Four new10Be-26Al isochron burial ages ranging from 4.4 to 2 Ma on ancestral Colorado River deposits in the lower Colorado River corridor (LCRC) help constrain the river's evolution during the Pliocene and early Pleistocene. They help fill a gap between previous work that focused on older and younger deposits: Older dated deposits include the 5 Ma Bouse Formation, which records the integration of the Colorado River through a series of preexisting basins to the Gulf of California and the ca. 4.5-3.5 Ma Bullhead Alluvium, a 200 to 300 m thick aggregational package that immediately followed integration. The much younger, 100-70 ka, Chemehuevi Formation is another major aggradation package mapped throughout the LCRC. The new burial ages on the facies of Santa Fe Railway (4.37 +/- 0.71 Ma), boulder conglomerate ofBat Cave Wash (2.12 +/- 0.26 and 2.05 +/- 0.31 Ma), and the Palo Verde alluvium (3.03 +/- 0.26 Ma) partially fill in a 3.5 M.y. gap between the deposition of the Bullhead Alluvium and the Chemeheuvi Formation and document the timescales over which the Colorado River was able to remove the Bullhead aggradational package and initiate newer and smaller aggradational pulses.

The Chemehuevi Formation is a distinctive 50-150-m-thick wedge-shaped Pleistocene sedimentary unit deposited by the Colorado River. It lines the perimeters of the river's floodplains and bedrock canyons for more than 600 km between the mouth of the Grand Canyon and the delta region in the Gulf of California. The formation is composed of a basal tan to light-yellowish-brown and pale-orange mud-dominated facies overlain and interbedded by a light-yellow-brown sand-dominated facies. The unit is one of two extensively exposed aggradational packages in the Lower Colorado River corridor, in addition to a series of other smaller alluvial terrace deposits. The Chemehuevi Formation appears to represent the response of a fully integrated Colorado River system to a significant perturbation, in contrast to the Bullhead Alluvium, which is likely a unique result of Pliocene river integration. The aggradation of the Chemehuevi Formation in the Lower Colorado River corridor may be similarly due to a unique event in the Colorado River system, or it may instead be a well-preserved sedimentary sequence recording typical behavior of the Colorado River below the Grand Canyon in the late Pleistocene. As such, multiple causal mechanisms have been proposed, but no study to date has conclusively explained the Chemehuevi Formation. To help resolve its timing, duration, and origin, we applied post-infrared infrared stimulated luminescence, carbonate U-Th series, and zircon sensitive high-resolution ion microprobe U-Th series geochronology to determine the ages of key exposures of the unit over a wide spatial area. These new data demonstrate that the Chemehuevi Formation was deposited ca. 110-90 ka. The depositional ages collectively overlap, suggesting that deposition occurred rapidly relative to the resolution of the geochronometers. The new depositional timing coincides with a shift from glacial to interglacial conditions after the marine isotope stage 5-6 transition. This observation is consistent with a climate-induced sediment pulse as a causal mechanism, yet correlations with similar deposits in the Colorado River headwaters or in neighboring catchments appear elusive. Potentially, climate transitions between glacial and interglacial periods induced a sediment pulse from hillslopes of the Colorado River system that resulted in the Chemehuevi Formation. An alternative or additional explanation is that the Chemehuevi Formation represents release of lava dam-impounded sediment in the Grand Canyon. The surface geometry of the Chemehuevi Formation projects upstream to the approximate location of lava dams, and the largest possible lava dam impoundment (the Upper Prospect dam) is comparable in volume to the formation. The lava dam hypothesis appears to be a possible explanation for the Chemehuevi Formation. However, tying deposition to a specific lava dam or series of lava dams remains challenging due to discrepancies in timing and volume. The combined effects of a series of lava dams may have led to the Chemehuevi Formation, as the last Pleistocene lava dam eruption coincides with the onset of deposition. Alternatively, the formation may result from the combined effects of both regional climate transitions and the lava dams that created a transient reservoir to compound a climate transition-driven sediment pulse. The geochronologic data presented here do not allow us to distinguish between the lava dam or climate transition hypotheses but will need to be reconciled with any future proposed depositional model.

The evolution of strain in nascent continental plate boundaries commonly involves distributed deformation and transitions between different styles of deformation as the plate boundary matures. Distributed NW-striking faults, many with km-scale right-lateral separation, are prevalent near Blythe, California, and have been variably interpreted to have accommodated either Middle Miocene NE-SW extension as normal faults or Late Miocene to Pliocene dextral shear as strike-slip faults. However, with poor timing and kinematic constraints, it is unclear how these faults relate to known domains of Neogene deformation and the evolution of the Pacific–NorthAmerica plate boundary. We present kinematic data (n = 642 fault planes, n = 512 slickenlines) that demonstrate that these faults dominantly dip steeply northeast; ~96% of measured faults record normal, dextral, or oblique dextral-normal kinematics that likely reflect a gradational transition between normal and dextral oblique kinematic regimes. We constrain fault timing with 11.7 Ma and 7.0 Ma 40Ar/39Ar dates of rocks cut by faults, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of calcite mineralized during oblique dextral faulting that demonstrates fault slip at ca. 10–7 Ma and perhaps as late as ca. 4 Ma. This Late Miocene dextral oblique faulting is best compatible with a documented regional transition from Early to Middle Miocene NE-directed extension during detachment fault slip to subsequent NW-directed dextral shear. We estimate 11–38 km of cumulative dextral slip occurred across a 50-km-wide zone from the Palen to Riverside mountains, including up to 20 km of newly documented dextral shear that may partly alleviate the regional discrepancy of cumulative dextral shear along this part of the Late Miocene Pacific–North America plate boundary.

Spatiotemporal constraints for Late Cretaceous tectonism across the Colorado Plateau and southern Rocky Mountains (northern Arizona-New Mexico, USA) are interpreted in regards to Laramide orogenic mechanisms. Onset of Laramide arch devel-opment is estimated from cooling recorded in representative thermochronologic sam-ples in a three-step process of initial forward models, secondary HeFTy inverse models with informed constraint boxes, and a cus-tom script to statistically estimate timing of rapid cooling from inverse model results. Onset of Laramide basin development is interpreted from increased rates of tectonic subsidence. Onset estimates are compared to published estimates for Laramide timing, and together suggest tectonism commenced ca. 90 Ma in northwestern Arizona and pro-gressed eastward with later onset in north -central New Mexico by ca. 75-70 Ma. The interpreted sweep of onset progressed at a rate of similar to 50 km/m.y. and was approximately half the 100-150 km/m.y. rate estimated for Late Cretaceous Farallon-North America convergence during the same timeframe. Previous suggestions that the Laramide tec-tonic front progressed at a rate similar to convergence via basal traction are not sup-ported by our results. We thereby suggest that (1) a plate margin end load established far field compression and that (2) sequen-tial Laramide-style strain was facilitated by progressive weakening of North American lithosphere from the dehydrating Farallon flat slab. Results are compared to models of sweeping tectonism and magmatism in other parts of the Laramide foreland. Discussions of the utility of the custom script and the potential for stratigraphic constraints to represent only minimum onset estimates are also presented.

Despite over 160 years of effort, no detailed seamless geologic map of the entire Grand Canyon region exists, and new findings and field observation warrant new mapping at a range of scales. We are pursuing both simultaneously as we also convert mapping into modern digital formats. Much of this is done with the help of a series of newly developed Python-based GIS scripts. Truly seamless mapping of the region only exists at the 1:500,000 scale. More detailed mapping is segmented at arbitrary boundaries where map scale, map making methodology, map objectives, and/or unit definitions change. To help merge these disparate maps, we have developed a Python-based GIS tool. It merges maps in the GeMS map standard by harmonizing disparate map units and line types across maps. The tool removes unneeded lines when merging units, while retaining a summary of the original map unit designations that were combined. The tool can be run at any time and allows for the incorporation of new isolated observations such that map compilation and new mapping efforts can proceed simultaneously. Seamless geologic mapping has many uses as it is a framework product that supports understanding about geologic evolution, hazards, and resources. One specific and non-traditional use of the map compilation that we are pursuing is a geologic map-based guide to the geology as observed from the river in the Grand Canyon, which we hope will bring geologic maps and understanding to new communities, such as those on raft trips. Currently, new mapping efforts are focused within the Grand Canyon 30’ x 60’ quadrangle, which is the center of nine 1:100,000 scale quadrangles covering the entire Grand Canyon region. Future efforts will focus on better mapping Grand Canyon lava dams, where new lidar data is being collected, and incorporating non-digital and legacy mapping into the compilation. These new mapping efforts will be aided by additional field-based investigations, high-resolution lidar-derived base data, and the latest digital mapping techniques (e.g., digitizing in 3D environments with the aid of multi-spectral data). Together, these detailed and regional compilation maps will help improve our understanding of the geology of this iconic landscape.

New detrital sanidine geochronology from the Bouse Formation in Blythe basin supports recent research that redefined the age of lower Colorado River integration. Sanidine dating along with magnetostratigraphy indicates that the Colorado River arrived at Cottonwood Valley after 5.24 Ma and reached the proto–Gulf of California between 4.80 and 4.63 Ma. Earlier studies had proposed that initial integration to the evolving proto–Gulf of California occurred at 5.3 Ma; this was based largely on magnetostratigraphy tied to ~2 Ma old ashes ~4 km higher in the section than the deposits containing the first Colorado River sand. The discrepancy in ages was ascribed to duplication of section across newly identified faults in the Fish Creek-Vallecito basin. Detrital sanidine dating from near the first arriving Colorado River sand in this basin placed a much tighter constraint on the timing of integration than was previously available. The new detrital sanidine results reported here come from near the base of a thick section of siliciclastic Bouse Formation in the northern Blythe basin, which was the last in a series of basins that the Colorado River filled before spilling into the proto–Gulf of California. New single grain sanidine ages indicate a maximum depositional age of 4.8 ± 0.1 Ma for this section. This age should approximately correspond with the first arrival of Colorado River sediment to that basin. Previous dating of single sanidine grains from the Bouse Formation indicated a similar age; currently 6 grains have been dated at younger than 5.3 Ma with increasing precision. Because the Colorado River had to pass through this basin prior to reaching the proto-Gulf of California, deposition should predate not postdate integration, supporting the new younger age. These new results and previous work are consistent with downward integration of the Colorado River through a series of closed basins in the lower Colorado River corridor, culminating in integration to the proto–Gulf of California between 4.8 and 4.63 Ma. During and after integration, the Colorado River quickly incised through sediment-filled basins and the liberation of sediment from throughout the entire system led to deposition of hundreds of meters of Bullhead Al­luvium starting around 4.6 Ma as the river adjusted to its lower base level.

The Packard Well fault zone (PWFZ) is likely one of the larger-displacement NW-striking Neogene faults in SE California, with variable dextral offset estimates in the literature of up to ~24 km, and may have formed a critical component of the Late Miocene to Pliocene paleo-eastern California shear zone (ECSZ). However, attempts to understand fault linkage from better-studied ECSZ faults in the Mojave Desert to a wider paleo-ECSZ system that included faults farther E are hampered by uncertainty that PWFZ deformation was compatible with paleo-ECSZ timing and kinematics. We use geologic mapping, fault kinematic analysis, and 40Ar/39Ar dating of faulted rocks at Palen Pass, California to constrain the slip history of the PWFZ. N-dipping strands of the PWFZ bound a 400 m-wide belt of asymmetrically folded alluvial fan conglomerate at Palen Pass. The northern fault strand thrust Mesozoic plutonic rocks over the conglomerate and preserves oblique dextral-reverse slickenlines consistent with a WNW-ESE restraining bend in a NW-SE dextral system. Sanidine 40Ar/39Ar dating of a steeply-dipping, lightly reworked ash bed in the deformed conglomerate indicates a population of young grains with a weighted mean average of 11.7 ± 0.1 Ma (2σ). A package of thin basalt flows ~5 km SW of the folded conglomerate records minor NNE-SSW shortening via conjugate strike-slip faults, and a published paleomagnetic study indicates the basalt experienced ~31° of clockwise rotation, both of which likely stem from local block rotation adjacent to the dextral PWFZ. New groundmass 40Ar/39Ar dating of this basalt yields a 7.0 ± 0.2 Ma (2σ) recoil model age. These findings suggest that significant dextral shear occurred on the PWFZ in the Late Miocene or Pliocene, prior to deposition of undeformed late Quaternary alluvial fans. The PWFZ likely connects along strike to concealed faults to the NW (e.g., Cadiz Lake and Iron Mountains faults), and towards Blythe, California to the SE (e.g., Cibola Pass fault zone), where steep horizontal gravity gradients bound a NW-SE-elongate gravity low. Our data demonstrate that the PWFZ was kinematically and temporally compatible with a paleo-ECSZ dextral system active in the Late Miocene to Pliocene and may have played a major role in accommodating Pacific-North America dextral shear.

Additional details on methods, summary of previous Ar/Ar dating relevant to the timing of Colorado River integration, sample locations, and full analytical results.

Structural evidence presented here documents that deformation was ongoing within the lower Colorado River corridor (southwestern USA) during and after the latest Miocene Epoch, postdating large-magnitude extension and metamorphic core complex formation. Geometric and kinematic data collected on faults in key geologic units constrain the timing of deformation in relation to the age of the Bouse Formation, a unit that records the first arrival and integration of the Colorado River. North-south–striking extensional, NW-SE–striking oblique dextral, NE-SW–striking oblique sinistral, and east-west–striking contractional faults and related structures are observed to deform pre– (>6 Ma), syn– (6–4.8 Ma), and post–Bouse Formation (<4.8 Ma) strata. Fault displacements are typically at the centimeter to meter scale, and locally exhibit 10-m-scale displacements. Bouse Formation basalt carbonate locally exhibits outcrop-scale (tens of meters) syndepositional dips of 30°–90°, draped over and encrusted upon paleotopography, and has a basin-wide vertical distribution of as much as 500 m. We argue that part of this vertical distribution of Bouse Formation deposits represents syn- and post-Bouse deformation that enhanced north-south–trending depocenters due to combined tectonic and isostatic subsidence in a regional fault kinematic framework of east-west diffuse extension within an overall strain field of dextral transtension. Here we (1) characterize post-detachment tectonism within the corridor, (2) show that diffuse tectonism is cumulatively significant and likely modified original elevations of Bouse Formation outcrops, and (3) demonstrate that this tectonism may have played a role in the integration history of the lower Colorado River. We suggest a model whereby intracontinental transtension took place in a several hundred kilometers-wide area inboard of the San Andreas fault within a diffuse Pacific–North America plate margin since the latest Miocene.