ALBERT

Description: Fault rock low-temperature thermochronometry can inform the timing, temperature, and significance of hydrothermal fluid circulation in fault systems. We demonstrate this with combined hematite (U-Th)/He (He) dating, and sandstone apatite fission-track (AFT) and apatite and zircon (U-Th)/He (He) thermochronometry from fault-related fissures on the Gower Peninsula, Wales. Hematite He dates from 141 ± 5.1 Ma to 120 ± 5.0 Ma overlap with a 131 ± 20 Ma sandstone infill AFT date. Individual zircon He dates are 402–260 Ma, reflecting source material erosion, and imply a maximum Late Permian infill depositional age. Burial history reconstruction reveals modern exposures were not buried sufficiently in the Triassic–Early Cretaceous to have caused reheating to temperatures necessary to reset the AFT or hematite He systems, and thus these dates cannot reflect cooling due to erosion alone. Hot fluids circulating through fissures in the Early Cretaceous reset the AFT system. Hematite was either also reset by fluids or precipitated from these fluids. Similar hematite He dates from fault-related mineralization in south Glamorgan (Wales) and Cumbria (England) imply concomitant regional hot groundwater flow along faults. In this example, hydrothermal fluid circulation, coeval with North Atlantic rifting, occurred in higher-permeability fissures and fault veins long after they initially formed, directly influencing local and regional geothermal gradients.

Description: Physical and numerical simulations of the development of mountain topography predict that asymmetric distributions of precipitation over a mountain range induce a migration of its drainage divide toward the driest flank in order to equilibrate erosion rates across the divide. Such migration is often inferred from existing asymmetries, but direct evidence for the migration is often lacking. New low-temperature apatite cooling ages from a transect across the northern North Cascades range (Washington, NW USA) and from two elevation profiles in the Skagit River valley record faster denudation on the western, wetter side of the range and lower denudation rates on the lee side of the range. This difference has already been documented further south along another transect across the range; however, in the south, the shift from young cooling ages to older ages occurs across the modern drainage divide. Here, further north, the shift occurs along a range-transverse valley within the Skagit Gorge. It has been proposed that the upper Skagit drainage was once a part of the leeward side of the range but started to drain toward the western side of the range across the Skagit Gorge in Quaternary time. Age-elevation profiles along the former drainage and in the Skagit Gorge restrict the onset of Skagit Gorge incision to the last 2 m.y., in agreement with 4 He/ 3 He data for the gorge floor. Breaching of the range drainage resulted in its displacement 40 km further east into the dry side of the range. In the 2000-m-deep, V-shaped Skagit Gorge, river stream power is still high, suggesting that incision of the gorge is still ongoing. Several other similar events have occurred along the range during the Pleistocene, supporting the proposed hypothesis that the repeated southward incursions of the Cordilleran ice sheet during this period triggered divide breaching and drainage reorganization by overflow of ice-dammed lakes at the front of the growing ice sheet. Since these events systematically rerouted streams toward the wet side of the range and resulted in leeward migration of the divide, we propose that in fact the Cordilleran ice sheet advance essentially catalyzed the adjustment of the mountain chain topography to the current orographic precipitation pattern.

Description: Geometrical modeling of radiation damages zones from α recoil and fission that accounts for their elongate nature provides new estimates of the doses required to reach percolation and full connectivity in zircon. Alpha recoil track damage percolates at doses from 2.5–3.1 x 10 16 α/g based on our calculations, about two orders of magnitude lower than previous estimates, with the difference partially due to elongation and partially due to decay chains creating pre-made networks of connected tracks. This dose level is far below that required for metamictization, and suggests that α recoil track percolation has no effect on macroscopic or unit-cell properties, at least as measured to date. However, fission tracks percolate at a dose of approximately 1.9 x 10 18 α/g, the approximate level formerly ascribed to α recoil damage percolation and correlating with various transitions in material properties, such as an inflection in the relationship between dose and macroscopic swelling. Consideration of the undamaged regions between damage zones indicates that c -axis-parallel channels are frequently interrupted, at the micrometer scale at very low doses and tens of nanometers at usual doses in natural zircon, with the probable effect of decreasing diffusivity anisotropy. The percolation and further interconnectivity of α recoil damage corresponds with a general minimum in diffusivity and maximum in closure temperature in zircon, indicating that α recoil damage percolation does not make a grain "leaky", but instead quite the opposite. Instead, the onset of poor He retentivity at high damage levels correlates with fission-track percolation. Some of these results are non-intuitive with respect to the trapping model of He diffusivity reduction, and the alternative mechanism of tortuosity is discussed.

Description: We identify olivine grains with compositions up to Fo 99 ·8 , which are found in multiple primitive basaltic lava flows from a monogenetic volcano in the Big Pine Volcanic Field, California, USA. In this study, we show that the forsterite in these basalts formed by subsolidus recrystallization in a high- f O 2 environment. Olivine compositions are bimodal, with flows having either all normal compositions (Fo 74 ·9 – 94 ·4 ) or highly forsteritic (Fo 97 ·2 – 99 ·8 ) compositions. In many grains, the subhedral forsteritic olivine has a hematite and clinopyroxene rim, and internal parallel-oriented planes of hematite, clinopyroxene and orthopyroxene. Results of isotopic, chemical, crystallographic, petrographic and mineralogical analyses show that the forsterite formed through subsolidus oxidation of olivine phenocrysts. The forsteritic olivines generally occur in the thinner flows. We infer that a rapidly emplaced sequence of thin, vesicular, spatter-fed flows allowed the original olivine phenocrysts to become repeatedly reheated while exposed to air. Our study required sampling each flow, so it was difficult to avoid the altered portions of the thinner flows. Other studies would tend to avoid such flows, which may account for why such forsteritic olivines have not been more widely recognized.

Description: Synconvergent extension within orogenic systems is often interpreted as gravitational spreading of thickened crust or as a response to thrust belt dynamics. However, the processes that spatially localize extension during orogenesis are not fully understood. Here, a case study from the United States Cordillera demonstrates that localized upper-crustal thickening can exert a first-order control on the spatial location of synorogenic extension. The Eureka culmination, a 20-km-wide, north-trending anticline with 4.5 km of structural relief in the hinterland plateau of the Sevier orogenic belt (or "Nevadaplano") in eastern Nevada was deformed by two sets of north-striking normal faults that pre-date late Eocene volcanism. (U-Th)/He and fission-track thermochronology data collected from Paleozoic quartzite in the footwalls of two normal faults demonstrate rapid (10 °C/m.y.), Late Cretaceous to Paleocene (75–60 Ma) cooling, which we interpret as tectonic exhumation during extension, and which was concurrent with late-stage shortening in the frontal Sevier thrust belt. This example illustrates that structural and topographic relief generated within zones of localized upper-crustal thickening can spatially focus extension during orogenesis, and adds to a growing body of evidence that Late Cretaceous–Paleocene extension in the Nevadaplano occurred at both mid- and upper-crustal levels.

Description: Oxide minerals in diagenetic cements, concretions, and fracture fill reflect episodes of ancient groundwater flow that have the potential to record tectonic, geomorphic, and climatic changes through time. To better understand the ages, conditions of formation, and potential geologic significance of these diagenetic materials, we have measured (U-Th)/He ages and element concentrations in hematite, goethite, and Mn-oxide in Mesozoic sandstones from several locations in the Colorado Plateau. Most (U-Th)/He ages are Pliocene–Pleistocene, but some samples are as old as 25 Ma, the age of previously determined 40 Ar/ 39 Ar ages on Mn-oxide cement. In one region, texturally diverse Mn- and Fe-oxides yield relatively reproducible ages of ca. 2–3 Ma, and in another region, vitreous Fe-oxide fracture coatings yield ages of ca. 300 ka. Elsewhere, most cements and concretions show a wide range of ages among aliquots taken over millimeter length scales. Hematite-dominated samples show a broad positive correlation between age and U-Th concentrations, whereas most goethite-dominated samples show a negative correlation. Exterior portions of spherical goethite concretion rinds have higher U-Th concentrations and younger (U-Th)/He ages than interior portions. Taken together, the age-composition relationships of these samples suggest that wide ranges of ages in some samples largely reflect relatively recent (Pliocene–Pleistocene) U-Th addition, recrystallization, or later oxide growth that affected precursor cements that may have been oxides or other minerals. In some cases, these precursors may have formed as early as 25 Ma. In at least two areas, the (U-Th)/He dates (or in cases where samples show a wide range in ages, the minimum dates) are associated with major climate or local incision rate changes. We speculate that diagenetic oxides, and possibly other types of fine-grained secondary oxide minerals, remain open to U-Th uptake, recrystallization, or continued growth through contact with groundwater long after initial formation. This may provide opportunities to understand groundwater compositions and flow regimes as samples are exhumed through the shallow crust, or as surface conditions change through time in diagenetic systems in the critical zone of Earth’s crust and on Mars.

Description: Apatite thermochronology is, in principle, uniquely suited to document the Cenozoic erosion of the Colorado Plateau (southwestern United States) and settle generations of debate regarding the region’s history of uplift, erosion, and fluvial incision. The protracted near-surface history of the Colorado Plateau bedrock, however, complicates the temperature sensitivity of apatite thermochronometers. This has confounded efforts to see clear evidence of late Cenozoic erosion, especially in the central Colorado Plateau, where this problem is compounded by the diverse detrital apatite grains in the region’s sedimentary bedrock. We overcome this problem in the thermal aureole of the Oligocene Henry Mountains intrusive complex, where these sandstones have apatite (U-Th-Sm)/He ages younger than 26 Ma with positive-slope age–effective U trends (3–25 Ma, 5–180 ppm eU) that resolve a distinctive late Cenozoic history. Thermal modeling results strongly suggest that the central Colorado Plateau was a stable Miocene landscape that was rapidly exhumed ~1.5–2 km during the past 5 m.y., likely in the past 3–2 m.y., at time-averaged rates of ~250–700 m/m.y. This demonstrates that substantial late Cenozoic erosion of the north-central plateau interior postdates the ca. 5.6 Ma integration of the Colorado River that lowered regional base level.

Description: The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24–25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely reflecting a subcritically tapered orogenic wedge. The first eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12–10 Ma, the thrust front jumped ~75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated 〉100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of shortening at this latitude is ~142 km, which is moderate in magnitude compared to the 250–350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can influence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities.

Description: Differential exhumation in the Puna Plateau and Eastern Cordillera of NW Argentina is controlled by inherited paleostructures and resulting paleotopography related to the Cretaceous Salta Rift paleomargins. The Cenozoic deformation front related to the development of the Andean retro-arc orogenic system is generally associated with 〉4 km of exhumation, which is recorded by Cenozoic apatite fission-track (AFT) and (U-Th-[Sm])/He ages (He ages) in the Eastern Cordillera of NW Argentina. New AFT ages from the top of the Nevado de Cachi document Oligocene (ca. 28 Ma) cooling, which, combined with existing data, indicates exhumation of this range between ca. 28 Ma and ca. 14 Ma. However, some of the highest ranges in the Eastern Cordillera preserve Cretaceous ages indicative of limited Cenozoic exhumation. Samples collected from an ~3-km-elevation transect along the northern part of the Sierra de Quilmes paleorift flank (Laguna Brava) show AFT ages between ca. 80 and ca. 50 Ma and He ages between ca. 45 and ca. 10 Ma. Another set of samples from an ~1-km-elevation transect farther to the southwest (La Quebrada) shows Cretaceous AFT ages between ca. 116 Ma and ca. 76 Ma, and mainly Cretaceous He ages, in agreement with AFT data. Analysis of existing AFT and He ages from the area once occupied by the Salta Rift reveals a pattern characterized by Cretaceous ages along paleorift highs and Cenozoic ages within paleorift hanging-wall basins and later foreland basin depocenters. This pattern is interrupted by the Sierras Pampeanas at ~28°S, which record mid-Cenozoic ages. Our data are consistent with a complex inherited pattern of pre-Andean paleostructures, likely associated with paleotopography, which was beveled by the Cenozoic regional foreland basin and reactivated during the late Neogene (ca. 〈10 Ma), strongly controlling the magnitude of Cenozoic uplift and exhumation and thus cooling age distribution. This, combined with variable lithologic erodibility, resulted in an irregular distribution of themochronological ages.