Transform Fault System Research Articles

A new geological framework for the Middle Ordovician Carillon Formation (uppermost Beekmantown Group, Ottawa Embayment): onset of Taconic foreland deposition and tectonism within the Laurentian platform interior

The Carillon Formation underlies the Ottawa Embayment, a craton-interior extension of the regional Laurentian paleoshelf, defined today by the Ottawa–Bonnechere Graben. Facies within the embayment during the Middle Ordovician identify a mixed-sediment (carbonate, siliciclastic), tide-influenced, peritidal and schizohaline basin, host to a mosaic of dolomitic tidal mudflats, shallow lagoons or ponds, and channels. Skeletal metazoans were rare, but bioturbation increased with episodic influx of less saline water carrying sand and mud, possibly in response to regional climate change. Stratigraphic analysis reveals that the formation coincides with a third-order eustatic sea level cycle, but higher order stratigraphic patterns show no obvious eustatic link. Instead, a stepwise onlap geometry coincided with local faulting that produced widespread syndepositional sediment deformation. In particular, stages of faulting and post-Carillon erosion are mapped along the present-day trace of a northwest–southeast-oriented lineament, the Gloucester Fault, within the present graben. Other present-day northwest–southeast-oriented faults of the graben may also possess Middle Ordovician histories. Platform-interior tectonism and development of the Carillon Formation coincided with the onset of Taconic deformation along the outer paleoshelf (western Newfoundland). The formation defines the initial stage of foreland basin sedimentation in the embayment. Platform-interior faulting identifies a predisposed structural weakness associated with underlying Neoproterozoic structure, possibly enhanced by a regional structural connection (an ocean-to-onland transform fault system) with the distal (~1200 km), convergent, plate boundary.

Coulomb Stress Transfer and Tectonic Loading Preceding the 2002 Denali Fault Earthquake

Pre-2002 tectonic loading and Coulomb stress transfer are modeled along the rupture zone of the M 7.9 Denali fault earthquake (dfe) and on adjacent segments of the right-lateral Denali–Totschunda fault system in central Alaska, using a three-dimensional boundary-element program. The segments modeled closely follow, for about 95°, the arc of a circle of radius 375 km centered on an inferred asperity near the northeastern end of the intersection of the Patton Bay fault with the Alaskan megathrust under Prince William Sound. The loading model includes slip of 6 mm/yr below 12 km along the fault system, consistent with rotation of the Wrangell block about the asperity at a rate of about 1°/m.y. as well as slip of the Pacific plate at 5 cm/yr at depth along the Fairweather–Queen Charlotte transform fault system and on the Alaska megathrust. The model is consistent with most available pre-2002 Global Positioning System (gps) displacement rate data. Coulomb stresses induced on the Denali–Totschunda fault system (locked above 12 km) by slip at depth and by transfer from the M 9.2 Prince William Sound earthquake of 1964 dominated the changing Coulomb stress distribution along the fault. The combination of loading (∼70–85%) and coseismic stress transfer from the great 1964 earthquake (∼15–30%) were the principal post-1900 stress factors building toward strike-slip failure of the northern Denali and Totschunda segments in the M 7.9 earthquake of November 2002. Postseismic stresses transferred from the 1964 earthquake may also have been a significant factor. The M 7.2–7.4 Delta River earthquake of 1912 (Carver et al. , 2004) may have delayed or advanced the timing of the dfe, depending on the details and location of its rupture. The initial subevent of the 2002 dfe earthquake was on the 40-km Susitna Glacier thrust fault at the western end of the Denali fault rupture. The Coulomb stress transferred from the 1964 earthquake moved the Susitna Glacier thrust fault uniformly away from thrust failure by about 100 kPa. The initiation of the Denali fault earthquake was advanced by transfer of 30–50 kPa of positive Coulomb stress to the Susitna Glacier fault (Anderson and Ji, 2003) by the nearby M 6.7 Nenana Mountain foreshock of 23 October 2002. The regional tectonic loading model used here suggests that the Semidi (Alaska Peninsula) segment of the megathrust that ruptured in 1938 ( M 8.2) may be reloaded and approaching failure.

Introduction: Evolution of continental extensional tectonics of western Turkey

(2005). Introduction: Evolution of continental extensional tectonics of western Turkey. Geodinamica Acta: Vol. 18, No. 3-4, pp. 153-165.

Stress Distribution along the Fairweather-Queen Charlotte Transform Fault System

Tectonic loading and Coulomb stress transfer are modeled along the right-lateral Fairweather–Queen Charlotte transform fault system using a three-dimensional boundary element program. The loading model includes slip below 12 km along the transform as well as motion of the Pacific plate, and it is consistent with most available Global Positioning System (gps) displacement rate data. Coulomb stress transfer is shown to have been a weak contributing factor in the failure of the southeastern (Sitka) segment of the Fairweather fault in 1972, hastening the occurrence of the earthquake by only about 8 months. Failure of the Sitka segment was enhanced by a combination of cumulative loading from below (95%) by slip of about 5 cm/yr since 1848, by stress transfer (about 1%) from major earthquakes on straddling segments of the Queen Charlotte fault ( M 8.1 in 1949) and the Fairweather fault ( M 7.8 in 1958), and by viscoelastic relaxation (about 4%) following the great 1964 Alaska earthquake, modeled by Pollitz et al. (1998). Cumulative stress increases in excess of 7 MPa at a depth of 8 km are projected prior to the M 7.6 earthquake. Coulomb stress transferred by the rupture of the great M 9.2 Alaska earthquake in 1964 (Bufe, 2004a) also hastened the occurrence of the 1972 event, but only by a month or two. Continued tectonic loading over the last half century and stress transfer from the M 7.6 Sitka event has resulted in restressing of the adjacent segments by about 3 MPa at 8 km depth. The occurrence of a M 6.8 earthquake on the northwestern part of the Queen Charlotte fault on 28 June 2004, the largest since 1949, also suggests increased stress. The Cape St. James segment of the fault immediately southeast of the 1949 Queen Charlotte rupture has accumulated about 6 MPa at 8 km through loading since 1900 and stress transfer in 1949. A continued rise in earthquake hazard is indicated for the Alaska panhandle and Queen Charlotte Islands region in the decades ahead as the potential for damaging earthquakes increases.

Cenozoic volcanism in the Middle East: petrogenesis of alkali basalts from northern Lebanon

The Cenozoic volcanic field of the Akkar region in northern Lebanon consists of a thick succession (200 m) of basaltic lava flows, erupted at the junction between a restraining bend (the Yammouneh transform fault) and its northern extension (the Ghab transform) in Syria. Both faults are part of the Dead Sea transform fault system, which represents the boundary between the Arabian and African plates and the Levantine subplate. The lavas are made up of about 15–25 vol. % olivine (Fo79–84), 30–40 % clinopyroxene (salite), 40–50 % plagioclase (An58–67), and opaque Fe–Ti oxides (∼ 5 %). Geochemically, they exhibit a narrow range of SiO2 (44.6 to 47.0 wt %), and MgO (2.9 to 7.5 wt %), are relatively enriched in TiO2 (2.0 to 2.9 wt %), and are classified as alkali basalts. Mg-numbers range from 0.32 to 0.59, with an average of 0.47. The rocks are enriched in incompatible trace elements such as Zr (98–184 ppm), Nb (16–39 ppm) and Y (25–34 ppm). The REE patterns are fractionated ((La/Yb)N=8.2), and are generally parallel to subparallel. Such compositions are typical of those of HIMU-OIB and plume-related magmas. Elemental ratios such as K/P (2.9), La/Ta (21.8), La/Nb (0.80), Nb/Y (0.92) and Th/Nb (0.35), and the low average SiO2 content (46.1 wt %) suggest that the magma was subjected to minimal crustal contamination. This may be related to a rapid ascent of the parental magma, in agreement with the nature (mafic, oceanic crust-like) and the thickness (only about 12 km) of the crust of the Eastern Mediterranean region. Cenozoic volcanism in this region is interpreted to have occurred in association with an episode of localized extension, particularly at the junction between the Yammouneh restraining bend and the Dead Sea–Ghab Transform (that is, in a transtensional tectonic regime). The 143Nd/144Nd isotopic composition of the basaltic rocks of northern Lebanon ranges from 0.512842 to 0.512934 (εNd=4.0 to 5.8), and 87Sr/86Sr from 0.703317 to 0.703579, suggesting a HIMU-like mantle source. Modelling indicates that the magma was produced by a small degree of partial melting (F=2 %) of a primitive, garnet lherzolitic mantle source, possibly containing a minor spinel component.

Evaluation of geodynamic activity of the Dead Sea transform fault by radon gas concentrations

Twelve radon lines of dosimeters (detectors) were placed across the Jordan Valley active fault, which is a segment of the active Dead Sea transform fault system. Each line of the dosimeters shows one or more peaks of radon anomaly concentrations. Some of these peaks prove the intersection of the fault trace with these lines in areas where the fault plane is inferred. In other lines, the peaks correspond to the arrangement of faults in areas of pull-apart basins (sag ponds) or pressure ridges, formed due to the left or right step of the fault. Sag ponds usually show low radon emanations, because they are the place for the accumulation of very fine sediments, which decreases their porosity and hence the upward migration of the radon gas. The northern part of the Jordan Valley relatively shows high radon emanation, which could be attributed to the presence of a seismic gap in the upper Jordan valley.

Late Cretaceous tectonic history of the Sierra‐Salinia‐Mojave arc as recorded in conglomerates of the Upper Cretaceous and Paleocene Gualala Formation, northern California

Upper Cretaceous and Paleogene conglomerates in the Gualala basin of northern California likely preserve the westernmost record of sedimentation in California during the interval between cessation of Sierran batholith magmatism and inception of Neogene transform activity on the San Andreas fault system. Detailed chemical and H, O, Sr, Nd, and Pb isotope analyses of conglomerate clasts, as well as U/Pb zircon ages, identify two source types: (1) evolved granite, like those found in the Salinian block or eastern Sierra Nevada batholith, and (2) primitive oceanic arcs that were restricted to the western portions of the Mesozoic arc system. The provenance of the Gualala clasts can be entirely explained through derivation from the Sierra‐Salinian‐Mojave portion of the Mesozoic Cordilleran arc, and do not support models for extensive large‐scale translation of the basin from low latitudes along a pre‐Neogene transform fault system. Contemporaneous deposition of the contrasting lithologies in the Gualala basin is interpreted to reflect tectonic juxtaposition of the source terranes approximately 80 m.y. ago. Tectonic processes responsible for juxtaposition of the two source regions in the Late Cretaceous most probably involved zones of lithospheric weakness that later localized the San Andreas fault system in the Neogene. Chemical, isotopic, and age data tie the Gualala basin to North American continental basement in the Mojave‐Salinian segment of the Mesozoic Cordilleran arc and imply minimal pre‐Neogene translation of the basin contrary to the low latitude origin for the basin inferred from previous paleomagnetic and paleontologic studies.

Active deformation and shallow structure of the Wagner, Consag, and Delfín Basins, northern Gulf of California, Mexico

Oblique rifting began synchronously along the length of the Gulf of California at 6 Ma, yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern Gulf. Instead, multichannel seismic data show a broad shallow depression, ∼70 × 200 km, marked by active distributed deformation and six ∼10‐km‐wide segmented basins lacking well‐defined transform faults. We present detailed images of faulting and magmatism based on the high resolution and quality of these data. The northern Gulf crust contains a dense (up to 18 faults in 5 km) complex network of mainly oblique‐normal faults, with small offsets, dips of 60–80° and strikes of N‐N30°E. Faults with seafloor offsets of tens of meters bound the Lower and two Upper Delfín Basins. These subparallel basins developed along splays from a transtensional zone at the NW end of the Ballenas Transform Fault. Twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone. A structural connection between the two Upper Delfín Basins is evident in the switching of the center of extension along axis. Sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material, atypical of mid‐ocean ridges. On the basis of the near‐surface manifestations of active faulting and magmatism, seafloor spreading will likely first occur in the Lower Delfín Basin. We suggest the transition to seafloor spreading is delayed by the lack of strain‐partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover.

Multi-stage evolution of the Tertiary Mineoka ophiolite, Japan: new geochemical and age constraints

Abstract The Mineoka ophiolite in the southern Boso Peninsula is situated in a unique tectonic setting in the collisional zone between the Izu and Honshu arcs in Japan. The ophiolitic rocks are composed mainly of tholeiitic pillow basalts and dolerites, alkali-basaltic sheet flows, and calc-alkaline dioritic to gabbroic rocks. The tholeiitic basalts show variable trace element compositions ranging from mid-ocean ridge basalt to island-arc basalt, whereas the alkali-basalts have a within-plate affinity. High-Fe and -Ti tholeiitic basalt and within-plate alkali-basalt have Ar/Ar ages of 49 ± 13 Ma and 19.62 ± 0.90 Ma, respectively. Three plutonic rocks have K-Ar ages of c. 25, 35 and 40 Ma. These ages are inconsistent with the known ages from the Pacific or Philippine Sea Plate. We infer that the Mineoka ophiolitic assemblage was part of another Tertiary oceanic plate, the ‘Mineoka Plate’, which underwent island-arc volcanism in the Miocene as a result of subduction initiation at a fracture zone or a transform fault system due to a change in the position of the Euler rotation pole of the Pacific Plate at c. 43 Ma. Eruption of within-plate type alkali basalts on the Mineoka Plate took place near the palaeo-Japan continental arc just before the emplacement of the Mineoka ophiolite into the Japanese continental margin.

What is the reason for the Dead Sea Transform fault system segmentation?

What is the reason for the Dead Sea Transform fault system segmentation?

Regional 2D seismic interpretation and exploration potential of offshore deepwater Sierra Leone and Liberia, West Africa

The offshore region of Sierra Leone and Liberia has been underexplored on the continental shelf and unexplored in deepwater. However, a reconnaissance interpretation of new 2D seismic data, in conjunction with a review of the reservoir and geochemical information available to date, is positive about the hydrocarbon potential of the region. Three distinct prospective hydrocarbon basins are present—Sierra Leone Basin, Liberia Basin, and Harper Basin (Figure 1). Figure 1. Location of Sierra Leone, Liberia, and Harper basins. Fault lines (blue) indicate major strike-slip fault systems. Nine exploratory wells have been drilled and abandoned on the shelf, at the limits of the study area, considerably shoreward of the potential deepwater basins. In terms of exploration maturity, considering a well density of one well per 6700 km2, the area should be classified as “frontier.” However, the regional potential is far from unknown. The nine wells provide an abundance of key data, which contribute to the identification of several hydrocarbon systems. The primary petroleum system contains at least three oil-prone, marine, and lacustrine source rock zones deposited during Lower Cretaceous time (Early Albian to Early Ceno-manian). A secondary petroleum system may have a Late Cenomanian to Turonian shale source rock. Trap types are numerous and relatively widespread, in particular those associated with Cretaceous syn-rift and transform related faulting and unconformities. In addition, traps on the slope and in the basin are ideally located for access to oil migration. The basins developed in two phases, a syn-rift phase and a passive margin phase, both significantly overprinted by wrenching associated with Atlantic transform fault systems. The main rift phase, accompanied by continental to marginally marine sedimentation, took place from Lower Cretaceous Aptian to Middle Albian time. The passive margin and wrenching phase, initiated with seafloor spreading, began in Late Albian time and continues to the …

Seamounts at the continental margin of California: A different kind of oceanic intraplate volcanism

Davidson, Guide, Pioneer, Gumdrop, and Rodriguez Seamounts, located at the continental margin offshore California between latitudes 37.5°N and 34.0°N, may represent a previously unrecognized type of intraplate oceanic volcanism. Morphologically unlike typical oceanic-island volcanoes or near-ridge seamounts, they are complex, northeast-trending ridges that reflect the ridge-parallel structure of the underlying oceanic crust. 40Ar/39Ar laser fusion ages of mineral separates indicate at least two episodes of volcanism at ca. 16 and ca. 12 Ma, younger by 7–11 m.y. than the underlying ocean crust. Volcanic rocks are predominantly differentiated alkalic basalt, hawaiite, and mugearite. The lack of coherent liquid- lines-of-descent is consistent with small batches of magma forming, fractionally crystallizing, and erupting in isolation from the previous and subsequent batches. The presence of mantle xenoliths suggests that magmas originated in the upper mantle. Xenoliths of alkalic cumulates and xenocrysts of feldspar, amphibole, and titanomagnetite are consistent with fractionation in the upper mantle as well. Sr, Nd, and Pb isotope ratios of some samples are from depleted-mantle sources like those of MORB (mid-oceanic-ridge basalt); others indicate more variably enriched mantle sources. Magmas formed by small- percentage partial melting of variably enriched MORB-source-type mantle, according to the samples' high abundances of incompatible elements such as Nb, Ta, and Th. Coeval middle Miocene basalts in onshore coastal California have a similar isotopic range, but their trace elements show a subduction-related signature. If the coeval volcanic rocks offshore originated in a slab window, as proposed for those onshore, they were not influenced by a slab component. Although a slab-window origin is plausible for the middle Miocene volcanism, later episodes of small, sporadic eruptions on- and offshore probably resulted from decompression melting of mantle rising along existing zones of weakness undergoing extension related to continued movement along transform-fault systems.

Newly identified segments of the Pacific–Australia plate boundary along the North Fiji transform zone

The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific–Australia plate boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise (March 2000) on board the R/V L’Atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100° has been mapped over a distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature. The uplifted Futuna–Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna spreading center trending 20–30° is composed of a series of en échelon left-stepping spreading segments. It represents a 200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones. The three active extensional zones have an ENE–WSW to NNE–SSW orientation, while compressive features along the Futuna transform fault are NW–SE to NNW–SSE oriented, in accordance with the present-day left-lateral transform motion along this part of the Pacific–Australia plate boundary.

Seismic reflection profiles across the southern Dead Sea basin

The main structural and stratigraphic elements that form the Southern Dead Sea basin (SDSB), which is part of the Dead Sea transform fault system, have been broadly delineated with various degrees of certainty by a number of researchers. The recent exchange of seismic data between Israel and Jordan enable us to fill in some of the dashed lines and eliminate some of the question marks that appear on the previously published geological cross-sections. Two east–west seismic time sections, one from each side of the international border that dissect the basin, have been selected, composed and interpreted. A remarkable correlation of the three major sedimentary sequences that form the basin-fill (Miocene clastics, Pliocene salt and Pleistocene–Holocene clastics and evaporites) has been established between the lines and, hence, across the entire width of the SDSB. The Miocene and Pliocene series are cut off in the east by the Ghor–Safi fault that is the equivalent of the western Sedom fault, thus, indicating that all three major structural steps, known to exist in the west: (1) rim block, (2) intermediate block and (3) deep block, occurs also in the east, and that the SDSB is indeed a full graben. The Ghor Safi fault is the northern extension of the major Arava strike-slip fault. The very thin Miocene (if at all) and lack of Pliocene salt in the eastern intermediate block, unlike the western one, suggests that the Ghor–Safi fault was initiated earlier then the Sedom fault. Salt is present over the entire width of the basin but does not exceed the 900 m that were penetrated by the Sedom Deep-1 borehole. The salt is conformable with the overlying thick Pleistocene and except for the Sedom diapir, no halokinesis phenomena have been recognized to be associated with both units.

Source Characteristics of a 5.5 Magnitude Earthquake that Occurred in the Transform Fault System of the Delfin Basin in the Gulf of California

Portable and permanent broadband seismic stations in the neighborhood of the Gulf of California recorded a moment magnitude Mw 5.5 event on 26 Novem- ber 1997. This is the first time that a moderate event located in the Gulf of California extensional province was well recorded by local broadband seismic stations. The event was located at 29.754 N and 113.708 W and at a focal depth of 5.0 km in the southeastern end of the transform fault that connects the lower and upper Delfin basins. The hypocentral location and the results of the wave modeling indicate that this is a complex event that originated in the pull-apart Delfin basin. The focal mech- anism estimated from first motions ( 310, d 83, k 97) and body-wave modeling of P waves in the frequency band 0.05-0.5 Hz suggests that the rupture started with dip-slip (reverse faulting) motion and ended releasing the bulk of energy through strike-slip motion. Synthetics of surface waves in the frequency band 0.05- 0.1 Hz were also calculated using a triangular source-time function of 3 sec. The best match between the synthetics and observed surface waves recorded at 90 km from the epicenter was obtained using a fault geometry defined by a strike of 330 15, dip 85 5, and slip of 165 15. The spectral analysis of the Lg phase recorded at stations in the Peninsular Ranges gives a seismic moment of 1.28 10 17 N m (1.28 10 24 dyne cm), a source radius of 6.3 km and a stress drop of 0.22 MPa (2.2 bar). The source parameters inferred with S-wave spectra and the same model (Brune, 1970) give similar values.

Structure and tectonic development of the Ghab basin and the Dead Sea fault system, Syria

We examine the structure and evolution of the Ghab basin that formed on the active, yet poorly understood, northern segment of the Dead Sea transform fault system. The basin formed in Plio-Quaternary time at a complex step-over zone on the fault. Subsidence occurred along cross-basin and transform-parallel faults in two asymmetric depocentres. The larger depocentre in the south of the basin is asymmetric towards the east, the margin along which most active transform displacement apparently occurs. The Syrian Coastal Ranges, located directly west of the Ghab basin, are a consequence of Late Cretaceous and Cenozoic regional compression, heavily modified by the Dead Sea fault system and Ghab basin formation. We prefer a model whereby the Dead Sea fault system in northwest Syria developed in Plio-Quaternary time, consistent with previously proposed models of two-phase Dead Sea fault system movement and Red Sea spreading.

Viscoelastic‐coupling model for the earthquake cycle driven from below

In a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself (viscoelastic‐coupling model) and a solution that provides the driving force. We consider two cases, one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle (i.e., the schizosphere and upper mantle, respectively, act as stress guides in the lithosphere). In both cases the driving stress is attributed to steady motion of the stress guide, and the upper crust is assumed to be elastic. The surface deformation that accumulates during the interseismic interval depends solely upon the earthquake‐cycle solution (viscoelastic‐coupling model) not upon the driving source solution. Thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system. In particular, the suggestion of Bourne et al. [1998] that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems.

Structural controls on the continent‐ocean transition in the northern Gulf of California

In the Gulf of California the Pacific‐North America plate boundary changes character from an oceanic‐type spreading center and transform fault system (to the south) to a region of diffuse continental deformation (to the north). The presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric, heat flow, gravity, or seismic data which indicate significant differences north and south of latitude ∼30°N. We suggest instead that north of ∼30°N a continent‐ocean transition begins which we name the Wagner Transition Zone (WTZ). Diffuse deformation characterizes the WTZ where slip occurs along reactivated north to NNW striking normal faults developed during late Miocene or Pliocene ENE directed extension. Transtensional deformation varies from ENE directed extension along dip‐slip faults in the west to dextral shear along the coast to dextral‐oblique slip along inferred north to NNW striking faults submerged in the northern gulf. By accounting for rotational and extensional plate motion deformation in northeastern Baja California, vector constraints require that submerged structures accommodate ∼30 mm/yr of slip in a direction slightly clockwise of the relative plate motion direction. The juxtaposition of the discrete spreading center system in the central gulf with the diffuse WTZ appears to have been a stable configuration since 4–6 Ma, perhaps controlling the evolution of spreading center jumps between Upper and Lower Tiburón and Delfin basins due to the juxtaposition of kinematically partitioned structural domains. Different histories of prerift extension and subduction‐related arc magmatism along the length of the gulf, partly related to the migration of the Rivera triple junction, may explain the location of the continent‐ocean transition.

Seismic properties of the Eltanin Transform System, South Pacific

We present a compilation of the seismic properties of the Eltanin Transform Fault (TF) system, compelled by the recent discovery of the Hollister Ridge and the possibility of a change of plate kinematics pattern in the region. The Hollister Ridge is a major volcanic system located on the Western flank of the Pacific–Antarctic Ridge (PAR), immediately south of the Eltanin Fracture Zones (FZs). We find or confirm two anomalous characteristics: the occurrence of seven normal faulting events on the transform segments, expressing extension across the plate boundary in the azimuth N26°W, and more than 90% deficiency in the seismic moment released during strike–slip events on the transforms, as compared to the rate expected from kinematic models. Other seismic properties are typical of the seismicity of a fast-spreading mid-ocean ridge (MOR) system. In particular, we could not document a single teleseismically recorded event on the Hollister Ridge; earthquakes are confined to narrow TFs with no activity present on the ridge segments. The transform events have regular frequency–moment statistics, and we could not document any significantly slow sources. These seismic properties generally support conventional plate tectonics models such as NUVEL-1, and cannot be reconciled with a proposed reorientation of the Pacific plate 4 Ma ago.

Distribution of displacement on and evolution of a young transform fault system: The northern San Andreas fault system, California

This paper presents a working model for the spatial and temporal distribution of dextral slip on the northern San Andreas fault system of coastal California, based primarily on field relations in the San Francisco Bay area, and offers insight into the evolution of a young transform fault system. A fundamental difference between this and previous models of slip distribution is that this model assigns negligible slip to the Pilarcitos fault, which has been suggested to have 120 to 250 +km of post early‐Miocene dextral slip, in previous models. Because separation on the San Francisco Peninsula reach of the San Andreas fault is about 25 km, and displacement on the San Andreas fault in central California is 310 to 320 km, more than 250 km of late Cenozoic dextral slip must be accommodated east of the San Francisco Peninsula. The distribution of this dextral slip is constrained by offset late Cenozoic and basement units. Slip distribution, combined with ages of offset features and plate boundary kinematic constraints, show that the distribution of slip rates on groups of faults along the transform boundary has changed in an irregular fashion through time, in contrast to existing models that propose progressive eastward migration of active faulting in the San Andreas system. In addition to the shifting patterns of displacement, a migrating transition zone connecting the eastern faults of the strike‐slip system to the Mendocino triple junction may have resulted in distributed dextral faulting and shortening in the northernmost and youngest part of the transform fault system.