Km Of Dextral Slip Research Articles

Whole-lithosphere shear during oblique rifting

Abstract Processes controlling the formation of continental whole-lithosphere shear zones are debated, but their existence requires that the lithosphere is mechanically coupled from base to top. We document the formation of a dextral, whole-lithosphere shear zone in the Death Valley region (DVR), southwest United States. Dextral deflections of depth gradients in the lithosphere-asthenosphere boundary and Moho are stacked vertically, defining a 20–50-km-wide, lower lithospheric shear zone with ~60 km of shear. These deflections underlie an upper-crustal fault zone that accrued ~60 km of dextral slip since ca. 8–7 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (~90 km, ca. 13–0 Ma) and total upper-crustal extension (~250 km, ca. 16–0 Ma). We show that, before ca. 8–7 Ma, weak middle crust decoupled upper-crustal deformation from deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma, detachment slip thinned, uplifted, cooled, and thus strengthened the middle crust, which is exposed in metamorphic core complexes collocated with the whole-lithosphere shear zone. Midcrustal strengthening coupled the layered lithosphere vertically and therefore enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre–16 Ma DVR), midcrustal strengthening is probably a necessary condition for whole-lithosphere shear.

Evidence for 40–41 km of dextral slip on the southern Death Valley fault: Implications for the Eastern California shear zone and extensional tectonics

Abstract Recognition of a pair of pre-Neogene markers together with analysis of published data indicate ∼40 km of dextral slip across the southern Death Valley fault zone, California, USA. Stratigraphic overlaps on fault rocks indicate much of the dextral slip predates the late Miocene, placing a significant fraction of the dextral slip in the same time window as regional extension and challenging interpretations that the modern strike-slip system became active post–6–3 Ma. However, these results are consistent with regional evidence that dextral transtension began by ca. 12 Ma.

Path and amount of dextral fault slip in the Eastern California shear zone across the central Mojave Desert

New total fault slip estimates for the central Mojave Desert portion of the Eastern California shear zone support through-going dextral shear of ∼18 km that is transferred northward to the Garlock fault. The Eastern California shear zone accommodates ∼25% of the plate-boundary shear, with the transversely oriented Garlock fault forming its northern boundary with the Walker Lane belt. Total fault slip estimates for the major faults in the central Mojave Desert were determined using detailed geologic mapping supplemented with aeromagnetic data. Offset data for the Blackwater fault show a consistent 1.8 ± 0.1 km of dextral slip along 55 km of strike length, and they indicate that slip initiated at or after 3.8 Ma. The faults adjacent to the Blackwater fault have dextral slip of: 4.8 ± 0.3 km (Harper Lake fault); 2.9 ± 0.5 km (Mount General fault); 1.0 ± 0.7 km (Lockhart fault); and 3.2 ± 0.3 and 0.53 ± 0.05 km (Paradise fault). These slip data augment published data for the central Mojave Desert and allow evaluation of the strain path in the central Mojave Desert, showing consistent along-strike, through-going dextral slip. Contractional and extensional step-overs in the central Mojave Desert accommodate a change from focused slip in the southern central Mojave Desert to a wider and more dispersed fault system northward with a shift in the locus of dextral shear westward. We interpret these changes in the fault system to be a response to the Garlock fault, which inhibits through-going faulting but allows through-going dextral shear via clockwise deflection of its trace.

Slip history of the La Cruz fault: Development of a late Miocene transform in response to increased rift obliquity in the northern Gulf of California

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in northwestern México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn-tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn-tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between ~19–17Ma and ~12.2Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5±2km of dextral slip ~8–4Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~8–6Ma onset of transform faulting and basin formation along >1000km of the reconstructed Pacific-North America plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins.

Paleotectonics of a complex Miocene half graben formed above a detachment fault: The Diligencia basin, Orocopia Mountains, southern California

The Diligencia basin in the Orocopia Mountains of southeastern California has been one of the primary areas used to test the hypothesis of more than 300 km of dextral slip along the combined San Andreas/San Gabriel fault system. The Orocopia Mountains have also been the focus of research on deposition, deformation, metamorphism, uplift and exposure of the Orocopia Schist, which resulted from fl at-slab subduction during the latest Cretaceous/Paleogene Laramide orogeny. The uppermost Oligocene/Lower Miocene Diligencia Formation consists of more than 1500 m of nonmarine strata, including basalt fl ows and intrusions dated at 24-21 Ma. The base of the Diligencia Formation sits nonconformably on Proterozoic augen gneiss and related units along the southern basin boundary, where low-gradient alluvial fans extended into playa-lacustrine environments to the northeast. The northern basal conglomerate of the Diligencia Formation, which was derived from granitic rocks in the Hayfield Mountains to the north, sits unconformably on the Eocene Maniobra Formation. The northern basal conglomerate is overlain by more than 300 m of mostly red sandstone, conglomerate, mudrock and tuff. The basal conglomerate thins and fines westward; paleocurrent measurements suggest deposition on alluvial fans derived from the northeast, an interpretation consistent with a NW-SE-trending normal fault (present orientation) as the controlling structure of the half graben formed during early Diligencia deposition. This fault is hereby named the Diligencia fault, and is interpreted as a SW-dipping normal fault, antithetic to the Orocopia Mountains detachment and related faults. Deposition of the upper Diligencia Formation was infl uenced by a NE-dipping normal fault, synthetic with, and closer to, the exposed detachment faults. The Diligencia Formation is nonconformable on Mesozoic granitoids in the northwest part of the basin. Palinspastic restoration of the Orocopia Mountain area includes the following phases, each of which corresponds with microplate-capture events along the southern California continental margin: (1) Reversal of 240 km of dextral slip on the San Andreas fault (including the Punchbowl and other fault strands) in order to align the San Francisquito-Fenner-Orocopia Mountains detachment-fault system at 6 Ma. (2) Reversal of N-S shortening and 90° of clockwise rotation of the Diligencia basin and Orocopia Mountains, and 40 km of dextral slip on the San Gabriel fault between 12 and 6 Ma. (3) Reversal of 40° of clockwise rotation of the San Gabriel block (including Soledad basin and Sierra Pelona) and 30 km of dextral slip on the Canton fault between 18 and 12 Ma. These palinspastic restorations result in a coherent set of SW-NE-trending normal faults, basins (including Diligenica basin) and antiformal structures consistent with NW-SE-directed crustal extension from 24 to 18 Ma, likely resulting from the unstable configuration of the Mendocino triple junction. © 2014 Geological Society of America.

Deciphering the early history of a multiply reactivated fault using strain analysis in an adjacent fold: Evidence for early brittle-plastic slip on the Picuris-Pecos fault, New Mexico, USA

Determining the earliest history of a multiply reactivated fault is difficult, particularly when the rock record is incomplete and brittle deformation reorients strain markers. We present a method that incorporates strain analysis of an adjacent fold, inferred deformation temperatures, and published thermochronology to find the initiation age of the north-striking Picuris-Pecos fault of the southern Rocky Mountains (USA). The Picuris-Pecos fault has accommodated at least 37 km of dextral slip. The older east-trending Hondo syncline (New Mexico) is refolded adjacent to the fault and has a map pattern consistent with dextral drag. We analyzed quartz grain shapes, deformation mechanisms, and fractures within the Hondo syncline to determine if the refold is genetically related to the Picuris-Pecos fault. Nine calculated grain-shape ellipsoids predate refolding and are rotated about a vertical axis subparallel to strike of the refold limbs. Healed fractures and semibrittle microstructures are most abundant in the hinge of the refold and adjacent to the fault. We interpret refolding and slip on the Picuris-Pecos fault to have initiated during unroofing through the brittle-plastic transition (∼250–310 °C). Regional thermochronologic studies suggest that the rocks of the Hondo syncline were at this temperature range during the Grenville orogeny. The refold is probably a fault-propagation fold that predated throughgoing faulting, or possibly formed during dextral drag on the Picuris-Pecos fault. Refolding in the Picuris Range and the Truchas Mountains accounts for at least 2.6 km of the total dextral shear on the fault.

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.

Late Cenozoic shortening in the west-central Alborz Mountains, northern Iran, by combined conjugate strike-slip and thin-skinned deformation

The west-central Alborz Mountains of northern Iran have deformed in response to the Arabia-Eurasia collision since ca. 12 Ma and have accommodated 53 ± 3 km of shortening by a combination of range-parallel, conjugate strike-slip faulting and range-normal thrusting. By our interpretation, ∼17 km of shortening across the Alborz is accommodated by westward relative motion of a crustal wedge bounded by conjugate dextral and sinistral strike-slip fault systems. The Nusha, Barir, and Tang-e-Galu fault zones strike west-northwest, constrain the north side of the wedge, and, prior to ca. 5 Ma, accumulated a total of ∼25 km of dextral slip. The south side of the wedge is bounded by the active sinistral reverse Mosha and Taleghan faults, which merge northwest of Tehran and have a total slip estimate of 30–35 km. A restored cross section across the range indicates a minimum of 36 ± 2 km of fold-and-thrust–related, range-normal shortening. Combined, wedge motion, thrusting, and folding yield a net shortening of 53 ± 3 km across the range, which is within the error of the shortening estimate predicted by assuming that the present-day shortening rate (5 ± 2 mm/yr) has been constant since ca. 12 Ma (∼60 km of predicted shortening). A second restored cross section farther west, which includes the wedge, gives a total shortening of 15–18 km and a long-term shortening rate of 1.25–1.5 mm/yr (constant shortening rate since ca. 12 Ma). These strong along-strike finite-strain and long-term strain-rate gradients are important for our understanding of how long-term strain rates compare with instantaneous strain rates derived from global positioning system (GPS) data, and should be considered when planning mountain belt–scale GPS surveys. Finally, a 60-km-long right-hand bend in the Mosha-Taleghan fault system has driven the development of a transpressional duplex south of the fault. The southern boundary of the duplex is the active Farahzad–Karaj–North Tehran thrust system. The kinematic development of this strike-slip duplex has implications for seismic hazard assessment in the heavily populated Karaj and Tehran areas.

Evidence for 65 km of dextral slip across Owens Valley, California, since 83 Ma

The Golden Bear dike in the Sierra Nevada and the Coso dikes in the Coso Range crop out on opposite sides of Owens Valley, California, and strike roughly perpendicular to it. Neither dike reappears along strike across the valley. New data demonstrate that the dike sets are ca. 83 Ma in age, share nearly identical mineralogy and petrography, and intrude similar wall rocks including distinctive 102 Ma leucogranite. Dike bulk-chemical and Sr and Nd isotope compositions are nearly indistinguishable. These data suggest that the dike sets were originally continuous and were offset dextrally by ∼65 km. This displacement estimate is consistent with other recent estimates of total slip across Owens Valley. If faulting began during the Pliocene, the average slip rate was significantly faster than the current rate. Alternatively, motion could have been episodic and have begun as early as the Late Cretaceous.

Metamorphism at a late Mesozoic accretionary margin: a study from the Coast Belt of the North American Cordillera

AbstractThe late Mesozoic and Cenozoic metamorphic evolution of the western North American continental margin is recorded in a belt of homogeneous metapelitic rocks, the Kluane metamorphic assemblage (KMA), in the northern Coast Belt of Yukon Territory. A record of Late Cretaceous medium‐pressure and ‐temperature (c. 7 kbar, 500 °C) metamorphism, M1, is preserved in Ca‐rich garnet and Na‐rich plagioclase cores in rocks that were little affected by later events. M1 was synchronous with mylonitization and is attributed to accretion of the KMA to the ancient continental margin. Isothermal decompression during rapid uplift was followed by early Eocene emplacement of the Ruby Range Batholith (RRB), part of a magmatic arc produced by subduction of the Kula plate. The intrusion of the RRB led to a contact metamorphic overprint, M2, producing a 5–6 km wide aureole in which the grade ranges from subgarnet zone to garnet–cordierite–K‐feldspar zone. Pressure and temperature estimates for M2, calculated from mineral equilibria, are 3.5–4.5 kbar and 530–720 °C, generally consistent with the stability limits of the observed mineral assemblages. Comparison of mineral assemblages and P–T conditions in the KMA with those in the Mclaren Glacier metamorphic belt in Alaska does not support the correlation of the two metamorphic sequences. This weakens the hypothesis proposing 400 km of dextral slip along the Denali fault zone.

Large-scale coast-parallel displacements in the Cordillera: a granitic resolution to a paleomagnetic dilemma

Resolution of the `Paleomagnetic dilemma', the discrepancy between large paleomagnetically determined dextral displacement of outboard portions of the northern Cordillera, and much smaller offsets implied by mapping and stratigraphic correlations, is fundamental to understanding the tectonic evolution of the Cordillera. This paper presents structural orientation data from the middle Cretaceous Dawson Range batholith of west central Yukon and its wallrocks, and suggests that some of the `missing' displacement may be found in intrusions. The elongate northwest-trending batholith has a margin-parallel foliation, a sub-horizontal stretching lineation, and records syn-intrusive dextral shearing. In country rocks adjacent to the batholith, north-trending lineations are deflected clockwise into near parallelism with the batholith's margins; lineations from wallrock screens within the batholith are all aligned parallel with the batholith's long axis. The Big Creek strike-slip fault forms the north-margin of the batholith and accommodated a minimum of 20 km of dextral slip. These observations imply that the batholith invaded an active dextral shear zone, accommodated shearing while crystallizing, and focused post-crystallization fault development. The batholith is conservatively estimated to have accommodated 45 km of syn-intrusive shearing. Collectively, middle Cretaceous intrusions of the northern Cordillera may account for >400 km of previously unrecognized dextral displacement.

Oblique to Orthogonal Convergence Across the Turan Block in the Post-Miocene

The 2-17-km-thick, post-Triassic sediments of the Turan continental block accumulated in a south-dipping basin characterized by fault-controlled facies and thickness variations. Since the late Miocene, the thickest part of the basin fill, on the southern margin of the Turan block, has been folded and uplifted as the Kopet Dag range, in response to the Iran-Turan convergence. The Apsheron sill, which separates the northern continental from the southern oceanic Caspian Sea basins, links the Kopet Dag-Greater Balkan ranges (Turkmenistan) to the Caucasus (Azerbaijan). The large-scale east-west- to west-northwest-east-southeast-oriented arrays of periclinal folds of the Kopet Dag range, which indicate a north-south oriented compression, were generated by a fault propagation mechanism. The migration and accumulation of hydrocarbons has been controlled by the deformation pattern. The 75 km of north-south shortening in the western Kopet Dag-Greater Balkan area can be resolved into 70 km of pure compression, orthogonal to the N120°-oriented Ashgabat fault, and 35 km of dextral slip along this fault. The north-south Iran-Asia relative motion has produced the oblique convergent northwest-southeast structures along the dextral Ashgabat fault and the pure convergent east-west structures in the western Kopet Dag-Greater Balkan region. The orientation of the structures has been controlled by the angular relationship between the relative motion of two blocks and the orientation of their boundaries. The Ashgabat fault as a major crustal anisotropy has concentrated the deformation into a narrow fold and thrust belt, whereas in the west the deformation is distributed over a wider area. The pattern of deformation has been controlled by the Iran-Turan boundary conditions.

Seven million years of strike‐slip and related off‐fault deformation, northeastern Marlborough fault system, South Island, New Zealand

The Marlborough fault system (MFS), South Island, New Zealand is remarkable for the oblique‐slip nature of its active faulting and can be placed in a framework of Pacific‐Australian (Pac‐Aus) plate motions and geodetic strain. New Zealand illustrates changing degrees of upper plate slip partitioning in an obliquely convergent margin that varies along strike in crustal type and subduction geometry. In this paper, we focus on the NE part of this transcurrent fault system, especially the Awatere fault, which is the only Marlborough fault to offset a major Neogene basin. Strike slip on an ancestral eastern strand of this fault began after ∼7.4–6.4 Ma; later vertical axis rotation may have caused it to be abandoned in favor of the currently active strand. Since ∼7 Ma, at least 34±10 km of dextral slip has accrued on the Awatere fault, <6 km of slip has accrued on two nearby strands, and <4 km has accrued on the currently active coastal strand. Comparison of Miocene with late Quaternary offsets suggests that rates of strike slip on the fault have been nearly constant or have possibly decelerated. We interpret spatial and temporal patterns of strike slip in the MFS to reflect southward migration of transform motion resulting from increased Pac‐Aus plate convergence. Late Neogene to present‐day shortening directions in NE South Island are indicative of a partially slip‐partitioned margin. Slip partitioning diminishes southward as the underlying subduction interface becomes strongly coupled, suggesting a simple mechanical relationship between these processes.

Paleomagnetic evidence of localized vertical axis rotation during Neogene extension, Sierra San Fermín, northeastern Baja California, Mexico

Paleomagnetic data from Sierra San Fermín in the Gulf of California Extensional Province indicate that localized clockwise rotations about vertical axes occurred during Pliocene through Recent extension and dextral shear. Relative declination discordances in upper Miocene and Pliocene ash flow tuffs indicate a net clockwise rotation of 30°±16°. Clockwise rotation between 12.5 and 6 Ma is statistically insignificant (11°±17°). Structural observations and geochronological data suggest that rotations in this area began post‐6 Ma, comprising uniform‐sense block rotations (oblique divergence) associated with extension and dextral slip in the northwest striking boundary between the Pacific and North American plates. Northeast striking sinistral‐slip faults and north striking normal faults accommodate distributed dextral shear in this area, allowing fault blocks to rotate in a clockwise sense. A model for oblique divergence predicts ∼21 km of shear in the direction of relative plate motion and ∼20% (∼7 km) ENE directed extension, perpendicular to the Main Gulf Escarpment. A broad region of northeastern Baja California may have undergone similar distributed shear. Two possible dynamic models may explain this shear. In one model, rotation accumulates above a deep, subhorizontal, basal shear zone. Rotating blocks may extend downward to a detachment beneath the extensional province, either a low‐angle eastward continuation of the San Pedro Mártir fault or to a basal shear surface on top of a subducted remnant of the Farallon plate. Alternatively, distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults. In either case, shear may have transferred northward onto faults west of the San Andreas fault, contributing to late Miocene to Recent clockwise rotation of the Western Transverse Ranges. This shear is not accounted for in the 300 km of dextral slip computed from cross‐gulf geologic tie points.

Latest Cretaceous to early Tertiary dextral strike-slip faulting on the southeastern Yalakom fault system, southeastern Coast Belt, British Columbia

The Yalakom fault system cuts the Cadwallader, Bridge River, and Methow terranes for 300 km along the northeast side of the Coast Belt in southwestern British Columbia. The fault system had major dextral slip after middle Cretaceous contractional deformation and early Late Cretaceous volcanism, which ceased by late Eocene time. At discrete times, the Yalakom fault system had major changes in the configuration of active faulting, which resulted in four stages of faulting over ≈40 m.y. The latest Cretaceous dextral slip occurred on the associated Castle Pass-Downton Creek fault zone and possibly the Yalakom fault. Strike slip recurred on the Yalakom fault during the Paleocene(?)-middle Eocene, and there is an associated series of left-stepping faults with zones of contraction and uplift in the stepovers. This associated deformation included early to middle Eocene dextral shear, and south-vergent thrusting within the Shulaps metamorphic belt on the southwest side of the Yalakom fault. The Marshall Creek fault became an important component of the system during middle Eocene time, and this led to tectonic unroofing of the Shulaps belt along normal faults localized in the right-stepping transfer zone linking the Marshall Creek and Yalakom faults. Magmatism was widespread but minor during faulting and was commonly localized along faults. Magmatism may have aided uplift of the Shulaps belt. No basinal deposition accompanied the strike-slip faulting. An estimate of 115 km dextral, strike-slip displacement on the Yalakom fault is based on offset of a unique three-part structural succession of the Methow, Cadwallader, and Bridge River terranes. About 5 km of dextral slip occurred on subsidiary faults that do not merge with the Yalakom fault; therefore, 120 km of dextral offset is estimated for the Yalakom fault system. All but ≈10 km of dextral offset occurred in the Eocene (and Paleocene?). Thus our interpretations preclude the Yalakom fault from being a major contributor to large-scale northward translation of the northwestern Cordillera (the Baja British Columbia hypothesis) interpreted from paleomagnetic data to have occurred largely before Eocene time. On the basis of our correlations across the Yalakom fault, such large faults must lie east of the Methow terrane. The Yalakom fault system probably was linked to the Hozameen and Ross Lake faults (now exposed more than 100 km to the south on the east side of the Fraser and Straight Creek faults). An internally consistent regional model of strike-slip faulting based on offset of geologic units and timing on faults suggests ≥250 km of dextral displacement between ca. 80 and 35 Ma within the southeast Coast Belt. This model resolves the apparent conflict in fault data from Washington and British Columbia and gives a template for comparison of pre-Late Cretaceous geology.

Mesoscale and regional kinematics of the northwestern Yalakom fault system: Major Paleogene dextral faulting in British Columbia, Canada

The northwestern Yalakom fault system (YFS) lies on the eastern margin of the Coast Belt in southwestern British Columbia. The Yalakom fault is the most significant structure in the fault system, with at least 115 km of dextral slip, mostly of early Tertiary age. The NW YFS is up to 25 km wide and is bounded on the southwest by the Tchaikazan fault zone, which had 5 km or more of dextral slip. Kinematic and paleostress analysis of mesofaults and map‐scale structures shows that secondary faults within the NW YFS had both dextral and sinistral oblique slip. Three large synclines had increasing amounts of clockwise, vertical axis rotation from southeast to northwest. Block rotation between two synthetic strike‐slip faults with large offset (Yalakom) and moderate offset (Tchaikazan) is similar to current kinematics between the San Andreas and San Jacinto faults in southern California. The NW YFS was active from latest Cretaceous(?) to (mainly) Eocene time and postdates early Late Cretaceous thrust faulting to the southwest. Early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike‐slip faulting commenced. The abrupt northwest termination of the YFS may be related to extension within the adjacent Tatla Lake metamorphic complex. Some dextral slip may have been transferred farther northwest through the Coast Belt, but these structures have not been documented in west central British Columbia. Most of the 120 km of dextral slip on the YFS was transferred to the north via a mosaic of normal and strike‐slip faults in central British Columbia. Thus the YFS forms the southwestern margin of a broad zone of transtensional structures that extends to the Tintina‐Rocky Mountain Trench fault zone; these structures accommodated northwestward displacement of the northwestern Cordillera in Paleogene time.

Seismic reflection/refraction mapping of faulting and regional dips in the Eastern Alaska Range

We present the results of a Trans‐Alaska Crustal Transect (TACT) investigation of the upper 2–5 km of the eastern Alaska Range in the vicinity of the Denali fault based on seismic reflection/refraction data, laboratory measurements of rock velocities, and structural mapping. The Denali fault is a major dextral slip structure mappable for more than 2000 km separating the Wrangellia and adjacent terranes to the south from the Yukon‐Tanana and adjacent terranes to the north. Geologic mapping suggests over 400 km of dextral slip has occurred on the fault, yet within the upper 1.5 km of the crust along the TACT corridor, basement rocks juxtaposed along the Denali fault reveal no significant seismic velocity differences, although the fault zone itself is associated with a minor lowering of velocity. The lack of seismic velocity contrast adjacent to the fault is in agreement with laboratory measurements of elastic wave velocities of samples from terranes bordering the fault. Laboratory measurements of elastic wave velocities of the metasedimentary mica‐quartz schists comprising the Yukon‐Tanana basement are highly anisotropic because of preferred orientation of mica and predict significant variations in velocity accompanying variations in foliation dip. Although other interpretations are possible, the northward shallowing of foliation dip of basement rocks in the Yukon‐Tanana terrane combined with the strong anisotropy associated with these highly foliated rocks can explain an observed northward increase in seismic velocity within this terrane. Seismic reflections from basement rocks within the Yukon‐Tanana terrane may originate from variations in anisotropy with depth and/or changes in composition reflecting different proportions of sandstone and shale in the protolith.