Change In Relative Plate Motion Research Articles

The impact of oblique inheritance and changes in relative plate motion on the development of rift-transform systems

In transform margins, oblique structural inheritance and plate motion vector changes have a direct impact on the margin's morphology and duration of transform activity. We investigate the effect of these two factors using numerical modelling. To simulate oblique inheritance in continental lithosphere, we model an initial rift-transform-rift configuration oriented at a range of angles (−45o to +45o) with respect to the extension direction. In a second suite of calculations, we first extend a rift-transform-rift system orthogonally and then vary the extension direction to simulate rotation of the far-field stress directions, and hence the relative plate motion. We found that transpressional deformation on the transform results in increased duration of fault activity and diffuse, longer transform zones at higher angles, while the opposite is true for the transtensional case. These observations are in good agreement with natural examples such as the Ungava Transform Zone, the Gulf of California and the Gulf of Aden, indicating that relative plate rotation plays an important role in the structural evolution of transform margins. Finally, we present a metric that links current transform margin morphology to past evolution.

Geochemical and geochronological records of tectonic changes along a flat-slab arc-transform junction: Circa 30 Ma to ca. 19 Ma Sonya Creek volcanic field, Wrangell Arc, Alaska

AbstractThe Sonya Creek volcanic field (SCVF) contains the oldest in situ volcanic products in the ca. 30 Ma–modern Wrangell Arc (WA) in south-central Alaska, which commenced due to Yakutat microplate subduction initiation. The WA occurs within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte–Fairweather and Denali–Duke River fault systems to the east. New 40Ar/39Ar geochronology of bedrock shows that SCVF magmatism occurred from ca. 30–19 Ma. New field mapping, physical volcanology, and major- and trace-element geochemistry, coupled with the 40Ar/39Ar ages and prior reconnaissance work, allows for the reconstruction of SCVF magmatic evolution. Initial SCVF magmatism that commenced at ca. 30 Ma records hydrous, subduction-related, calc-alkaline magmatism and also an adakite-like component that we interpret to represent slab-edge melting of the Yakutat slab. A minor westward shift of volcanism within the SCVF at ca. 25 Ma was accompanied by continued subduction-related magmatism without the adakite-like component (i.e., mantle-wedge melting), represented by ca. 25–20 Ma basaltic-andesite to dacite domes and associated diorites. These eruptions were coeval with another westward shift to anhydrous, transitional-tholeiitic, basaltic-andesite to rhyolite lavas and tuffs of the ca. 23–19 Ma Sonya Creek shield volcano; we attribute these eruptions to intra-arc extension. SCVF activity was also marked by a small southward shift in volcanism at ca. 21 Ma, characterized by hydrous calc-alkaline lavas. SCVF geochemical compositions closely overlap those from the <13 Ma WA, and no alkaline lavas that characterize the ca. 18–10 Ma eastern Wrangell volcanic belt exposed in Yukon Territory are observed. Calc-alkaline, transitional-tholeiitic, and adakite-like SCVF volcanism from ca. 30–19 Ma reflects subduction of oceanic lithosphere of the Yakutat microplate beneath North America. We suggest that the increase in magmatic flux and adakitic eruptions at ca. 25 Ma, align with a recently documented change in Pacific plate direction and velocity at this time and regional deformation events in southern Alaska. By ca. 18 Ma, SCVF activity ceased, and the locus of WA magmatism shifted to the south and east. The change in relative plate motions would be expected to transfer stress to strike-slip faults above the inboard margin of the subducting Yakutat slab, a scenario consistent with increased transtensional-related melting recorded by the ca. 23–19 Ma transitional-tholeiitic Sonya Creek shield volcano between the Denali and Totschunda faults. Moreover, we infer the Totschunda fault accommodated more than ∼85 km of horizontal offset since ca. 18 Ma, based on reconstructing the initial alignment of the early WA (i.e., 30–18 Ma SCVF) and temporally and chemically similar intrusions that crop out to the west on the opposite side of the Totschunda fault. Our results from the SCVF quantify spatial-temporal changes in deformation and magmatism that may typify arc-transform junctions over similar time scales (>10 m.y.).

Provenance of bentonite layers in the Palaeocene strata of the Central Basin, Svalbard: implications for magmatism and rifting events around the onset of the North Atlantic Igneous Province

A fold-and-thrust belt developed between Greenland and Svalbard during the Palaeogene, with an associated foreland basin forming in what is now Spitsbergen. This Central Basin is comprised of the Van Mijenfjorden Group, a ~2.3km thick sandstone-shale dominated succession that contains prominent and laterally continuous bentonite layers in the lower formations. These altered tephra layers can be used as stratigraphic markers that connect the basin development with regional explosive volcanism and changes to relative plate motions. We sampled and analysed bentonites from nine borehole cores across the Central Basin. Each layer shows evidence of alteration, with mobile elements such as alkali and alkali earth metals particularly disrupted. However, immobile elements including rare earth elements (REE) and preserved igneous minerals retain a magmatic signature, allowing for comparisons with potential volcanic sources to be made. The majority of bentonites are both evolved and strongly alkaline, with chemical signatures that are much closer to the continental rift events around Ellesmere Island and North Greenland than to the early activity of the North Atlantic Igneous Province (NAIP). There is a clear difference between tephra layers in the mid Palaeocene versus late Palaeocene strata. The early bentonites have a REE signature comparable to the volcanics of the Kap Washington Group exposed in North Greenland. The later bentonites have likely come from volcanic centres in the Nares Strait that are also the source of abundant volcaniclastic sediments in the Judge Daly Promontory, Ellesmere Island. These findings suggest that a mid to late Palaeocene change in locus of volcanic provenance may reflect changes in relative plate motions related to the formation of the West Spitsbergen fold-and-thrust belt and the emplacement of the NAIP. However, the lack of bentonites matching NAIP sources suggests that explosive volcanism was of insufficient magnitude to lead to deposition in the Central Basin at this time.

Seamounts and oceanic igneous features in the NE Atlantic: a link between plate motions and mantle dynamics

Abstract A new regional compilation of seamount-like oceanic igneous features (SOIFs) in the NE Atlantic points to three distinct oceanic areas of abundant seamount clusters. Seamounts on oceanic crust dated 54–50 Ma are formed on smooth oceanic basement, which resulted from high spreading rates and magmatic productivity enhanced by higher than usual mantle plume activity. Late Eocene–Early Miocene SOIF clusters are located close to newly formed tectonic features on rough oceanic crust in the Irminger, Iceland and Norway basins, reflecting an unstable tectonic regime prone to local readjustments of mid-ocean ridge and fracture zone segments accompanied by extra igneous activity. A SOIF population observed on Mid-Miocene–Present rough oceanic basement in the Greenland and Lofoten basins, and on conjugate Kolbeinsey Ridge flanks, coincides with an increase in spreading rate and magmatic productivity. We suggest that both tectonic/kinematic and magmatic triggers produced Mid-Miocene–Present SOIFs, but the Early Miocene westwards ridge relocation may have played a role in delaying SOIF formation south of the Jan Mayen Fracture Zone. We conclude that Iceland plume episodic activity combined with regional changes in relative plate motion led to local mid-ocean ridge readjustments, which enhanced the likelihood of seamount formation. Supplementary material: Figures detailing NE Atlantic seamounts and SOIF distribution, and the location of earthquake epicentres are available at https://doi.org/10.6084/m9.figshare.c.3459729

Changes in Relative Plate Motion during the Isan Orogeny (1670–1500 Ma) and Implications for Pre‐Rodinia Reconstructions

AbstractFoliation inflexion/intersection axes (FIAs) preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE–SW, N–S, E–W to NW–SE. This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny. Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined. Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE–SW shortening, 1645±7 Ma for N–S shortening, and 1591±10 Ma for that directed E‐W. Inclusion trail asymmetries indicate shear senses of top to the SW for NW–SE FIAs and dominantly top to the N for E–W FIAs, reflecting thrusting towards the SW and N. No evidence for tectonism related to early NE–SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier. Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE–SW and N–S‐directed thrusting as the ages of tectonism are similar. Overlapping dates, tectonic, metamorphic, and metallogenic similarities between eastern Australia (Mount Isa and Broken Hill terranes) and the southwest part of Laurentia imply a most probable connection between both continental masses. Putting Australia in such position with respect to North America during the Late‐Paleo‐to‐Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent.

Strain partitioning in accretionary orogens, and its effects on orogenic collapse: Insights from western North America

Changes in relative plate motions during the construction of accretionary orogens generally result in varying structural styles along the length of the orogen. These disparate structural styles can be interpreted as having been formed by different tectonic regimes along the orogenic axis that formed at the same time. If the orogen is considered at the large scale, the differences in the way in which the crust responds during accretion can be explained by large-scale strain partitioning within the same overall tectonic environment. The westernmost Canadian Cordillera records the transition from Late Cretaceous dextral strike-slip faulting to near-orthogonal compression along the orogenic axis. We postulate that the transition between strike-slip–dominated to compression-dominated tectonics represents a Late Cretaceous partitioning of strain that resulted in a significant difference in crustal rheology along strike of the orogeny. This had a dramatic effect on subsequent Tertiary orogen-scale extension. We propose that plate readjustments in the Tertiary led to orogen-perpendicular collapse in portions of the orogen, facilitated by decoupling between the middle and lower crusts along thermally weakened layers. In contrast, localized, orogen-parallel extension occurred in other portions of the orogen, along kinematically linked, large dextral strike-slip faults where the upper crust remained coupled to the middle and lower crust. New data indicate that partitioning of strain occurs across very large regions within any orogenic system, and that the way in which strain is partitioned can lead to dramatic differences in future orogenic processes. It becomes apparent from these data that orogens must be examined as a whole and that differing structural styles of similar ages are likely responses to the same overall tectonic regime.

The role that plate tectonics, inferred stress changes and stratigraphic unconformities have on the evolution of the West and Central African Rift System and the Atlantic continental margins

The Muglad rift basin of Sudan, is a good example of polyphase rifting, with at least three major phases of basin development. Each phase has resulted in the generation of source rock, reservoir and seal geology with structural traps often closely linked to basement highs. In this paper we investigate on a regional scale the tectonic processes that have contributed to rift basin development.On a regional scale, the evolution of the Africa-wide Mesozoic rift system is intimately linked to relative movements of African sub-plates and to global plate tectonic processes and plate interactions. Changes in plate interactions are observed in the oceanic crust as azimuth changes of fracture zone geometries and by inference have caused significant modifications to both the orientation and magnitude of the motions of the African sub-plates. Such plate motion processes have controlled the polyphase development of the West and Central African Rift System. On the basinal scale, changes of sub-plate motions have resulted in changes in the stress field which have had a clear impact on the deformation and fault geometries of rift basins and on the resulting stratigraphy. The construction of the first unified stratigraphic chart for the West and Central African Rift System shows a close correlation in the timing of the major unconformities with the timing of changes in relative plate motion as observed in the changes of the azimuthal geometry of the oceanic fracture zones in the Central Atlantic. Since similarly timed unconformities exist along the continental margins of Africa and South America, we propose that the causative mechanism is change in relative plate motion which leads to an increase or decrease in the tension on the plate and thus controls the strength or effective elastic thickness, Te, of the crust/plate beneath the margins. This results in a focused change in isostatic response of the margin during short-period changes in relative plate motion; i.e. more tension will mean that loads are not compensated locally resulting in local uplift of the margin.

Greenland Fracture Zone–East Greenland Ridge(s) revisited: Indications of a C22‐change in plate motion?

Changes in the lithospheric stress field, causing axial rift migration and reorientation of the transform, are generally proposed as an explanation for anomalously old crust and/or major aseismic valleys in oceanic ridge‐transform‐ridge settings. Similarly, transform migration of the Greenland Fracture Zone and separation of the 200‐km‐long, fracture‐zone‐parallel continental East Greenland Ridge from the Eurasia plate is thought to be related to a major change in relative plate motions between Greenland and Eurasia during the earliest Oligocene (Chron 13 time). This study presents a reinterpretation of the Greenland Fracture Zone – East Greenland Ridge based on new and existing geophysical data. Evidence is shown for two overstepping ridge segments (Segments A and B) of which Segment A corresponds to the already known East Greenland Ridge while Segment B was not detected previously. Interpretation of sonobuoy data and revised modeling of existing OBS data across Segment B indicate a continental composition of the segment. This interpretation is supported by magnetic anomaly data. The Segments A and B are bounded by portions of the Greenland Fracture Zone with a distinct ∼10° difference in strike. This is suggested to relate to an early episode of transform migration and reorientation of the lithospheric stress field around Chron 22 time, i.e., shortly after the Eocene breakup in the northern NE Atlantic. These findings contradict with previous interpretations of the fracture zone, which infer simple pre‐C13 strike‐slip kinematics.

Postspreading rifting in the Adare Basin, Antarctica: Regional tectonic consequences

Extension during the middle Cenozoic (43–26 Ma) in the north end of the West Antarctic rift system (WARS) is well constrained by seafloor magnetic anomalies formed at the extinct Adare spreading axis. Kinematic solutions for this time interval suggest a southward decrease in relative motion between East and West Antarctica. Here we present multichannel seismic reflection and seafloor mapping data acquired within and near the Adare Basin on a recent geophysical cruise. We have traced the ANTOSTRAT seismic stratigraphic framework from the northwest Ross Sea into the Adare Basin, verified and tied to DSDP drill sites 273 and 274. Our results reveal three distinct periods of tectonic activity. An early localized deformational event took place close to the cessation of seafloor spreading in the Adare Basin (∼24 Ma). It reactivated a few normal faults and initiated the formation of the Adare Trough. A prominent pulse of rifting in the early Miocene (∼17 Ma) resulted in normal faulting that initiated tilted blocks. The overall trend of structures was NE–SW, linking the event with the activity outside the basin. It resulted in major uplift of the Adare Trough and marks the last extensional phase of the Adare Basin. Recent volcanic vents (Pliocene to present day) tend to align with the early Miocene structures and the on‐land Hallett volcanic province. This latest phase of tectonic activity also involves near‐vertical normal faulting (still active in places) with negligible horizontal consequences. The early Miocene extensional event found within the Adare Basin does not require a change in the relative motion between East and West Antarctica. However, the lack of subsequent rifting within the Adare Basin coupled with the formation of the Terror Rift and an on‐land and subice extension within the WARS require a pronounced change in the kinematics of the rift. These observations indicate that extension increased southward, therefore suggesting that a major change in relative plate motion took place in the middle Miocene. The late Miocene pole of rotation might have been located north of the Adare Basin, with opposite opening sign compared to the Eocene‐Oligocene pole.

Pacific‐North America plate boundary reorganization in response to a change in relative plate motion: Offshore Canada

The transition from subduction in Cascadia to the transform Queen Charlotte fault along western Canada is often drawn as a subduction zone, yet recent studies of GPS and earthquake data from northern Vancouver Island are not consistent with that model. In this paper we synthesize seismic reflection and gravity interpretations with microseismicity data in order to test models of (1) microplate subduction and (2) reorganization of the preexisting strike‐slip plate boundary. We focus on the critical region of outer Queen Charlotte Sound and the adjacent offshore. On much of the continental shelf, several million years of subsidence above thin crust are a counterindicator for subduction. An undated episode of compression uplifted the southernmost shelf, but subsidence patterns offshore show that recent subduction is unlikely to be responsible. Previously unremarked near‐vertical faults and a mix of extensional and compressional faults offshore indicate that strike‐slip faulting has been a significant mode of deformation. Seismicity in the last 18 years is dominantly strike‐slip and shows large amounts of moment release on the Revere‐Dellwood fault and its overlap with the Queen Charlotte fault. The relative plate motion between the Pacific and North American plates rotated clockwise ∼6 Ma and appears to have triggered formation of an evolving array of structures. We suggest that the paleo‐Queen Charlotte fault which had defined this continental margin retreated northward as offshore distributed shear and the newly formed Revere Dellwood fault propagated to the northwest.

Comment on "Major Australian-Antarctic Plate Reorganization at Hawaiian-Emperor Bend Time"

Whittaker et al. (Reports, 5 October 2007, p. 83) presented reconstructions for Australia and Antarctica showing a change in relative plate motion approximately 53 million years ago, coincident with an inferred major global plate reorganization. This comment addresses problematic areas in their assumptions and the geological consequences of their reconstructions.

Temperature distribution of the upper surface of the subducted Philippine Sea Plate along the Nankai Trough, southwest Japan, from a three-dimensional subduction model: relation to large interplate and low-frequency earthquakes

The Nankai subduction fault in southwest Japan is a plate boundary along which large megathrust earthquakes have repeatedly occurred. A previous 2-D thermal model suggested that the updip and downdip limits of the seismogenic zone are closely related to the temperature of the plate interface. We estimated a new temperature distribution for the plate interface based on a 3-D thermal convection model that incorporates revised governing equations, temporal changes in relative plate motion, the shape of the subducted slab, temperature variations related to basin evolution and more heat flow data and model regions than have been used in previous models. The margin-parallel spatial variation in the temperature of the plate interface was caused by spatio-temporal changes associated with the cooling of the Shikoku Basin. The updip limit of the thermally delineated seismogenic zone (thermal seismogenic zone) was approximately parallel to the trough axis due to a spreading rate that increased with distance from a fossil ridge, although the margin-parallel age dependency of the subducting plate was taken into account. The downdip limit of the thermal seismogenic zone was determined by temperature, except in eastern Kyushu, where it was determined by a boundary between the continental Moho depth and the upper surface of the subducting plate. The maximum coseismic slip associated with the 1944 Tonankai (M 7.9) and the 1946 Nankai (M 8.0) earthquakes, and a strongly coupled region estimated from GPS data inversion obtained by previous studies, were both located in the thermal seismogenic zone on the plate interface. Low-frequency earthquakes located at the downdip of the thermal seismogenic zone took place at temperatures of about 400–500 °C. This may be related to the phase transformation of hydrous minerals in oceanic crust from lawsonite + blueschist + jadeite to amphibole + eclogite and the associated decreases in water content of approximately 2.4 wt%. The thermal seismogenic zone is narrowest off the Kii Peninsula, where the hypocentres of the Tonankai and Nankai earthquakes were located. The narrow seismogenic zone may have determined the rupture initiation points for these earthquakes, as was previously demonstrated by numerical simulations of the earthquake cycle.

Using thermochronology to determine the timing and rates of tectonic processes

The Earth’s crustal evolution is preserved in the mineral assemblages, textures and isotopic composition of exhumed rocks. Thermochronology provides a set of tools to determine rates of geologic and tectonic processes. Using a range of minerals with different closure temperatures, integrated with petrologic and fabric analysis, we present three case studies from the obliquely convergent Australia(AUS)–Pacific(PAC) plate boundary zone that constrain rates of subduction, timing of peak metamorphism, and timing/rates of rock exhumation. In each case changes in exhumation rates can be linked to changes in relative plate motions. The HP terrane of New Caledonia preserves a record of Paleogene subduction and exhumation within the AUS-PAC plate boundary zone. Volcanic and sedimentary protoliths were subducted at rates of 0.6–1.6 cm/yr, and metamorphosed under HP conditions at 44 Ma. Subsequent rapid cooling from 40 to 34 Ma occurred at rates of 0.5 cm/yr as the HP terrane was exhumed as a coherent block to relatively shallow crustal levels, via ductile shearing associated with crustal extension. Since the mid Oligocene ( 1 cm/yr) within the lower plates of metamorphic core complexes. Counter-clockwise rotation of the Woodlark plate relative to the Australian plate led to reactivation of a former subduction thrust, and resulted in extensional eversion of the youngest known HP and UHP rocks on Earth.

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Seismicity in the Eltanin transform system region reflects the current relative motion between the Pacific and Antarctica plates. As such, the seismicity provides little or no constraints on models for late Neogene Pacific absolute plate motion changes that in turn forced a synchronous change in relative plate motions resulting in the current relative plate motion.

Autosomal-dominant dilated cardiomyopathy with atrioventricular block: a lamin A/C defect-related disease

Seismicity in the Eltanin transform system region reflects the current relative motion between the Pacific and Antarctica plates. As such, the seismicity provides little or no constraints on models for late Neogene Pacific absolute plate motion changes that in turn forced a synchronous change in relative plate motions resulting in the current relative plate motion.

Fault kinematics and past plate motions at a convergent plate boundary: Tertiary Shimanto Belt, southwest Japan

Early‐ and late‐stage, outcrop‐scale faults have been documented in Paleogene strata of the Muroto Peninsula of southwest Japan. The former were active early in the consolidation history of the sediment and are crosscut by the latter, which were active after regional‐scale imbrication, folding, and penetrative deformation (i.e., regional, spaced pressure solution cleavage). Orientation distributions of striae and kinematic axes indicate that the two generations of faults reflect distinct kinematic regimes. The orientations and crosscutting relations indicate a counterclockwise (viewed downward) rotation of the principal shortening direction between early‐ and late‐stage faulting. Existing fossil and radiometric age data suggest this transition occurred during Eocene‐Oligocene time. The fault kinematic data are interpreted to reflect a change in relative plate motions between Eurasia and an oceanic plate subducting beneath Eurasia in Paleogene time. These results, coupled with existing paleothermal and paleopressure data, are consistent with the presence of a spreading ridge in the western Pacific basin during this time.

Comments on “seismic properties of the Eltanin transform system, south Pacific” by Emile A. Okal and Amy R. Langenhorst

Seismicity in the Eltanin transform system region reflects the current relative motion between the Pacific and Antarctica plates. As such, the seismicity provides little or no constraints on models for late Neogene Pacific absolute plate motion changes that in turn forced a synchronous change in relative plate motions resulting in the current relative plate motion.

Ontong Java Plateau and late Neogene changes in Pacific plate motion

Aided by a new geometrical technique called hot spotting, we recently proposed a new model for the absolute motion of the Pacific plate. This model, called WK97, assumed the Hawaiian hotspot is under Loihi and that there was a change in plate motion at 3–6 Ma. The model located the Louisville hotspot close to the Hollister Ridge, a shallow volcanic ridge associated with a geoid high, seismicity, and recent volcanism. Most of the tectonic implications of WK97, however, were not addressed. Furthermore, although several recent studies suggest that the geochemistry of Hollister lavas may indicate a mixture between Louisville melt material and Pacific‐Antarctic Ridge mid‐ocean ridge basalt, these studies strongly disagree with WK97 and consider it seriously flawed. Here we discuss numerous tectonic predictions of WK97 and present a large amount of observational evidence in support of it. We find that a late Neogene collision between the Ontong Java Plateau and the northern margin of the Australia plate appears to have altered the motion of the Pacific plate, as inferred from hotspot volcanism, by forcing it to rotate counter‐clockwise. This rotation, about a low‐latitude pole, appears to have induced right‐lateral shear stress along the entire Pacific plate divergent boundary, which resulted in the formation of extensional transform faults, microplates, and propagating ridges. This change in motion also appears to have triggered concomitant circum‐Pacific tectonism, including trench migration and back arc rifting. The WK97 suggests that long‐lived, plume‐fed volcanism on the Pacific plate appears very limited, perhaps only to five to six hotspots: Hawaii, Louisville, Cobb, Caroline, and perhaps Marquesas and Bowie; the geometry of these chains are consistent with our model. Despite some evidence for volcanic age progressions, most other Pacific Neogene hotspots are most likely “crack spots”, i.e., sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses. Indeed, several volcanic ridges, such as Hollister Ridge near the Louisville hotspot, appear to have formed in response to stresses following the change in absolute plate motion. WK97 also obviates the need for simultaneous, different directions of mantle flow in the south central Pacific. Transpression at the San Andreas and Alpine strike‐slip faults and Aleutian Arc explosivity since 6 Ma also appear to be consequences of the changes in Pacific plate motion. Furthermore, both the WK97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion. Our findings support slab pull as the dominant plate tectonic driving force, highlight the rheological difference between strong oceanic lithosphere and weak continental crust, and suggest that WK97 presently provides a unifying, albeit preliminary, framework for relating both intraplate and circum‐Pacific tectonism and volcanism in the late Neogene.

Walker Creek fault zone, central Rocky Mountains, British Columbia-southern continuation of the Northern Rocky Mountain Trench fault zone

Walker Creek fault zone (WCFZ), well exposed in the western Rocky Mountains of central British Columbia near 54°, comprises a 2 km wide zone of variably deformed Neoproterozoic and Cambrian strata in fault-bounded slivers and lozenges. Extensional shear bands, subhorizontal extension lineations, slickensides, mesoscopic shear bands, and other minor structures developed within and immediately adjacent to the fault zone consistently indicate right-lateral displacement. Offset stratigraphic changes in correlative Neoproterozoic strata indicate at least 60 km of right-lateral displacement across the zone. WCFZ is the southern continuation of the Northern Rocky Mountain Trench (NRMT) fault zone. It shows a through going, moderate displacement, strike-slip fault system structurally links the NRMT and the north-central part of the Southern Rocky Mountain Trench. Strike-slip motion on the WCFZ occurred in the Late Cretaceous to Early Eocene at the same time as northeast-directed shortening in the fold-and-thrust belt. Thus, oblique convergence in the eastern part of the south-central Canadian Cordillera was apparently resolved into parallel northwest-striking zones of strike-slip and thrust faulting during the Late Cretaceous to Early Eocene. The change in the net Late Cretaceous to Early Eocene displacement direction for rocks in the Rocky Mountain trenches from north (56-54°N) to northeast (52-49°N) suggests that the disappearance of strike-slip displacement and increase in fold-and-thrust belt shortening in the eastern Cordillera between 56° and 49°N is largely the result of a north-south change in relative plate motion or strain partitioning across the Cordillera, rather than the southward transformation of right-lateral strike-slip displacement on the Tintina - NRMT fault system into compressional deformation.

Distribution, nature, and origin of Neogene-Quaternary magmatism in the northern Cordilleran volcanic province, Canada

The northern Cordilleran volcanic province encompasses a broad area of Neogene to Quaternary volcanism in northwestern British Columbia, the Yukon Territory, and adjacent eastern Alaska. Volcanic rocks of the northern Cordilleran volcanic province range in age from 20 Ma to ca. 200 yr B.P. and are dominantly alkali olivine basalt and hawaiite. A variety of more strongly alkaline rock types not commonly found in the North American Cordillera are locally abundant in the northern Cordilleran volcanic province. These include nephelinite, basanite, and peralkaline phonolite, trachyte, and comendite. The most MgO-rich nephelinites, basanites, and alkaline basalts from throughout the northern Cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post-5 Ma alkaline basalts from the Basin and Range. Our petrologic observations help constrain the origin of northern Cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province. Results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath Stikinia, which underlies the southern part of the northern Cordilleran volcanic province, than beneath the Cache Creek and Yukon-Tanana terranes, which underlie the northern part of the northern Cordilleran volcanic province. Our results support a model for initiation of northern Cordilleran volcanic province magmatism due to incipient rifting of the northern Cordillera, driven by changes in relative plate motion between the Pacific and North American plates ca. 15–10 Ma.