ALBERT

Description: Upper Cretaceous–Eocene forearc strata deposited along the California continental margin record a complex history of plate convergence that shaped the tectonic development of the U.S. Cordillera. Synthesis of new and published detrital zircon U-Pb ages over a 2000 km length of the southern Oregon–California–northern Baja forearc clearly demonstrates spatial and temporal changes in sandstone provenance that reflect evolving sediment dispersal patterns associated with the extinction of continental margin arc magmatism and transfer of deformation to the continental interior during latest Cretaceous–early Cenozoic Laramide low-angle subduction. Measured age distributions from Cenomanian to Campanian forearc strata indicate the existence of a drainage divide formed by a high-standing mid-Cretaceous Cordilleran arc that crosscut older, Late Permian–Jurassic arc segments. Progressive influx of 125–85 Ma detrital zircon in the Great Valley forearc reflects ongoing denudation of the Sierra Nevada batholith throughout Late Cretaceous–early Paleogene time. In contrast, age distributions in the Peninsular Ranges forearc indicate early denudation of the Peninsular Ranges batholith that is hypothesized to have resulted from the initial collision of an oceanic plateau with the southern California margin; as a result, these age distributions exhibit little change over time until delivery of extraregional detritus to the margin in Eocene time. Maastrichtian through middle Eocene strata preserved south of the Sierra Nevada record a pronounced shift from local to extraregional provenance caused by the development of drainages that extended across the breached mid-Cretaceous continental margin batholith to tap the continental interior. This geomorphic breaching of the mid-Cretaceous arc, and associated inland drainage migration, represents the culminating influence of Laramide low-angle subduction on the continental margin and likely occurred following subduction of the Shatsky conjugate plateau beneath the western United States.

Description: The Pungarehu debris avalanche deposit was emplaced by the largest known collapse of the proto–Taranaki volcano, ca. 25,000 calibrated (cal.) years ago. This debris avalanche deposit displays a highly contrasting sedimentary character between its proximal and distal reaches. Examination of the deposit granulometry, sedimentary structures, and microscopic particle attributes provides new insights into debris avalanche transport and internal evolution processes. Initial collapse of the proto–Taranaki volcano during this event occurred near the Last Glacial Maximum, with snow and ice cover and substantial groundwater present. The collapsing, sliding large blocks of edifice material, "megaclasts," were highly fractured by the landslide generation and the depressurization event, forming pervasive jigsaw textures. As the megaclasts moved, shear was focused in softer domains between the hardest, lava-dominated lithologies. These crush and shear zones developed a complex pattern of relative motion between horizontal and vertical parts of the landslide, rather than a simple basal shear zone that supported an upper pluglike mass. The sheared zones, concentrated in soft, pyroclastic lithologies, were areas of intense synflow fragmentation, producing a proto-interclast matrix between large blocks of coherent (albeit jigsaw fractured) lavas. Down flow, the interclast matrix component increased to become pervasive by ~23–25 km from the source, enveloping and preserving large megaclasts out to at least 30 km. The most distal exposures, limited by coastal erosion to ~25–27 km, show that the matrix was not completely water saturated, with only superficial penetration of the sand-dominated material into the margins of fractured lava domains, which still contained central void space. Evidence of multiple generations of particle fracturing is seen under scanning electron microscopy of sand-grade clasts, with initial decompression fractures crosscut by later cracks, pits, and scratches produced by collisional and frictional processes during transport. The findings from this study help to explain the formation of the highly irregular topography of debris avalanche deposits, with chaotically distributed (and probably temporary) zones of shear developing where softer lithologies occur in a collapsing mass, thus leading to differential velocity profiles of portions of the flowing mass in vertical and horizontal planes.

Description: Three-dimensional (3-D) finite strain analyses from across the central Andes are used to document the contribution of grain-scale strain in quartzites and sandstones to the total shortening budget. The results are compared to thermal, stratigraphic, and strain data from other fold-and-thrust belts to determine the influence of lithologic strength and deformation temperature on strain accommodation during orogenic evolution. In the central Andes, 3-D best-fit ellipsoids are inconsistently oriented relative to structural trends, have short axes at high angles to bedding ( Z , mean plunge = 78° ± 21°), and have bedding-parallel long axes ( X , mean plunge = 6° ± 24°). Ellipsoid shapes are dominantly oblate ( X = Y 〉 Z ), indicate low natural octahedral shear strains ( S = 0.03–0.19), and have axial ratios that range from 1.02:1:0.81 ( S = 0.19) to 1.02:1:0.97 ( S = 0.03). Highly variable R f - data ( R f = 1.0–5.0, fluctuations exceeding 100°) indicate detrital grain shapes may overwhelm any measurable tectonic strain fabric recorded by grain geometry. The best-fit ellipsoids may reflect either weak compaction strain, or they may be related to a depositional fabric. At a minimum, granular strain was insufficient to reset the detrital grain fabric, and therefore grain-scale strain in quartzites and sandstones is not a significant factor in deformation. We suggest that the nonstrained nature of these stiffer lithologies indicates a lack of regional, penetrative strain in the central Andes like that quantified in similar lithologies in other orogens. The regional lack of strain may be due to deformation temperatures 〈180 °C and the presence of five ≥1-km-thick shale detachments. In the Sevier and Appalachian orogens, granular strain fabrics are best developed where temperatures exceeded ~180 °C, but they are also found where temperatures were 〈180 °C. The lack of distributed detachments in Appalachian and Sevier stratigraphy may have favored minor layer-parallel shortening at temperatures 〈180 °C rather than formation of numerous, low-offset faults as in the central Andes. Minimal slip on individual central Andean thrusts would have limited footwall burial and maintained low deformation temperatures.

Description: A general lack of consensus about the origin of Himalayan gneiss domes hinders accurate thermomechanical modeling of the orogen. To test whether doming resulted from tectonic contraction (e.g., thrust duplex formation, antiformal bending above a thrust ramp, etc.), channel flow, or via the buoyant rise of anatectic melts, this study investigates the depth and timing of doming processes for Gianbul dome in the western Himalaya. The dome is composed of Greater Himalayan Sequence migmatite, Paleozoic orthogneiss, and metasedimentary rock cut by multiple generations of leucogranite dikes. These rocks record a major penetrative D2 deformational event characterized by a domed foliation and associated NE-SW–trending stretching lineation, and they are flanked by the top-down-to-the-SW (normal-sense) Khanjar shear zone and the top-down-to-the-NE (normal sense) Zanskar shear zone (the western equivalent of the South Tibetan detachment system). Monazite U/Th-Pb geochronology records (1) Paleozoic emplacement of the Kade orthogneiss and associated granite dikes; (2) prograde Barrovian metamorphism from 37 to 33 Ma; (3) doming driven by upper-crustal extension and positive buoyancy of decompression melts between 26 and 22 Ma; and (4) the injection of anatectic melts into the upper levels of the dome—neutralizing the effects of melt buoyancy and potentially adding strength to the host rock—by ca. 22.6 Ma on the southwestern flank and ca. 21 Ma on the northeastern flank. As shown by a northeastward decrease in 40 Ar/ 39 Ar muscovite dates from 22.4 to 20.2 Ma, ductile normal-sense displacement within the Zanskar shear zone ended by ca. 22 Ma, after which the Gianbul dome was exhumed as part of a rigid footwall block below the brittle Zanskar normal fault, tilting an estimated 5°–10°SW into its present orientation.

Description: Quantifying how hillslopes respond to river incision and climate change is fundamental to understanding the evolution of uplifting landscapes during glacial-interglacial cycles. We investigated the interplay among uplift, river incision, and hillslope response in the nonglacial Waipaoa River catchment located in the exhumed inner forearc of an active subduction margin on the East Coast of the North Island of New Zealand. New high-resolution topographic data sets (light detection and ranging [lidar] and photogrammetry) combined with field mapping and tephrochronology indicate that hillslopes adjusted to rapid latest Pleistocene and Holocene river incision through the initiation and reactivation of deep-seated landslides. In the erodible marine sedimentary rocks of the Waipaoa sedimentary system, postincision deep-seated landslides can occupy over 30% of the surface area. The ages of tephra cover beds identified by electron microprobe analysis on 80 tephra samples from 173 soil test pits and 64 soil auger sites show that 4000–5000 yr after the initiation of river incision, widespread hillslope adjustment started between the deposition of the ca. 14,000 cal. yr B.P. Waiohau Tephra and the ca. 9420 cal. yr B.P. Rotoma Tephra. Tephrochronology and geomorphic mapping analysis indicate that river incision and deep-seated landslide slope adjustment were synchronous between main-stem rivers and headwater tributaries. Hillslope response in the catchment can include the entire slope, measured from river to ridgeline, and, in some cases, the interfluves between incising subcatchments have been dramatically modified through ridgeline retreat and/or lowering. Using the results of our landform tephrochronology and geomorphic mapping, we derive a conceptual time series of hillslope response to uplift and climate change–induced river incision over the last glacial-interglacial cycle.

Description: The Sallys Bend swamp and marsh area on the central Oregon coast onshore of the Cascadia subduction zone contains a sequence of buried coastal wetland soils that extends back ~4500 yr B.P. The upper 10 of the 12 soils are represented in multiple cores. Each soil is abruptly overlain by a sandy deposit and then, in most cases, by greater than 10 cm of mud. For eight of the 10 buried soils, times of soil burial are constrained through radiocarbon ages on fine, delicate detritus from the top of the buried soil; for two of the buried soils, diatom and foraminifera data constrain paleoenvironment at the time of soil burial. We infer that each buried soil represents a Cascadia subduction zone earthquake because the soils are laterally extensive and abruptly overlain by sandy deposits and mud. Preservation of coseismically buried soils occurred from 4500 yr ago until ~500–600 yr ago, after which preservation was compromised by cessation of gradual relative sea-level rise, which in turn precluded drowning of marsh soils during instances of coseismic subsidence. Based on grain-size and microfossil data, sandy deposits overlying buried soils accumulated immediately after a subduction zone earthquake, during tsunami incursion into Sallys Bend. The possibility that the sandy deposits were sourced directly from landslides triggered upstream in the Yaquina River basin by seismic shaking was discounted based on sedimentologic, microfossil, and depositional site characteristics of the sandy deposits, which were inconsistent with a fluvial origin. Biostratigraphic analyses of sediment above two buried soils—in the case of two earthquakes, one occurring shortly after 1541–1708 cal. yr B.P. and the other occurring shortly after 3227–3444 cal. yr B.P.—provide estimates that coseismic subsidence was a minimum of 0.4 m. The average recurrence interval of subduction zone earthquakes is 420–580 yr, based on an ~3750–4050-yr-long record and seven to nine interearthquake intervals. The comparison of the Yaquina Bay earthquake record to similar records at other Cascadia coastal sites helps to define potential patterns of rupture for different earthquakes, although inherent uncertainty in dating precludes definitive statements about rupture length during earthquakes. We infer that in the first half of the last millennia, the northern Oregon part of the subduction zone had a different rupture history than the southern Oregon part of the subduction zone, and we also infer that at ca. 1.6 ka, two earthquakes closely spaced in time together ruptured a length of the megathrust that extends at least from southwestern Washington to southern Oregon.

Description: The Garlock fault is an integral part of the plate-boundary deformation system inboard of the San Andreas fault (California, USA); however, the Garlock is transversely oriented and has the opposite sense of shear. The slip history of the Garlock is critical for interpreting the deformation of the through-going dextral shear of the Walker Lane belt–Eastern California shear zone. The Lava Mountains–Summit Range (LMSR), located along the central Garlock fault, is a Miocene volcanic center that holds the key to unraveling the fault slip and development of the Garlock. The LMSR is also located at the intersection of the NNW-striking dextral Blackwater fault and contains several sinistral WSW-striking structures that provide a framework for establishing the relationship between the sinistral Garlock fault system and the dextral Eastern California shear zone. New field mapping and geochronology data ( 40 Ar/ 39 Ar and U-Pb) show five distinct suites of volcanic-sedimentary rock units in the LMSR overlain by Pliocene exotic-clast conglomerates. This stratigraphy coupled with fifteen fault slip markers define a three-stage history for the central Garlock fault system of 11–7 Ma, 7–3.8 Ma, and 3.8–0 Ma. Pliocene to recent slip occurs in a ~12-km-wide zone and accounts for ~33 km or 51% of the total 64 km of left-lateral offset on the Garlock fault in the vicinity of the LMSR since 3.8 Ma. This history yields slip rates of 6–9 mm/yr for the younger stage and slower rates for older stages. The LMSR internally accommodates northwest-directed dextral slip associated with the Eastern California shear zone–Walker Lane belt via multiple processes of lateral tectonic escape, folding, normal faulting, and the creation of new faults. The geologic slip rates for the Garlock fault in the LMSR match with and explain along-strike variations in neotectonic rates.

Description: Debris avalanches caused by the collapse of volcanic flanks pose a great risk to inhabited areas and may permanently change the surrounding landscape and its drainage systems. In this research, we explored the interplay between a debris avalanche and a tectonically uplifting surrounding landscape, providing insights into the long-term consequences of volcanic edifice failures. Exposures of coarse volcaniclastic sediments along the Hautapu River ~50 km southeast of Mount Ruapehu, New Zealand, show evidence of the largest known collapse event of the stratovolcano, which was followed by a vigorous regrowth phase that produced numerous pyroclastic eruptions and pumice-rich lahars. Similar diamicton deposits are exposed within the river catchment adjacent to the west. Cover-bed stratigraphy and geochemical correlation of andesitic lava blocks within the debris-avalanche deposit with dated lavas exposed on the cone indicate that deposition occurred between 125 and 150 ka. The collapse took place during the shift from a glacial to an interglacial climate, when glaciers on the cone were in retreat, and high pore-water pressures combined with deep hydrothermal alteration weakened the cone. In addition, collapse may have been accompanied by magmatic unrest. The ~2–3 km 3 debris avalanche inundated an area of 〉260 km 2 and entered the proto-Hautapu catchment, where it was channelized within the deeply entrenched valley. Mass-wasting events associated with postcollapse volcanism continued to be channeled into the proto–Hautapu River for another ~10 k.y., producing long-runout lahars. Subsequently, the river catchment was isolated from the volcano by incision of the intervening Whangaehu River into the proximal volcaniclastic sediments, accompanied by regional faulting and graben deepening around Ruapehu. At present, the volcaniclastic deposits form a distinctive plateau on the highest topographic elevation within the Hautapu Valley, forming a reversed topography caused by preferred incision of the Hautapu River into softer Late Tertiary sediments concurrent with constant uplift.

Description: Combined petrofabric, microstructural, stable isotopic, and 40 Ar/ 39 Ar geochronologic data provide a new perspective on the Cenozoic evolution of the northern Snake Range metamorphic core complex in east-central Nevada. This core complex is bounded by the northern Snake Range detachment, interpreted as a rolling-hinge detachment, and by an underlying shear zone that is dominated by muscovite-bearing quartzite mylonite and interlayered micaschist. In addition to petrofabric, microstructural analysis, and 40 Ar/ 39 Ar geochronology, we use hydrogen isotope ratios (D) in synkinematic white mica to characterize fluid-rock interaction across the rolling-hinge detachment. Results indicate that the western flank of the range preserves mostly Eocene deformation (49–45 Ma), characterized by coaxial quartz fabrics and the dominant presence of metamorphic fluids, although the imprint of meteoric fluids increases structurally downward and culminates in a shear zone with a white mica 40 Ar/ 39 Ar plateau age of ca. 27 Ma. In contrast, the eastern flank of the range displays pervasive noncoaxial (top-to-the-east) fabrics defined by white mica that formed in the presence of meteoric fluids and yield Oligocene–Miocene 40 Ar/ 39 Ar ages (27–21 Ma). Evolution of the Oligocene–Miocene rolling-hinge detachment controlled where and when faulting was active or became inactive owing to rotation, and therefore where fluids were able to circulate from the surface to the brittle-ductile transition. On the western flank (rotated detachment), faulting became inactive early, while continued active faulting on the eastern flank of the detachment allowed surface fluids to reach the mylonitic quartzite. The combined effects of synkinematic recrystallization and fluid interaction reset argon and hydrogen isotope ratios in white mica until the early Miocene (ca. 21 Ma), when the brittle-ductile transition was exhumed beneath the detachment.

Description: The timing and pattern of Tibetan Plateau rise provide a critical test of possible mechanisms for the development and support of high topography, yet views range widely on the history of surface uplift to modern elevations of ~4.5 km. To address this issue we present clumped isotope thermometry data from two well-studied basins in central and southwestern Tibet, for which previous carbonate 18 O data have been used to reconstruct high paleoelevations from late Oligocene to Pliocene time. Clumped isotope thermometry uses measurements of the 13 C- 18 O bond ordering in carbonates to constrain the temperature [ T ( 47 )] and 18 O value of the water from which the carbonate grew. These data can be used to infer paleoelevation by exploiting the systematic decrease of surface temperature and the 18 O value of meteoric water with elevation, provided samples record original depositional conditions and appropriate context exists for interpreting T ( 47 ) and 18 O values. Previous calcite 18 O and 13 C values for Oligocene-age marls from the Nima Basin in central Tibet are thought to reflect original depositional conditions; however, T ( 47 ) values exceed Earth-surface temperatures, indicating that the samples have been diagenetically altered. Maximum burial temperatures were not high enough to cause solid-state C-O bond reordering. Instead, the elevated T ( 47 ) and water 18 O values are consistent with recrystallization of the samples in a rock-buffered system. Miocene–Pliocene aragonitic gastropod shells from the Zhada Basin in southwestern Tibet record primary environmental temperatures, which we interpret in the context of modern shell and tufa T ( 47 ) values and lake water temperatures. Modern shell and tufa T ( 47 ) values are similar to warm-season water temperatures. The ca. 9–4 Ma shell temperatures are significantly colder, suggesting a 9 ± 3 °C (2) average increase in warm-season lake water temperatures since the late Miocene. If the temperature increase is due entirely to elevation change and the modern June-July-August (JJA) surface water lapse rate of 6.1 °C/km applies, it implies 〉1 km of elevation loss since the late Miocene–Pliocene—corresponding to an average basin floor paleoelevation of 5.4 ± 0.5 km (2). A warmer mid-Pliocene climate would make this a minimum estimate. Our finding of cold paleotemperatures contrasts with previous conclusions based on Pliocene snail shell T ( 47 ) data interpreted in the absence of modern shell and water temperature data, but is consistent with 18 O-based paleoaltimetry and with paleontological and isotopic data indicating the presence of cold-adapted mammals living in a cold, high-elevation climate. We suggest that late Neogene elevation loss across the Zhada Basin catchment probably related to local expression of east-west extension across much of the southern Tibetan Plateau at this time.