David A. Sawyer

Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion 40 Ar/ 39 Ar isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of ∼0.3% (1σ), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intra-caldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paint-brush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climactic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space

Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-Turonian boundary interval, Western Interior Basin, USA

We develop an intercalibrated astrochronologic and radioisotopic time scale for the Cenomanian-Turonian boundary (CTB) interval near the Global Stratotype Section and Point in Colorado, USA, where orbitally influenced rhythmic strata host bentonites that contain sanidine and zircon suitable for 40Ar/39Ar and U-Pb dating. Paired 40Ar/39Ar and U-Pb ages are determined from four bentonites that span the Vascoceras diartianum to Pseudaspidoceras flexuosum ammonite biozones, utilizing both newly collected material and legacy sanidine samples of J. Obradovich. Comparison of the 40Ar/39Ar and U-Pb results underscores the strengths and limitations of each system, and supports an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. The radioisotopic data and published astrochronology are employed to develop a new CTB time scale, using two statistical approaches: (1) a simple integration that yields a CTB age of 93.89 ± 0.14 Ma (2σ; total radioisotopic uncertainty), and (2) a Bayesian intercalibration that explicitly accounts for orbital time scale uncertainty, and yields a CTB age of 93.90 ± 0.15 Ma (95% credible interval; total radioisotopic and orbital time scale uncertainty). Both approaches firmly anchor the floating orbital time scale, and the Bayesian technique yields astronomically recalibrated radioisotopic ages for individual bentonites, with analytical uncertainties at the permil level of resolution, and total uncertainties below 2‰. Using our new results, the duration between the Cenomanian-Turonian and the Cretaceous-Paleogene boundaries is 27.94 ± 0.16 Ma, with an uncertainty of less than one-half of a long eccentricity cycle.

Integrating 40Ar/39Ar, U-Pb, and astronomical clocks in the Cretaceous Niobrara Formation, Western Interior Basin, USA

This study revises and improves the chronostratigraphic framework for late Turonian through early Campanian time based on work in the Western Interior U.S. and introduces new methods to better quantify uncertainties associated with the development of such time scales. Building on the unique attributes of the Western Interior Basin, which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles, we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the Niobrara Formation. New 40Ar/39Ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples, as well as newly collected material. These results are complemented by new U-Pb (zircon) chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages from four biozones in the study interval. When combined with published radioisotopic data from the Cenomanian-Turonian boundary, paired 206Pb/238U and 40Ar/39Ar ages spanning Cenomanian to Campanian time support an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. Stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the Niobrara Formation. The ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the Niobrara Formation to radioisotopic ages with the lowest uncertainty proximal to the boundary, and adding or subtracting time by parsing the 405 k.y. cycles. The new stage boundary age determinations are: 89.75 ± 0.38 Ma for the Turonian-Coniacian, 86.49 ± 0.44 Ma for the Coniacian-Santonian, and 84.19 ± 0.38 Ma for the Santonian-Campanian boundary. The 2σ uncertainties for these estimates include systematic contributions from the radioisotopic measurements, astrochronologic methods, and geologic uncertainties (related to stratigraphic correlation and the presence of hiatuses). The latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations. Each methodological approach employed in this study—new radioisotopic analysis, stratigraphic correlation, astrochronology, and ammonite and inoceramid biostratigraphy—was critical for achieving the final result.

Crustal and magmatic evolution in a large multicyclic caldera complex: isotopic evidence from the central San Juan volcanic field

The Sr, Nd and Pb isotope compositions of ash-flow tuffs and lavas from the central caldera cluster of the San Juan volcanic field, Colorado, suggest that the silicic magmas were derived by fractional crystallization of mantle-derived basalts, coupled with extensive assimilation of both lower-and upper-crustal components. Temporal trends of increasing eNd values and decreasing 87Sr/86Sr ratios of the ash-flow tuffs suggest that extensive crustal hybridization of both upper-and lower-crustal reservoirs occurred as a result of magmatism. Mantle-derived basalts are envisioned to have initially crystallized and significantly interacted with crust near the crust-mantle boundary, creating a hybrid crust that is a mixture of mantle and lower-crustal components. Evolved magmas ascended into the upper crust, where they continued to assimilate and crystallize, modifying the bulk upper-crustal composition through transfer of both lower-crustal and mantle components into the upper crust, strongly affecting the isotopic compositions of the lower and upper crust. Lower-crustal eNd values and 87Sr/86Sr ratios are calculated to have shifted > 30–40% toward mantle compositions during these processes, although Pb isotope compositions of the lower crust are not as strongly affected (< 30% shift to mantle compositions). Depending on the extent of upper crust-lower crust recycling, upper-crustal eNd values shift 15–30% toward mantle compositions, 87Sr/86Sr ratios shift 20–55% toward lower-crustal and mantle ratios and 206Pb/204Pb ratios decrease 15–25% toward lower-crustal ratios. Crustal hybridization during evolution of the central caldera cluster was intense, although hybridization in the upper crust was localized to the region immediately underlying the calderas. Two distinct isotopic cycles are recognized in the ash-flow tuffs, one from 27.8 to 26.9 Ma (first caldera cycle) and the second from 26.4 to 26.1 Ma (San Luis caldera cycle). The break between the two cycles may be due to a shift in the locus of magmatism to the northwest, into an area of relatively unhybridized, original Proterozoic crust. Isotopic compositions of the caldera-related lavas are variable, and unlike the ash-flow tuffs, there are no distinctive temporal trends. The lavas are believed to undergo late-stage evolution in smaller magma chambers surrounding the caldera cluster, where the crust has not been as intensely hybridized. The comparatively large isotopic variability of the lavas reflects the high susceptibility of small magma chambers to isotopic modification during interaction with heterogeneous crust. In contrast, ash-flow magmas evolve in much larger magma chambers which tend to average isotopic heterogeneity of crust and mantle end-members, and the isotopic compositions of the tuffs serve as a better monitor of changes in the average composition of the magmatic system and the crustal column.

Mesozoic ash-flow caldera fragments in southeastern Arizona and their relation to porphyry copper deposits

Jurassic and Upper Cretaceous volcanic and associated granitic rocks in southeast Arizona are remnants of large composite silicic volcanic fields, characterized by voluminous ash-flow tuffs and associated calderas. Presence of 10–15 large caldera fragments is inferred primarily from (1) ash-flow deposits more than 1 km thick, having features of intracaldera ponding; (2) “exotic-block” breccias within a tuff matrix, interpreted as caldera-collapse megabreccias; and (3) local granitic intrusions along arcuate structural boundaries of the thick volcanic sequences. Several porphyry copper deposits are associated with late granitic intrusions within the calderas or along their margins.

Paleomagnetism and rotation constraints for the middle Miocene southwestern Nevada volcanic field

Middle Miocene rocks of the southwestern Nevada volcanic field (SWNVF) lie across the projection of the Walker Lane belt within the Basin and Range province and thus provide an interesting opportunity to test for late Cenozoic vertical-axis rotation. Paleomagnetic data from individual ash flow sheets document no significant relative vertical-axis rotation among localities within central SWNVF, an area of relatively low stratal tilts and widely spaced faults. A time-averaged mean paleomagnetic direction (D = 351.4°, I = 52.7°, α95 = 4.5°) calculated from data from numerous separate rock units suggests that the central SWNVF underwent minimal counterclockwise vertical-axis rotation (R = −7.1° ± 6.6°) with respect to the North American craton. No clockwise vertical-axis rotation is found to support projection of dextral faults of the Walker Lane beneath the central SWNVF. Clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field. These clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central SWNVF block and more strongly extended areas to the south and southwest of the field. Negligible rotation of 11.45-Ma to 13.25-Ma tuffs relative to the central SWNVF was found at the southeast margin of the field where 90° clockwise rotation at the northwest termination of the Las Vegas Valley shear zone had been postulated. Any clockwise rotation in this area must predate 13.25 Ma, and thus dextral shear within this part of the Walker Lane belt was not synchronous or connected across the southern margin of the field. Small counterclockwise vertical-axis rotation relative to the craton, as found for the central SWNVF block, might be a regional feature in the western Great Basin.