western North American margin: Paleocene-Eocene: Laramide primarily amagmatic compression related to flat-slab subduction; Oligocene: intermediate to silicic volcanism related to collapse of the slab; Miocene to present: rifting related to complex plate interactions overprinted on the previous history. Cenozoic paleogeographic and paleotectonic characteristics are consistent with a passive-mantle mode of rifting. North-central New Mexico provides a unique opportunity to constrain models for rift initiation and evolution. It is one of the few locations within any rift where excellently exposed pre-rift and syn-rift basin fill has been studied thoroughly enough to allow detailed paleogeographic reconstruction for almost the entire Cenozoic. Cenozoic paleogeography for the study area is summarized as follows: (1) Eocene (58-37 Ma): a single amagmatic sedimentary basin (El Rito-Galisteo) trended northwest-southeast with Laramide basement uplifts on three sides; (2) early to late Oligocene (37-28 Ma): intermediate magmatism with volcaniclastic aprons derived from the San Juan and Ortiz-Cerrillos volcanic fields, and residual Laramide uplifts; (3) late Oligocene-early Miocene (28-21 Ma): initiation of bimodal volcanism with widely dispersed volcaniclastic aprons derived from the predominantly silicic San Juan and Latir volcanic fields; (4) early to middle Miocene (21-15 Ma): continued volcaniclastic dispersal from silicic volcanic centers, concurrent with initiation of block faulting to form half grabens, internal drainage, and erosion of Phanerozoic strata and Precambrian basement; (5) middle to late Miocene (15-8 Ma): continued deepening of half grabens, widespread exposure of Precambrian terranes, formation of complex depositional environments in basin centers, and continued bimodal volcanism; (6) late Miocene to present (8-0 Ma): concentration of extension in central grabens linked by accommodation zones, major bimodal volcanism (for example, Taos Plateau and Jemez Mountains), regional uplift, and integration of Rio Grande drainage. Dispersal patterns, petrofacies analysis, K-Ar ages, and chemical analyses of volcanic clasts provide details concerning three primary volcanic centers: (1) San Juan Mountains (27-29 m.y., high-K andesite and rhyodacite); (2) Latir volcanic field (25-28 m.y., high-K andesite and rhyolite); and (3) the previously unrecognized Servilleta Plaza center (22-23 m.y., latite, high-K andesite and rhyodacite), which may have been a southern extension of the Latir field. These three volcanic centers provided detritus to the following units, respectively: (1) Esquibel Mbr. of Los Pinos Fm., upper Abiquiu Fm., middle Picuris Fm., Bishops Lodge Mbr. of Tesuque Fm. and volcaniclastic units in the northern Albuquerque basin (Zia and Abiquiu); (2) Cordito Mbr. of Los Pinos Fm. and uppermost Abiquiu Fm.; and (3) Chama-El Rito Mbr. of Tesuque Fm and upper Picuris Fm. Use of petrofacies (1. Esquibel, 2. Cordito, and 3. Plaza) simplifies the chaos of stratigraphic nomenclature and promotes regional correlations of poorly exposed units.

Implications of paleomagnetic data on Miocene extension near a major accommodation zone in the Basin and Range Province, northwestern Arizona and southern Nevada

Paleomagnetic data from volcanic and crystalline rocks elucidate the evolution of a major Miocene accommodation zone in the northern Colorado River extensional corridor. The accommodation zone is a 10-km-wide belt of intermeshing conjugate normal faults that facilitates reversals in the dominant tilt direction of fault blocks and dip direction of major normal fault systems. Tilt-corrected means (e.g., N = 28 sites, D = 353.4°, I = 61.3°, α95 = 6.7°, k = 17.4) from Miocene volcanic strata overlap expected Miocene directions at the 95% confidence level. These data and geologic relations suggest that at exposed structural levels the accommodation zone did not facilitate distributed strike-slip displacement between opposing tilt block domains. Vertical axis rotations are probably negligible in most of the corridor, as the Miocene structural grain generally mimics that in the zone. Discrepancies between characteristic remanent magnetizations (ChRM) in crystalline rocks and expected directions are therefore attributed to rotations about horizontal (i.e., tilting or flexing) axes. ChRMs from Cretaceous and Miocene intrusions suggest 50°–90° of tilting of large crystalline terranes on either side of the accommodation zone. The magnitude of tilting inferred from the paleomagnetic data is similar to that of Tertiary strata in nearby fault blocks, implying that these crystalline terranes are parts of highly tilted fault blocks rather than flexed lower plate rocks. Major low-angle normal faults that bound highly tilted parts of these crystalline terranes probably nucleated at steep dips and were rotated to gentle dips by block tilting. Paleomagnetic data indicative of negligible tilting (e.g., Miocene intrusions, northern Black Mountains crystalline terrane, N = 13 sites, D = 359°, I = 55°, α95 = 9°, k = 24) and geologic relations imply that lower plate rocks may surface in both the east and west tilted domains 35–50 km away from the zone. The trend toward shallower structural levels, with respect to Miocene extension, and tapering of highly extended terrane toward the accommodation zone imply that the magnitude of upper crustal extension decreases toward the zone. Temporal patterns of major extension, especially an apparently continuous northward younging across both the east and west tilted domains, further suggest that the accommodation zone served as a long-lived rupture barrier between conjugate normal fault systems rather than as a short-term boundary between opposing systems that propagated toward and converged at the zone.

Early Miocene mylonitization and detachment faulting, South Mountains, central Arizona

The South Mountains of central Arizona are one of the geologically simplest metamorphic core complexes of the North American Cordillera. An early Miocene age of mylonitization is indicated by crosscutting relationships between mylonitic fabric and a composite pluton dated at 22-25 Ma by Rb-Sr, U-Th-Pb, and K-Ar techniques. The kinematic agreement and close temporal association of mylonitization and detachment faulting support models in which the two processes are related to an evolving crustal shear zone that accommodated mid-Tertiary continental extension. 19 references, 3 figures, 2 tables.

Late Cenozoic volcanism of the southeastern Colorado Plateau: I. Volcanic geology of the Lucero area, New Mexico

The Lucero area is a region of isolated, lava-capped mesas and buttes located in the transition zone of the southeastern Colorado Plateau adjacent to the central Rio Grande rift. Basaltic volcanism (basanites, alkali-olivine basalts, tholeiites, and evolved alkalic basaltic rocks) in the area began 8.3 m.y. ago and continued into Quaternary time, coinciding with much of the classic Basin and Range deformation and with the latest phase of rifting. Volcanism occurred in 3 distinct pulses 8.3−6.2, 4.3−3.3, and 1.1−0 m.y. ago. Compositions of rocks erupted during each cycle comprise distinctly different, although partially overlapping, populations. They define a trend (from oldest to youngest) toward relatively larger volumes of tholeiites and an increasingly bimodal distribution with respect to alkalis versus silica. The alkaline basalts could have been derived from significantly deeper (50–70 km) than were the tholeiites (40–50 km) and thus suggest that the depth from which magmas were derived became shallower with time. Alternatively, the alkaline magmas may have been derived from the same depths as were the tholeiites by smaller amounts of partial melting. In either case, the relatively large volumes of tholeiites erupted during the intermediate and youngest cycles and the (relatively) low alkali content of the youngest tholeiites require an increase of melting at shallow mantle depths through time beginning ∼8 m.y. ago, accompanying the late Cenozoic reactivation of rifting. A lull in magmatism from ∼7 to 4 m.y. ago between the oldest and intermediate cycles was widespread on the southeastern Colorado Plateau and in the central Rio Grande rift, although not on the Great Plains. A second period of quiescence 3−1 m.y. ago was of only local importance, correlating with a shift in the locus of volcanism to the Mount Taylor area. Such shifts in volcanism probably occur in response to local stresses in the lithosphere in an overall extensional stress field. The temporal and compositional patterns that we observe in the Lucero area, although not yet understood, will provide constraints on models of rift evolution and guide future studies of volcanic rocks in this region.

SOIL-CARBONATE GENESIS IN THE PINACATE VOLCANIC FIELD, NORTHWESTERN SONORA, MEXICO

Uranium-series methods were used to date and evaluate pedogenic CaCO3 genesis in the Pinacate volcanic field, northwestern Sonora, Mexico. Soils are developed in eolian deposits on lava flows. 230Th234U dates of pedogenic carbonate are mininum soil ages because of (1) the time needed to yield clasts from flows and to accrete enough carbonate to sample, (2) subsequent additions of uranium, and (3) continued solution and reprecipitation of carbonate rinds. KAr dates of basalt flows are maximum soil ages. Maximum and minimum rates of CaCO3 accumulation are calculated from the Th/U dates and KAr dates, respectively. The mean maximum rate is 0.13 g CaCO3/cm2/1000 yr and the mean minimum rate is 0.05 g CaCO3/cm2/1000 yr. Least-squares regressions of pedogenic carbonate and clay content and of Th/U ages against KAr ages suggest additions to soils from atmospheric sources throughout the late Quaternary. Morphology of pedogenic carbonate and laboratory data for soluble salts indicate that the climate of the Pinacate has not changed significantly during the past 150,000 yr. Soil variability is influenced by proximity of the eolian source. Near the periphery of the Pinacate, carbonate and clay are evenly distributed throughout soil profiles. Within the volcanic field, carbonate and clay are concentrated in soil horizons, suggesting that additions from atmospheric sources are slow enough to allow translocation.

Eruptive style and location of volcanic centers in the Miocene Washington Cascade Range: Reconstruction from the sedimentary record

Primary and reworked pyroclastic material in the Ellensburg Formation of central Washington records middle and late Miocene volcanism in the Cascade Range despite the absence of correlative volcanics within the volcanic chain. Volcanics marking sources for Ellensburg detritus were eroded during late Neogene uplift. Facies patterns and paleocurrent data suggest that the bulk of the volcaniclastics were derived from a source near Bumping Lake; a K-Ar date for an intrusion in this area supports this conclusion. Depositional patterns and characteristics of the detritus allow hypothetical reconstruction of the style of volcanism during this period. Eruptive episodes began with modest-sized Plinian eruptions followed by extended periods of dome growth. Aggradation in adjacent sedimentary basins occurred principally in response to introduction of large volumes of lithic pyroclastic material during eruptive episodes.

Isotopic provenance of Paleogene sandstones from the accretionary core of the Olympic Mountains, Washington

Conventional modal sandstone data from Paleogene units of the Pacific Northwest are not precise enough to pinpoint source areas and constrain displacement histories of accreted sedimentary terranes. Isotopic provenance study, used in conjunction with traditional basin-analysis techniques, provides a powerful means of identifying source areas. Analysis of Rb-Sr data in both wholerock and detrital white micas of Paleogene sandstones from allochthonous units in the eastern core of the Olympic Mountains and coeval autochthonous sandstones from coastal Pacific Northwest shows that the Needles-Gray Wolf and Grand Valley lithic assemblages of the core came from the same source as sandstones of the Chuckanut Formation and Puget Group in northern Washington. The source of these units is isotopically distinct from the source for units farther south, such as the Tyee Formation in Oregon. Chemical compositions, conventional K-Ar age determinations, and oxygen- and hydrogen-isotope compositions of white micas support this conclusion. Similar analyses of sandstones from the Western Olympic lithic assemblage and the Yakutat terrane of southeastern Alaska suggest that these two units have a similar source, but that they differ slightly from sandstones of the eastern Olympic core and autochthonous Washington units. The overall composition of sandstones (lithic arkosic), and the very high initial-Sr values (>0.710), moderately high δ 18 O values (∼+9) and Mesozoic age for detrital white mica of Olympic core rocks and sandstone of the Yakutat terrane suggest a source in the high-rank metamorphic and plutonic rocks from the eastern part of the Omineca Crystalline Belt in southeastern British Columbia. Furthermore, rapid uplift of this source area during Eocene time is consistent with the depositional age of the Olympic rocks. Sediment derived from this source area was transported westward by major river systems that crossed the low-lying North Cascade Range and supplied the deposits of the autochthonous units of the northern Washington coast and the offshore equivalents before the latter were accreted to form the Olympic core. Limited data from the Yakutat terrane suggest that it lay offshore of southern British Columbia sometime during middle Eocene to early Oligocene time, consistent with paleomagnetic and some paleontologic interpretations, and subsequently migrated northward by plate motions.