Timothy F. Lawton

Detrital-zircon record of major Middle Triassic–Early Cretaceous provenance shift, central Mexico: demise of Gondwanan continental fluvial systems and onset of back-arc volcanism and sedimentation

New stratigraphic and petrographic data and zircon U–Pb geochronology from sandstones and volcanic rocks in the states of Queretaro and Guanajuato in central Mexico indicate an important provenance change between Late Triassic and latest Jurassic–Early Cretaceous time. The Upper Triassic El Chilar Complex consists of pervasively deformed, deep-marine olistostromes, and debris-flow deposits of arkosic and subarkosic composition. Detrital-zircon populations range from latest Palaeoproterozoic (1.65 Ga) to Middle Triassic (240 Ma), all predating the depositional age of the strata. The detrital-zircon populations are similar to those previously reported from turbidites of the Potosi fan complex of north-central Mexico and interpreted as derived from Grenville and Pan-African (Maya block) basement and Permo-Triassic arc of continental Mexico directly to the east of the basin. A single sample with a dominant Proterozoic population at ∼1.65–1.30 Ga was likely derived either from the Rio Negro-Juruena province of the Amazonian craton or from a local source in the Huiznopala Gneiss, and indicates that El Chilar strata were likely deposited in the proximal part of a submarine-fan system separate from the Potosi fan. Uppermost Jurassic–Lower Cretaceous strata of the San Juan de la Rosa Formation unconformably overlie the El Chilar Complex and likewise consist of deep-marine olistostromes, slump deposits, debris-flow deposits, and proximal fan-channel fills, but are volcanogenic litharenites with abundant felsic and vitric volcanic lithic fragments. Detrital-zircon populations are dominated by Early Cretaceous grains (150–132 Ma) with no known sources in eastern Mexico. Abundant young grains indicate a maximum depositional age of ∼134 Ma (Valanginian–Hauterivian). The San Juan de la Rosa Formation is overlain by deepwater carbonates with interbedded siliciclastic beds of the Peña Azul Formation, which contains detrital-zircon ages as young as ∼130 Ma, indicating possible equivalence with similar strata of the Las Trancas Formation, with a maximum depositional age of ∼127 Ma and lying to the east in the Zimapan Basin, now part of the Sierra Madre Oriental fold and thrust belt. Decreasing content of volcaniclastic strata eastward indicates a volcanic source to the west. Upper Cretaceous marine strata in the Mineral de Pozos area to the northwest in the state of Guanajuato contain litharenites with a maximum depositional age near 92 Ma, and are thus part of a younger depositional system. Composition and detrital-zircon content of the Upper Triassic and Lower Cretaceous successions in central Mexico indicates an important shift from Gondwanan continental sediment sources in the Triassic to western volcanic sources, probably on the edge of the newly opened Arperos basin, by the end of the Jurassic. This important sediment-dispersal change records the break-up of Pangea and concomitant development of arc-related sedimentary basins on the western edge of Mexico.

U-Pb geochronology of the type Nazas Formation and superjacent strata, northeastern Durango, Mexico: Implications of a Jurassic age for continental-arc magmatism in north-central Mexico

U-Pb ages of ignimbrites and detrital-zircon analyses from Middle Jurassic through lowermost Cretaceous strata in northeastern Durango, Mexico, indicate the age of local volcanism and the age range, respectively, of the Jurassic continental-margin arc in north-central Mexico, where it is termed the Nazas arc. The type Nazas Formation along the Rio Nazas consists of red continental sandstone and siltstone, ignimbrites, and intermediate flows that provide an important record of the extensional Jurassic Nazas magmatic arc. Overlying shallow-marine sandstone and shale of Late Jurassic to earliest Cretaceous age represent deposits of younger extensional basins that are part of the Border rift system, which extended from southern Arizona to the Gulf of Mexico. The younger strata constitute volcanic litharenite and lithic arkose derived from the Jurassic volcanic rocks and from basement rocks of the Coahuila block to the north. Two ignimbrites in the upper part of the Nazas Formation have statistically indistinguishable ages of 170 ± 2 and 169 ± 2 Ma. The ages are consistent with their stratigraphic order and position above volcanic-lithic sandstone with a maximum depositional age of ca. 180–178 Ma calculated from detrital zircons. Six detrital-zircon age populations were defined from four sandstone samples ( n = 377 zircon grains) of the Nazas Formation and overlying La Casita and Carbonera Formations. The most prominent age clusters (≥10% of the entire grain population) have ranges ca. 1277–890 Ma (Grenville, 22% of total), ca. 486–310 Ma (early Paleozoic, 11%), and ca. 202–159 Ma (Jurassic, two populations constituting 54% of total grains). The Jurassic age cluster indicates that Nazas arc magmatism in the region took place during the Early and Middle Jurassic and ended by early Late Jurassic time. The Nazas Formation correlates with arc-related Middle Jurassic strata in Sonora, southern Arizona, and southeastern California that contain interbedded eolian sandstone derived from the Jurassic ergs of the Colorado Plateau; absence of interbedded eolianite in the Nazas Formation thus indicates that the outcrops of north-central Mexico were likely not translated to their present locations along a regional, Late Jurassic transcurrent fault system, as postulated by some workers. Detrital-zircon data from the Upper Jurassic and lowermost Cretaceous strata indicate that Jurassic volcanic rocks covered the Coahuila block, directly north of the study area, at the end of Middle Jurassic time. Sandstone composition and detrital-zircon ages record erosional unroofing of the Coahuila block, whereby removal of Lower and Middle Jurassic volcanic and volcaniclastic strata exposed Grenville, Pan-African, and peri-Gondwanan basement of the block during Late Jurassic time.

Location, location, location: The variable lifespan of the Laramide orogeny

The Laramide orogeny had a spatially variable lifespan, which we explain using a geodynamic model that incorporates onset and demise of flat-slab subduction. Laramide shortening and attendant uplift began in southeast California (USA) at ca. 90 Ma, swept to the northeast to arrive in the Black Hills of South Dakota (USA) at ca. 60 Ma, and concluded in South Dakotawithin ∼10 m.y. During subsequent slab rollback, the areal extent of Laramide deformation decreased as the eastern edge of active deformation retreated to the southwest rapidly from ca. 55 to 45 Ma and more slowly from ca. 45 to 40 Ma, with deformation ultimately ceasing in the southwestern part of the orogen at ca. 30 Ma. Geodynamic modeling of this process suggests that changes in the strength of the North America plate and densifcation of the Farallon plate played important roles in controlling the areal extent of the Laramide orogen and hence the lifespan of the orogenic event at any particular location in western North America.

Provenance of a Permian erg on the western margin of Pangea: Depositional system of the Kungurian (late Leonardian) Castle Valley and White Rim sandstones and subjacent Cutler Group, Paradox Basin, Utah, USA

Consideration of petrographic and U-Pb provenance data and paleocurrent analysis of Kungurian (upper Leonardian) Cutler Group strata in the salt anticline province of the Paradox Basin of Utah demonstrates striking contrasts in composition and inferred sources of stratigraphically adjacent eolian and fluvial facies. Eolian strata, termed here the Castle Valley Sandstone, exposed in the Castle Valley northeast of Moab, Utah, and long correlated with the White Rim Sandstone, were deposited on the southwestern flank of a NW-trending diapiric salt wall. The eolian strata, which overlie red fluvial sandstone and conglomerate of the undifferentiated Cutler Formation, are as much as 183 m thick in outcrop and consist of two eolianite members separated by a thin sheet-flood deposit that contains pebbles derived from the salt wall and upturned conglomeratic strata adjacent to it. Both eolian and underlying fluvial deposits thin and onlap eastward onto the now-collapsed salt wall. Fluvial strata at Castle Valley and in exposures to the northeast were transported northwestward, parallel to the salt wall. Large-scale foresets in the lower eolianite member indicate dominant northeasterly wind directions (present coordinates) and transport directly away from the contemporary Uncompahgre uplift, whereas foresets in the upper member indicate variable northeasterly and northwesterly paleowinds. The eolian strata thus accumulated on the lee side of the salt wall, but sandstone composition and northwesterly wind components indicate net transport from the northwest, comparable with dominant southeastward sand transport, away from the Pangean shoreline, documented for the greater White Rim erg to the west and northwest. The NW and NE winds are both predicted by late Paleozoic atmospheric circulation models for western Pangea. Cutler fluvial sandstones are compositional arkoses (mean Qt 56 F 42 L 2 ) containing basement-derived detrital components that include potassium feldspar, plagioclase, biotite, and zircons with a restricted, bimodal age distribution of ∼1790–1689 Ma and ∼1466–1406 Ma. These grain ages exactly match known basement ages in the nearby Uncompahgre uplift. In contrast, the Castle Valley Sandstone ranges from quartz-rich arkose to subarkose and exhibits a consistent upsection decrease in feldspar content, from Qt 71 F 27 L 2 in the lower eolianite member to Qt 90 F 10 L 0 in the upper member. Like the underlying fluvial arkose, the lower eolianite member contains potassium feldspar, plagioclase, and mica derived from the Uncompahgre uplift, but the locally derived zircon age groups constitute only 23%–37% and 13% of the zircon grain ages in the lower and upper eolianite members, respectively; whereas older Archean and Paleoproterozoic grains, including ca. 1.5 Ga grains uncommon in the Laurentian detrital-zircon record, and Grenville, Neoproterozoic, and early Paleozoic grains constitute the bulk of the zircons. Quartzarenite of the greater White Rim erg contains detrital-zircon populations similar to those of the upper eolianite member. The Grenville and younger grains are interpreted as having an eastern Laurentian (Appalachian) source, whereas the ca. 1.5 Ga grains probably had an ultimate source in Baltica. Sediment-transport directions indicate that zircon grains not directly attributable to local basement of the Ancestral Rocky Mountains, including grains with a likely Baltica source, were transported to the western shoreline of Laurentia by transcontinental fluvial systems and then southeastward to their depositional site at the erg margin in salt-withdrawal minibasins.

The late Paleozoic Southwestern Laurentian Borderland

A broad region of late Paleozoic tectonism along the Laurentian margin from central Nevada (United States) to Sonora (Mexico), including offshore western terranes, is herein termed the Southwestern Laurentian Borderland (SLaB). Carboniferous–middle Permian sinistral translation within the SLaB explains a wide range of apparently disparate geologic observations, including (1) diachronous spatial distribution of regional structures and continental- margin sedimentary basins; (2) sub-regional unconformities and associated structures in central Nevada and Sonora; (3) displacement of the southwest part of the early Paleozoic passive margin, which now comprises the Caborca block in Sonora; and (4) distribution from central Nevada to Sonora of allochthonous lower Paleozoic deep-water sandstones with distinctive detrital zircons derived from northern Laurentia. Northward latitudinal translation of Laurentia by ~3000 km proposed by recent plate-tectonic models provides a mechanism for displacement of the continent relative to outboard western lithospheric domains. The SLaB was a sinistral transpressive plate boundary active from Mississippian to middle Permian time. As defined here, it directly followed the Antler collisional event and ended prior to the Sonoma orogeny.

Impact of the North American monsoon on isotope paleoaltimeters: Implications for the paleoaltimetry of the American southwest

Paleoaltimetric studies have characterized in detail the relationship between carbonate oxygen isotope ratios (δ18Oc) and elevation in orogens with simple, single-moisture-source hydrological systems, and applied this relationship to ancient continental carbonates to provide constraints on their past elevation. However, mixing of different atmospheric moisture sources in low-elevation orogens should affect δ18Oc values, but this effect has not yet been confirmed unequivocally. In the American Southwest, summer monsoonal moisture, sourced in the Equatorial Pacific and the Gulf of Mexico, and winter moisture, sourced in the East Pacific, both contribute to annual rainfall. We present stable isotope results from Quaternary carbonates within the American Southwest to characterize the regional δ18Oc-elevation relationship. We then provide stable isotope results from local Eocene carbonates to reconstruct late Laramide paleoelevations. The Quaternary δ18Oc-elevation relationship in the American Southwest is not as straightforward as in more simple hydrological systems. δ18Oc changes with altitude are non-linear, scattered, and display an apparent isotopic lapse rate inversion above 1200 m of elevation. We speculate that decreasing surface temperatures at high altitudes limit the duration of carbonate growth to the summer months, biasing δ18Oc values toward higher values typical of the summer monsoon and leading to lapse rate inversion. δ18Oc-elevation relationships based on modern water isotope data or distillation models predict paleoelevations that range up to as much as 2 km higher than the modern elevations of 2000 to 2400 m for our late Eocene sites located at the southern edge of the Colorado Plateau. By contrast, our δ18Oc-elevation relationship for the American Southwest yields lower paleoelevation estimates. These alternate estimates nonetheless suggest that significant elevation (at least ∼1 km) had already been attained by the Eocene, but are also compatible with < 1 km of uplift by post-Laramide mechanisms. Our results show the limitations of standard δ18Oc-elevation models in complex hydrological systems and suggest that similar mechanisms may have led to summer-biased paleoaltimetry estimates for the initial stages of other orogenies —in the American Southwest and elsewhere.

U-Pb ages of igneous xenoliths in a salt diapir, La Popa basin: Implications for salt age in onshore Mexico salt basins

Crystallization ages of igneous xenoliths entrained in a salt diapir in La Popa basin, northeastern Mexico, indicate the timing of magmatism in onshore Mexico salt basins and elucidate igneous rock-salt relations. Three equigranular phaneritic mafic to intermediate xenoliths have U-Pb zircon ages from 158 to 154 Ma (Oxfordian–Kimmeridgian). Phaneritic textures, hydrothermal alteration, zircon zonation, and previously published 40Ar/39Ar cooling ages from a nearby diapir, which are younger than Upper Jurassic strata overlying the salt, combine to suggest that these samples were intruded into salt and exhumed during diapirism. A porphyritic mafic rock with a U-Pb zircon age of 150 Ma (Tithonian) has a crystallization age coeval with marine strata overlying the salt. It is interpreted as a shallow intrusion into salt emplaced after the onset of salt diapirism. Magmatism thus took place from 158 to 150 Ma. The Oxfordian–Kimmeridgian ages of the three older xenoliths overlap with published biostratigraphic ages in carbonate strata intercalated with evaporites of the Minas Viejas Formation in the Sierra Madre Oriental south of Monterrey, Mexico. The entire time interval represented by the xenolith ages is coeval with mafic flows and pillow basalts in Border rift basins in northern Mexico and the southwestern United States. Comparison of the new ages with 40Ar/39Ar ages of igneous xenoliths from coastal Louisiana diapirs implies that rift magmatism began later in northeastern Mexico than in the Gulf of Mexico. The magmatic time interval indicated by the diapiric xenoliths is younger than accepted ages for Jurassic arc magmatism in northern and central Mexico and supports the inference that the Late Jurassic magmatism was not a result of subduction. The U-Pb ages and biostratigraphy are consistent in indicating a late Oxfordian–early Kimmeridgian salt age in northeastern Mexico, younger than the Louann Salt of the Gulf of Mexico. The contrasting salt ages indicate incursion of water into La Popa basin and adjacent onshore salt basins of northeastern Mexico from a fully marine Gulf of Mexico. LITHOSPHERE; v. 9; no. 5; p. 745–758 | Published online 30 June 2017 https://doi.org/10.1130/L658.1

Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera as revealed in the Little and Big Hatchet Mountains, southwest New Mexico, USA

A Mesozoic to Paleogene polyphase tectonic model presented here for the southern United States (U.S.) Cordillera provides new insight into style and timing of Mesozoic–Paleogene deformation and basin formation in the region south of the Colorado Plateau and Mogollon-Datil volcanic field. The model proposes reverse reactivation of Jurassic normal faults during Late Cretaceous Laramide shortening. It also recognizes late Paleogene east-west– and northwest-southeast–trending normal faults formed during a north-south extensional event that postdated Laramide shortening and preceded Neogene Basin and Range extension. Late Jurassic to Early Cretaceous extension generated northwest-southeast normal faults that formed part of the Border rift system that extended from southern California to the northwestern Gulf of Mexico. The normal faults cut Mesoproterozoic basement rocks, and localized subsequent uplift of basement rocks during Late Cretaceous fault reactivation that formed northwest-southeast–trending Laramide uplifts of southwest New Mexico and southeastern Arizona. The Hidalgo uplift, reconstructed here from structural relations in the Little Hatchet and Big Hatchet Mountains of southwestern New Mexico, is bound by bivergent reverse faults that resulted from tectonic inversion of a Jurassic–Early Cretaceous graben. The Hidalgo uplift is flanked to the north by the Campanian to earliest Maastrichtian Ringbone basin, which accumulated synorogenic continental strata and basaltic andesite flows from ca. 75 to 70 Ma. The Ringbone basin was converted from a subsiding basin in the Little Hatchet Mountains to a volcanic center by ca. 69 Ma, the emplacement age of an assemblage of shallow, subvolcanic intrusions termed the Sylvanite plutonic complex. The basement-involved structural style and yoked intermontane basin resemble other Laramide uplifts and basins in the Rocky Mountain Cordillera and refute alternative Laramide models of strikeslip faulting or regionally extensive horizontal thrust faults in southwestern New Mexico, the latter of which fail to account for basement-cored uplifts. A significant difference with the Rocky Mountain Laramide province is the size of the uplifts and basins and the close association of southern U.S. Cordilleran structures to nearby Late Cretaceous magmatic centers, which contributed to interstratified volcanic and volcaniclastic rocks in the basin fill. Upper Eocene–Oligocene ignimbrites and volcaniclastic rocks of the Boot Heel volcanic field of southwestern New Mexico unconformably overlie Laramide syntectonic strata and bury eroded Laramide structures. The distribution of the Paleogene volcanic rocks in the Little Hatchet and Big Hatchet Mountains is in part controlled by synmagmatic east-west and northwest-southeast normal faults active from ca. 34 to 27 Ma, the age range of rhyolite dikes intruded along the faults. Two generations of intrusive rocks occupy these normal faults in the Little Hatchet Mountains: (1) older (ca. 34 Ma) phaneritic stocks and dikes in the central and southern parts of the range, and (2) younger (31–27 Ma) aphanitic latite and rhyolite dikes. East-west–trending faults and dikes are cut by north-south faults formed during Basin and Range extension. The late Eocene–early Oligocene north-south extension provides an important minimum age limit for Laramide shortening, which ended prior to ca. 34 Ma.