North America Plate Research Articles

Controls on the genesis of a giant sand injection complex; insights into the Paleogene evolution of the stress of northern and central California

Giant sand injection complexes and localized swarms of sandstone intrusions are common in Upper Cretaceous to Miocene sedimentary successions of central and northern California within a distance of less than 100 km from the Pacific margin of the North America plate. One of the best preserved and extensively exposed injection complexes is the late Eocene Tumey Giant Injection Complex. The emplacement of sand injectites was driven by overpressure generated by thermal diagenesis of biosiliceous and smectite-rich mudstone host-rocks. The orientation and size distribution of sandstone intrusions was controlled by stress in which σ 1 and σ 3 were horizontal and, respectively, parallel and perpendicular to the present trace of the San Andreas Fault, and σ 2 was vertical. A strike-slip tectonic regime is inferred. Our analysis documents margin-orthogonal extension and provides support for a late Eocene phase of increase of strain, and possibly active slip, along a syn-subduction strike-slip fault zone. Comparison with other injection complexes in the region indicates that the near-field maximum principal stress rotated through time, from normal to parallel with respect to the plate margin, probably in relation to variations of the relative motion vector of the converging plates.

Late Miocene transition from extension to dextral transtension in the southern Coastal Sonora fault zone, Gulf of California rift, Mexico

The Coastal Sonora fault zone (CSFZ) in northwestern Mexico records the Pacific–North America plate boundary reorganization during the Miocene from subduction to a transform boundary. This study presents the structural architecture of the Sierra El Aguaje-San Carlos-Guaymas region along the southern part of the CSFZ. The study area shows a complex fault pattern divided into four tectonic domains: ENE–WSW left-lateral, normal, and oblique faults in the Sierra El Aguaje and Cerro El Vigía domains and N–S domino-type normal faults in the El Parral and San Carlos domains, which are separated by the NW-striking dextral San Carlos fault. Based on a new fault-slip dataset and K-Ar ages, we propose a structural evolution model in which deformation progressively evolved from extension to dextral transtension. The deformation history involved ∼E–W pure extension associated with N–S normal faults from ∼15.3 Ma to ∼11–10 Ma, followed by transtension and the development of strike-slip faults after ∼11–10 Ma. That change induced the reactivation of preexisting faults and local strain partitioning between normal, oblique, and strike-slip faults. Transtension became strike-slip dominated during the latest Miocene and the Pliocene, accumulating ∼8 km of post-6.3 Ma dextral slip along the San Carlos fault. Paleostress analysis demonstrates that during the extensional phase, σ2 and σ3 were horizontal and oriented ∼N–S and ∼E–W, respectively. The transition to transtension required that σ2 reached the lithostatic stress value, its magnitude approached σ1, and its orientation remained ∼N–S. These conditions implied a horizontal tectonic force directed towards the north began after ∼11–10 Ma in northwestern Mexico. We propose that this new stress regime was related to the end of the Farallon–North America convergence and the inception of the Gulf of California oblique rifting with σ1 oriented N–S.

Evolution of Miocene normal and dextral faulting in the lower Colorado River region near Blythe, California, USA

Abstract The evolution of strain in nascent continental plate boundaries commonly involves distributed deformation and transitions between different styles of deformation as the plate boundary matures. Distributed NW-striking faults, many with km-scale right-lateral separation, are prevalent near Blythe, California, and have been variably interpreted to have accommodated either Middle Miocene NE-SW extension as normal faults or Late Miocene to Pliocene dextral shear as strike-slip faults. However, with poor timing and kinematic constraints, it is unclear how these faults relate to known domains of Neogene deformation and the evolution of the Pacific–NorthAmerica plate boundary. We present kinematic data (n = 642 fault planes, n = 512 slickenlines) that demonstrate that these faults dominantly dip steeply northeast; ~96% of measured faults record normal, dextral, or oblique dextral-normal kinematics that likely reflect a gradational transition between normal and dextral oblique kinematic regimes. We constrain fault timing with 11.7 Ma and 7.0 Ma 40Ar/39Ar dates of rocks cut by faults, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of calcite mineralized during oblique dextral faulting that demonstrates fault slip at ca. 10–7 Ma and perhaps as late as ca. 4 Ma. This Late Miocene dextral oblique faulting is best compatible with a documented regional transition from Early to Middle Miocene NE-directed extension during detachment fault slip to subsequent NW-directed dextral shear. We estimate 11–38 km of cumulative dextral slip occurred across a 50-km-wide zone from the Palen to Riverside mountains, including up to 20 km of newly documented dextral shear that may partly alleviate the regional discrepancy of cumulative dextral shear along this part of the Late Miocene Pacific–North America plate boundary.

Seismicity at the intersection of the Coast Shear Zone and Anahim Volcanic Belt near Bella Coola, British Columbia, Canada

In the Coast Mountains of western British Columbia, an anomalous seismicity concentration exists near the intersection of the Coast Shear Zone, a major northwest–southeast trending Eocene-age shear zone that accommodated deformation between the Pacific and North America plates, with the Anahim Volcanic Belt, an east-northeast–west-northwest trending zone of volcanic features that decrease in age to the east. To better characterize seismicity in the Coast Mountains, we augment the existing Natural Resources Canada seismicity catalogue by applying an automatic detection and location algorithm to both permanent Canadian National Seismic Network stations and temporary stations from the 2005–2006 BATHOLITHS deployment, resulting in 837 relocated events with at least three paired P- and S-phase picks. Double-difference relocation reveals several small-scale linear strands subparallel to the Coast Shear Zone and within the Anahim Volcanic Belt and three clusters of events striking at a high angle to the Coast Shear Zone that occurred as swarms in 2015 and 2017. First-motion focal mechanisms exhibit extensional and strike-slip faulting. Our observations indicate that most of these events are not associated with surficial processes such as landslides, but rather, we hypothesize that the interaction of the Anahim Volcanic Belt and Coast Shear Zone has weakened the lithosphere in this region, leading to current-day strain localization and high heat flow that manifest seismicity, including swarm-like activity.

Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range

AbstractThe uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or breakaway zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligoceneaged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal faultslip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slabdriven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere preconditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transtensional plate motion along the Pacific–North America plate boundary, and likely a growing slab window, continued to drive extension along the SNNF and the western Basin and Range, but not until ca. 11 Ma when the Mendocino triple junction reached the latitude of our northernmost (U-Th)/He transect.

Double‐Difference Adjoint Tomography of the Crust and Uppermost Mantle Beneath Alaska

AbstractWe perform adjoint waveform tomography to reveal the P‐wave velocity structure of the crust and uppermost mantle in Alaska by using common‐source double‐difference traveltime data. Our underlying forward modeling tool is a 3D seismic‐wave solver called SPECFEM3D_GLOBE. We select ∼13,000 high‐quality P‐wave arrivals from 147 earthquakes recorded by more than 600 stations. The waveforms are filtered between 15 and 8 s and used to generate about 100,000 differential traveltimes via the cross‐correlation technique. We invert these common‐source differential traveltimes for a P‐wave velocity model down to 150 km depth in Alaska. In the upper mantle, a high‐velocity zone is imaged along the Aleutian volcanic arc, which represents the Pacific plate subducting beneath the North America plate. On the eastern edge of the subducting Pacific plate, the imaged high‐velocity anomaly extends distinctly beyond the Wadati–Benioff zone, indicating an aseismic slab edge. We observe an absence of a low‐velocity mantle wedge beneath the Denali Volcanic Gap (DVG) in our tomographic model. This finding suggests that minimal melt accumulation exists beneath the DVG, which may explain the cessation of magmatism there. Our model also reveals a high‐velocity anomaly near the Wrangell Volcanic Field (WVF), suggesting the possible existence of the Wrangell slab or the high‐velocity slab edge of the Yakutat slab. A potential slab gap shown as a low‐velocity body is detected at 95–125 km depth near the WVF, which could act as a channel to transport mantle materials to feed the cluster of volcanoes in the WVF.

Estimating Coseismic Deformation of Southwestern Puerto Rico from the 7 January 2020 Mw 6.4 Earthquake: Constraints from Campaign and Continuous GPS

ABSTRACT The Puerto Rico–Virgin Islands (PRVI) block lies within the Northern Caribbean Plate Boundary Zone—a zone accommodating stresses between the larger North America and Caribbean plates. Data from Global Positioning System (GPS) sites throughout the PRVI block have been used to confirm the existence of a distinct microblock in the southwest. It is no coincidence that this portion of the PRVI block is the epicentral region of the 7 January 2020 Mw 6.4 earthquake and the ensuing seismic sequence. Prior to the mainshock, the southwestern Puerto Rico (SWPR) region exhibited most of the onland seismic activity. The 2020–2021 SWPR earthquake seismic sequence has been characterized by having an atypical aftershock decay distribution occurring along multiple faults. As a result, fault parameters of the 7 January 2020 mainshock have been poorly defined by conventional seismic methods. Here, we present results from campaign and continuous GPS sites in SWPR, and compare GPS-derived displacements to those computed from the U.S. Geological Survey National Earthquake Information Center (NEIC) focal mechanism. We conclude that irrespective of which nodal plane is used, the observed coseismic displacements from GPS differ from those predicted using a simple elastic model and the NEIC focal mechanism. We infer based on these observations that the complex mainshock rupture resulted in a suboptimal double-couple solution.

Middle-late Miocene rift-related magmatism in Guaymas, Sonora, Mexico at the eastern margin of the Gulf of California: Petrogenetic implications associated to the Pacific-north American plate boundary

Miocene volcanism in the Guaymas region at the eastern margin of the Gulf of California, in coastal Sonora, NW Mexico, was dominated by arc magmatism. However, the presence of rift-related magmatism is also exposed. Rocks associated with subduction of the Farallon plate beneath North America plate are of Oligocene-Miocene ages, while the magmatic rocks with ages between ca. 12.5 to ca. 6 Ma occur in a post-subduction tectonic setting. This period is sometimes referred to as the proto-Gulf phase. The proto-Gulf magmatic rocks are exposed as part of a volcanic igneous plumbing system, herein named Guaymas Group. The Guaymas Group has an age range between ∼12 and 10 Ma and is defined by two magmatic events: an intermediate compositional pulse (Unit 1) and a felsic pulse (Unit 2).The lithological units from the Guaymas Group have similar geochemical features, exhibiting LILE and LREE enrichment and Nb, Ta and Ti depletion. These rocks show a transitional to tholeiitic composition affinity that differs from the arc magmatism from coastal Sonora. The geochemical data suggest that assimilation and fractional crystallization (AFC) processes played an important role in the geochemical variations observed in volcanic and subvolcanic rocks. The geochemical characteristics of the Guaymas Group rocks also could indicate an origin by decompression melting of a mantle source metasomatized by subduction processes.Finally, considering the geological, geochronological and geochemical study, we suggest that the emplacement of the Guaymas Group rocks is contemporaneous with the oblique rifting environment associated with an incipient Pacific-North American plate boundary.

Kinematics and paleogeology of the western United States and northern Mexico computed from geologic and paleomagnetic data: 0 to 48 Ma

Abstract Fault traces and offsets, cross-section length changes, paleomagnetic inclination and declination anomalies, and stress-direction indicators with ages back to 90 Ma are collected from the geologic literature on the western United States and northern Mexico. Finite-element program Restore simulates paleokinematics by weighted least squares and integrates displacements, strains, and rotations back in time, producing paleogeologic maps, as well as maps of velocity, heave rate, strain rate, and stress direction at 6 m.y. intervals. After calibrating three program parameters against neotectonic velocities from geodesy, all classes of data except inclination anomalies are fit reasonably well. The kink in the San Andreas fault near San Gorgonio Pass has been gradually restored by slip on adjacent faults and automated smoothing. Piercing-point pairs successfully restored along the San Andreas–Gulf of California plate boundary include the Pelona and Orocopia Schists at 6 Ma, the Pinnacles and Neenach Volcanics at 21 Ma, and the Jolla Vieja and Poway conglomerates adjacent to their Sonoran source at 48–42 Ma. During 18–6 Ma, rapid extension on the Oceanside detachment fault system was restored, placing present San Nicolas Island adjacent to present Rosarito, Baja California, at 18 Ma. Since ca. 18 Ma, the western Transverse Ranges have rotated 70° clockwise, restoration of which implies that sinistral faults in this province originated with NNE trends. The first contact between the Pacific and North America plates at ca. 28 Ma was not associated with any dramatic increase in dextral faulting on land; instead, the primary result was extension in the Plush Ranch–Vasquez-Diligencia basins and Colorado River corridor, probably driven by an unstable triple-junction and accelerated by heating and uplift of North America above enlarging slab windows.

The triple junction of the North America, Cocos, and Caribbean plates. What we know, what we don’t

We present a summary of the tectonic elements of the North America-Caribbean-Cocos triple junction area. In the vicinity of the triple junction, displacement related to the North America-Caribbean plate boundary takes place along the left-lateral Motagua-Polochic fault system, and convergence between the Cocos and the other two plates occurs along the Middle America trench.
 The trace of the Motagua-Polochic system is lost at its westernmost end and does not reach the convergent boundary. Deformation of the plate boundary in this location is then distributed along a system of reverse faults (the Reverse-faults tectonic province), a system of left-lateral faults (Strike-slip faults province), two or more large NW-SE oriented left-lateral faults (Angostura and Concordia faults), and a left-lateral fault (Tonalá) that might be construed as the continuation of the Polochic fault along the southern border of the Chiapas Massif.
 Somewhere within this deformation zone, transition in overriding plate between North America and Caribbean takes place, but it is not clear exactly where. It is probably at about longitude 96° W because both the dip and the shape of the subducted Cocos slab change significantly at this longitude.

Complex Rupture and Triggered Aseismic Creep During the 14 August 2021 Haiti Earthquake From Satellite Geodesy

AbstractThe 2021 August 14 MW 7.2 Nippes, Haiti earthquake occurred 75 km west of the epicenter of the 2010 Leogane earthquake (Haiti) on the transpressive Caribbean ‐ North America plate boundary. We present an updated fault map for Hispaniola and model coseismic and early postseismic fault slip using Interferometric Synthetic Aperture Radar and pixel offsets. We find the earthquake ruptured multiple segments of the Enriquillo‐Plantain Garden Fault Zone. Slip occurred in two main sub‐events on either side of a restraining bend at Pic Macaya, with ∼2.7 m of peak reverse‐slip east of the bend. To the west, slip jumps the restraining bend and further ruptures with ∼1.2 m of left‐lateral slip. Afterslip in the 4 days following the event occurred at shallow depth and adjacent to the coseismic rupture areas and reached the surface east of Pic Macaya.

GPS-derived interseismic fault locking along the Jalisco–Colima segment of the Mexico subduction zone

SUMMARYNortheastward subduction of the oceanic Rivera and Cocos plates in western Mexico poses a poorly understood seismic hazard to the overlying areas of the North America plate. We estimate the magnitude and distribution of interseismic locking along the northern ∼500 km of the Mexico subduction zone, with a series of elastic half-space inversions that optimize the fits to the velocities of 57 GPS stations in western Mexico. All velocities were corrected for the co-seismic, afterslip and viscoelastic rebound effects of the 1995 Colima–Jalisco and 2003 Tecomán earthquakes. We explore the robustness of interseismic locking estimates to a variety of mantle Maxwell times that are required for the viscoelastic corrections, to the maximum permitted depth for locking of the subduction interface and to the location assigned to the Rivera–Cocos–North America plate triple junction offshore from western Mexico. The best-fitting locking solutions are associated with a maximum locking depth of 40 km, a triple junction location ∼50 km northwest of the Manzanillo Trough and a mantle Maxwell time of 15 yr (viscosity of 2 × 1019 Pa s). Checkerboard tests show that the locking distribution is best resolved at intermediate depths (10–40 km). All of our inversions define a gradual transition from strong locking (i.e. 70–100 per cent) of most (70 per cent) of the Rivera–North America subduction interface to strong but less uniform locking below the Manzanillo Trough, where oceanic lithosphere transitional between the Cocos and Rivera plate subducts, to weak to moderate locking (averaging 55 per cent) of the Michoacán segment of the Cocos–North America interface. Strong locking of the ∼125-km-long trench segment offshore from Puerto Vallarta and other developed coastal areas, where our modelling indicates an average annual elastic slip-rate deficit of ∼20 mm yr−1, implies that ∼1.8 m of unrelieved plate slip has accrued since the segment last ruptured in 1932, sufficient for an M ∼ 8.0 earthquake.

Geodynamic evaluation of the pacific tectonic model for chortis block evolution using 3D numerical models of subduction

Past position and subsequent tectonic evolution of the Chortis Block represents one of the major uncertainties related to the geodynamic evolution of southern Mexico and northern Central America. This uncertainty has led to different hypotheses and evolutionary models based on tectonic, geological and paleographic studies that attempt to explain the origin and paleoposition of the Chortis Block. Among these models, a challenging hypothesis suggests the Chortis Block originates from a west-southwest position in the Pacific rather than attached to the Mexican margin. Additionally, this model proposes a high rate (200 km/20 Myr) of trench erosion along the Mexican margin just north of the present-day Chortis Block. In the Pacific model, the Chortis Block is being pushed eastward by the subduction movement of the Farallon and Cocos oceanic plates, and is finally captured by the Caribbean plate. In this study, the evolution of subduction along the Mexican and Central America margins is investigated numerically taking into account kinematic reconstruction of the Pacific model. The main goal is to investigate validity of the Pacific model for Chortis Block evolution by comparing our predicted slab geometries with present-day tomographic images. The kinematic 3D numerical models are integrated 45 Myr in time and use a mantle reference frame for tectonic plate evolution of the study area. These models generate subducting plate geometries that are only partial consistent with what is observed from seismic tomography, they reproduce realistic slab geometries only for the region located beneath the North America Plate, where the Cocos slab is flat at shallow depths and also stagnates along the mantle transition zone. On the other hand, for the region located beneath the Chortis Block the slab penetrates vertically through the upper mantle and even curls back in the upper portion of the lower mantle, a geometry that is inconsistent with seismic tomography images which shows the presence of east-dipping slab that tends to stagnate at 1000 km depth. This study indicate that the Pacific model is not an appropriate hypothesis for explaining the Chortis Block origin, and future studies should focus on alternative tectonic models.

Evidence for asthenospheric flow rotation in northwest Canada: insights from shear wave splitting

SUMMARYThe Mackenzie Mountains (MM) of northwest Canada are an actively uplifting, seismogenic salient of the northern Canadian Cordillera that lie 750 km NE of the main Pacific–North America plate boundary. We present new shear wave splitting measurements from a linear array transecting the region to characterize upper-mantle anisotropy and the relative role of the lithosphere and asthenosphere in MM uplift. A gradual rotation in anisotropy occurs across the Canadian Cordillera, with stations nearest to the craton yielding NE fast-axis orientations that are subparallel to North America absolute plate motion (∼230°). Moving SW from the craton, across the MM and towards the plate boundary, fast-axis orientations rotate to become aligned with major lithospheric fabrics (NW–SE). Previous work has shown that the Cordilleran lithosphere is thin (≈50 km) in this region. We therefore interpret these results to primarily reflect sublithospheric flow. Three possibilities for asthenospheric flow are presented, but none of them fit the data well. Therefore, we believe that flow in the SW is due to upwelling from the slab window that opened up around 45–25 Ma, although it is possible the lithosphere is more than 50 km in thickness, in which case these splits reflect the tectonic fabric of the region.

Heterogenous late Miocene extension in the northern Walker Lane (California-Nevada, USA) demonstrates vertically decoupled crustal extension

AbstractThe spatial distribution and kinematics of intracontinental deformation provide insight into the dominant mode of continental tectonics: rigid-body motion versus continuum flow. The discrete San Andreas fault defines the western North America plate boundary, but transtensional deformation is distributed hundreds of kilometers eastward across the Walker Lane–Basin and Range provinces. In particular, distributed Basin and Range extension has been encroaching westward onto the relatively stable Sierra Nevada block since the Miocene, but the timing and style of distributed deformation overprinting the stable Sierra Nevada crust remains poorly resolved. Here we bracket the timing, magnitude, and kinematics of overprinting Walker Lane and Basin and Range deformation in the Pine Nut Mountains, Nevada (USA), which are the westernmost structural and topographic expression of the Basin and Range, with new geologic mapping and 40Ar/39Ar geochronology. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation. Conformable volcanic and sedimentary rocks, with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest by east-dipping normal faults. This relationship demonstrates that dip-slip deformation initiated after ca. 6.8 Ma. A retrodeformed cross section across the range suggests that the range experienced 14% extension. Subsequently, Walker Lane transtension initiated, and clockwise rotation of the Carson domain may have been accommodated by northeast-striking left-slip faults. Our work better defines strain patterns at the western extent of the Basin and Range province across an approximately 150-km-long east-west transect that reveals domains of low strain (∼15%) in the Carson Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude extension (∼150%–180%) in the Singatse and Wassuk Ranges. There is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic, prior to extension, or today—which suggests that strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ∼150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable.

The Cuban staircase sequences of coral reef and marine terraces: A forgotten masterpiece of the Caribbean geodynamical puzzle

The emerged sequences of coral reef and marine terraces of the Cuban Archipelago have been recognized since the end of the 19th century but with noticeable exceptions, their bio-constructions and/or deposits are not dated. The northern Caribbean islands and associated archipelagos are located in a left-lateral strike-slip tectonic setting, at the boundary between the North America and Caribbean plates. Cuba is the only landmass located on the American Plate directly adjacent to this transform fault zone. Quantifying upper Pleistocene coastal uplift is thus key to elucidate the recent vertical deformation of the Caribbean geodynamic puzzle with regards to the active tectonic segmentation of this area. We compiled bibliographic data and present new measurements concerning the Cuban sequences of coral reef and marine terraces; maximum elevations, minimum number of successive strandlines and elevation of the lowermost terrace. The Cuban Archipelago exhibits five main uplifting coastal stretches separated by subsiding areas, with at least 23 emerged staircase sequences of coastal terraces. At four sites, the lowest coral reef terrace has been previously correlated to the Last Interglacial Maximum (MIS 5e, 122 ± 6 ka). At nine sites, we extended the morpho-stratigraphy to derive Upper Pleistocene apparent and eustasy-corrected uplift rates. Alongshore Cuba, MIS 5e coastal terraces and associated shoreline angles occur at elevations ranging from 7 m to 40 m, yielding eustasy-corrected uplift rates ranging from 0.06 ± 0.01 mm.yr−1 (NW Cuba) to 0.33 ± 0.01 mm.yr−1 (SE Cuba). More than 400 km northward of the transform fault, eustasy-corrected uplift rates (0.13 mm.yr−1) suggest that the whole Cuban Archipelago is affected by the North America/Caribbean plate motion, with a partitioned compressive component resulting in block tectonics with tilting controlled by regional faults.

Multi-stage, Upper Eocene-Oligocene anatexis in the Xolapa metamorphic belt (Puerto Escondido, Mexico): Dynamics of the Xolapa Complex as the decoupled lower crust of the Chortís Block upper crust during its tectonic migration

Three successive strain regimes spanning up to ~16 million years were identified from the field, structural and geochronological analyses in Upper Eocene-Oligocene migmatites of the Xolapa Complex around the Puerto Escondido longitude (~97°W), in southern Mexico. A west-facing asymmetric folding affecting diatexites at ca. 38–31 Mad defines Regime A. Regimes B and C affected younger metatexites. Regime B recorded an outward extension at ca. 31–25 Ma, whereas Regime C occurred at ca. 25–22 Ma as NE-directed thrusting. By coupling the Upper Cretaceous-Oligocene tectonic framework of southern Mexico with the dynamics and timing of the identified strain regimes, we interpret that Regimes A to C recorded distinct stages of the separation and eastward migration of the Chortís Block, viewed as a continuous geodynamic episode. According to our proposal, the upper and lower crust of Chortís adjacent to nuclear Mexico were vertically decoupled before its separation by the presence of pre-Eocene crustal anisotropies. Regime A occurred when the Chortís Block rode over the Xolapa Complex during its initial eastward escape. Since this episode, the Chortís Block (upper crust) was captured by the Caribbean plate, whereas the Xolapa Complex (lower crust) remained as the southernmost North America plate. Chortís eastward migration and the consequent unloading of its decoupled lower crust triggered anatexis, isostatic adjustments, and diachronous uplift of the Xolapa Complex (Regime B). The progressive substitution of Chortís by its thinner lower crust in the tectonic configuration next to nuclear Mexico gradually enhanced the local convergence rate between the Farallon/Cocos plate and just-severed North America. The Farallon/Cocos plate was accreted to North America (Regime C) after a critical lengthening of the uprising lower crust along the southern Mexican margin. The local increase of the convergence rate promoted the landward consumption of the Xolapa Complex by subduction erosion since 22 Ma.

Exploring GPS Observations of Postseismic Deformation Following the 2012 MW7.8 Haida Gwaii and 2013 MW7.5 Craig, Alaska Earthquakes: Implications for Viscoelastic Earth Structure

AbstractThe Queen Charlotte‐Fairweather Fault (QC‐FF) system off the coast of British Columbia and southeast Alaska is a highly active dextral strike‐slip plate boundary that accommodates ∼50 mm/yr of relative motion between the Pacific and North America plates. Nine MW ≥ 6.7 earthquakes have occurred along the QC‐FF system since 1910, including a MS(G‐R)8.1 event in 1949. Two recent earthquakes, the October 28, 2012 Haida Gwaii (MW7.8) and January 5, 2013 Craig, Alaska (MW7.5) events, produced postseismic transient deformation that was recorded in the motions of 25 nearby continuous Global Positioning System (cGPS) stations. Here, we use 5+ yr of cGPS measurements to characterize the underlying mechanisms of postseismic deformation and to constrain the viscosity structure of the upper mantle surrounding the QC‐FF. We construct forward models of viscoelastic deformation driven by coseismic stress changes from these two earthquakes and explore a large set of laterally heterogeneous viscosity structures that incorporate a relatively weak back‐arc domain; we then evaluate each model based on its fit to the postseismic signals in our cGPS data. In determining best‐fit model structures, we additionally incorporate the effects of afterslip following the 2012 event. Our results indicate the occurrence of a combination of temporally decaying afterslip and vigorous viscoelastic relaxation of the mantle asthenosphere. In addition, our best‐fit viscosity structure (transient viscosity of 1.4–2.0 × 1018 Pa s; steady‐state viscosity of 1019 Pa s) is consistent with the range of upper mantle viscosities determined in previous studies of glacial isostatic rebound and postseismic deformation.

Tectonostratigraphic record of late Miocene–early Pliocene transtensional faulting in the Eastern California shear zone, southwestern USA

AbstractThe Eastern California shear zone (ECSZ; southwestern USA) accommodates ~20%–25% of Pacific–North America relative plate motion east of the San Andreas fault, yet little is known about its early tectonic evolution. This paper presents a detailed stratigraphic and structural analysis of the uppermost Miocene to lower Pliocene Bouse Formation in the southern Blythe Basin, lower Colorado River valley, where gently dipping and faulted strata provide a record of deformation in the paleo-ECSZ. In the western Trigo Mountains, splaying strands of the Lost Trigo fault zone include a west-dipping normal fault that cuts the Bouse Formation and a steeply NE-dipping oblique dextral-normal fault where an anomalously thick (~140 m) section of Bouse Formation siliciclastic deposits filled a local fault-controlled depocenter. Systematic basinward thickening and stratal wedge geometries in the western Trigo and southeastern Palo Verde Mountains, on opposite sides of the Colorado River valley, record basinward tilting during deposition of the Bouse Formation. We conclude that the southern Blythe Basin formed as a broad transtensional sag basin in a diffuse releasing stepover between the dextral Laguna fault system in the south and the Cibola and Big Maria fault zones in the north. A palinspastic reconstruction at 5 Ma shows that the southern Blythe Basin was part of a diffuse regional network of linked right-stepping dextral, normal, and oblique-slip faults related to Pacific–North America plate boundary dextral shear. Diffuse transtensional strain linked northward to the Stateline fault system, eastern Garlock fault, and Walker Lane, and southward to the Gulf of California shear zone, which initiated ca. 7–9 Ma, implying a similar age of inception for the paleo-ECSZ.

Stratigraphy and Geological Structures of the Miura Group in the Ubajima Islands, Northernmost Sagami Bay: Multiple Duplexes and Fault-propagation Folds Based on Tracing Tephra Key Beds

Mapping was carried out of well exposed folded and faulted strata in the Ubajima Islands off Chigasaki City, Kanagawa Prefecture, central Japan of the northernmost part of Sagami Bay, where the present Philippine Sea plate subduction boundary extends beneath the central part of the North America plate. The strata were dated with radiolarian fossils as middle to late Miocene, and correlated with part of the Misaki Formation of the Miura Group of the southern Miura Peninsula. The strata are composed of dominant basaltic pyroclastic or volcaniclastic sedimentary rocks with sporadic felsic tephra layers and rare silty sedimentary rocks, and were defined as the Ubajima Formation. Detailed field mapping of the Ubajima Formation verified that the strata are deformed in a complicated way into duplex structures having two stages. The first stage is characterized by bedding parallel to subparallel thrust faults in the SW direction of the horizontal reference frame. This might be of the antiformal stack type known by many parallel repetitions of the same horizons. At the second stage, the same parts are further duplicated into other antiformal stack-type duplex structures, forming multi-duplex structures. Finally those structures are folded and faulted by a fault-propagation fold system that has strong shear zones at propagation fault zones. The Ubajima Formation is thought to be deposited on the forearc area of the Izu volcanic arc, then accreted to the Honshu side by accretionary prism formation. These structures represent good examples of accretionary prism toes along subduction zones.