ALBERT

Description: Zircon U-Pb analyses of 18 orthogneisses from the Clearwater and Priest River complexes of northern Idaho have identified important exposures of Neoarchean and Paleoproterozoic basement rocks in northwest Laurentia. All samples have ages that fall into two tightly defined age ranges: in the Neoarchean from 2.67 to 2.65 Ga and in the Paleoproterozoic from 1.88 to 1.84 Ga. The Neoarchean orthogneisses show no evidence of older components, whereas some of the Paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the Neoarchean gneisses. Collectively, these two packages of rocks represent the two main periods of crust formation in northwest Laurentia, one wholly juvenile in the Neoarchean, and the other a mixture of juvenile Paleoproterozoic and inherited Neoarchean components. Based on existing data, the ages of orthogneisses in the Clearwater and Priest River complexes are identical and probably represent one continuous crustal block; we refer to this basement domain as the Clearwater block. The Paleoproterozoic 1.86 Ga magmatism described here is distributed throughout the northwest Laurentian margin and does not coincide with the proposed trend of the Great Falls tectonic zone. Therefore, a model of a single linear arc preserved within the Great Falls tectonic zone is inadequate in describing the majority of the known 1.86 Ga crust in the region.

Description: Current models of landscape response to Holocene climate change in midcontinent North America largely reconcile Earth orbital and atmospheric climate forcing with pollen-based forest histories on the east and eolian chronologies in Great Plains grasslands on the west. However, thousands of sand dunes spread across 12,000 km2 in eastern upper Michigan (EUM), more than 500 km east of the present forest-prairie ecotone, present a challenge to such models. We use 65 optically stimulated luminescence (OSL) ages on quartz sand deposited in silt caps (n = 8) and dunes (n = 57) to document eolian activity in EUM. Dune building was widespread ca. 10–8 ka, indicating a sharp, sustained decline in forest cover during that period. This decline was roughly coincident with hydrologic closure of the upper Great Lakes, but temporally inconsistent with most pollen-based models that imply canopy closure throughout the Holocene. Early Holocene forest openings are rarely recognized in pollen sums from EUM because faint signatures of non-arboreal pollen are largely obscured by abundant and highly mobile pine pollen. Early Holocene spikes in nonarboreal pollen are recorded in cores from small ponds, but suggest only a modest extent of forest openings. OSL dating of dune emplacement provides a direct, spatially explicit archive of greatly diminished forest cover during a very dry climate in eastern midcontinent North America ca. 10–8 ka.

Description: Fault rocks can function as barriers to subsurface fluid flow and affect the storage of CO2 in geological structures. Even though flow across faults often involves more than one fluid phase, it is typically modeled using only single-phase functions due to a lack of fault rock relative permeability data and complexities in incorporating two-phase flow properties into flow simulations. Here we present two-phase fluid flow data for cataclastic fault rocks in porous sandstone from the 90-Fathom fault (northeast England). The study area represents a field analogue for North Sea saline aquifers of Permian–Triassic age that are currently being considered for CO2 storage. We use the experimental data to populate a synthetic model of a faulted saline aquifer to assess the impact of these fault rocks on CO2 injection. We show that even fault rocks with low clay contents and very limited quartz cementation can act as major baffles to the flow of a non-wetting phase if realistic two-phase properties are taken into account. Consequently, pressure may increase far more rapidly in the storage compartment during CO2 injection than anticipated based on models that only incorporate absolute fault rock permeabilities. To avoid high pressures, which may lead to hydrofracturing and CO2 leakage, either more complex injection strategies need to be adopted or seismic data acquired to ensure the absence of faults in aquifers selected for CO2 storage.

Description: 〈span〉Observations of ancient subduction fault zones, combined with mechanical models of the plate interface, indicate that many first-order features of shallow subduction seismicity can result from failure of areas of the interface that nucleate and strengthen during interseismic periods at rates determined by silica kinetics. In the Shimanto belt, Japan, and in the Kodiak archipelago in Alaska, shear zones of tectonic mélange were accreted at a range of depths representative of the seismogenic zone. The scaly fabric and crack-seal veins in these mélanges record fluctuations in crack porosity at rates controlled by silica kinetics. Temperature-dependent healing of cracks impacts the critical stiffness for slip stability through increases in contact area and contact junction strength—a macroscopic analogue to aging in slide-hold-slide experiments on gouge. The potential for portions of the interface to strengthen through mineral redistribution during the interseismic period forms the basis for a two-dimensional numerical block-slider model for slip behavior of the subduction interface, where aging follows a temperature-dependent rate law. The model responds to a population balance equation, with nucleation, strengthening, and failure of patches of the interface that have greater static frictional strength, defined in the model as “asperities.” An exponential rate law for nucleation and strengthening, based on Arrhenius equation silica kinetics, leads to: (1) supercycles of buildup and release of elastic strain, (2) a temperature-based updip limit to genesis of large earthquakes, and (3) a power-law size distribution of earthquakes that varies as a function of temperature. Asperities in this case arise by stochastic nucleation and strengthening based on a temperature-dependent rate law and are unrelated to roughness of the plate interface. Over a temperature range typical of the seismogenic zone, variations in effective stress along the plate interface lead to heterogeneous frictional characteristics in the interseismic period that could control the location, recurrence time, and magnitude of earthquakes.〈/span〉

Description: Along the Middle America Trench in southern Costa Rica, flat slab subduction of the aseismic Cocos Ridge has uplifted and exposed the outer forearc, shortened the Térraba forearc basin sequence in the inner forearc (i.e., the Fila Costeña thrust belt), and uplifted the magmatic arc. The Osa Peninsula, an outer forearc high ~20 km inboard of the Middle America Trench and ~3 km to ~10 km above the plate interface at its trenchward edge, is deforming in response to variations in the bathymetry of the subducting aseismic Cocos Ridge where relief locally exceeds 1 km. Modern topography of the Osa Peninsula, elevation of the basement rocks (Early to Middle Tertiary Osa mélange), elevations of Quaternary marine deposits (Marenco formation), and distribution of late Quaternary uplift rates directly mirror the bathymetry on the Cocos Ridge outboard of the Middle America Trench. Rates of late Quaternary uplift are calculated from eight new radiocarbon ages, five new optically stimulated luminescence ages, and 10 previously published radiocarbon ages. Rates of uplift range from 1.7 m/k.y. to 8.5 m/k.y. The Osa Peninsula is fragmented into small (~5 km), independently deforming blocks bounded by trench-parallel and trench-perpendicular, subvertical, normal and reverse faults that extend down to the plate interface, allowing for greatly different deformation histories over short distances. Quaternary deformation on the Osa Peninsula is modeled as a thin, outer-margin wedge that deforms in response to subduction of short-wavelength, high-relief asperities on the downgoing plate. Permanent deformation is largely accomplished by simple shear on a complex array of subvertical faults that allow the upper plate to adjust to variations in the slope of incoming ridges and seamounts. Currently, permanent deformation of the outer forearc does not appear to involve significant subhorizontal shortening of the margin wedge, although the global positioning system velocity field records elastic shortening related to locking of the plate interface. Permanent uplift and uplift rates in the outer forearc in southern Costa Rica are driven, to the first order, by the bathymetry associated with the subducting Cocos Ridge and not by the basal shear stress on the plate interface.

Description: Mapping the age and trace element and Sm-Nd isotope compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation–split stream analysis reveals a wide range in Nd isotope and rare earth element (REE) compositions within and between single grains. These data corroborate isotopic variability indicated by Hf isotope analysis of zircon in the same granite sample. The REE variations indicate that monazite grew during fractional crystallization. Hf and Nd isotopes indicate that the granitic magma was generated from at least two distinct Proterozoic sources of approximately the same age: one component that had highly radiogenic initial 176 Hf/ 177 Hf and 143 Nd/ 144 Nd and a second component that was notably less radiogenic. This study highlights the utility of in situ REE and Sm-Nd isotope data in monazite in magmatic systems. Further, it refines the zircon-based constraints on magmatic processes because of sensitivity of light REEs to fractional crystallization, lower probability of complications owing to inheritance, and smaller analytical volumes required.

Description: New geochronologic and geomorphic constraints on the Little Lake fault in the Eastern California shear zone reveal steady, modest rates of dextral slip during and since the mid-to-late Pleistocene. We focus on a suite of offset fluvial landforms in the Pleistocene Owens River channel that formed in response to periodic interaction with nearby basalt flows, thereby recording displacement over multiple time intervals. Overlap between 40 Ar/ 39 Ar ages for the youngest intracanyon basalt flow and 10 Be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the Little Lake channel at ca. 64 ka. Older basalt flows flanking the upper and lower canyon margins indicate localization of the Owens River in its current position between 212 ± 14 and 197 ± 11 ka. Coupled with terrestrial light detection and ranging (lidar) and digital topographic measurements of dextral offset, the revised Little Lake chronology indicates average dextral slip rates of at least ~0.6–0.7 mm/yr and 〈1.3 mm/yr over intervals ranging from ~10 4 to 10 5 yr. Despite previous geodetic observations of relatively rapid interseismic strain along the Little Lake fault, we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles. Instead, our results indicate that accelerated fault loading may be transient over much shorter periods (~10 1 yr) and perhaps indicative of time-dependent seismic hazard associated with Eastern California shear zone faults.

Description: In this paper, stratigraphy, geochronology, and geologic mapping are used to characterize a sequence of Quaternary deposits associated with edifice failure of Barú Volcano, which, together with balanced cross sections, illustrate the upper plate’s response to along-strike variations in subduction properties that occur across the tear in the subducting slab located at the Panama fracture zone. The subducting Panama fracture zone is an active transform that separates disparate styles of subduction between the thick, rapid, and flat subduction of the Cocos plate to the west and the thinner, more oblique, and steeper subduction of the Nazca plate to the east. We focus on the arc-forearc region above the Cocos-Nazca slab tear, where both the Fila Costeña inner forearc fold-and-thrust belt and the exhumed Cordillera de Talamanca terminate along strike to the southeast. The Fila Costeña thrust belt imbricates an Eocene–late Miocene forearc basin sequence with up to 40 km of shortening inboard of Cocos plate subduction and crosscuts a sequence of Pleistocene and younger volcano-sedimentary units on the southwestern flanks of the active Barú Volcano at its southeastern termination. These units, constrained in age by radiocarbon dating ( n = 11), soil color, and 40 Ar/ 39 Ar ( n = 5) data, include a sequence of late Pleistocene to Holocene debris-avalanche and lahar deposits. Landscape morphology, the areal distribution of units, and new shortening estimates from balanced cross sections collectively suggest that the southeastern termination of the Fila Costeña thrust belt actively propagates to the southeast coeval with migration of the Panama triple junction. The prevailing map pattern suggests that the forearc’s response to southeastward migration of the slab tear varies with distance to the trench. In contrast to deformation patterns in the outermost forearc, plate-boundary tractions associated with shallow subduction and Cocos-Caribbean convergence are more influential in the development of arc-forearc deformation than either oblique subduction of the ~2 km bathymetric scarp, or right-lateral shear, which both occur astride the subducting Panama fracture zone.

Description: The Jim Sage volcanic suite (JSVS) exposed in the Jim Sage and Cotterel Mountains of southern Idaho (USA) consists of two volcanic members composed of ~240 km 3 of Miocene rhyolite lavas separated by an interval of lacustrine sediments. It is capped by rheomorphic ignimbrite and as much as 100 m of basaltic lava flows probably derived from the central Snake River Plain (SRP) province to the north. The occurrence of volcanic vents in the JSVS links the lava flows to their local eruptive centers, while the adjacent Albion–Raft River–Grouse Creek metamorphic core complex exposes ~3000 km 2 of once deep-seated rocks that offer constraints on the composition of the potential crustal sources of these rhyolites. U-Pb zircon ages from the rhyolite lavas of the JSVS range from 9.5 to 8.2 Ma. The Miocene basalt of the Cotterel Mountains has an 87 Sr/ 86 Sr i composition of 0.7066–0.7075 and Nd (i) = –3.7, and the rhyolite lavas of the JSVS have 87 Sr/ 86 Sr i = 0.7114–0.7135 and Nd (i) values that range from –6.7 to –7.1. Zircon from the rhyolites of the JSVS range in 18 O zr (Vienna standard mean ocean water, VSMOW) from –0.5 to 5.7 and have Hf (i) values ranging from –0.8 to –6.8. Based on geochronology, whole-rock major elements, trace elements, isotopes (Sr and Nd), and in situ zircon O and Hf isotopic compositions, we infer that the JSVS is genetically related to the central SRP province. The eruption of the low- 18 O rhyolites of the JSVS, outside of the main topographic extent of the SRP province (without the large calderas inferred for the SRP rhyolites) implies that there might be an alternative mechanism for the formation of the low- 18 O signature other than the proposed assimilation of hydrothermally altered caldera blocks. One suggestion is that the north to south propagation of SRP-type low- 18 O rhyolitic melt along the Albion fault led to off-axis magmatism. Another possibility is that there was prior and widespread (across a region wider than the SRP) hydrothermal alteration of the crust related to its earlier magmatic and faulting history. The eruption of SRP-type lavas in the hanging wall of an evolving metamorphic core complex helps us outline the role of the SRP magmatic province in the extensional evolution of the northeastern Basin and Range. The lavas of the JSVS imply the addition of basalt, related to the SRP hotspot, to the crust beneath the Raft River Basin that provided a heat source for crustal melting and weakening of the deep crust; this led to a vertical component of crustal flow and doming during extension, after the eruption of the 9.5–8.2 Ma JSVS rhyolites. This younger than 8.2 Ma component of vertical motion during faulting of the Miocene stratified sequence of the Raft River Basin and the rotation of the Albion fault to shallower angles collectively resulted in the subhorizontal detachment structure imaged seismically beneath the Raft River Basin.

Description: Landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation. Whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography, no consensus currently exists concerning their respective influences. To address this question, denudation rates at centennial to millennial time scales were deduced from 10 Be concentrations in detrital sediments derived from 30 small basins (10–600 km 2 ) in an ~200-km-wide region in central Nepal. Along a northward, strike-perpendicular transect, average denudation rates sharply increase from 〈0.5 mm/yr in the Lesser Himalayas to ~1 mm/yr when crossing the Physiographic Transition, and then accelerate to 2–3 mm/yr on the southern flank of the high peaks in the Greater Himalayas. Despite a more than five-fold increase in denudation rate between the southern and northern parts of this transect, the corresponding areas display similar precipitation rates. The primary parameter that presents a significant co-variation with denudation is the long-term rock-uplift rate that is interpreted to result from the ramp-flat transition along the Main Himalayan Thrust. We propose that, in this rapidly uplifting mountain range, landscapes adjust quickly to changing climatic conditions, such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes. In this particular context, variations in precipitation appear to have only a second-order role in modulating the denudation signal that is primarily set by the background rock-uplift rate.