Axial Valley Research Articles

Permanently enhanced dynamic triggering probabilities as evidenced by two M ≥ 7.5 earthquakes

The 2012 M7.7 Haida Gwaii earthquake radiated waves that likely dynamically triggered the 2013 M7.5 Craig earthquake, setting two precedents. First, the triggered earthquake is the largest dynamically triggered shear failure event documented to date. Second, the events highlight a connection between geologic structure, sedimentary troughs that act as waveguides, and triggering probability. The Haida Gwaii earthquake excited extraordinarily large waves within and beyond the Queen Charlotte Trough, which propagated well into mainland Alaska and likely triggering the Craig earthquake along the way. Previously, focusing and associated dynamic triggering have been attributed to unpredictable source effects. This case suggests that elevated dynamic triggering probabilities may exist along the many structures where sedimentary troughs overlie major faults, such as subduction zones’ accretionary prisms and transform faults’ axial valleys. Although data are sparse, I find no evidence of accelerating seismic activity in the vicinity of the Craig rupture between it and the Haida Gwaii earthquake.

Surficial permeability of the axial valley seafloor: Endeavour Segment, Juan de Fuca Ridge

Hydrothermal systems at mid‐ocean spreading centers play a fundamental role in Earth's geothermal budget. One underexamined facet of marine hydrothermal systems is the role that permeability of the uppermost seafloor veneer plays in the distribution of hydrothermal fluid. As both the initial and final vertical gateway for subsurface fluid circulation, uppermost seafloor permeability may influence the local spatial distribution of hydrothermal flow. A method of deriving a photomosaic from seafloor video was developed and utilized to estimate relative surface permeability in an active hydrothermal area on the Endeavour Segment of the Juan de Fuca Ridge. The mosaic resolves seafloor geology of the axial valley seafloor at submeter resolution over an area greater than 1 km2. Results indicate that the valley walls and basal talus slope are topographically rugged and unsedimented, providing minimal resistance to fluid transmission. Elsewhere, the axial valley floor is capped by an unbroken blanket of low‐permeability sediment, resisting fluid exchange with the subsurface reservoir. Active fluid emission sites were restricted to the high‐permeability zone at the base of the western wall. A series of inactive fossil hydrothermal structures form a linear trend along the western bounding wall, oriented orthogonal to the spreading axis. High‐temperature vent locations appear to have migrated over 100 m along‐ridge‐strike over the decade between surveys. While initially an expression of subsurface faulting, this spatial pattern suggests that increases in seafloor permeability from sedimentation may be at least a secondary contributing factor in regulating fluid flow across the seafloor interface.

Accommodation Space Controls on the Latest Pleistocene and Holocene (16–0 ka) Sediment Size and Bypassing in the Lower Columbia River Valley: A Large Fluvial–Tidal System in Oregon and Washington, USA

Peterson, C.D.; Gates, E.B.; Minor, R., and Baker, D.L., 2013. Accommodation space controls on the latest Pleistocene and Holocene (16–0 ka) sediment size and bypassing in the Lower Columbia River Valley: a large fluvial–tidal system in Oregon and Washington, USA.In this study, we establish the roles that increasing basin accommodation space have on sediment size and bypassing in the transgressive fill (16–0 ka) in the submerged Lower Columbia River Valley (LCRV). The antecedent forearc valley (225 km in length, 4–8 km in width, and 60–115 m in axial valley depth) is characterized by high sediment supply rates (10–15 million t y−1) but no delta at its mouth to the Pacific Ocean. Core sample sediment textures (N = 1600) are analyzed from 3000 m of borehole sections in 58 representative boreholes to characterize the ancestral valley fill: 57% sand, 17% muddy sand, 12% sandy mud, and 14% mud (total fill volume = 77 km3). Decreasing mud endmember texture from between 30 and 60% (early Holocene) to 18% (late Holocene) is directly related to (1) declining rates of increasing basin accommodation space and (2) increasing sediment bypassing through the LCRV. Both conditions result from decreasing rates of sea level rise (15.0–1.5 mm y−1) in Holocene time. The overall coarsening of sediments, both upvalley and upsection, is controlled by declining rates of increasing basin accommodation space (5.66 million–0.57 million m3 y−1) during middle to late Holocene time. Differences between middle and late Holocene sediment accumulation rates yield bedload bypassing rates of 5– 6 million m3 y−1, under assumed conditions of constant sediment supply rates in middle to late Holocene time.

Atmospheric boundary layer structures associated with the Ora del Garda wind in the Alps as revealed from airborne and surface measurements

The paper investigates a coupled lake-breeze and valley-wind system, known as Ora del Garda. The latter typically originates on clear-sky days over the northern shore of Lake Garda in the Alps. After channelling into the nearby Sarca Valley and Lakes Valley, this airflow finally breaks out, through an elevated saddle, into the adjacent Adige Valley, where it strongly interacts with the local valley wind. Two flights of an instrumented motorglider explored, under different synoptic conditions, the thermal structure of the atmospheric boundary layer (ABL) associated with this wind at selected vertical sections—namely over the lake shore, at mid-valley, and at the junction with the Adige Valley. Data from airborne measurements, as well as from weather stations disseminated along the valley floor, provided the basis for mapping 3D fields of potential temperature over high-resolution grids by means of a Residual Kriging (RK) technique. This representation allowed the identification of site-specific ABL features associated with the Ora del Garda. In particular, a typical daytime coastal-breeze structure is detected in the lake shore region, where the advection of colder air tends to stabilize the atmosphere throughout the ABL depth. Mid-valley vertical profiles from both flights display shallow convective mixed layers, surmounted by deeper weakly stable layers. On the other hand, RK-gridded temperature maps show cross-valley thermal asymmetries, amenable to the complex topography and to the inhomogeneous surface coverage, as well as to a curvature of the valley axis. Finally, in the area where the upper Lakes Valley joins the Adige Valley, specific features associated with the complex interaction between the Ora del Garda and the local up-valley wind are found.

Sulfide geochronology along the Endeavour Segment of the Juan de Fuca Ridge

Forty‐nine hydrothermal sulfide‐sulfate rock samples from the Endeavour Segment of the Juan de Fuca Ridge, northeastern Pacific Ocean, were dated by measuring the decay of 226Ra (half‐life of 1600 years) in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years. This dating method is effective for samples ranging in age from ∼200 to 20,000 years old and effectively bridges an age gap between shorter‐ and longer‐lived U‐series dating techniques for hydrothermal deposits. Results show that hydrothermal venting at the active High Rise, Sasquatch, and Main Endeavour fields began at least 850, 1450, and 2300 years ago, respectively. Barite ages of other inactive deposits on the axial valley floor are between ∼1200 and ∼2200 years old, indicating past widespread hydrothermal venting outside of the currently active vent fields. Samples from the half‐graben on the eastern slope of the axial valley range in age from ∼1700 to ∼2925 years, and a single sample from outside the axial valley, near the westernmost valley fault scarp is ∼5850 ± 205 years old. The spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship, with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift. These relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years.

Algunos criterios topográficos para identificar el origen antrópico de cárcavas

In semi-arid or sub-humid environments, there are isolate gullies or badland areas that could be caused by human activity. In some cases, particularly in cultivated areas, it is possible to identify artificial disturbances of the drainage network whose incision is the responsible for gully development. A detailed study of the gullies and the basic layout of the drainage can show anomalies that are incompatible with a natural evolution. The anomalous layout of a gully that does not follow the axis of a small valley, the existence of gullies that cross obliquely a hillslope, or a sudden change in the path of the gully use to be the most frequent criteria. The identification of one of several anomalies can allow us to deduce the anthropic origin of badland networks originally attributed to a natural origin.

Waning magmatic activity along the Southern Explorer Ridge revealed through fault restoration of rift topography

We combine high‐resolution bathymetry acquired using the Autonomous Underwater Vehicle ABE with digital seafloor imagery collected using the remotely operated vehicle ROPOS across the axial valley of the Southern Explorer Ridge (SER) to infer the recent volcanic and tectonic processes. The SER is an intermediate spreading ridge located in the northeast Pacific. It hosts the Magic Mountain hydrothermal vent. We reconstruct the unfaulted seafloor terrain at SER based on calculations of the vertical displacement field and fault parameters. The vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief‐forming processes over the last 11,000–14,000 years. The restored topography indicates that the axial morphology evolved from a smooth constructional dome >500 m in diameter, to a fault‐bounded graben, ~500 m wide and 30–70 m deep. This evolution has been accompanied by changes in eruptive rate, with deposition of voluminous lobate and sheet flows when the SER had a domed morphology, and limited‐extent low‐effusion rate pillow eruptions during graben development. Most of the faults shaping the present axial valley postdate the construction of the dome. Our study supports a model of cyclic volcanism at the SER with periods of effusive eruptions flooding the axial rift, centered on the broad plateau at the summit of the ridge, followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes, with minor low‐effusion rate eruptions confined to the axial graben. The asymmetric shape of the axial graben supports an increasing role of extensional processes, with a component of simple shear in the spreading processes.

New approach to hydrogeological modelling and source data collecting in a small mountainous hard rock basin – experiences from the Sudetes Mts. (SW Poland)

We demonstrate a new research methodology into flow paths and groundwater resources with in small hard rock basins , where little hydrogeological data is available, a picture may be obtained by modeling. Data has been collected in the Złoty Potok river catchment (area 4.4 sq km), located in the eastern Sudety Mts. (SW Poland). The study area, as for most small hard rock basins in the Sudety Mts. is characterized by: i) steeply sloping terrain, ii) a complex flow system connected to several media, ii) poorly constrained hydrogeological parameters. In such conditions, groundwater models are difficult to implement. To overcome these difficulties, we applied a concept of mixed flow ruled by laws of Darcy (porous media) and Hagen-Poiseuille (fractured media), and the concept of three water-bearing zones, a classic solution for scales of above several sq km, combined with a discrete fracture model. Field data applied for modeling were collected over one year, measuring all manifestations of groundwater occurrence: i) effective infiltration (lysimeter), ii) fracture mapping, iii) stream flow and flow disappearance. As a result of modelling, specific flow domains were identified; which form a system of zones, characterized by different geometries and flow velocities. A new, previously unrecognized zone of considerable importance for water extraction, i.e. fractures in the river valley axis, reaching a deep part of the orogen (up to 300 m b.g.l.) was defined. The relationships investigated allowed prepation of a prognosis for deep groundwater intake locations in poorly described mountainous areas. The success of the solutions obtained in this typical mountainous river basin suggests that this method may be come efficiently and widely used in other hard rock areas. The research undertaken offers an innovative, efficient approach to groundwater resource assessment in hard rock.

High‐resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid‐Atlantic Ridge axial valley

AbstractAxial valleys are found along most slow‐spreading mid‐ocean ridges and are one of the most prominent topographic features on Earth. In this paper, we present the first deep‐tow swath bathymetry for the axial valley walls of the Mid‐Atlantic Ridge. These data allow us to analyze axial valley wall morphology with a very high resolution (0.5 to 1 m compared to ≥ 50 m for shipboard multibeam bathymetry), revealing the role played by landslides. Slow‐spreading ridge axial valleys also commonly expose mantle‐derived serpentinized peridotites in the footwalls of large offset normal faults (detachments). In our map of the Ashadze area (lat. 13°N), ultramafic outcrops have an average slope of 18° and behave as sliding deformable rock masses, with little fragmentation. By contrast, the basaltic seafloor in the Krasnov area (lat. 16°38′N) has an average slope of 32° and the erosion of the steep basaltic rock faces leads to extensive fragmentation, forming debris with morphologies consistent with noncohesive granular flow. Comparison with laboratory experiments suggests that the repose angle for this basaltic debris is > 25°. We discuss the interplay between the normal faults that bound the axial valley and the observed mass wasting processes. We propose that, along axial valley walls where serpentinized peridotites are exposed by detachment faults, mass wasting results in average slopes ≤ 20°, even in places where the emergence angle of the detachment is larger.

Active positioning of vent larvae at a mid-ocean ridge

The vertical position of larvae of vent species above a mid-ocean ridge potentially has a strong effect on their dispersal. Larvae may be advected upward in the buoyant vent plume, or move as a consequence of their buoyancy or by active swimming. Alternatively, they may be retained near the bottom by the topography of the axial trough, or by downward swimming. At vents near 9°50′N on the axis of the East Pacific Rise, evidence for active larval positioning was detected in a comparison between field observations of larvae in the plankton in 2006 and 2007 and distributions of non-swimming larvae in a two-dimensional bio-physical model. In the field, few vent larvae were collected at the level of the neutrally buoyant plume (~75m above the bottom); their relative abundances at that height were much lower than those of simulated larvae from a near-bottom release in the model. This discrepancy was observed for many vent species, particularly gastropods, suggesting that they may actively remain near the bottom by sinking or swimming downward. Near the seafloor, larval abundance decreased from the ridge axis to 1000m off axis much more strongly in the observations than in the simulations, again pointing to behavior as a potential regulator of larval transport. We suspect that transport off axis was reduced by downward-moving behavior, which positioned larvae into locations where they were isolated from cross-ridge currents by seafloor topography, such as the walls of the axial valley—which are not resolved in the model. Cross-ridge gradients in larval abundance varied between gastropods and polychaetes, indicating that behavior may vary between taxonomic groups, and possibly between species. These results suggest that behaviorally mediated retention of vent larvae may be common, even for species that have a long planktonic larval duration and are capable of long-distance dispersal.

Influence of basin connectivity on sediment source, transport, and storage within the Mkabela Basin, South Africa

Abstract. The management of sediment and other non-point source (NPS) pollution has proven difficult, and requires a sound understanding of particle movement through the drainage system. The primary objective of this investigation was to obtain an understanding of NPS sediment source(s), transport, and storage within the Mkabela Basin, a representative agricultural catchment within the KwaZulu–Natal Midlands of eastern South Africa, by combining geomorphic, hydrologic and geochemical fingerprinting analyses. The Mkabela Basin can be subdivided into three distinct subcatchments that differ in their ability to transport and store sediment along the axial valley. Headwater (upper catchment) areas are characterized by extensive wetlands that act as significant sediment sinks. Mid-catchment areas, characterized by higher relief and valley gradients, exhibit few wetlands, but rather are dominated by a combination of alluvial and bedrock channels that are conducive to sediment transport. The lower catchment exhibits a low-gradient alluvial channel that is boarded by extensive riparian wetlands that accumulate large quantities of sediment (and NPS pollutants). Fingerprinting studies suggest that silt- and clay-rich layers found within wetland and reservoir deposits of the upper and upper-mid subcatchments are derived from the erosion of fine-grained, valley bottom soils frequently utilized as vegetable fields. Coarser-grained deposits within these wetlands and reservoirs result from the erosion of sandier hillslope soils extensively utilized for sugar cane, during relatively high magnitude runoff events that are capable of transporting sand-sized sediment off the slopes. Thus, the source of sediment to the axial valley varies as a function of sediment size and runoff magnitude. Sediment export from upper to lower catchment areas was limited until the early 1990s, in part because the upper catchment wetlands were hydrologically disconnected from lower parts of the watershed during low to moderate flood events. The construction of a drainage ditch through a previously unchanneled wetland altered the hydrologic connectivity of the catchment, allowing sediment to be transported from the headwaters to the lower basin where much of it was deposited within riparian wetlands. The axial drainage system is now geomorphically and hydrologically connected during events capable of overflowing dams located throughout the study basin. The study indicates that increased valley connectivity partly negated the positive benefits of controlling sediment/nutrient exports from the catchment by means of upland based, best management practices.

Megafaunal distribution and assessment of total methane and sulfide consumption by mussel beds at Menez Gwen hydrothermal vent, based on geo-referenced photomosaics

The Menez Gwen hydrothermal vents, located on the flanks of a small young volcanic structure in the axial valley of the Menez Gwen seamount, are the shallowest known vent systems on the Mid-Atlantic Ridge that host chemosynthetic communities. Although visited several times by research cruises, very few images have been published of the active sites, and their spatial dimensions and morphologies remain difficult to comprehend. We visited the vents on the eastern flank of the small Menez Gwen volcano during cruises with RV Poseidon (POS402, 2010) and RV Meteor (M82/3, 2010), and used new bathymetry and imagery data to provide first detailed information on the extents, surface morphologies, spatial patterns of the hydrothermal discharge and the distribution of dominant megafauna of five active sites. The investigated sites were mostly covered by soft sediments and abundant white precipitates, and bordered by basaltic pillows. The hydrothermally-influenced areas of the sites ranged from 59 to 200m2. Geo-referenced photomosaics and video data revealed that the symbiotic mussel Bathymodiolus azoricus was the dominant species and present at all sites. Using literature data on average body sizes and biomasses of Menez Gwen B. azoricus, we estimated that the B. azoricus populations inhabiting the eastern flank sites of the small volcano range between 28,640 and 50,120 individuals with a total biomass of 50 to 380kg wet weight. Based on modeled rates of chemical consumption by the symbionts, the annual methane and sulfide consumption by B. azoricus could reach 1760molCH4yr−1 and 11,060molH2Syr−1. We propose that the chemical consumption by B. azoricus over at the Menez Gwen sites is low compared to the natural release of methane and sulfide via venting fluids.

Geological context and vents morphology of the ultramafic‐hosted Ashadze hydrothermal areas (Mid‐Atlantic Ridge 13°N)

Recent ROV dives and high‐resolution bathymetric data acquired over the Ashadze fields on the Mid‐Atlantic Ridge (13°N) allow us to derive constraints on the regional and local geological setting of ultramafic‐hosted hydrothermal fields. The active vent fields of Ashadze hydrothermal fields are located in the western axial valley wall, downslope from the termination of a prominent corrugated surface and in a transitional domain with respect to ridge segmentation. The study of the shipboard and ROV bathymetry shows that decameter (100 m by 60 m) to kilometer‐scaled rockslides shape the axial valley wall slopes in this region. The Ashadze 1 vent field occurs on a coherent granular landslide rock mass that is elongated in an E‐W direction. The Ashadze 1 vent field comprises hundreds of active and inactive sulfide chimneys. The Ashadze 2 vent field is located in a NNE‐trending linear depression which separates outcrops of gabbros and serpentinized peridotites. Active black smokers in the Ashadze 2 field are located on ultramafic substratum in a 40‐m diameter crater, 5‐m deep. This crater recalls similar structures described at some vents of the Logatchev hydrothermal field (Mid‐Atlantic Ridge 15°N). We discuss the mode of formation for these craters, as well as that for a breadcrust‐like array of radial fissures identified at Ashadze 1. We propose that hydrothermalism at Ashadze can be an explosive phenomena associated with geyser‐like explosions. Our study also constrains the geological and geophysical context of the ultramafic‐hosted Ashadze hydrothermal system that may use the oceanic detachment fault as a preferred permeability conduit.

Effusive and explosive volcanism on the ultraslow‐spreading Gakkel Ridge, 85°E

We use high‐definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 Arctic Gakkel Vents Expedition (AGAVE) to evaluate the volcanic characteristics of the 85°E segment of the ultraslow spreading Gakkel Ridge (9 mm yr−1full rate). Our seafloor imagery reveals that the axial valley is covered by numerous, small‐volume (order ∼1000 m3) lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm. The valley floor contains two prominent volcanic lineaments made up of axis‐parallel ridges and small, cratered volcanic cones. The lava flows appear to have erupted from a number of distinct source vents within the ∼12–15 km‐wide axial valley. Only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site, and these are associated with the Oden and Loke volcanic cones. We model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of CO2 that accumulated in (possibly deep) crustal melt reservoirs. The energy released during explosive discharge, combined with the buoyant rise of hot fluid, lofted fragmented clasts of rapidly cooling magma into the water column, and they subsequently settled onto the seafloor as fall deposits surrounding the source vent.

Seismicity of the Atlantis Massif detachment fault, 30°N at the Mid‐Atlantic Ridge

At the oceanic core complex that forms the Atlantis Massif at 30°N on the Mid‐Atlantic Ridge, slip along the detachment fault for the last 1.5–2 Ma has brought lower crust and mantle rocks to the seafloor. Hydroacoustic data collected between 1999 and 2003 suggest that seismicity occurred near the top of the Massif, mostly on the southeastern section, while detected seismicity along the adjacent ridge axis was sparse. In 2005, five short‐period ocean bottom seismographs (OBS) were deployed on and around the Massif as a pilot experiment to help constrain the distribution of seismicity in this region. Analysis of six months of OBS data indicates that, in contrast to the results of the earlier hydroacoustic study, the vast majority of the seismicity is located within the axial valley. During the OBS deployment, and within the array, seismicity was primarily composed of a relatively constant background rate and two large aftershock sequences that included 5 teleseismic events with magnitudes between 4.0 and 4.5. The aftershock sequences were located on the western side of the axial valley adjacent to the Atlantis Massif and close to the ridge‐transform intersection. They follow Omori's law, and constitute more than half of the detected earthquakes. The OBS data also indicate a low but persistent level of seismicity associated with active faulting within the Atlantis Massif in the same region as the hydroacoustically detected seismicity. Within the Massif, the data indicate a north‐south striking normal fault, and a left‐lateral, strike‐slip fault near a prominent, transform‐parallel, north‐facing scarp. Both features could be explained by changes in the stress field at the inside corner associated with weak coupling on the Atlantis transform. Alternatively, the normal faulting within the Massif might indicate deformation of the detachment surface as it rolls over to near horizontal from an initial dip of about 60° beneath the axis, and the strike‐slip events may indicate transform‐parallel movement on adjacent detachment surfaces.

The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex

Oceanic core complexes (OCCs) are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults. We combine previously reported U‐Pb zircon crystallization ages with (U‐Th)/He zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the Atlantis Massif OCC (30°N, MAR) and help establish cooling rates, as well as depths of detachment faulting and gabbro emplacement. We present nine new (U‐Th)/He zircon ages for samples from IODP Hole U1309D ranging from 40 to 1415 m below seafloor. These data paired with U‐Pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at Atlantis Massif as 2895 (+1276/−1162) °C Myr−1 (from ∼780°C to ∼250°C); the lower 600 m of the borehole cooled more slowly at mean rates of ∼500 (+125/−102) °C Myr−1(from ∼780°C to present‐day temperatures). Rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of ∼300°C Myr−1, possibly due to hydrothermal circulation to ∼4 km depth through the detachment fault zone. Assuming a fault slip rate of 20 mm/yr (from U‐Pb zircon ages of surface samples) and a rolling hinge model for the sub‐surface fault geometry, we predict that the 780°C isotherm lies at ∼7 km below the axial valley floor, likely corresponding both to the depth at which the semi‐brittle detachment fault roots and the probable upper limit of significant gabbro emplacement.

Combining HVSR microtremor observations with the SPAC method for site resonance study of the Tamar Valley in Launceston (Tasmania, Australia)

SUMMARY The presence of the deep and narrow Tamar Valley in the City of Launceston (Tasmania, Australia), in-filled with soft sediments above hard dolerite bedrock, induces a complex pattern of resonance across the city. Horizontal to vertical spectrum ratio (HVSR) microtremor observations are combined with 1-D shear wave velocity (SWV) profiles evaluated from spatially averaged coherency spectra (SPAC) observations of the vertical component of the microtremor wavefield to complete a site resonance study in a valley environment such as the Tamar Valley. Using the methodology developed in a previous paper, 1-D SWV profiles are interpreted from observed coherency spectra (axial-COH) above the deepest point of the Tamar Valley, using pairs of sensors spatially separated parallel to the valley axis. The 1-D SWV profiles interpreted at five sites suggest the depth to bedrock interface varies from approximately z= 25 m north of the city, to z= 250 m above the deepest point of the valley. Numerical simulations of the propagation of surface waves in a 2-D model representation of the Tamar Valley compare well with HVSR observations recorded on two profiles transverse to the valley axis. HVSR observations can identify the in-plane shear (SV) frequency of resonance above the deepest part of the valley on two separate profiles transverse to the valley axis. By computing the ellipticity curves from the preferred SWV profiles interpreted by the SPAC method, the antiplane shear (SH) modes of resonance expected to develop in the Tamar Valley are identified; modes which HVSR observations alone fail to locate with precision. HVSR observations suggest a complex mix of 1-D and 2-D patterns of resonance develops across the valley. The results from this paper suggest that HVSR microtremor observations can be combined with SPAC microtremor method to characterize the geology and the pattern of resonance in a 2-D narrow structure such as the Tamar Valley.

Deformation associated with the denudation of mantle‐derived rocks at the Mid‐Atlantic Ridge 13°–15°N: The role of magmatic injections and hydrothermal alteration

Outcrops of deeply derived ultramafic rocks and gabbros are widespread along slow spreading ridges where they are exposed in the footwall of detachment faults. We report on the microstructural and petrological characteristics of a large number of samples from ultramafic exposures in the walls of the Mid‐Atlantic Ridge (MAR) axial valley at three distinct locations at lat. 13°N and 14°45′N. One of these locations corresponds to the footwall beneath a corrugated paleo‐fault surface. Bearing in mind that dredging and ROV sampling may not preserve the most fragile lithologies (fault gouges), this study allows us to document a sequence of deformation, and the magmatic and hydrothermal history recorded in the footwall within a few hundred meters of the axial detachment fault. At the three sampled locations, we find that tremolitic amphiboles have localized deformation in the ultramafic rocks prior to the onset of serpentinization. We interpret these tremolites as hydrothermal alteration products after evolved gabbroic rocks intruded into the peridotites. We also document two types of brittle deformation in the ultramafic rocks, which we infer could produce the sustained low magnitude seismicity recorded at ridge axis detachment faults. The first type of brittle deformation affects fresh peridotite and is associated with the injection of the evolved gabbroic melts, and the second type affects serpentinized peridotites and is associated with the injection of Si‐rich hydrothermal fluids that promote talc crystallization, leading to strain localization in thin talc shear zones. We also observed chlorite + serpentine shear zones but did not identify samples with serpentine‐only shear zones. Although the proportion of magmatic injections in the ultramafic rocks is variable, these characteristics are found at each investigated location and are therefore proposed as fundamental components of the deformation in the footwall of the detachment faults associated with denudation of mantle‐derived rocks at the MAR.

Airborne Doppler Lidar Measurements of Valley Flows in Complex Coastal Terrain

AbstractThree-dimensional winds obtained with an airborne Doppler lidar are used to investigate the spatial structure of topographically driven flows in complex coastal terrain in Southern California. The airborne Doppler lidar collected four hours of data between the surface and 3000 m MSL along a 40-km segment of the Salinas Valley during the afternoon of 12 November 2007. The airborne lidar measurements, obtained at horizontal and vertical resolutions of approximately 1500 and 50 m, respectively, reveal a detailed spatial structure of the atmospheric flows within the valley and their associated aerosol features. Clear skies prevailed on the flight day with northwesterly synoptic flows around 10 m s−1. The data document a shallow sea breeze making a transition into an upvalley flow in the Salinas Valley that accelerates in the upvalley direction. Along with the acceleration of the upvalley wind, the lidar data indicate the presence of enhanced sinking motions. No return flows associated with the sea-breeze or upvalley flows are observed. While synoptic flows are aligned along the valley axis in the upvalley direction, lidar data indicate the presence of a northerly cross-valley flow around the height of the surrounding ridges. This flow intrudes into the valley atmosphere and induces, along with thermally driven slope flows on the sunlit valley sidewall, a cross-valley circulation that causes an asymmetric distribution of the aerosols. This study demonstrates the large potential of airborne Doppler lidar data in describing flows in complex terrain.

Detrital zircon U-Pb ages of a late-Variscan Carboniferous succession associated with the Porto-Tomar shear zone (West Portugal): Provenance implications

Laser ablation ICP-MS U-Pb zircon ages are used to investigate the sediment provenance of a late-Variscan Carboniferous succession from Buçaco Basin. The Buçaco Basin is associated with the Porto-Tomar Shear Zone (PTSZ), which is a major transcurrent Variscan structure and tectonic boundary within Iberia. The clastic succession is interpreted to be deposited within a pull-apart basin, where alluvial fans radiating from surrounding range fronts grade into an axial valley. Coarse-grained sediments attributed to short streams that drained the eastern basin margin exhibit mostly Pan-African ages. In contrast, the sandy sediments associated with longer fluvial systems tend to have higher proportions of Grenvillian and Paleozoic ages. Limited amounts of Variscan zircons were found in upper beds. These results suggest that the intermontaine Buçaco Basin had a catchment area that increased progressively eventually exceeding the range front. It is proposed that rivers striking parallel to the PTSZ played a major role in supplying Grenvillian and Paleozoic ages while lateral supply sourced mainly Pan-African ages. Given the prevalence of short sediment supply, the proportion of Variscan grains is always limited. The higher proportion of Mesoproterozoic ages in some Carboniferous beds than has been presented for central west Iberia indicates that nearby basement units enriched in Grenviilian zircons were exhumed during the Variscan orogeny.