Gorda Escarpment Research Articles

Uplift, Emergence, and Subsidence of the Gorda Escarpment Basement Ridge Offshore Cape Mendocino, CA

AbstractThe Gorda Escarpment is a topographic step that characterizes the south side of the Mendocino Transform Fault east of 126oW and forms the northern edge of the Vizcaino Block. Seismic reflection data suggest that the basement beneath the northern edge of the Vizcaino is composed of east‐west trending slivers of oceanic crust that form a 15 km wide band of buried ridges we call the Gorda Escarpment Basement Ridge (GEBR) to distinguish it from the northwest‐trending basement structure that characterizes most of the Vizcaino Block. The history of uplift and subsidence of the GEBR is reconstructed by combining the seismic reflection profiles with age and lithological constraints from ODP Site 1022, gravity cores, and grab samples from the northern face of the Escarpment recovered using a remotely operated vehicle. Uplift of the GEBR began prior to 6 Ma, and it was above sea level 3.7–2.5 Ma. GEBR uplift and emergence coincided with sediment deposition on the southern flank of the GEBR that we interpret as indicative of strong upwelling and turbulence in the lee of a shallow ridge and island chain. A bright reflection, interpreted to be a sill, is observed south of the shallowest part of the GEBR. We speculate that this sill may reflect a larger, hidden intrusion at depth and that thermal expansion of the crust combined with tectonic forces to drive enhanced uplift of this segment of the plate boundary. The GEBR has been subsiding since 2.7 Ma, and its shallowest point is now 1,400 m below sea level.

The Morphometry of the Deep-Water Sinuous Mendocino Channel and the Immediate Environs, Northeastern Pacific Ocean

Mendocino Channel, a deep-water sinuous channel located along the base of Gorda Escarpment, was for the first time completely mapped with a multibeam echosounder. This study uses newly acquired multibeam bathymetry and backscatter, together with supporting multichannel seismic and sediment core data to quantitatively describe the morphometry of the entire Mendocino Channel and to explore the age and possible causes that may have contributed to the formation and maintenance of the channel. The first 42 km of the channel is a linear reach followed for the next 83.8 km by a sinuous reach. The sinuous reach has a sinuosity index of 1.66 before it changes back to a linear reach for the next 22.2 km. A second sinuous reach is 40.2 km long and the two reaches are separated by a crevasse splay and a large landslide that deflected the channel northwest towards Gorda Basin. Both sinuous reaches have oxbow bends, cut-off meanders, interior and exterior terraces and extensive levee systems. The lower sinuous reach becomes more linear for the next 22.2 km before the channel relief falls below the resolution of the data. Levees suddenly decrease in height above the channel floor mid-way along the lower linear reach close to where the channel makes a 90° turn to the southwest. The entire channel floor is smooth at the resolution of the data and only two large mounds and one large sediment pile were found on the channel floor. The bathymetry and acoustic backscatter, together with previously collected seismic data and box and piston cores provide details to suggest Mendocino Channel may be no older than early Quaternary. A combination of significant and numerous earthquakes and wave-loading resuspension by storms are the most likely processes that generated turbidity currents that have formed and modified Mendocino Channel.

Plume 1400 Meters High Discovered at the Seafloor off the Northern California Margin

On 17 May 2009, the Kongsberg EM302 multibeam echo sounder on board the U.S. National Oceanic and Atmospheric Administration's (NOAA) Okeanos Explorer was collecting bathymetry and water column acoustic data offshore of northern California when it suddenly imaged a previously undiscovered 1400‐meter‐high plume (Figure 1) rising from the seafloor at 40°32.13′N, 124°47.01′W. The ship was mapping in water depths of approximately 1830 meters and heading east up the northern California continental margin 20 kilometers north of the Gorda escarpment. The continental shelf in this area is known to have subsurface and water column thermogenic and methane gas, although no plumes from this area previously have been reported from deeper than the continental shelf.

Geochemical and age constraints on the formation of the Gorda Escarpment and Mendocino Ridge of the Mendocino transform fault in the NE Pacific

The Mendocino transform fault is an active, dextral strike-slip zone that separates the Gorda plate from the Pacifi c plate in the NE Pacifi c Ocean. The compositions of the igneous rocks exposed along the southern margin of the Mendocino transform fault include tholeiitic and alkaline basalts. Majorelement, trace-element, and radiometric data suggest that the rocks were generated through fractionation of different parental melts, derived by varying degrees of partial melting from different depths, at or near the intersection of the Mendocino transform fault with the Gorda Ridge. There is evidence for extensive cooling and fractionation reminiscent of the transform-fault effect of Langmuir and Bender (1984). Alkaline and highAl compositions also argue for melts from a deeper source than a normal mid-ocean-ridge environment. The preferred geochemical analogue for the Mendocino transform fault is a failed rift system where mid-ocean-ridge basalt (MORB) compositions likely represent basalts created at a waning spreading center before its abandonment. The MORB compositions were subsequently buried by younger enriched (E-MORB) and alkaline basalts derived from deeper melting and/or a more enriched source. We suggest that a period of rift failure, abandonment, and continued alkaline volcanism occurred on the southernmost Gorda Ridge, or on a series of short intratransform spreading-center segments during plate reorganization. Thus, the Mendocino transform fault provides a record of ridge migration, abandonment, and residual volcanism of the southern Gorda Ridge spreading system from 23 to 11 Ma.

Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino, California. In spite of high sedimentation rates and high biological productivity, direct or indirect indicators of gas hydrate presence had not previously been recognized in this region, or along transform margins in general. Gas is indicated by a bottom simulating reflection (BSR) observed near the Gorda Escarpment, by “bright spots” and “gas curtains” scattered throughout the sedimentary basin to the south, and by δ13C and δ18O isotopes of carbonates, which are similar to those recovered from other hydrate‐bearing regions. The BSR reflection coefficient of −0.13 ± 0.04 and interval velocities as low as 1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR toward the north facing Gorda Escarpment and beneath a channel near the crest suggests fluid flow toward the seafloor. Integrating these various observations, we suggest a scenario in which methane is formed in thick Miocene and Pliocene deposits of organic‐rich sediments that fill the marginal basin south of the transform fault. Dissolved and free gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate formation and in channels, slumps and chemosynthetic communities on the face of the escarpment. We conclude that the BSR appears where hydrate‐bearing sediments are uplifted because of current triple junction tectonics.

Mass wasting, methane venting, and biological communities on the Mendocino transform fault

Chemosynthetic cold-seep vestimentiferan tubeworms and vesicomyid clams inhabiting oceanic basaltic rock have been discovered on the Gorda Escarpment sector of the Mendocino transform fault 73 km west of Cape Mendocino, northern California. The sparse cold-seep animals are biomarkers that identify zones of focused fluid venting from a methane gas horizon seismically imaged as a bottom-simulating reflector (BSR) within sediments on the southern flank of the escarpment. This is the first example of a methane-based cold-seep community on exposed oceanic basement rock within an area dominated by transform tectonics. This discovery extends the range of known environments in which the subsurface flow and venting of methane-rich fluids are linked both with geological activity and chemosynthetic communities. Observations by remotely operated vehicle of the distribution of the animals, sediment, basaltic talus, and basement outcrop delineate a large slump headscarp that channels subsurface fluid. Seismic surveys of the southern flank of the Mendocino transform fault (the Vizcaino block) define a BSR hosted in the thick sedimentary sequence that projects to the wall of the Gorda Escarpment at the same depth as the chemosynthetic community. The well-defined BSR in the marine sediment of the Vizcaino block results from an accumulation of methane gas possibly capped by methane hydrate. The isotopic composition of Mg-calcite found along the headscarp (δ 1 3 C = -65‰; δ 1 8 O = 4.8‰) is consistent with fluids derived from dissociated methane hydrate. We propose that the tectonic uplift along this transform margin has resulted in the lateral, northward movement of methane from the Vizcaino sedimentary sequence to the east-trending wall of the north-facing Gorda Escarpment.

Evolution of the Gorda Escarpment, San Andreas Fault and Mendocino triple junction from multichannel seismic data collected across the northern Vizcaino block, offshore northern California

The Gorda Escarpment is a north facing scarp immediately south of the Mendocino transform fault (the Gorda/Juan de Fuca‐Pacific plate boundary) between 126°W and the Mendocino triple junction. It elevates the seafloor at the northern edge of the Vizcaino block, part of the Pacific plate, ∼1.5 km above the seafloor of the Gorda/Juan de Fuca plate to the north. Stratigraphy interpreted from multichannel seismic data across and close to the Gorda Escarpment suggests that the escarpment is a relatively recent pop‐up feature caused by north‐south compression across the plate boundary. Close to 126°W, the Vizcaino block acoustic basement shallows and is overlain by sediments that thin north toward the Gorda Escarpment. These sediments are tilted south and truncated at the seafloor. By contrast, in a localized region at the eastern end of the Gorda Escarpment, close to the Mendocino triple junction, the top of acoustic basement dips north and is overlain by a 2‐km‐thick wedge of pre‐11 Ma sedimentary rocks that thickens north, toward the Gorda Escarpment. This wedge of sediments is restricted to the northeast corner of the Vizcaino block. Unless the wedge of sediments was a preexisting feature on the Vizcaino block before it was transferred from the North American to the Pacific plate, the strong spatial correlation between the sedimentary wedge and the triple junction suggests the entire Vizcaino block, with the San Andreas at its eastern boundary, has been part of the Pacific plate since significantly before 11 Ma.

Crustal structure of the northwestern Vizcaino block and Gorda Escarpment, offshore northern California, and implications for postsubduction deformation of a paleoaccretionary margin

The Vizcaino block is an anomalously shallow region of the western U.S. continental margin located southwest of the Mendocino triple junction. It originated as part of the accretionary prism of the North America plate and was transferred to the Pacific plate in the Miocene as the Pacific‐North America plate boundary migrated ∼130 km eastward, forming the Gorda Escarpment at its northern boundary. We present hybrid crustal models for the northwestern part of the Vizcaino block derived from marine seismic and gravity data. The velocity and density structure of the northwestern Vizcaino block are compatible with paleoaccretionary complex material similar to San Simeon/Patton terrane overlying oceanic crust or a mafic layer. The most significant result of our modeling is an abrupt increase in Moho dip from ∼5° to ∼20–30° beneath the western edge of the Oconostota ridge along the northwestern margin of the Vizcaino block. This Moho dip is steeper than observed anywhere along the Cascadia subduction zone, indicating postsubduction deformation. We suggest that the paleotrench was deformed by compression, which reactivated preexisting thrust faults in the upper crust and thickened the crust within this apparent weak zone. At least part of the deformation predates late Pliocene Pacific‐North America plate convergence and may result mainly from north‐south compression between the Pacific‐Juan de Fuca plates across the Mendocino transform fault. North‐south compression continues today and may dynamically support the uplifted northern margin of the Vizcaino block, although the primary locus of deformation shifted to the relatively weak Gorda plate sometime prior to 3 Ma.

Recent movements and stress field in the San Andreas fault system by the results of modelling

Tectonophysical studies have established that the presence of recent active tectonic faults causes a regular redistribution of the initial tectonic stress field and formation of sections of increased shear stresses. The modern regional tectonic stress field of California is characterized by a horizontal, meridional direction of the axis of maximum compression. These peculiarities allowed of performing modelling of the stress distribution in the San Andreas fault system on photoelastic models, using the methods of the Laboratory of Tectonophysics, Institute of Physics of the Earth, Academy of Sciences, U.S.S.R. The location of zones of increased shear stresses in elastic models, imitating the San Andreas fault system, agrees with the dislocation of seismically active regions, for instance, near the Gorda Escarpment, in the southern part of the San Andreas zone and within the angle between the San Andreas and Garlock faults. The study performed also confirms and explains certain regularities of right-lateral displacement in the San Andreas zone and the occurrence of rifts and spreading at its ends at the transition to the Pacific mid-oceanic Rise.

Microearthquake seismicity and tectonics of Coastal Northern California

Abstract This paper demonstrates that high-gain, high-frequency portable seismographs operated for short intervals can provide unique data on the details of the current tectonic activity in a very small area. Five high-frequency, high-gain seismographs were operated at 25 sites along the coast of northern California during the summer of 1968. Eighty per cent of 160 microearthquakes located in the Cape Mendocino area occurred at depths between 15 and 35 km in a well-defined, horizontal seismic layer. These depths are significantly greater than those reported for other areas along the San Andreas fault system in California. Many of the earthquakes of the Cape Mendocino area occurred in sequences that have approximately the same magnitude versus length of faulting characteristics as other California earthquakes. Consistent first-motion directions are recorded from microearthquakes located within suitably chosen subdivisions of the active area. Composite fault plane solutions indicate that right-lateral movement prevails on strike-slip faults that radiate from Cape Mendocino northwest toward the Gorda basin. This is evidence that the Gorda basin is undergoing internal deformation. Inland, east of Cape Mendocino, a significant component of thrust faulting prevails for all the composite fault plane solutions. Thrusting is predominant in the fault plane solution of the June 26 1968 earthquake located along the Gorda escarpement. In general, the pattern of slip is consistent with a north-south crustal shortening. The Gorda escarpment, the Mattole River Valley, and the 1906 fault break northwest of Shelter Cove define a sharp bend that forms a possible connection between the Mendocino escarpment and the San Andreas fault. The distribution of hypocenters, relative travel times of P waves, and focal mechanisms strongly indicate that the above three features are surface expressions of an important structural boundary. The sharp bend in this boundary, which is concave toward the southwest, would tend to lock the dextral slip along the San Andreas fault and thus cause the regional north-south compression observed at Cape Mendocino. The above conclusions support the hypothesis that dextral strike-slip motion along the San Andreas fault is currently being taken up by slip along the Mendocino escarpment as well as by slip along northwest trending faults in the Gorda basin.

Active rift system in the basin and range province

The active rift system along the postulated landward extension of the East Pacific Rise into western North America (Heezen, 1960; Menard, 1960; Cook, 1961) consists of several active rift zones which follow belts of seismicity and probable high heat flow and include: (a)the belt of grabens along the eastern margin of the Sierra Nevada, the Walker Lane, and the Churchill Arc; (b)the system of great trenches extending through western Arizona, central Utah, southeastern Idaho, western Wyoming, western Montana, and British Columbia; and (c)the Rio Grande rift belt. The present paper reviews the following additional recent geophysical evidence that defines in more detail the active rift system in the eastern part of the Basin and Range province: 1. (1) In north central Utah, gravity surveys, together with refraction and/ or reflection seismic data along some profiles, indicate that the Wasatch Trench is a northward-trending belt of grabens about 160 miles long and up to 13 miles wide. 2. (2) In south central and southwestern Utah, gravity surveys indicate grabens adjacent to the northward-trending Sevier and Hurricane fault zones, which lie along the rift belt. 3. (3) In New Mexico, several gravity profiles across the Rio Grande rift belt indicate the width and depth of the grabens. 4. (4) Sparse data tentatively indicate high heat flow in the rift belts. The possible mechanism of the Late Cenozoic vulcanism and diastrophism in the Basin and Range province is explained by a postulated mantle convection current (associated with the formation of the Rise) that rose beneath the eastern part of the Basin and Range province (or perhaps the Colorado Plateau) and flowed westward (Cook, 1961). The active rift system is in the region of tension over the upwelling convection currents. To the west, in the region of translation and compression, it is postulated that the westward-moving mantle convection current dragged along the continental crust in the California region. The crustal drag caused a large continental-type block to move westward into the Pacific Ocean about 50–100 km in the California region between the landward projections of the Mendocino and Murray fracture zones, which were probably major lines of weakness existing before the formation of the East Pacific Rise. On the northern and southern margins of the block, respectively, the movement occurred along transcurrent and/or transform faults in: 1. (1) the Cape Mendocino area (Gorda escarpment). 2. (2) the Garlock fault, Big Pine fault, Channel Islands. Transverse Ranges, and Pinto Mountain fault areas. The westward movement of the block, which occurred along transcurrent and/or transform faults associated with these two fracture zones, probably caused: (a) the folding of the Coast Ranges in California along the frontal (i.e., west) segment of the block and (b) the bending of the San Andreas fault zone in two areas along the southern margin of the block. The right-lateral movement of the San Andreas fault zone is explained by a postulated second, northwestward-trending mantle convection current with a velocity gradient increasing southwest; this movement has apparently moved into the western Basin and Range province recently to cause transcurrent faulting there (Billings, 1960), and a change in the direction of the mantle convection currents there is postulated. The concept that a westward-moving continental-type crustal block has overridden the oceanic crust in the Pacific Ocean and that the driving force probably originated in the upper-mantle spreading beneath the Basin and Range province (or perhaps the Colorado Plateau) is fundamental to the interpretation of the Late Cenozoic tectonics in the southwestern United States. If the United States, along with the North American continent, has drifted westward as a consequence of the sea-floor spreading along the Mid-Atlantic Ridge and overriden the East Pacific Rise, as postulated by Vine (1966), the westward movement of the above-mentioned crustal block would constitute about 50–100 km of horizontal displacement in addition to the “normal” westward drift of the North American continent as a whole.

Seismological evidence on the tectonics of central and northern California and the Mendocino escarpment

ABSTRACT Some 300 regional earthquakes have been located for 1962 through June 1965 using readings from local stations. The three greatest concentrations occur close to the north-facing Gorda escarpment off Cape Mendocino (latitude 40.°4 N); in the Coast Ranges west of the San Andreas fault (latitude 36.°8N) between Watsonville and Salinas; and 40 km southwest, in the Gabilan Range west of the San Andreas fault near the Pinnacles (latitude 36.°5 N). For 280 km northward of a small cluster which straddles the San Andreas off San Francisco, earthquakes were not detected near to the San Andreas but eastward in the Coast Ranges. The bulk of earthquakes with magnitudes in excess of 2 1 2 in the Coast Ranges have focal depths less than 5 km; no reliable depths were estimated greater than 20 km. Seismicity on the Gorda scarp is high; it decreases westward along the Mendocino escarpment. Submarine earthquakes were detected westward to the northerly-trending (Escanaba) trough which traverses the Gorda ridge and then northward along the Gorda ridge. Earthquakes also were located within the Gorda basin between the California coast and the Gorda ridge. The first motions of P were read for 29 earthquakes using mainly the high-resolution near-stations. The patterns are consistent with dominant right-lateral strike-slip along the San Andreas-Calaveras fault system from Parkfield to San Francisco Bay; some exceptional motion occurs near the San Andreas fault in the vicinity of the San Francisco Peninsula. In the Coast Ranges north of San Francisco Bay, right-lateral strike-slip appears again to be the rule. Along the Gorda wall, the P polarities are consistent with the local escarpment trend of about N70°W. Earthquakes on the continental shelf north of Cape Mendocino and in the Gorda basin have fault-plane solutions which again indicate right-lateral strike-slip with strikes of about N40°W. The motions are consistent with a broad system of branching faults whose trend at the northern extremity becomes aligned with the Blanco fracture zone. On the Gorda ridge, the shock of 18 April 1965 shows normal fault characteristics. Mechanisms for four earthquakes on the Blanco fracture zone are not uniform; right-lateral strike-slip motion is again found for the earthquakes of 8 May 1968 at 128° west and of 14 June 1965 at nearly 130° west, while an earthquake nearly midway between them has normal faulting. Overall, from the Gulf of California to the Juan de Fuca ridge, seismicity and earthquake mechanism agree well with a mechanical model based on the theory of J. Tuzo Wilson. Some motion is transferred by transcurrent faulting from the San Andreas fault across the Gorda basin to the Blanco transform-fault system.

SAN ANDREAS FAULT NORTH OF POINT ARENA, CALIFORNIA

Continuous reflection profiling north of Point Arena, California, shows that the San Andreas fault bends toward Shelter Cove rather than continuing in a straight line from the southeast. North of Shelter Cove, the fault probably remains near-shore until it reaches the vicinity of Punta Gorda, and may then curve westward along the Gorda Escarpment.

Earthquakes off the north Pacific Coast of the United States*

Abstract Epicenters of most of the larger earthquakes occurring off the coast of northern California and Oregon lie in two zones. One zone includes the north-facing submarine Gorda Escarpment, which extends nearly due west from Punta Gorda, and the south-facing Mendocino Escarpment as far offshore as long. 127° W. The other zone extends northwestward from the region of Cape Mendocino, indicating a linear extension of the San Andreas Rift zone as far as lat. 44° or 45° N.