Pacific North America Research Articles

Role of Sea Surface Temperatures in Forcing Circulation Anomalies Driving U.S. Great Plains Pluvial Years

Abstract In the U.S. Great Plains (GP), diagnosing precipitation variability is key in developing an understanding of the present and future availability of water in the region. Building on previous work investigating U.S. GP pluvial years, this study uses ERA twentieth century (ERA-20C) reanalysis data to investigate key circulation anomalies driving GP precipitation anomalies during a subset of GP pluvial years (called in this paper Pattern pluvial years). With previous research showing links between tropical Pacific sea surface temperature (SST) anomalies and GP climate variability, this study diagnoses the key circulation anomalies through an analysis of SSTs and their influence on the atmosphere. Results show that during Pattern southern Great Plains (SGP) pluvial years, central tropical Pacific SST anomalies are coincident with key atmospheric anomalies across the Pacific basin and North America. During northern Great Plains (NGP) Pattern pluvial years, no specific pattern of oceanic anomalies emerges that forces the circulation anomaly feature inherent in specific NGP pluvial years. Utilizing the results for SGP pluvial years, a conceptual model is developed detailing the identified pathway for the occurrence of circulation patterns that are favorable for pluvial years over the SGP. Overall, results from this study show the importance of the identified SGP atmospheric anomaly signal and the potential for predictability of such events.

Statistical Characteristics of Major Sudden Stratospheric Warming Events in CESM1-WACCM: A Comparison with the JRA55 and NCEP/NCAR Reanalyses

Using the historical simulation from the CESM1-WACCM coupled model and based on the JRA55 and NCEP/NCAR reanalyses, the general statistical characteristics of the major sudden stratospheric warmings (SSWs) in this stratosphere-resolving model are assessed. The statistical and diagnostic results show that CESM1-WACCM can successfully reproduce the frequency of SSW events. As in the JRA55 and NCEP/NCAR reanalyses, five or six SSW events, on average, occur in a model decade. The seasonal distribution of SSWs is also well simulated with the highest frequency in January (35%). The unprecedented low SSW frequency observed in 1990s from the two reanalyses is also identified in a model decade (1930s). In addition, the overestimated duration of SSW events in the earlier WACCM version is not identified in CESM1-WACCM when compared with the two reanalyses. The model can well reproduce the downward propagation of the stratospheric anomaly signals (i.e., zonal wind, height, temperature) following SSWs. Both the modelling and observational evidences indicate that SSWs are proceeded by the positive Pacific–North America (PNA) and negative Western Pacific (WP) pattern. The negative North Atlantic Oscillation (NAO) develops throughout the SSW life cycle, which is successfully modeled. A cold Eurasian continent–warm North American continent pattern is observed before SSWs at 850 h Pa, while the two continents are anomalously cold after SSWs in both the reanalyses and the model.

Distributed Neogene faulting across the western to central Arizona metamorphic core complex belt: Synextensional constriction and superposition of the Pacific–North America plate boundary on the southern Basin and Range

Abstract We present fault data from a belt of Miocene metamorphic core complexes in western and central Arizona (USA) to determine patterns of brittle strain during and after large-magnitude extension, and to evaluate the magnitude of postextensional dextral shear across the region. In the White Tank Mountains, coeval WNW- to NW-striking dextral, normal, and oblique dextral-normal faults accommodated constrictional strain with extension subparallel to the direction of ductile stretching during core complex development. Northwest-striking oblique dextral-normal faults locally accommodated similar strain in the Harquahala Mountains, whereas in the South Mountains, constriction was primarily partitioned on NE-dipping normal faults and conjugate NW- and north-striking strike-slip faults. We interpret brittle constrictional strain to have developed during the late stages of large-magnitude extension associated with core complex development and folding of detachment fault corrugations. The oblique orientation of the Arizona core complex belt with respect to the extension direction likely resulted in a minor component of dextral transtension, accounting for much of the constrictional strain. In addition, far-field stresses associated with the transtensional Pacific–North America plate boundary may have contributed to constriction, which characterizes most Neogene detachment fault systems in the southwest Cordillera. Following cessation of detachment fault slip across the Arizona core complex belt (ca. 14–12 Ma), distributed NW-striking dextral and oblique dextral–NE-side-up (reverse) faults modified the topographic envelope of corrugations to an orientation clockwise of the core complex extension direction. Based on our analysis of this misalignment, we interpret the postdetachment fault dextral shear strain to increase northwestward from 0.03 across the South Mountains (0.5–0.6 km total slip across 18 km) to >0.03–0.07 across the Harquahala and Harcuvar Mountains (1.2–2.5 km of total slip across ∼35 km) and ∼0.2 across the Buckskin-Rawhide Mountains (7–8 km across 36 km). This along-strike variation in dextral shear is consistent with the regional pattern of distributed strain associated with the Pacific–North America plate boundary, as cumulative dextral offset in the lower Colorado River region increases toward the eastern Mojave Desert region to the northwest.

North Atlantic Rossby Wave Breaking during the Hurricane Season: Association with Tropical and Extratropical Variability

Abstract This study explores the connection of Rossby wave breaking (RWB) with tropical and extratropical variability during the Atlantic hurricane season. The exploration emphasizes subtropical anticyclonic RWB events over the western North Atlantic, which strongly affect tropical cyclone (TC) activity. The first part of the study investigates the link between RWB and tropical sea surface temperature (SST) variability. Tropical SST variability affects tropical precipitation and modulates the large-scale atmospheric circulation over the subtropical Atlantic, which influences the behaviors of Rossby waves and the frequency of RWB occurrence. Meanwhile, RWB regulates surface heat fluxes and helps to sustain SST anomalies in the western North Atlantic. The second part of the study explores the connections between RWB and extratropical atmosphere variability by leveraging weather regime analysis. The weather regimes over the North Atlantic are closely associated with RWB over the eastern North Atlantic and western Europe, but show weak associations with RWB over the western North Atlantic. Instead, RWB over the western basin is closely related to the weather regimes in the North Pacific–North America sector. The finding helps clarify why the correlation between the Atlantic TC activity and the summertime North Atlantic Oscillation is tenuous. The relations between the extratropical weather regimes and tropical climate modes are also discussed. The findings suggest that both tropical and extratropical variability are important for understanding variations of RWB events and their impacts on Atlantic TC activity.

Sequence stratigraphy and depositional history of the Baranof Fan: Insights for Cordilleran Ice Sheet outflow to the Gulf of Alaska

AbstractThe Baranof Fan is one of three large Alaska deep-sea fans that preserve sedimentary records reflecting both tectonic and climatic processes. However, lack of drill sites in the Baranof Fan makes the depositional history across the southeastern Alaska margin still poorly understood. Sequence correlation from the adjacent Surveyor Fan to the Baranof Fan provides updated age constraints on the Baranof Fan evolution history. Results show that both the Baranof and Surveyor Fans are dominantly glacial and initiated ca. 2.8 Ma and expanded rapidly since ca. 1.2 Ma in response to the major glaciation events; these results place the deposition of the Baranof Fan younger than previously thought (ca. 7 Ma). The glacially influenced Baranof Fan contains two sub-fans that are laterally stacked with their depocenters migrating southeastward. Each sub-fan developed multiple channels that young southeastward as channel avulsion, coevolution, and tectonic beheading progressed over the past ∼2.8 m.y. Tectonic reconstruction suggests that the Baranof Fan is sourced from the Coast Range via shelf-crossing troughs near the Chatham Strait and Dixon Entrance and thus represents a major outflow for the Cordilleran Ice Sheet during glaciations; the Chatham Strait is the major conduit that has fed most of the Baranof Fan channels. Comparatively, the Surveyor Fan is sourced predominantly from the St. Elias Range where a confluence of orogenesis and glaciations are a coupled system and only partly from the Coast Range via the Icy Strait. It is concluded that the formation and expansion of the Cordilleran Ice Sheet has determined the timing of the Baranof Fan deposition, yet Pacific–North America strike-slip motion has influenced the Baranof Fan sediment distribution, as previously suggested, via a series of southeastward avulsing channels and resultant southeastward migration of deep-sea depocenters.

Hazardous waves from winter trade winds?

One of the weather-related shipping hazards in the Caribbean Islands is an intense anticyclone during winter. Surges of trade winds cause steeply sloped short-period (<7 s, >2 m) waves to penetrate the shipping lanes near Puerto Rico. In addition to the large-scale air pressure gradient (5 hPa/300 km), the Antilles Islands funnel the winds (>10 m/s) and accentuate the surface heat fluxes (>200 W/m2) and air-sea momentum transfer. Here, the weather scenario surrounding a case of damage to a supply ship 30–31 December 2015 is presented, when the wave height-to-length ratio exceeded 3%. This case is placed in context by statistical analysis of climate and weather features contributing to intensified trade winds. A negative Pacific-North America pattern during La Nina enhances an anticyclonic ridge north of Puerto Rico, contributing to steep waves. The research highlights the hazard to shipping of short-period waves produced by surges of trade winds.

Subduction control on the curie isotherm around the Pacific-North America plate boundary in northwestern Mexico (Gulf of California). Preliminary results

Spectral analysis of NAMAG magnetic data enabled us to estimate depths to the Curie isotherm along the Pacific-North America plate boundary. The estimated depths for the Gulf of California, and southern Baja California peninsula, correlate quite well with the depths to the Moho available from previous studies based on receiver function, and seismic refraction. In northernmost Baja California, and southwestern USA no correlation is observed between depths to the Curie isotherm and depths to the mantle obtained from receiver function analysis. Depths to the Curie isotherm are shallower. Along the coastal plains of Sonora and Sinaloa, differences between receiver function based Mantle depths and Curie depths are equally large. However, there is more consistency with depths established by seismic refraction studies. Four zones with shallow Curie isotherm are identified in southern Gulf of California (GS1, GS2, GS3, and GS4). GS1 is located at the Gulf of California mouth. The Alarcón rise is located between GS1 and GS2. Pescaderos Basin is located northwest of GS2. GS3 stretches from the Carmen Basin to the Farallón Basin. GS4 extends from Tiburón Island up to northern Guaymas Basin. Northernmost shallow Curie depth zone (GN1) comprises Upper and Lower Delfín Basins, as well as the Consag and Wagner Basins. A northern prolongation also covers Pinacate Volcanic center. The southern shallow Curie isotherm zones are associated with lithospheric-asthenospheric material upwelling zones. GN1 also includes a convective component related to the younger magmatism of the Pinacate Volcanic Field.Deeper Curie depths are found at southern Baja California peninsula, interpreted as associated with thicker continental or stretched crust. It is possible that the crust is underlain or underplated by magnetic oceanic crust as suggested by satellite magnetic anomalies centered at the Magdalena shelf. A contrasting pattern features northern Baja California: deep Curie zones to the west contrasting with relative shallower Curie isotherm areas to the east. The prolongation towards the peninsula of the extinct Shirley transform fracture zone roughly separates the two domains observed along the peninsula, and suggests that the Curie isotherm along the peninsula is controlled by differences subduction of the Guadalupe and Magdalena microplates. Major differences pointed out between northern and southern Gulf of California might indicate that subduction also played a key role at the beginning of the opening of the Gulf of California, however, currently, the Curie pattern is controlled by the rifting processes (shallow Curie zones correlating quite well with the rift basins and the lithosphere-asthenosphere upwelling material).

The complete mitochondrial and plastid genomes of Corallina chilensis (Corallinaceae, Rhodophyta) from Tomales Bay, California, USA

Genomic analysis of the marine alga Corallina chilensis from Tomales Bay, California, USA, resulted in the assembly of its complete mitogenome (GenBank accession number MK598844) and plastid genome (GenBank MK598845). The mitogenome is 25,895 bp in length and contains 50 genes. The plastid genome is 178,350 bp and contains 233 genes. The organellar genomes share a high-level of gene synteny to other Corallinales. Comparison of rbcL and cox1 gene sequences of C. chilensis from Tomales Bay reveals it is identical to three specimens from British Columbia, Canada and very similar to a specimen of C. chilensis from southern California. These genetic data confirm that C. chilensis is distributed in Pacific North America.

International Epidemiological Differences in Acute Poisonings in Pediatric Emergency Departments.

Identifying international differences in the epidemiology of acute poisonings in children may help in improving prevention. We sought to evaluate the international epidemiological differences in acute poisonings in children presenting to emergency departments (EDs) from 8 different global regions. This was an international multicenter cross-sectional prospective study including children younger than 18 years with acute poisonings presenting to 105 EDs in 20 countries was conducted. Data collection started at each ED between January and September 2013, and continued for 1 year. During the study period, we registered 363,245 pediatric ED presentations, of which 1727 were for poisoning (0.47%; 95% confidence interval, 0.45%-0.50%), with a significant variation in incidence between the regions. Full data were obtained for 1688 presentations. Most poisonings (1361 [80.6%]) occurred at home with either ingestion (1504 [89.0%]) or inhalation of the toxin (126 [7.6%]). Nonintentional exposures accounted for 1157 poisonings (68.5%; mainly in South America and Eastern Mediterranean region), with therapeutic drugs (494 [42.7%]), household products (310 [26.8%]), and pesticides (59 [5.1%]) being the most common toxins. Suicide attempts accounted for 233 exposures (13.8%; mainly in the Western Pacific region and North America), with therapeutic drugs (214 [91.8%], mainly psychotropics and acetaminophen) being the most common toxins. Significant differences between regions were found in both types of poisonings. Recreational poisonings were more common in Europe and Western Pacific region. No patient died. There are substantial epidemiological differences in acute poisonings among children in different countries and regions of the globe. International best practices need to be identified for prevention of acute poisonings in childhood.

Linear and nonlinear links of winter European precipitation to Northern Hemisphere circulation patterns

The connections between European rainfall variability in winter (from January to March) and Northern Hemisphere circulation patterns, as represented by 500-hPa geopotential height, are investigated in the period 1900–2014. Initially, three statistically significant pairs of linearly-related circulation and precipitation patterns, explaining 45% of the variance in the latter field, are identified. The first two essentially represent the North Atlantic Oscillation (NAO) and the East Atlantic (EA) patterns, while the third corresponds to a structure described in recent literature as a blend of the Pacific North America (PNA) pattern and the so-called Asia Bering North America (ABNA) pattern. The residual precipitation field is then examined for patterns that can introduce non-linear modulations into the circulation-precipitation links. It is so found that the NAO impact on European rainfall is modulated by North American and Pacific factors controlling cyclogenesis over Newfoundland. The positive-phase EA rainfall anomalies over Central Europe and the British Isles seem to be markedly affected by the NAO phase. Finally, a possible signature from the Circumglobal Teleconnection Pattern disrupting the NAO influence on East Mediterranean precipitation anomalies is detected.

Can ENSO-Like Convection Force an ENSO-Like Extratropical Response on Subseasonal Time Scales?

A daily El Niño–Southern Oscillation (ENSO) index is developed based on precipitation rate and is used to investigate subseasonal time-scale extratropical circulation anomalies associated with ENSO-like convective heating. The index, referred to as the El Niño precipitation index (ENPI), is anomalously positive when there is El Niño–like convection. Conversely, the ENPI is anomalously negative when there is La Niña–like convection. It is found that when precipitation becomes El Niño–like (La Niña–like) on subseasonal time scales, the 300-hPa geopotential height field over the North Pacific and western North America becomes El Niño–like (La Niña–like) within 5–10 days. The composites show a small association with the MJO. These results are supported by previous modeling studies, which show that the response over the North Pacific and western North America to an equatorial Pacific heating anomaly occurs within about one week. This suggests that the mean seasonal extratropical response to El Niño (La Niña) may in effect simply be the average of the subseasonal response to subseasonally varying El Niño–like (La Niña–like) convective heating. Implications for subseasonal to seasonal forecasting are discussed.

Interdecadal modulation of El Ni\xf1o\u2013tropical North Atlantic teleconnection by the Atlantic multi-decadal oscillation

It has been well documented that El Niño (La Niña) is able to induce sea surface temperature (SST) warming (cooling) in the tropical North Atlantic (TNA) in the following spring. Based on the observational analysis, in this study we found that such a lagged El Niño–TNA SST relationship is non-stationary and depends on the phase of Atlantic multi-decadal oscillation (AMO). During the AMO negative phase, the influence of El Niño on TNA-SST becomes intensified and lasts longer till the following summer to fall, and vice versa. We demonstrated that such a modulation is through atmospheric teleconnection change because El Niño during the AMO negative phase tends to have a stronger and eastward-shifting SST warming over the eastern Pacific and the SST warming persists longer throughout late spring. It is further shown that both extratropical and tropical teleconnections are important in causing the non-stationary El Niño effect according to the AMO phase. The extratropical teleconnection is through the Pacific–North America pattern, while the tropical teleconnection is via anomalous Walker circulation. The extratropical teleconnection acts primarily during winter to early spring, while the tropical teleconnection appears more important in spring. If AMO remains in its positive phase in the near future, a weak influence of El Niño on TNA-SST is expected to ensue.

Control of Storminess over the Pacific and North America by Circulation Regimes

Circulation regimes are identified from a cluster analysis applied to 5-day means of the combined anomalies of 500 hPa geopotential height and the 250 hPa zonal wind over the extended Pacific-North America region obtained from ERA-Interim reanalyses. Five regimes are identified: the Arctic Low (AL), Pacific wavetrain (WT), Alaskan Ridge (AR) and Pacific Trough (PT) and Arctic High (AH). Regime dependent anomalies of the frequency of atmospheric rivers and measures of the storm tracks (850 hPa covariance between high pass filtered meridional wind and temperature and 300 hPa variance of high pass filtered meridional wind) are shown to be significant. The circulation regimes provide statistically significant modulation of observed precipitation (obtained from the Multi-Source Weighted-Ensemble data set). We examine regime-specific anomalies, normalized anomalies and the occurrence of extreme values of precipitation. We identify regions in which the ratio of frequency of occurrence of extremes (5th and 95th percentile) to the climatological frequency in each regime is significantly high. Precipitation is decreased over the Pacific Northwest in the AL regime, but is increased over the central and Eastern US, with a large increase in the probability of extreme precipitation, enhanced atmospheric river occurrence and stronger storm tracks. In the WT regime, Pacific atmospheric rivers are shifted towards Alaska, accompanied by a decrease in storm track magnitude over the Eastern North Pacific. The AR regime is characterized by an increase in moisture flux into the Southwest, increases in extreme precipitation and atmospheric rivers and storm track related heat transport. Increases in precipitation, and atmospheric rivers along the Southwest coast are noted in the PT regime, along with increased precipitation over Texas and Florida. The Northeastern region experiences increases in extremely dry conditions. Significant drought conditions occur over the Midwest US. Significant increases in precipitation over a broad region of the Northwest occur during the AH regime.

Quantifying the agreement between observed and simulated extratropical modes of interannual variability

Using historical simulations of the Coupled Model Intercomparison Project-5 (CMIP5) and multiple observationally-based datasets, we employ skill metrics to analyze the fidelity of the simulated Northern Annular Mode, the North Atlantic Oscillation, the Pacific North America pattern, the Southern Annular Mode, the Pacific Decadal Oscillation, the North Pacific Oscillation, and the North Pacific Gyre Oscillation. We assess the benefits of a unified approach to evaluate these modes of variability, which we call the common basis function (CBF) approach, based on projecting model anomalies onto observed empirical orthogonal functions (EOFs). The CBF approach circumvents issues with conventional EOF analysis, eliminating, for example, corrections of arbitrarily assigned, but inconsistent, signs of the EOF’s/PC’s being compared. It also avoids the problem that sometimes the first observed EOF is more similar to a higher order model EOF, particularly if the simulated EOFs are not well separated. Compared to conventional EOF analysis of models, the CBF approach indicates that models compare significantly better with observations in terms of pattern correlation and root-mean-squared-error (RMSE) than heretofore suggested. In many cases, models are doing a credible job at capturing the observationally-based estimates of patterns; however, errors in simulated amplitudes can be large and more egregious than pattern errors. In the context of the broad distribution of errors in the CMIP5 ensemble, sensitivity tests demonstrate that our results are relatively insensitive to methodological considerations (CBF vs. conventional approach), observational uncertainties in pattern (as determined by using multiple datasets), and internal variability (when multiple realizations from the same model are compared). The skill metrics proposed in this study can provide a useful summary of the ability of models to reproduce the observed EOF patterns and amplitudes. Additionally, the skill metrics can be used as a tool to objectively highlight where potential model improvements might be made. We advocate more systematic and objective testing of simulated extratropical variability, especially during the non-dominant seasons of each mode, when many models are performing relatively poorly.

Prediction and predictability of Northern Hemisphere persistent maxima of 500-hPa geopotential height eddies in the GEFS

This study analyzes the predictability of the persistent maxima of 500-hPa geopotential height (Z500; PMZ) zonal eddies over the Northern Hemisphere in the long-term forecast datasets of the Global Ensemble Forecast System (GEFS) version 10. PMZ patterns, which potentially extending the predictability of severe weather events, include not only closed blocking anticyclones that occur more frequently in the Euro-Atlantic-Asia sector (EAAS) but also persistent open ridges and omega-shape blockings that prevail more often over the Pacific-North America sector (PNAS). The predicted PMZ occurrence frequencies in both the EAAS and the PNAS generally decrease with the lead time, which is consistent with classical blockings in early studies but different from the nearly invariant frequencies of blockings in a recent relevant diagnosis by Hamill and Kiladis. The Brier skill score associated with PMZ frequencies is generally higher in the PNAS than in the EAAS, indicating better predictions in the former. The forecast reliability decreases with the lead time in both sectors, particularly at the tails of probability distributions, suggesting some limitations of the GEFS. PMZ events longer than 1 week with anomaly correlation coefficients (ACCs) exceeding 0.6 in the Northern Hemisphere have a mean useful skill of nearly 10 lead days, which is approximately 0.5–1 day more than the average skill of all cases. Among these events, 50% extend useful ACC skills up to 12 days, and 25% extend the useful skill even further. A discussion is provided regarding how the better PMZ prediction skill in the PNAS can help improve 2 to 3-week predictions over North America.

Global evaluation of atmospheric river subseasonal prediction skill

Subseasonal-to-Seasonal (S2S) forecasts of weather and climate extremes are being increasingly demanded by water resource managers, operational forecasters, and other users in the applications community. This study uses hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) S2S forecast system to evaluate global subseasonal prediction skill of atmospheric rivers (ARs), which are intense lower tropospheric plumes of moisture transport that often project strongly onto extreme precipitation. An aggregate quantity is introduced to assess AR subseasonal prediction skill, defined as the number of AR days occurring over a week-long period (AR1wk occurrence). The observed pattern of seasonal mean AR1wk occurrence strongly resembles the general pattern of daily AR frequency. The ECMWF S2S forecast system generally shows positive (negative) biases relative to reanalysis in the mid-latitude regions in summer (winter) of up to 0.5–1.0 AR days in AR1wk occurrence in regions of highest AR activity. ECMWF AR1wk occurrence forecast skill outperforms a reference forecast based on monthly climatology of AR1wk occurrence at week-3 (14–20 days) lead over a number of subtropical to midlatitude regions, with slightly better skill evident in wintertime. The magnitude and subseasonal forecast skill of AR1wk occurrence are shown to vary interannually, and both quantities are modulated during certain phases of the El Nino–Southern Oscillation, Arctic Oscillation, Pacific–North America teleconnection pattern, and Madden–Julian Oscillation.

Vertical-displacement history of an active Basin and Range fault based on integration of geomorphologic, stratigraphic, and structural data

Knowledge of the last reactivation age of faults in the western Basin and Range province (western North America) is key to understanding the recent evolution of the province in the context of the Pacific-North America plate boundary region. The Dixie Valley fault system (DVFS;Nevada, USA) is an important active normal fault in this region, but its slip history is not well known. Our goal is to determine the age of faulting initiation and the history of vertical displacement at the million-year time scale. We used well data from the Dixie Valley geothermal field and geologic maps to calculate offsets of geologic units. Tectonic events and their ages were determined from knickpoint analysis in the Stillwater Range. We then matched basin unconformities and knickpoints to obtain a vertical displacement history. The age of inception of the modern DVFS is likely between 13 and 7 Ma. The total vertical displacement of the geologic units is between 3 and 3.5 km, and most of it occurred in the last 2-3 Ma. Assuming an erosion rate between 0.03 and 0.07 m/ka, the maximum vertical displacement rate (similar to 1-2 mm/a) occurred between 3 and 0.5 Ma, and resulted in similar to 2 km of vertical displacement. Finally, the age-versus-distance profile of the Stillwater Range crest is tapered at both ends, as expected for a propagating range-bounding normal fault, but it also has two maxima matching the deepest parts of the Dixie Valley basin. This indicates that the present-day DVFS may have started out as two separate strands that connected within 2-3 Ma of inception. Our work shows that the combination of stratigraphic and structural data from the basin with geomorphological data from the adjacent mountain range is suitable for placing constraints on the displacement history at the million-year time scale of an active range-bounding normal fault. The resulting displacement-time paths for normal faults may not be as uniform as commonly assumed.

Slope failure and mass transport processes along the Queen Charlotte Fault, southeastern Alaska

Abstract The Queen Charlotte Fault defines the Pacific–North America transform plate boundary in western Canada and southeastern Alaska for c. 900 km. The entire length of the fault is submerged along a continental margin dominated by Quaternary glacial processes, yet the geomorphology along the margin has never been systematically examined due to the absence of high-resolution seafloor mapping data. Hence the geological processes that influence the distribution, character and timing of mass transport events and their associated hazards remain poorly understood. Here we develop a classification of the first-order shape of the continental shelf, slope and rise to examine potential relationships between form and process dominance. We found that the margin can be split into six geomorphic groups that vary smoothly from north to south between two basic end-members. The northernmost group (west of Chichagof Island, Alaska) is characterized by concave-upwards slope profiles, gentle slope gradients (&lt;6°) and relatively low along-strike variance, all features characteristic of sediment-dominated siliciclastic margins. Dendritic submarine canyon/channel networks and retrogressive failure complexes along relatively gentle slope gradients are observed throughout the region, suggesting that high rates of Quaternary sediment delivery and accumulation played a fundamental part in mass transport processes. Individual failures range in area from 0.02 to 70 km 2 and display scarp heights between 10 and 250 m. Transpression along the Queen Charlotte Fault increases southwards and the slope physiography is thus progressively more influenced by regional-scale tectonic deformation. The southernmost group (west of Haida Gwaii, British Columbia) defines the tectonically dominated end-member: the continental slope is characterized by steep gradients (&gt;20°) along the flanks of broad, margin-parallel ridges and valleys. Mass transport features in the tectonically dominated areas are mostly observed along steep escarpments and the larger slides (up to 10 km 2 ) appear to be failures of consolidated material along the flanks of tectonic features. Overall, these observations highlight the role of first-order margin physiography on the distribution and type of submarine landslides expected to occur in particular morphological settings. The sediment-dominated end-member allows for the accumulation of under-consolidated Quaternary sediments and shows larger, more frequent slides; the rugged physiography of the tectonically dominated end-member leads to sediment bypass and the collapse of uplifted tectonic features. The maximum and average dimensions of slides are an order of magnitude smaller than those of slides observed along other (passive) glaciated margins. We propose that the general patterns observed in slide distribution are caused by the interplay between tectonic activity (long- and short-term) and sediment delivery. The recurrence (&lt;100 years) of M &gt; 7 earthquakes along the Queen Charlotte Fault may generate small, but frequent, failures of under-consolidated Quaternary sediments within the sediment-dominated regions. By contrast, the tectonically dominated regions are characterized by the bypass of Quaternary sediments to the continental rise and the less frequent collapse of steep, uplifted and consolidated sediments.

A kinematic model for the evolution of the Eastern California Shear Zone and Garlock Fault, Mojave Desert, California

The Eastern California shear zone in the Mojave Desert, California, accommodates nearly a quarter of Pacific–North America plate motion. In south-central Mojave, the shear zone consists of six active faults, with the central Calico fault having the fastest slip rate. However, faults to the east of the Calico fault have larger total offsets. We explain this pattern of slip rate and total offset with a model involving a crustal block (the Mojave Block) that migrates eastward relative to a shear zone at depth whose position and orientation is fixed by the Coachella segment of the San Andreas fault (SAF), southwest of the transpressive “big bend” in the SAF. Both the shear zone and the Garlock fault are assumed to be a direct result of this restraining bend, and consequent strain redistribution. The model explains several aspects of local and regional tectonics, may apply to other transpressive continental plate boundary zones, and may improve seismic hazard estimates in these zones.

Deformation of the Pacific/North America Plate Boundary at Queen Charlotte Fault: The Possible Role of Rheology

AbstractThe Pacific/North America (PA/NA) plate boundary between Vancouver Island and Alaska is similar to the PA/NA boundary in California in its kinematic history and the rate and azimuth of current relative motion, yet their deformation styles are distinct. The California plate boundary shows a broad zone of parallel strike slip and thrust faults and folds, whereas the 49‐mm/yr PA/NA relative plate motion in Canada and Alaska is centered on a single, narrow, continuous ~900‐km‐long fault, the Queen Charlotte Fault (QCF). Using gravity analysis, we propose that this plate boundary is centered on the continent/ocean boundary (COB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the COB is a boundary between materials of contrasting elastic properties, once a fault is established there, it will probably remain stable. We propose that deformation progressively shifted to the COB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by fault reactivation on Pacific crust and by an eastward dipping QCF. Underthrusting of Pacific slab under Haida Gwaii occurs at convergence angles &gt;14°–15° and may have been responsible for the emergence of the archipelago. The calculated slab entry dip (5°–8°) suggests that the slab probably does not extend into the asthenosphere. The PA/NA plate boundary at the QCF can serve as a structurally simple site to investigate the impact of rheology and composition on crustal deformation and the initiation of slab underthrusting.