Andrew P. Barth

Late Cretaceous–early Cenozoic tectonic evolution of the southern California margin inferred from provenance of trench and forearc sediments

During the Late Cretaceous to early Cenozoic, southern California was impacted by two anomalous tectonic events: (1) underplating of the oceanic Pelona-Orocopia-Rand schists beneath North American arc crust and craton; and (2) removal of the western margin of the arc and inner part of the forearc basin along the Nacimiento fault. The Pelona-Orocopia-Rand schists crop out along a belt extending from the southern Sierra Nevada to southwestern Arizona. Protolith and emplacement ages decrease from >90 Ma in the northwest to <60 Ma in the southeast. Detrital zircon U-Pb ages imply that metasandstones in the older schists originated primarily from the western belt of the Sierran–Peninsular Ranges arc. Younger units were apparently derived by erosion of progressively more inboard regions, including the southwestern edge of the North American craton. The oldest Pelona-OrocopiaRand schists overlap in age and provenance with the youngest part of the Catalina Schist of the southern California inner continental borderland, suggesting that the two units are broadly correlative. The Pelona-OrocopiaRand-Catalina schists, in turn, share a common provenance with forearc sequences of southern California and the associated Salinian and Nacimiento blocks of the central Coast Ranges. This observation is most readily explained if the schists were derived from trench sediments complementary to the forearc basin. The schists and forearc units are inferred to record an evolution from normal subduction prior to the early Late Cretaceous to flsubduction extending into the early Cenozoic. The transition from outboard to inboard sediment sources appears to have coincided with removal of arc and forearc terranes along the Nacimiento fault, which most likely involved either thrusting or sinistral strike slip. The strike-slip interpretation has not been widely accepted but can be understood in terms of tectonic escape driven by subduction of an aseismic ridge, and it provides a compelling explanation for the progressively younger ages of the PelonaOrocopia-Rand schists from northwest to southeast.

Temporal and spatial trends of Late Cretaceous-early Tertiary underplating Pelona and related schist beneath southern California and southwestern Arizona

The Pelona, Orocopia, and Rand Schists and the schists of Portal Ridge and Sierra de Salinas constitute a high-pressure/temperature terrane that was accreted beneath North American basement in Late Cretaceous-earliest Tertiary time. The schists crop out in a belt extending from the southern Coast Ranges through the Mojave Desert, central Transverse Ranges, southeastern California, and southwestern Arizona. Ion microprobe U-Pb results from 850 detrital zircons from 40 meta-graywackes demonstrates a Late Cretaceous to earliest Tertiary depositional age for the sedimentary part of the schists protolith. About 40% of the 2 0 6 Pb/ 2 3 8 U spot ages are Late Cretaceous. The youngest detrital zircon ages and post-metamorphic mica 4 0 Ar/ 3 9 Ar cooling ages bracket when the schists graywacke protolith was eroded from its source region, deposited, underthrust, accreted, and metamorphosed. This interval averages 13 ′ 10 m.y. but locally is too short (<∼3 m.y.) to be resolved with our methods. The timing of accretion decreases systematically (in palinspastically restored coordinates) from about 91 ′ 1 Ma in the southwesternmost Sierra Nevada (San Emigdio Mountains) to 48 ′ 5 Ma in southwest Arizona (Neversweat Ridge). Our results indicate two distinct source regions: (1) The Rand Schist and schists of Portal Ridge and Sierra de Salinas were derived from material eroded from Early to early Late Cretaceous basement (like the Sierra Nevada batholith); and (2) The Orocopia Schist was derived from a heterogeneous assemblage of Proterozoic, Triassic, Jurassic, and latest Cretaceous to earliest Tertiary crystalline rocks (such as basement in the Mojave/Transverse Ranges/southwest Arizona/northern Sonora). The Pelona Schist is transitional between the two.

Timing of Magmatism following Initial Convergence at a Passive Margin, Southwestern U.S. Cordillera, and Ages of Lower Crustal Magma Sources

Initiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkali‐calcic Fe‐ and Sr‐enriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonal‐miogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete Permo‐Triassic intrusion ages, and a prolonged postemplacement thermal history within the long‐lived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc.

Detrital zircon as a proxy for tracking the magmatic arc system: The California arc example

Coupled age and trace element geochemical analyses of detrital zircons provide criteria that may be applied to describe average melt compositions through virtually the entire history of a long-lived magmatic arc. Detrital zircon data suggest that the California (western United States) Cordilleran arc was characterized by five mean magmatic states. Three geochemically distinct pulses were characterized by high Th/U and progressively heavy rare earth element (HREE)–depleted melts. The first and last pulses were also characterized by higher than average U/Yb, suggesting that progressive crustal thickening coupled with variable fluid inputs from the subducting slab modulated pulse volumes. Lulls between pulses were states generally characterized by low magmatic volumes and low Th/U and U/Yb, suggesting fluid-poor conditions with minimized crustal involvement in magmatism.

U-Pb geochronology and geochemistry of the McCoy Mountains Formation, southeastern California: A Cretaceous retroarc foreland basin

The timing of deposition of fluvial sediments now forming the >7-km-thick McCoy Mountains Formation is one of the key uncertainties in reconstructing the Mesozoic paleogeography of southern California and western Arizona. Ion-microprobe U-Pb geochronologic data for individual zircons from nine sandstones from the McCoy Mountains type section and six associated igneous rocks provide significant new constraints on the tectonic setting and the timing of deposition within the northwest-trending McCoy basin. U-Pb zircon data from a metavolcanic rock of the underlying Dome Rock sequence in the Palen Mountains confirm that the McCoy Mountains Formation was deposited after regional Middle to Late Jurassic arc magmatism. U-Pb zircon data from a Late Cretaceous granodiorite intruding the formation in the Coxcomb Mountains confirm that the formation was deformed and metamorphosed prior to 73.5 ± 1.3 Ma. Populations of detrital zircons vary systematically with both rock type and stratigraphic height; lithic arkoses predominantly derived from the west have consistently more abundant younger zircons than do litharenite sandstones predominantly derived from the north, and the youngest zircons yield maximum depositional ages that decrease from 116 Ma near the base to 84 Ma near the top of the section. The detrital-zircon data permit a Late Jurassic age for the basal, comparatively quartz-rich sandstone. However, the data further suggest that >90% of the formation was deposited between middle Early and middle Late Cretaceous time. These results are consistent with the hypothesis that most of the McCoy Mountains Formation represents a retroarc foreland basin, deposited behind the active, evolving Cretaceous Cordilleran continental- margin magmatic arc that lay to the west and in the foreland of the actively deforming Cretaceous Maria fold-and-thrust belt.

Geochronology of the Proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone

Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 6 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded UPb ages of 2706 6 6M a to 2066 6 Ma. Eleven of these ages lie between 211 and 216 6 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 6 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide 206 Pb*/ 238 U ages from 214 6 2M a to 200 6 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 6 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada.

U-Pb zircon geochronology of rocks in the Salinas Valley region of California: A reevaluation of the crustal structure and origin of the Salinian block

The Salinian block in the Salinas Valley region of central California consists of arc granitic and metasedimentary rocks (schist of Sierra de Salinas) sandwiched between coeval high-pressure, low-temperature melange belts. U-Pb zircon ages of three granitic plutons from this region range from 88 to 82 Ma, and coexisting biotite yielded 4 0 Ar/ 3 9 Ar cooling ages of 76-75 Ma. The U-Pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117-81 Ma igneous provenance. Muscovite and biotite 4 0 Ar/ 3 9 Ar cooling ages of 72-68 Ma from the nearby schist are distinctly younger than those from the granitic plutons. These data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks, contradicting the prevailing view that the schist comprises the local framework for the Salinian arc. We propose that the schist of Sierra de Salinas was thrust beneath the Salinian magmatic arc along a Campanian thrust fault that has not been recognized. This hypothesis implies that the Salinian arc originated as a klippe of basement rocks derived from the vicinity of the western Mojave Desert. Thrusting initiated southeastward-migrating Laramide tectonism of a style similar to that which formed the Vincent thrust and the latest Cretaceous and Paleocene Pelona and Orocopia Schists of southern California and southwestern Arizona.

Crustal growth and tectonic evolution of the Mojave crustal province: Insights from hafnium isotope systematics in zircons

Coupled U-Pb ages and Hf isotopic ratios in zircons from Proterozoic basement and three siliciclastic cover sequences in southern California provide important insights into the formation of the southern Mojave crustal province and its incorporation into southwestern Laurentia. Hafnium isotopic ratios measured in >800 zircons, coupled with new and previously reported U-Pb ages, suggest that the crystalline basement of the Mojave crustal province formed from four main components: 1) mantle components ranging from depleted to moderately enriched; 2) metasedimentary framework rocks derived from 2.6 to 2.4 Ga and 2.0 to 1.8 Ga crust; 3) 1.79 to 1.64 Ga intrusive rocks that reflect mixing of mantle-derived melts and crust; and 4) Mesoproterozoic (1.4 to 1.2 Ga) anorthosite and granitic to syenitic intrusive rocks. Initial Hf isotopic ratios of detrital zircons in siliciclastic cover sequences suggest varying degrees of insularity of the Mojave province during assembly of southwestern Laurentia. The Mesoproterozoic Pinto Mountain Group appears entirely derived from Mojave province basement. In contrast, Neoproterozoic quartzites of the Big Bear Group had a distal provenance, either an unexposed, older, western subprovince of the Mojave crustal province lacking ca. 1.7 Ga magmatic rocks or from a distinctive Paleo- and Mesoproterozoic basement province far to the east within Laurentia. Zircons in latest Neoproterozoic to Cambrian quartzites reflect provenance from an integrated transcontinental drainage network delivering sediment to the craton edge and westward into the Cordilleran miogeocline.

SHRIMP‐RG U‐Pb Zircon Geochronology of Mesoproterozoic Metamorphism and Plutonism in the Southwesternmost United States

Mesoproterozoic intrusive and granulite‐grade metamorphic rocks in southern California have been inferred to be exotic to North America on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks. Ion microprobe geochronology indicates that zircons in granulite‐grade gneisses, dated at 1.4 Ga using conventional methods, are composed of 1.68–1.80‐Ga cores and 1.19‐Ga rims. These Early Proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1.19‐Ga San Gabriel anorthosite complex. The lack of a 1.4‐Ga metamorphic event suggests that Proterozoic rocks in this region, rather than being exotic to North America, may in fact be a midcrustal window into Mesoproterozoic crustal evolutionary processes in southwestern North America.