Geology topics

Barbara E. John

Publications and source records attributed to Barbara E. John.

14 recordsLinked to original sources

USGS2018

Geologic map of the Castle Rock 7.5' quadrangle, Arizona and California

The Castle Rock quadrangle is in the northeast corner of Chemehuevi Valley, California and Arizona. It includes the Colorado River’s entrance to the valley at the mouth of Topock Gorge and the northern outskirts of Lake Havasu City, Arizona, and the Chemehuevi Indian Tribe community of Havasu Lake, California. The map includes large parts of the Chemehuevi Indian Reservation and the Havasu National Wildlife Refuge. Upon its exit through the mouth of Topock Gorge, the Colorado River enters Chemehuevi Valley where its floodplain (now submerged under Lake Havasu) is flanked by alluvial piedmonts of the Chemehuevi and Mohave Mountains to the west and east, respectively. This abrupt transition offers a useful perspective into the structural evolution of the Colorado River extensional corridor and of the Colorado River itself. It contains key structural and stratigraphic elements recording a complex history of Cretaceous plutonism and deformation, significant tectonic extension, volcanism, and sedimentation in the Miocene, and, ultimately, the evolution of the Colorado River from the latest Miocene to the present. Lake Havasu submerged the axis of Chemehuevi Valley following the completion of Parker Dam in 1938, and the Colorado River now feeds a verdant delta marsh that composes part of the map. Important bedrock units include the Cretaceous Chemehuevi Mountains Plutonic Suite, the 18.78 Ma Peach Spring Tuff, and thick overlying sequences of interlayered Miocene megabreccia and fanglomerate. The exposure of these units is closely linked to extension along the Chemehuevi-Whipple Mountains detachment fault system. The complex bedrock geologic framework serves as the structural and topographic foundation for the key strata chronicling the evolution of the lower Colorado River. Important stratigraphic units that bear on its evolution to the present day include the Bouse Formation, the Bullhead Alluvium, and the Chemehuevi Formation. The map area also contains the river’s modern delta at the head of Lake Havasu.

Arizona, California↗

USGS2013

Geologic map of the Lead Mountain 15’ quadrangle, San Bernardino County, California

The Lead Mountain 15’ quadrangle in the Mojave Desert contains a record of Jurassic, Cretaceous, Tertiary, and Quaternary magmatism. Small amounts of Mesoproterozoic(?) augen gneiss and Paleozoic and Mesozoic(?) metasedimentary rocks are preserved in small patches; they are intruded by voluminous Jurassic plutons of quartz diorite to granite composition and by Late Cretaceous granite of the Cadiz Valley batholith. Jurassic intrusive rocks include part of the Bullion Mountain Intrusive Suite and also younger dikes inferred to be part of the Jurassic Independence dike swarm. A contact-metamorphosed aureole 2 km wide in the Jurassic plutonic rocks fringes the Cadiz Valley batholith. Early Miocene dacitic magmatism produced a dense swarm of dikes in the eastern Bullion Mountains and the volcanic-intrusive remnant of a volcano at Lead Mountain. Tilting of the dike swarm from inferred vertical orientations may have resulted from Miocene tectonic extension. Conglomerate of Pliocene and (or) Miocene age is also tilted. Younger volcanism is recorded by Pliocene basalt of the Deadman Lake volcanic field, basalt of Lead Mountain (approximately 0.36 Ma), and the even younger basalt of Amboy. Quaternary sedimentation built alluvial fans and filled playas in the map area. Faulting in the dextral eastern California shear zone produced several northwest-striking faults in the quadrangle, some of them active into the Pleistocene and some that may have many kilometers of right-lateral offset.

California↗

USGS2013

Geologic map of the Topock 7.5’ quadrangle, Arizona and California

The Topock quadrangle exposes a structurally complex part of the Colorado River extensional corridor and also exposes deposits that record landscape evolution during the history of the Colorado River. Paleoproterozoic gneisses and Mesoproterozoic granitoids and intrusive sheets are exposed through tilted cross-sectional thicknesses of many kilometers. Intruding them are a series of Mesozoic to Tertiary igneous rocks including dismembered parts of the Late Cretaceous Chemehuevi Mountains Plutonic Suite. Plutons of this suite in Arizona, if structurally restored for Miocene extension, formed cupolas capping the Chemehuevi Mountains batholith in California. Thick (1–3 km) Miocene sections of volcanic rocks, sedimentary breccias, conglomerate, and sandstone rest nonconformably on the Proterozoic rocks and record the structural and depositional evolution of the Colorado River extensional corridor. Four major Miocene low-angle normal faults and a steep block-bounding fault that developed during this episode divide the deformed rocks of the quadrangle into major structural plates and tilted blocks in and east of the Chemehuevi Mountains core complex. The low-angle faults attenuate crustal section, superposing supracrustal and upper crustal rocks against gneisses and granitoids originally from deeper crustal levels. The transverse block-bounding Gold Dome Fault Zone juxtaposes two large hanging-wall blocks, each tilted 90°, and the fault zone splays at its tip into folds in layered Miocene rocks. A synfaulting intrusion occupies the triangular zone where the folded strata detached from an inside corner along this fault between the tilt blocks. Post-extensional upper Miocene to Quaternary strata, locally deformed, record post-extensional landscape evolution, including several Pliocene and younger aggradational episodes in the Colorado River valley and intervening degradation episodes. The aggradational sequences include (1) the Bouse Formation, (2) fluvial deposits correlated with the alluvium of Bullhead City, (3) the younger fluvial boulder conglomerate of Bat Cave Wash, (4) the fluvial Chemehuevi Formation and related valley-margin deposits, and (5) fluvial Holocene deposits under the river and the valley floor. These fluvial records of Colorado River deposition are interspersed with piedmont alluvial fan deposits of several ages.

Arizona, California↗

USGS2011

Cooling rates and the depth of detachment faulting at oceanic core complexes: Evidence from zircon Pb/U and (U-Th)/He ages

Oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid‐ocean ridges, but many questions persist about the thermal evolution and depth of faulting. We present new Pb/U and (U‐Th)/He zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the Mid‐Atlantic Ridge (Ocean Drilling Program (ODP) holes 1270D and 1275D near the 15°20′N Transform, and Atlantis Massif at 30°N). Closure temperatures for the Pb/U (∼800°C–850°C) and (U‐Th)/He (∼210°C) isotopic systems in zircon bracket acquisition of magnetic remanence, collectively providing a temperature‐time history during faulting. Results indicate cooling to ∼200°C in 0.3–0.5 Myr after zircon crystallization, recording time‐averaged cooling rates of ∼1000°C–2000°C/Myr. Assuming the footwalls were denuded along single continuous faults, differences in Pb/U and (U‐Th)/He zircon ages together with independently determined slip rates allow the distance between the ∼850°C and ∼200°C isotherms along the fault plane to be estimated. Calculated distances are 8.4 ± 4.2 km and 5.0 ± 2.1 km from holes 1275D and 1270D and 8.4 ± 1.4 km at Atlantis Massif. Estimating an initial subsurface fault dip of 50° and a depth of 1.5 km to the 200°C isotherm leads to the prediction that the ∼850°C isotherm lies ∼5–7 km below seafloor at the time of faulting. These depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the TAG hydrothermal field and high‐temperature fault rocks recovered from many oceanic detachment faults.

Geochemistry, Geophysics, Geosystems↗

USGS2008

Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from 206 Pb/ 238 U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR)

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30°N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40–1415 m below seafloor in U1309D yield a weighted mean of 1.20 ± 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ± 0.07 Ma and 1.28 ± 0.05 Ma indicating crustal construction occurred over a minimum of 100–200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a “multiple sill” model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ± 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation.

Geochemistry, Geophysics, Geosystems↗

USGS2007

Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB. ?? 2007 The Geological Society of America.

Geology↗

USGS2003

Mechanism for generating the anomalous uplift of oceanic core complexes: Atlantis Bank, southwest Indian Ridge

Atlantis Bank is an anomalously uplifted oceanic core complex adjacent to the Atlantis II transform, on the southwest Indian Ridge, that rises >3 km above normal seafloor of the same age. Models of flexural uplift due to detachment faulting can account for ???1 km of this uplift. Postdetachment normal faults have been observed during submersible dives and on swath bathymetry. Two transform-parallel, large-offset (hundreds of meters) normal faults are identified on the eastern flank of Atlantis Bank, with numerous smaller faults (tens of meters) on the western flank. Flexural uplift associated with this transform-parallel normal faulting is consistent with gravity data and can account for the remaining anomalous uplift of Atlantis Bank. Extension normal to the Atlantis II transform may have occurred during a 12 m.y. period of transtension initiated by a 10?? change in spreading direction ca. 19.5 Ma. This extension may have produced the 120-km-long transverse ridge of which Atlantis Bank is a part, and is consistent with stress reorientation about a weak transform fault.

Geology↗

USGS1999

Geologic map of the Mohave Mountains area, Mohave County, western Arizona

Introduction The Mohave Mountains area surrounds Lake Havasu City, Arizona, in the Basin and Range physiographic province. The Mohave Mountains and the Aubrey Hills form two northwest-trending ranges adjacent to Lake Havasu (elevation 132 m; 448 ft) on the Colorado River. The low Buck Mountains lie northeast of the Mohave Mountains in the alluviated valley of Dutch Flat. Lowlands at Standard Wash separate the Mohave Mountains from the Bill Williams Mountains to the southeast. The highest point in the area is Crossman Peak in the Mohave Mountains, at an elevation of 1519 m (5148 ft). Arizona Highway 95 is now rerouted in the northwestern part of the map area from its position portrayed on the base map; it now also passes through the southern edge of the map area. Geologic mapping was begun in 1980 as part of a program to assess the mineral resource potential of Federal lands under the jurisdiction of the U.S. Bureau of Land Management (Light and others, 1983). Mapping responsibilities were as follows: Proterozoic and Mesozoic rocks, K.A. Howard; dikes, J.K. Nakata; Miocene section, J.E. Nielson; and surficial deposits, H.G. Wilshire. Earlier geologic mapping includes reconnaissance mapping by Wilson and Moore (1959). The present series of investigations has resulted in reports on the crystalline rocks and structure (Howard and others, 1982a), dikes (Nakata, 1982), Tertiary stratigraphy (Pike and Hansen, 1982; Nielson, 1986; Nielson and Beratan, 1990), surficial deposits (Wilshire and Reneau, 1992), tectonics (Howard and John, 1987; Beratan and others, 1990), geophysics (Simpson and others, 1986), mineralization (Light and McDonnell, 1983; Light and others, 1983), field guides (Nielson, 1986; Howard and others, 1987), and geochronology (Nakata and others, 1990; Foster and others, 1990).

Arizona↗

USGS1997