We investigated a Plio-Pleistocene alluvial succession in the Albuquerque Basin of the Rio Grande rift in New Mexico using geomorphic, stratigraphic, sedimentologic, geochronologic, and magnetostratigraphic data. New 40 Ar/ 39 Ar age determinations and magnetic-polarity stratigraphy refine the ages of the synrift Santa Fe Group. The Pliocene Ceja Formation lies on the distal hanging-wall ramp across much of the Albuquerque Basin. The Ceja onlapped and buried a widespread, Upper Miocene erosional paleosurface by 3.0 Ma. Sediment accumulation rates in the Ceja Formation decreased after 3.0 Ma and the Ceja formed broad sheets of amalgamated channel deposits that prograded into the basin after ca. 2.6 Ma. Ceja deposition ceased shortly after 1.8 Ma, forming the Llano de Albuquerque surface. Deposition of the Sierra Ladrones Formation by the ancestral Rio Grande was focused near the eastern master fault system before piedmont deposits (Sierra Ladrones Formation) began prograding away from the border faults between 1.8 and 1.6 Ma. Widespread basin filling ceased when the Rio Grande began cutting its valley, shortly after 0.78 Ma. Although the Albuquerque Basin is tectonically active, the development of through-going drainage of the ancestral Rio Grande, burial of Miocene unconformities, and coarsening of upper Santa Fe Group synrift basin fill were likely driven by climatic changes. Valley incision was approximately coeval with increased northern- hemisphere climatic cyclicity and magnitude and was also likely related to climatic changes. Asynchronous progradation of coarse-grained, margin-sourced detritus may be a consequence of basin shape, where the basinward tilting of the hanging wall promoted extensive sediment bypass of coarse-grained, margin-sourced sediment across the basin.

The Department of Earth and Planetary Sciences (EPS) at the University of New Mexico offers two field geology courses (EPS 319L, Introductory Field Geology, and EPS420L, Advanced Field Geology). Prior to summer 1986, these courses were taught during the academic year, on the weekends. Over a two year time span, despite some faculty consternation, the department converted both classes into full-blown summer field geology courses. These continue to be offered as two separate, independent classes for several reasons. Introductory Field Geology is required of all EPS geoscience majors and has attracted numerous students from institutions outside New Mexico. All mapping is done using a paper topographic map and/or an air photograph base, with, eventually, the aid of a handheld global positioning system (GPS) device. Given that topographic map skills remain essential for effective computer- and GPS-based mapping, we emphasize these traditional techniques within the limited time span (three weeks) of the course. Despite the fact that all students are expected (required) to have passed the standard array of core undergraduate courses in the geosciences, the backgrounds of the students, including level of previous field experience, vary considerably. Consequently, the approach taken in EPS 319L is one in which strong emphasis is placed on providing rapid feedback and focusing maximum instructor attention on the students who need it the most. As one means of providing rapid feedback to all of our students, we utilize a “postage stamp” map exercise as an essential component of each mapping project. After at least one day of introduction to the project, the entire class focuses on a morning of mapping in a small, yet very revealing project area. The maps are turned in after a group discussion of the postage stamp area, and detailed feedback, using several rubrics, is provided to all students by the end of the day (but these maps are not graded). In field geology courses, where the goal is to maximize student field learning within a limited time frame, the postage stamp exercises have proven to be an effective way to provide timely instructor input and reinforcement of burgeoning student skills. Student evaluations of the course support the use of the postage stamp exercises for each map project; these exercises improve the instructor’s ability to assess final map products in an even more rigorous and consistent fashion.

Integrated mapping, fault-kinematic, paleomagnetic, and gravity analyses around the Rhodes Salt Marsh extensional basin, located within the east-west–trending Mina deflection of the central Walker Lane, reveal that from 8.0 to 9.0 km of late Cenozoic displacement was accommodated on a curved array of faults. The dominant slip on the faults systematically varies from left-oblique, to normal, and to right-oblique as fault strike changes from east, to north-northeast, and to north-northwest, respectively. Kinematic consistency of fault slickenline rakes, preservation of displacement budget, and paleomagnetic data from a pluton and volcanic rocks in the fault-system hanging wall indicate that the curved fault geometry is primary and not due to superposition of two fault systems nor to later vertical-axis rotation. Large-magnitude extension was localized at the apex of the curved faults and resulted in the formation of an ~3.0-km-deep prismatic basin beneath Rhodes Salt Marsh. The offset geologic structures and geophysical basin models indicate that hanging-wall displacement diverged around the curved fault array and resulted in finite flattening, with primary and secondary extensional axes oriented west-northwest and north-northeast, respectively. Fault-slip inversion yields two directions of extension consistent with the finite strain axes, and slickenlines with mutually crosscutting relations indicate formation during incremental flattening. Although broadly contemporaneous, extension parallel to the primary and secondary extension axes alternated at periods ranging from months to as much as several hundred thousand years. Large through-going structures sustained extension directions recorded geodetically and seismologically through multiple seismic cycles. In contrast, the alternation between primary and secondary extension directions recorded by a strainmeter suggests that, on small structures contained within fault-bounded blocks, the two extension directions alternated over time scales of as little as 2 yr.

Paleomagnetic data from three regionally extensive Oligocene ignimbrite sheets, two sequences of Miocene andesite flows, and ten sequences of Upper Miocene to Pliocene basaltic andesite flows in the Candelaria Hills and adjacent areas, west-central Nevada, provide further evidence that, since the late Miocene, and possibly between latest Miocene and earliest Pliocene time, the broad region that initially facilitated Neogene displacement transfer between the Furnace Creek and central Walker Lane fault systems experienced some 20° to 30° of clockwise vertical-axis rotation. The observed sense and magnitude of rotation are similar to those previously inferred from paleo-magnetic data from different parts of the Silver Peak Range to the south. We propose that clockwise rotation within the transfer zone formed in response to horizontal components of simple and pure shear distributed between early-formed, northwest-striking right-lateral structures that initiated in mid- to late Miocene time. Notably, the spatial distribution of the early-formed transfer zone is larger and centered south of the presently active stepover, which initiated in the late Pliocene and is characterized by a trans-tensional deformation field and slip on east-northeast–oriented left-oblique structures that define the Mina deflection. The sense and magnitude of rotation during this phase of deformation, which we infer to be of pre–latest Pliocene age, are inconsistent with the geodetically determined regional velocity field and seismologically determined strain field for this area. As a consequence, the longer-term kinematic evolution of the stepover system, and the adjoining parts of the Furnace Creek and Walker Lane fault systems, cannot be considered as a steady-state process through the Neogene.

Paleomagnetic and geochronologic data from mafic intrusive rocks, inferred to contain magnetizations of early Late Cretaceous age, and upper Tertiary volcanic rocks, all part of the upper plate of the Silver Peak extensional complex in the southern Silver Peak Range, add to the growing body of results suggesting that Neogene displacement transfer within the central Walker Lane involved components of modest magnitude crustal tilting and, at least locally, rotation of structural blocks. Mesozoic intrusions and upper Tertiary volcanic rocks yield paleomagnetic data that are discordant to expected field directions. The data from 49 accepted sites in mafic dikes that cut granitic rocks, 4 sites in a single Oligocene(?) ash flow tuff, 20 sites in mid-Miocene andesite flows, and 28 sites in upper Miocene to lower Pliocene pyroclastic rocks may imply a systematic progression in the magnitude of vertical axis rotation and tilting with age. At a minimum, the data are consistent with at least some 20° of clockwise rotation of upper-plate rocks in this part of the Silver Peak Range and demonstrate a greater regional extent to the area affected by clockwise rotation during Neogene displacement transfer. Eight new 40 Ar/ 39 Ar age determinations from the mafic dikes and adjacent host rocks, all somewhat disturbed age spectra, imply that these rocks cooled below ∼300 °C during the Late Cretaceous between about 90 and 80 Ma. Four mafic dike groundmass concentrates yield integrated apparent ages between 86.31 Ma ± 0.12 Ma and 80.80 Ma ± 0.11 Ma, and four age spectra from biotite from the host granite yield integrated values between 93.6 ± 0.9 Ma and 78.6 Ma ± 0.2 Ma. The mafic dikes yield in situ exclusively normal polarity results consistent with an early Late Cretaceous age of magnetization acquisition, with an overall group mean (D = 25.1°, I = 55.4°, α 95 = 3.4°) that is discordant to an early Late Cretaceous expected field (D = 337°, I = 66°). Ten of 20 sites from steeply dipping mid-Miocene andesite flows and 21 of 28 sites in gently tilted upper Miocene ash flow tuffs yield overall stratigraphically corrected group means (D = 24.4°, I = 36.7°, α 95 = 7.1°) and (D = 16.5°, I = 53.5°, α 95 = 7.6°, respectively) that are discordant in a clockwise sense to the Miocene expected direction (D = 358°, I = 55°). The paleomagnetic data support a history of tilting and vertical axis rotation of the southern Silver Peak Range, most of which occurred coincidently with latest Miocene and Pliocene exhumation of the lower-plate rocks in the extensional complex. In addition, it is possible that the paleomagnetic data from Mesozoic intrusions record an additional, modest phase of deformation that predated development of the extensional complex. The observations are consistent with a tectonic model where deformation of upper-plate rocks in this area involved a small component of west- to southwest-side-down tilting, likely related to range-scale folding during the late Miocene and Pliocene, accompanied by modest clockwise vertical axis rotation.

Abstract Tertiary intrusive and extrusive igneous rocks and Upper Paleozoic sedimentary rocks were sampled at 47 sites in the Oquirrh Mountains. Paleomagnetic data from over 350 samples provide a grand mean direction for the Oquirrh Mountains derived from site means determined by averaging best-fit lines. The grand mean for the Oquirrh Mountains (D/I = 332.1/55.4, α95 = 4.1) compared with a late Eocene reference direction (D/I = 351.2/58.3, α95 = 2.4) provides a quantitative assessment of the magnitude of deformation related to extension. The discrepancy between the observed and expected late Eocene directions is interpreted to indicate that the Oquirrh Mountains have been tilted some 10.8 ± 4.0 degrees, down to the east, since late Eocene time. This contrasts with previous estimates of about 15 to 25 degrees of tilting based on orientations of volcaniclastic strata and the apparent asymmetry of the Bingham Canyon porphyry copper ore shell. A modest amount of tilt is consistent with a subhorizontal decoupling or anastomosing shear zone model for extension in the eastern Great Basin. Seismic reflection profiles, two-dimensional basin modeling using residual gravity profiles, and earthquake focal mechanisms corroborate this conclusion.

Abstract The Mojave Sonora megashear hypothesis proposes that the Caborca terrane, northwest Sonora, arrived at its present position with respect to the North America craton via left-lateral (southeastward) displacement along a strike slip fault system. Nonetheless, clear stratigraphic and paleomagnetic links exist between rocksof the Sonoran segment of the Jurassic Cordilleran arc (JCA), and lower Mesozoic strata of the Caborca and Antimonio terranes supporting an alternative Jurassic paleogeography for northwest Mexico. The characteristic “J” magnetizations in Jurassic rocks of the JCA givea (tilt corrected) mean of D=15.0°, 1=4.0° (a95=14.3°;k=12.4; N=10 sites). Magnetizations pass fold, conglomerate, and reversal tests and are interpreted to be primary in origin. The age of these rocks is roughly bracketed between about 190 and 160 Ma. Secondary “J*” magnetizations in Neoproterozoic and Jurassic rocks southof the f - megashear give an overall (in situ) mean of D=15.0°, 1=10.0° (a95=5.8°; k=23.0; N=28 sites). “J*” magnetizations fail a fold test but timing of acquisition is bracketed between about 120 and 190 Ma, based on the youngest age of remagnetized Jurassic strata andthe fact that acquisition must predate the Cretaceousnormal polarity superchron. The overall means of rocks south and north of the MSM are statistically indistin uishable, arguing against the existencer of a crustal discontinuity along the proposed locat on of the MSM. For the interval that brackets acquisition of secondar “J*” and primary “J*” magnetizations, rotation of both the JCA and the Caborca terrane with respect to North America is 12° to 50° clockwise, depending on the age assumed. Estimates oflatitudinal displacement vary from as little as l-250 km southward, for a Sinemurian age of the magnetization (195 Ma), toas much as -800 km northward for a 1' Callovian-Oxfordian age (155 Ma). If the Sonoran results are compared with high-latitude Middle Jurassic poles for North America, larger estimates of northward displacement(>1400 km) result. Although the timing of magnetization acquisition is based on reasonable geological arguments, an Early Cretaceous age for “J*” magnetizations is permissible. Such an interpretation would indicate significantly larger northward displacement (>2000 km) with respect to cratonic [North America. Together, the lack of clear evidence for large southward displacement, the observed clock-wise rotation, and the similarity of the Jurassic magnetizations in the Cordilleran arc with those ofthe Caborca block are not consistent with the Mojave-Sonora megashear model of significant Late Jurassic southeast motion of northern Mexico along a left-lateral strike-slip fault system.

Abstract Paleomagnetic data have been obtained from heterogeneous, shallow-water, miogeoclinal carbonate rocks of the Pogonip Group (Early Ordovician) in the Desert Range of southern Nevada, the Egan Range of east-central Nevada, and the southern House Range of western Utah. These rocks locally contain abundant replacive chert that preserves relict textures from the host limestones as well as clearly detrital grains (e.g., blue-luminescing feldspars). Stylolites are abundant and are interpreted as late diagenetic features, as they cut late cements and truncate bedding lamination. Differential compaction along stylolites wrapping around the chert masses has resulted in macroscopic deformation, as evidenced by tilting of bedding of over 25° about chert masses in some cases. We have used the differential compaction fabrics in these rocks to test for the age of acquisition of a generally well-grouped and well-defined characteristic magnetization. All three carbonate sections give a low-inclination, southerly to southeasterly magnetization residing in magnetite (e.g., Decl. = 152°, Incl. = -21°, α 95 = 3°, kl = -61, k2 = -21, N = 48 independent samples, site 12; Pogonip Group, Sawmill Canyon, Egan Range). The magnetization is interpreted to be secondary and acquired after local compaction because directions of magnetizations from different samples are not dispersed by the compaction deformation. The uniform reversed polarity in addition to the direction of the magnetization, moreover, is interpreted to suggest a late Paleozoic age of remagnetization. In the Desert Range, the remagnetization had been previously attributed to a viscous partial thermoremanent magnetization (VPTRM) from deep burial. Based on several observations, we now argue for a chemical origin from late diagenetic magnetite, such as is now well-documented in the Appalachians and mid-continent. The cherts are almost nonmagnetic, as would be expected from their impermeability if the magnetite were precipitated from late fluids. Abundant authigenic alkali feldspar in the Desert Range is also consistent with late metasomatism. Finally, in the Egan Range, the remagnetization extends through a section exceeding 3 km in thickness, into rocks as young as Mississippian, which were never buried as deeply and thus not heated to the same degree as lower Paleozoic strata. These results underscore the utility of integrating observations based on paleomagnetic data with carbonate textures. “Micro”-field tests can constrain both the timing of magnetization acquisition and of diagenetic events. The micro-fold tests discussed apply to features that are not formed by tectonic deformation. The availability of field tests from early formed textures in sedimentary rocks is especially important given the recent recognition of widespread remagnetization in ancient rocks.

Extensional accommodation zones, or tilt-block domain boundaries, facilitate reversals in the dominant tilt direction of fault blocks and possibly inversions in the dip of regional detachment systems in rifted continental crust. The amount and direction of movement of the footwall (lower plate) and hanging wall (upper plate) of the detachment terrane dictate the deformational style along accommodation zones. Various models of extension can potentially be evaluated by defining modes of deformation along accommodation zones. A 40-km-long, east-west-trending, middle Miocene accommodation zone bisects the central Black Mountains, northwestern Arizona, and southern Eldorado Mountains, southern Nevada. The Black and Eldorado Mountains lie within the northern Colorado River extensional corridor, a 50- to 100-km-wide region of severely extended crust. The generally sublinear, 5- to 10-km-wide accommodation zone separates more than 5,000 km 2 of east-tilted fault blocks to the north from 25,000 km 2 of dominantly west-tilted fault blocks to the south. The zone may also mark the join between regionally extensive, oppositely dipping detachment systems. Transversely oriented segments (i.e., perpendicular to strike of tilted blocks) of the accommodation zone in the upper-plate rocks correspond to areas of intermeshing conjugate normal faults. East- and west-dipping normal faults dominate the west- and east-tilted domains, respectively, whereas east- and west-dipping faults are equally common in the axial part of the zone. Some of the major normal faults in the west- and east-tilted domains terminate in drag folds within the axial part of the zone. Fault-block tilting on either side of the accommodation zone commonly exceeds 60°. Tilting decreases progressively toward the axis of the zone, where transversely oriented, obliqueslip normal faults accommodate scissors-like torsional offset between gently tilted (10 to 35°) individual fault blocks of opposing polarity. Concomitant with the decrease in tilting, fault spacing decreases, and average fault dip increases. Fault blocks within the zone were periodically tilted in opposite directions during the same episode of extension. Minor amounts of open to tight folding characterize along-strike segments (i.e., parallel to strike of tilted blocks) of the accommodation zone.

Early events during the formation of the Ries crater include ejection of moldavite tektites and deposition of the Bunte breccia outside of the uplifted inner crystalline ring of the crater. The restricted location of the moldavites may suggest an oblique impact at the Ries. The Bunte breccia ejecta was emplaced relatively cold and was probably a continuous ejecta blanket. Impact-melt-bearing breccias (suevite) were deposited inside the crater (crater suevite) and deposited on top of the Bunte breccia outside the crater (fallout suevite). The presence of aerodynamically shaped glass bombs in the fallout suevite indicates solidification during high-speed travel through the atmosphere, while the existence of fluid-drop chondrules and glass spherules in the fallout suevite and the upper sorted suevite within the crater indicates solidification of some of the melt in the absence of atmosphere or at low relative velocity to the atmosphere. Accretionary lapilli are also found in the upper sorted layer of the crater suevite, suggesting condensation of water vapor in a cloud over the center of the crater, similar to those produced by the rise of a nuclear fireball, or a Plinian eruption column. A thin, fine-grained layer at the base of the fallout suevite could be a lateral extension of the sorted crater suevite, or a depositional feature, similar to those seen in ignimbrite deposits. Following deposition of the suevite, formation of low-temperature (<100°C) clays by hydrothermal alteration associated with cooling of the melt-bearing deposits is supported by recently obtained paleomagnetic data. Several of the features inferred for the Ries crater, such as an oblique impact, the cloud over the crater, and the ignimbrite-like depositional features may also have been present during larger events such as the Cretaceous/Tertiary boundary event.

Abstract Geochemical studies of Leadville Formation dolomites provide information about the solutions from which they formed. Seven dolomite types can be grouped into six geochemical types on the basis of trace element contents and oxygen isotopes. Medium and coarse crystalline dolomite are indistinguishable on the basis of either trace element or oxygen isotope contents. Low trace element abundances and slightly negative δ 18 O PDB values are snggestive of diagenesis in solutions containing a significant meteoric component. Fine crystalline dolomite formed penecontemporaneously with deposition and contains heavier oxygen than either medium or coarse crystalline dolomite or undolomitized Leadville Limestone. The oxygen isotope data supports dolomitization of fine crystalline dolomite by evaporatively concentrated sea water followed by stabilization by meteoric water. Two varieties of zebra spar dolomite precipitated into cavities formed during karst dissolution. Trace element abundances and 18 O composition of earlier formed “cloudy zebra spar” are similar to those of medium and coarse crystalline dolomite and suggest that this dolomite formed as cement during karst solution erosion. Later formed “clear zebra spar” contains lighter oxygen and higher Fe and other trace elements, and fluid inclusions in it indicate precipitation above 130°C from evolved brines. This dolomite is interpreted to have formed from sedimeutary brine fluids after burial by Pennsylvanian sediments. Baroque dolomite occurring as cement in karst breccia bodies contains very light oxygen (δ 18 O PDB = —20%) and is high in Fe, Mn, and base metals. This dolomite is intimately associated with sphalerite and galena in some ore deposits and formed by recrystallization of detrital dolomite sand in karst breccias during ore deposition. Paleomagnetic studies reveal a well-defined late Paleozoic remanence in most dolomite types spatially removed from Laramide-Tertiary igneous activity. The remanences are carried in magnetite of possible diagenetic origin. Preservation of these remanences suggests that these rocks have not undergone significant aquatic alteration since late Paleozoic time.