Northwest Trending Research Articles

A new high-resolution aeromagnetic dataset over central Ayrshire: insights into the concealed geology

Synopsis High-resolution aeromagnetic data have been acquired over central Ayrshire as part of a multi-component environmental and resource survey. The data were recorded along easterly orientated flight lines at 200 m separation and show a very marked improvement in resolution over the pre-existing aeromagnetic data for the region. The distribution of the extensive volcanic and intrusive igneous rocks in the near sub-surface is defined more accurately, many new features are recognized and features distinguished in the pre-existing aeromagnetic data are now seen to be laterally continuous. Northwesterly trending lineaments are clearly defined and most can be attributed to Palaeogene dykes, the majority of which are reversely magnetized. Depth to source solutions and modelling suggest that even though the dykes have a limited width at outcrop, their widths increase with depth. Permo-Carboniferous easterly trending dykes are not well resolved in the new aeromagnetic data. This is most likely due to a low amplitude resultant magnetic vector that, combined with the easterly-orientated flight lines, results in few large anomalies. One significant northwesterly trending lineament is interpreted as the trace of the Cleveland Dyke across the Midland Valley. These new data should be of value to the extractive and water industries and assist in the planning and sustainable management of these resources.

Compositions of dikes and lavas from the Pito Deep Rift: Implications for crustal accretion at superfast spreading centers

The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ∼3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ∼8 km2 study areas. Most of the PDR samples are incompatible‐element‐depleted normal mid‐ocean ridge basalts (NMORB; (La/Sm)N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/204Pb, suggesting that an enriched component of the mantle was present in this region in the past ∼3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of ∼1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along‐axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers.

Estimating permeability from quasi-static deformation: Temporal variations and arrival-time inversion

Transient pressure variations within a reservoir can be treated as a propagating front and analyzed using an asymptotic formulation. From this perspective, one can define a pressure “arrival time” and formulate solutions along trajectories, in the manner of ray theory. We combine this methodology and a technique for mapping overburden deformation into reservoir volume change as a means to estimate reservoir flow properties, such as permeability. Given the entire “travel time” or phase field obtained from the deformation data, we can construct the trajectories directly, thereby linearizing the inverse problem. A numerical study indicates that, using this approach, we can infer large-scale variations in flow properties. In an application to Interferometric Synthetic Aperture Radar (InSAR) observations associated with a [Formula: see text] injection at the Krechba field, Algeria, we image pressure propagation to the northwest. An inversion for flow properties indicates a linear trend of high permeability. The high permeability correlates with a northwest trending fault on the flank of the anticline that defines the field.

Finding concealed active faults: Extending the southern Whidbey Island fault across the Puget Lowland, Washington

The southern Whidbey Island fault zone (SWIF), as previously mapped using borehole data, potential field anomalies, and marine seismic reflection surveys, consists of three subparallel, northwest trending strands extending ∼100 km from near Vancouver Island to the northern Puget Lowland. East of Puget Sound, the SWIF makes landfall between the cities of Seattle and Everett but is concealed beneath a thick mantle of young glacial deposits and vegetation. A ∼20‐km‐wide, northwest trending swath of subparallel, low‐amplitude aeromagnetic anomalies crosses this region of the Puget Lowland and is on strike with the SWIF. The most prominent aeromagnetic anomaly, the Cottage Lake lineament, extends at least 18 km and lies approximately on strike with the SWIF on Whidbey Island. Subtle scarps and topographic lineaments on Pleistocene surfaces, visible on high‐resolution lidar topography at a number of locations along the SWIF, lie on or near these magnetic anomalies. In the field, scarps exhibit northeast‐side‐up and vertical relief of 1 to 5 m. Excavations across several lidar scarps lying on or near magnetic anomalies show evidence for multiple folding and faulting events since deglaciation, most likely above buried reverse/oblique faults. Excavations in areas away from magnetic anomalies do not show evidence of tectonic deformation. In total, paleoseismological evidence suggests that the SWIF produced at least four earthquakes since deglaciation about 16,400 years ago, the most recent less than 2700 years ago.

Sedimentology of the fluvial and fluvio-marine facies of the Bahariya Formation (Early Cenomanian), Bahariya Oasis, Western Desert, Egypt

The Lower Cenomanian Bahariya Formation in the Bahariya Oasis, Western Desert, Egypt, was deposited under two coeval environmental conditions. A fully fluvial system occurs in the southern portion of the Bahariya Oasis, including depositional products of meandering and braided streams, and a coeval fluvio-marine setting is dominant to the north. These deposits are organized into four unconformity-bounded depositional sequences, whose architecture is shaped by a complex system of incised valleys. The fluvial portion of the lower two depositional sequences is dominated by low-energy, meandering systems with a tabular geometry, dominated by overbank facies. The fluvial deposits of the upper two sequences represent the product of sedimentation within braided streams, and consist mainly of amalgamated channel-fills. The braided fluvial systems form the fill of incised valleys whose orientation follows a southeast–northwest trending direction, and which truncate the underlying sequences. Four sedimentary facies have been identified within the braided-channel systems, namely thin-laminated sandstones (Sh), cross-bedded sandstones (Sp, St), massive ferruginous sandstones (Sm) and variegated mudstones (Fm). The exposed off-channel overbank facies of the meandering systems include floodplain (Fm) and crevasse splay (Sl) facies. The fluvio-marine depositional systems consist of interbedded floodplain, coastal and shallow-marine deposits. The floodplain facies include fine-grained sandstones (Sf), laminated siltstones (Stf) and mudstones (Mf) that show fining-upward cycles. The coastal to shallow-marine facies consist primarily of mudstones (Mc) and glauconitic sandstones (Gc) organized vertically in coarsening-upward prograding cyclothems topped by thin crusts of ferricrete (Fc). The four depositional sequences are present across the Bahariya Oasis, albeit with varying degrees of preservation related to post-depositional erosion associated with the formation of sequence boundaries. These unconformities may be overlain by braided-stream channel sandstones at the base of incised valleys, or marked by ferricrete paleosols (lithofacies Fc) in the interfluve areas.

Geophysical modeling of Devonian plutons in the southern Gulf of St. Lawrence: implications for Appalachian terrane boundaries in Maritime Canada

A prominent positive magnetic anomaly spans the 100 km distance between Prince Edward Island and Cape Breton Island in the southern Gulf of St. Lawrence. The anomaly occurs in an area of complex structure where Appalachian terrane boundaries are poorly resolved because of thick late Paleozoic sedimentary cover. Analysis of the magnetic anomaly led to the interpretation that it is produced by four separate, approximately circular, source bodies aligned along the northwesterly trend of the anomaly. Seismic data, physical property measurements, and magnetic and gravity anomalies were used to further investigate the anomaly sources through forward modeling techniques. The four source bodies have densities and magnetic susceptibilities compatible with dioritic to granitic compositions. Modeling also suggests that basement to the north of the plutons has higher density and susceptibility than basement to the south, and hence the source bodies are interpreted as plutons emplaced along the boundary between Ganderian composite terranes to the north and the Ganderian Brookville – Bras d’Or terrane to the south. This interpretation suggests that the Ganderia–Avalonia boundary is located farther south, and shows the need for re-evaluation of the location and role of the Canso fault in offsetting terranes between Cape Breton Island and southern New Brunswick.

Geology and petroleum potential of the Fairway Basin in the Tasman Sea

The Fairway Basin is a large, generally north – south-trending, sediment-filled structure in water 1500 – 3000 m deep, on the eastern slope of the Lord Howe Rise in the Tasman Sea, and is partly within Australian jurisdiction. It was poorly known until a few years ago, when seismic profiling and piston coring cruises were carried out. The basin, about 1100 km long and 120 – 200 km wide, can be divided into three segments—north, central and south—that trend northwest, north and north-northwest, respectively. All three segments probably formed by thinning of continental crust during breakup of Lord Howe Rise and surrounding aseismic continental ridges in the Late Cretaceous and Paleocene. Normal faulting, large inputs of terrigenous sediment and subsidence to bathyal marine depths occurred during that time. A period of compression, perhaps related to overthrusting on New Caledonia, occurred in the Eocene, leading to uplift (and in parts, erosion) of northern Lord Howe Rise, and reversal of faulting in the basin. By the Oligocene, the area was again in bathyal depths, and pelagic ooze and some turbidites accumulated. The basinal sequence is generally 2000 – 4000 m thick, with 1200 – 3200 m of Cretaceous to Eocene sediment concentrated in depocentres, capped by 500 – 800 m of Oligocene and younger sediment. In the depocentres, numerous sedimentary diapirs pierce sedimentary sequences. The sedimentary diapirs appear to be fed by Cretaceous muds deposited during rifting. Often, these diapirs are overlain by faults extending to the seafloor, and hummocky bathymetry is possibly caused by fluid escape. The overall geology suggests that the Fairway Basin may be a large frontier hydrocarbon province. Seismic profiles display a bottom-simulating reflector above many depocentres, 500 – 700 m below the seafloor. The bottom-simulating reflector has positive polarity, which could result from a diagenetic phase transformation, a thin gas hydrate layer with a sharp top, or from the sharp base of a gas layer (probably beneath gas hydrates). Standard piston cores taken above diapirs and apparent fluid-escape features have recovered little gas. Other than drilling, the next steps in assessing petroleum potential are to clearly document fluid-escape structures, and to sample any fluids emitted for hydrocarbons.

Magnetic fabrics and microstructures of the Jurassic Shah-Kuh granite pluton (Lut Block, Eastern Iran) and geodynamic inference

The Jurassic Shah-Kuh granite pluton was emplaced in the northeastern part of the Lut Block (Eastern Iran) while this block was a part of the active margin under which the Tethys ocean, that separated Arabia from Central Iran, was subducting. Since this time, the Lut Block has rotated and migrated northward up to its present position. This structural study of the Shah-Kuh aims at strengthening the evidence of its original geodynamical location. Thanks to a systematic collection of oriented samples in the field, well-defined fabric and microstructural patterns were deduced from magnetic fabric measurements and optical microscopy observations. The overall magnetic fabric of the pluton yields vertical, north–south striking foliations and shallow lineations plunging to the north. The corresponding microstructures attest to their formation in the magmatic stage, i.e. during emplacement. Subsequent deformation, characterized by (sub)mylonitic microstructures, has modified the original fabric into a northwest and locally a west–northwest trend inside a two kilometre-wide corridor. This corridor reflects the trace of a sinistral shear zone that offsets the previous north–south magmatic pattern of the pluton. The latter magmatic pattern is proposed to result from the shear component of strain that was parallel to the ancient active margin during pluton emplacement, as a result of strain partitioning, a situation frequently documented at convergent margins. Sinistral shear along an ∼ E–W directed south-facing active margin, assuming a northeastward slip vector for the Tethys ocean during the Jurassic, and accepting a ∼ 150° counterclockwise rotation of the Lut Block after emplacement of the Shah-Kuh pluton, best explains both the magmatic and the solid-state lineation patterns.

Lithofacies analysis of the Dezadeash Formation (Jura–Cretaceous), Yukon, Canada: The depositional architecture of a mud/sand-rich turbidite system

The Dezadeash Formation is a northwest trending belt of predominantly Jura–Cretaceous deep-marine siliciclastic rocks in southwestern Yukon, Canada. It is truncated to the southwest by the Denali Fault System, a major transcurrent fault along which ∼ 370 km of dextral slip occurred in post-Early Cretaceous time, and it is flanked to the northeast by the Kluane Schist, a mainly mica–quartz schist assemblage of unknown age and origin. This study provides a robust quantitative description of lithofacies comprising a point-source mud/sand-rich turbidite system: approximately 16 335 m of measured strata from 75 sections throughout the Dezadeash Formation indicate that up to 29 lithofacies types are present and these are dominated by thin- to thick-bedded sandstone–mudstone couplets and medium- to thick-bedded sandstone. The various lithofacies are grouped into conglomerate, sandstone, sandstone–mudstone, and siltstone–mudstone lithofacies associations, which are interpreted as slide, channel-lobe transition, lobe, and overbank architectural elements, respectively. Stacked channel-lobe transition and lobe elements predominate, but the depositional architecture displays no apparent, overall vertical trend. The Dezadeash Formation represents mainly the lower middle and lower subdivisions of a point-source, mud/sand-rich turbidite system that records a single stage of fan growth as it prograded transversely from a volcano-plutonic arc into the basin.

Biochronology and biostratigraphy of the Uquía Formation (Pliocene–early Pleistocene, NW Argentina) and its significance in the Great American Biotic Interchange

The Uquía Formation crops out in the Quebrada de Humahuaca in Jujuy province, Eastern Cordillera, NW Argentina. This unit is composed of a sequence of fluviatile sediments and water-laid air-fall tuff beds; it is approximately 260 m thick and unconformably overlain by Pleistocene conglomerates and Quaternary alluvium. The sediments have been folded into a syncline and broken by several faults that generally trend northwest–southeast. Following Castellanos stratigraphy, we characterize three units (Lower, Middle, and Upper) of the Uquía Formation. Biochronologically, the Lower Unit is assigned to the late Chapadmalalan, the Middle Unit (“Uquian fauna”) to the late Vorohuean and Sanandresian, and the Upper Unit to the Ensenadan. Biostratigraphic evidence provides a calibration of important biochronologic events in the Great American Biotic Interchange (GABI), namely, the first appearances of Erethizon, Hippidion, and proboscideans at 2.5 Ma (late Pliocene) in South America. Geological and paleobiological evidence suggest that during the late Pliocene, the area could have been a wide intermountain valley at 1400–1700 m elevation, with a more humid environment than that of the present day and some wet–dry seasonality that permitted the coexistence of forest and open areas. Uquian mammals also indicate that northwestern Argentina and the Pampean region have represented distinct biogeographical areas since at least the late Pliocene.

Gravity signals from the lithosphere in the Central European Basin System

We study the gravity signals from different depth levels in the lithosphere of the Central European Basin System (CEBS). The major elements of the CEBS are the Northern and Southern Permian Basins which include the Norwegian–Danish Basin (NDB), the North-German Basin (NGB) and the Polish Trough (PT). An up to 10 km thick sedimentary cover of Mesozoic–Cenozoic sediments, hides the gravity signal from below the basin and masks the heterogeneous structure of the consolidated crust, which is assumed to be composed of domains that were accreted during the Paleozoic amalgamation of Europe. We performed a three-dimensional (3D) gravity backstripping to investigate the structure of the lithosphere below the CEBS. Residual anomalies are derived by removing the effect of sediments down to the base of Permian from the observed field. In order to correct for the influence of large salt structures, lateral density variations are incorporated. These sediment-free anomalies are interpreted to reflect Moho relief and density heterogeneities in the crystalline crust and uppermost mantle. The gravity effect of the Moho relief compensates to a large extent the effect of the sediments in the CEBS and in the North Sea. Removal of the effects of large-scale crustal inhomogeneities shows a clear expression of the Variscan arc system at the southern part of the study area and the old crust of Baltica further north–east. The remaining residual anomalies (after stripping off the effects of sediments, Moho topography and large-scale crustal heterogeneities) reveal long wavelength anomalies, which are caused mainly by density variations in the upper mantle, though gravity influence from the lower crust cannot be ruled out. They indicate that the three main subbasins of the CEBS originated on different lithospheric domains. The PT originated on a thick, strong and dense lithosphere of the Baltica type. The NDB was formed on a weakened Baltica low-density lithosphere formed during the Sveco-Norwegian orogeny. The major part of the NGB is characterized by high-density lithosphere, which includes a high-velocity lower crust (relict of Baltica passive margin) overthrusted by the Avalonian terrane. The short wavelength pattern of the final residuals shows several north–west trending gravity highs between the Tornquist Zone and the Elbe Fault System. The NDB is separated by a gravity low at the Ringkøbing–Fyn high from a chain of positive anomalies in the NGB and the PT. In the NGB these anomalies correspond to the Prignitz (Rheinsberg anomaly), the Glueckstadt and Horn Graben, and they continue further west into the Central Graben, to join with the gravity high of the Central North Sea.

Sedimentology and paleontology of a Carboniferous log jam

A localized sandstone split in the Mary Lee coal (Early Pennsylvanian, Langsettian) of the Black Warrior Basin, Alabama, has a channel-form geometry and preserves a concentrated log-and-gravel (pebbles, cobbles, and boulders) assemblage at the top of the fill sequence. Gravel lithotypes within and amongst rooting structures of lycopsid, cordaitean, and calamitean trees are indicative of an Appalachian orogenic provenance, and support an allochthonous origin for some of the logs. In addition, a Skolithos ichnological assemblage within the channel is indicative of opportunist colonization during channel fill. A low sinuosity geometry characterizes the overall channel-form belt, exhibiting a general northwesterly trend. Paleocurrent measurements from the cross-bedded sandstone at the margin of the channel belt indicate flow was to the northwest. Log orientations at these sites are subperpendicular to perpendicular to the overall sediment transport direction and are interpreted to represent an ancient log jam. The genesis of the fluvial channel, the introduction and emplacement of the concentrated log assemblage, the effects of this floating log accumulation on sedimentation within the channel, and the relationship between paleocurrent and log orientation are discussed. The Red River, located in the southern United States, is used as a modern analog for comparative purposes.

Fault Orientation in the Eastern Tennessee Seismic Zone: A Study Using the Double-Difference Earthquake Location Algorithm

We use the double-difference earthquake location algorithm to relocate approximately 1,000 earthquakes in the eastern Tennessee seismic zone (ETSZ). We examine the earthquake hypocenter relocations in an effort to resolve fault orientations and thereby gain insights into the tectonics of the seismic zone. The analysis involves visual comparison of three-dimensional perspective plots of the hypocenter relocations oriented accord ing to focal mechanism nodal planes derived from events within several clusters of earthquakes. Relocations in the central, most seismically active, portion of the ETSZ indicate a diffuse west-striking, north-dipping zone of hypocenters. The orientation of this zone of hypocen ters is consistent with structure imaged by a previous seismic reflection profile that shows a reflective mid-to-upper crust with apparent dips of approximately 35° to the north. Relocation of a smaller cluster of earthquakes near the Tennessee-North Carolina border suggests a steeply dipping northwest trending fault in that area.

Historical population dynamics and demography of the eastern Atlantic pomacentrid Chromis limbata (Valenciennes, 1833)

Recent studies have focused on the relationship between the marine fauna of the eastern Atlantic and the Mediterranean Sea, but within the Atlantic, little is known about genetic relationships between populations of the Macaronesian islands. In this study, we tested whether the paleo-climatology and paleo-oceanography of the region could predict the genetic relationships among three eastern Atlantic populations (Azores, Madeira, and Canaries) of a damselfish, Chromis limbata, and compared our results with its Mediterranean and adjacent Atlantic sister species, Chromis chromis. We combined phylogeographic and coalescent approaches using the fast evolving mitochondrial control region gene. No population structure was found for the three archipelagos. The coalescence time estimated for C. limbata (0.857–1.17 Mya) was much greater than that estimated for C. chromis. We propose that this difference reflects differences in glaciating extents in the Northeastern Atlantic and the Mediterranean. Diversity indexes (Hd and genetic distances) together with historical demographic parameters of C. limbata ( Θ and g) revealed a more stable population history when compared to C. chromis. Our results suggest that the Macaronesian populations of C. limbata have probably been less affected by the last glaciation than the Mediterranean populations of C. chromis. Migration across the three archipelagos was estimated and a prevailing northwest trend was detected. This result supports the idea of a colonization of the Azores by warm water fish from Madeira or the westernmost Canary islands which acted as major glacial refugia for the tropical and subtropical marine fauna during the glaciations.

Rejuvenation of The Pre-Cambrien Najd Fault System and Its Importance In The Oil Province of Saudi Arabia

Structural interpretation covering the Central Saudi Arabia was carried out through Landsat, Radar images, topographic maps and seismic data. Existing geology maps provide materialized evidence for this study. A strong northwest trending lineament system has been mapped in the region and is interpreted as being rela- ted to the reactivated Najd Fault System. Elongate hills, strait-going creeks, offset along valley and ridges, and pull-apart basins are evident, indicating rejuvenation of the Najd transtensional sinistral movement. Geological map shows evidence indicating a northwestward dragging on the sedimentary cover along a NW trending regio- nal lineament in the region which extend from South Ghawar to An Nafud Basin in Saudi Arabia. Northeast trend- ing structures in Saudi Arabia are related to the pre-Permian structuring and Oman-Masirah stress regime. The Ghawar (Saudi Arabia), Dukhan (Qatar) and other north-northwest trending anticlines were reshaped by the reju- venated Najd stress regime. The northwest trending sinistral Najd Fault was initiated in Precambrian and was reactivated at various times, probably during Paleozoic and late Jurassic times, and continuously, though of vari- able intensity, from late Cretaceous to the late Tertiary time. The Carboniferous, Late Cretaceous and Tertiary events are overprinted on one another in the region. The Oman stress regime and the stress regime that is rela- ted to oblique obduction of the Masirah Ophiolite are the principal controlling event for the anticline structures in Central Arabia. Dominant structural elements of the area are N-S anticline axis that suggests approximate E-W compressional stress direction, which is consistent with Oman and reactivated Najd tectonic regimes. The effect of the Tertiary orogenic overprint of the Oman-Masirah stress regime in eastern Arabia was to favor a slight renewed movement along the old basement faults such as the Najd Trend. Reactivated Najd system, Oman prin- cipal horizontal stress regime and obduction of the Masirah Ophiolite onto the Arabian continent have produced a combined effect on the structures in Eastern Arabian Plate. The Zagros stress regime may have produced Mineral Res. Exp., Bull., 132, 73-82, 2006 little effect in the region.

Geophysical exploration for epithermal gold deposits at Pajingo, North Queensland, Australia

The Pajingo Epithermal System is an area of low-sulphidation epithermal veining and alteration, covering about 150 square km at the northern margin of the Drummond Basin. Tertiary and younger conductive sediments cover about 80% of the area. Gold mineralization occurs in thin quartz veins (generally 0.5–3 m wide), and most of the ore bodies discovered to date are along the north-west trending Vera-Nancy structureThe host intermediate volcanics are magnetic, but the epithermal alteration that extends up to 50 m from the veins along the Vera- Nancy structure is magnetite destructive. Results of a high-resolution magnetic survey clearly delineate the major structures, including the Vera-Nancy structure. The quartz veins occur within broader zones of silicification, and gradient-array resistivity surveying has been successfully used to map these zones. Generally, the high resistivity zones due to silicification are coincident with the structures identified in the magnetics. Gravity and seismic surveys have aided the interpretation of regional structure. These techniques along with stratigraphic data indicate that the Vera-Nancy structure is a major north-west striking extensional fault.High resolution magnetics and resistivity continue to be the most useful geophysical tools in the ongoing exploration for additional mineralised structures within the Pajingo Epithermal System.

Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau

We derive a detailed horizontal velocity field for the southeast borderland of the Tibetan Plateau using GPS data collected from the Crustal Motion Observation Network of China between 1998 and 2004. Our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes, separated by strike‐slip and transtensional faults. Most notably, the regional deformation includes 10–11 mm/yr left slip across the Xianshuihe fault, ∼7 mm/yr left slip across the Anninghe‐Zemuhe‐Xiaojiang fault zone, ∼2 mm/yr right slip across a shear zone trending northwest near the southern segment of the Lancang River fault, and ∼3 mm/yr left slip across the Lijiang fault. Deformation along the southern segment of the Red River fault appears not significant at present time. The region south and west of the Xianshuihe‐Xiaojiang fault system, whose eastward motion is resisted by the stable south China block to the east, turns from eastward to southward motion with respect to south China, resulting in clockwise rotation of its internal subblocks. Active deformation is detected across two previously unknown deformation zones: one is located ∼150 km northwest of and in parallel with the Longmenshan fault with 4–6 mm/yr right‐slip and another is continued south‐southwestward from the Xiaojiang fault abutting the Red River fault with ∼7 mm/yr left slip. While both of these zones are seismically active, the exact locations of faults responsible for such deformation are yet to be mapped by field geology. Comparing our GPS results with predictions of various models proposed for Tibetan Plateau deformation, we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger, highly fragmented upper crust.

Constraints on the Location of the Late Quaternary Reelfoot and New Madrid North Faults in the Northern New Madrid Seismic Zone, Central United States

The New Madrid North and Reelfoot Faults are believed to have ruptured during large-magnitude earthquakes on 23 January and 7 February 1812, respectively, based on the regional distribution of liquefaction, contemporary New Madrid seismic zone microseismicity, and historical accounts of earthquake-related damage. Although the location of the Reelfoot Fault generally is well constrained between the Kentucky Bend of the Mississippi River in Kentucky and the southeastern margin of the Mississippi Valley in Tennessee, the northern extent of the fault remains enigmatic in southeastern Missouri. This is equally true of the New Madrid North Fault, which has a postulated location based primarily on contemporary microseismicity and rupture scenario models of the 1811-1812 earthquake sequence. In this paper, we synthesize existing unpublished and published data with recently acquired subsurface and geomorphic information to clarify the locations of the Reelfoot and New Madrid North Faults in southeastern Missouri. On the basis of this data synthesis, we interpret that the Reelfoot Fault trends northwest across the Kentucky Bend of the Mississippi River as a northwest-facing scarp coincident with Des Cyprie Slough near New Madrid, Missouri, and anomalous elevated topography on southern Sikeston Ridge northwest of New Madrid. Furthermore, Quaternary faulting and folding imaged from seismic-reflection profiles across Des Cyprie Slough coincide with bedrock structural lineaments, a northeast-trending band of contemporary microseismicity, and a distinct northwest-trending post-Tertiary change in alluvial thickness. We trace the Reelfoot Fault as much as several kilometers northwest of the Mississippi River, where it either dies out or steps north-northeast to merge with the New Madrid North Fault. The New Madrid North Fault appears to be expressed geomorphically as left-stepping, en echelon northeast-trending fractures preserved in Pleistocene glacial outwash material comprising Sikeston Ridge. The fractures coincide with Quaternary faults and folds, as well as deeper Cretaceous and Paleozoic faults and flexures, imaged in geophysical profiles. In summary, the surface locations of the Reelfoot and New Madrid North Faults directly west-northwest of New Madrid, Missouri are constrained by geomorphic, geologic, geophysical, and historical seismological data sets and reflect transfer of strain from the northeast-verging Reelfoot reverse fault to the northeast-striking, dextral New Madrid North Fault.

U–Th–Pb geochronologic constraints on the structural evolution of the Selkirk fan, northern Selkirk Mountains, southern Canadian Cordillera

In the southern Canadian Cordillera a zone of structural divergence marks the eastward transition from penetrative ductile deformation and metamorphism in the Omineca belt to the more brittle ‘thin-skinned’ style of deformation typical of the Foreland belt. In the Selkirk Mountains of southern British Columbia, this zone includes a regional-scale structure termed the Selkirk fan. The fan trends northwest, consists primarily of medium- to high-grade metamorphic rocks, and comprises at least three generations of superposed structures. IDTIMS and SHRIMP analyses provide new U–Th–Pb age constraints for the structural evolution of the Selkirk fan. The data demonstrate that the thermo-structural development of the fan's west flank occurred principally in the Middle Jurassic (ca. 172–167 Ma), whereas in the east flank significant Cretaceous (ca. 104–84 Ma) deformation was superimposed on an early transposition fabric. These data require revision of previous models that concluded fan formation occurred primarily during Middle Jurassic time. Rather, the Selkirk fan is a composite structure comprising Middle Jurassic and Cretaceous deformation. Development of the fan during the Early–Middle Jurassic accretion of the Intermontane Superterrane was followed by extensive reworking and tightening of structures in the fan's east flank during the Cretaceous accretion of the Insular Superterrane.

Active Faulting and Seismicity in a Prefractured Terrane: Grand Canyon, Arizona

Structural and seismic data for the eastern Grand Canyon were analyzed to determine the relationship between basement fractures, surface faulting, and seismicity. Published structural data were compared with the results of focal mechanism analysis of tremors occurring in the eastern canyon region during the period 1989–2004. Previous studies of the geology of this area indicate that the fault and joint trends exposed in the surface rocks and through the entire sedimentary section were first established with predominant northeast and northwest trends in Precambrian time. The northwest trends of this prefractured terrane correlate with a dominant northwest trend of nodal planes in the seismic data, suggesting that the present stress system in the region is favoring the reactivation of the preferentially oriented pre-existing northwest-trending fractures.