Gorda Plates Research Articles

The state of stress near the Mendocino Triple Junction from inversion of earthquake focal mechanisms

Focal mechanisms of 70 earthquakes occurring in the region of the Mendocino triple junction between 1977 and 1995 are inverted to obtain the regional stress orientations and relative magnitudes in this technically complex area. A diverse set of earthquake geometries is consistent with a single stress field characterized by north‐northwest, horizontal, maximum principal compressive stress. Although this stress direction is almost perpendicular to convergence between the North American and Gorda plates, it is consistent with the stress direction inferred within the Gorda plate northwest of the triple junction. A maximum compressive stress direction nearly parallel to strike of the Cascadia subduction zone implies very low resolved shear stress across this plate boundary. Evidence for failure along the southernmost section of the Cascadia subduction zone comes from the occurrence of the recent 1992 (Ms=7.1) Cape Mendocino underthrusting earthquake as well as from measurements of Holocene surface uplift consistent with the 1992 coseismic uplift pattern. Rupture of the Cascadia subduction zone under this stress regime requires that the southernmost region of the Gorda‐North American Plate boundary is weak.

Summary of Coastal Geologic Evidence for past Great Earthquakes at the Cascadia Subduction Zone

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

A comparison of epicenter relocation techniques: a case study for the Juan de Fuca plate

The ‘master event relocation’ and ‘hypocentroid’ techniques are compared in an effort to determine their relative performance in evaluating the location of epicenters in a region hampered with recording station location bias and a poorly understood seismic velocity structure. The study areas chosen are the Explorer and Gorda plates, two small lithospheric subplates of the oceanic Juan de Fuca plate, which show a high degree of apparent intraplate seismicity. Previous studies have indicated systematic eastward errors in the location of epicenters due to the lack of recording stations to the west. Both techniques produce similar results in defining plate boundary vs. intraplate seismicity; however, noteworthy differences exist. The selection of a well-constrained master event has a clear effect on the alignment of relocated epicenters with plate boundaries. An event on the Gorda plate located by ocean bottom seismometer allows for a relocation of epicenters about the Gorda Rise. Using a more widely recorded master event on the Explorer plate produces parallelism of the events with the plate boundaries but an additional uniform shift in the relocated epicenters is required to align seismicity with the boundaries. Location of the master event, with respect to tectonic features of the plate, appears to affect the results, probably owing to differences in lithospheric conditions and their effect on wave travel paths through the plate. A comparison of results of the two techniques on intraplate seismicity on the Gorda plate shows opposing direction vectors for the relocations. The hypocentroid technique is less affected by changing conditions within the plate as events are clustered according to location. The relocations are still affected by the station location bias, as the events are relocated in relation to the weighted center of the event cluster. The technique produces greater definition of the trends of the relocated epicenters; however, in this situation an additional shift of the epicenters is required for alignment with the boundaries.

Lithospheric structure of northern California from teleseismic images of the upper mantle

Teleseismic P wave travel time residuals from 120 earthquakes recorded across the U.S. Geological Survey California seismic network were used to determine the lithosphere P wave velocity structure beneath northern California, a region characterized by complex interactions between the Pacific, North American, and Gorda plates. Lateral P wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals. Inversion results for the crust show strong correlations to volcanic features. The active volcanic fields, Shasta‐Medicine Lake, Lassen, and Clear Lake, are characterized by crustal low‐velocity anomalies that average approximately −6%, possibly identifying partially molten magma bodies. Cooled, solidified magma bodies beneath the extinct volcanic fields, Sonoma, southern Clear Lake, and Sutler Buttes, are denoted by relative velocity highs averaging +3%. The largest upper mantle velocity variations occur in the depth range 30–110 km, where velocities vary from −5.5% to +9.5%. These velocity variations reflect changes in the thickness and geometry of the Pacific, North American, and Gorda plates where they interact at the Mendocino Triple Junction. North of the Mendocino Triple Junction, the steep 70° east dipping portion of the Gorda plate is imaged as a +5% velocity high to depths near 270 km. A presumed segment of the Gorda plate, observed beneath the northern Great Valley and south of the inferred edge of the plate, is characterized by a +9% velocity high in the depth range 30–70 km. Beneath the northern Coast Ranges, shallow asthenosphere is imaged in the depth range 30–100 km as a pronounced southward tapering −4% low‐velocity zone, which we interpret as the slab window. Results from this study provide improved constraints on Gorda plate subduction, evolution of the San Andreas fault system, and development of the lithosphere beneath western North America.

A comparison of brittle deformation models for the Gorda plate

The Gorda plate is an actively deforming subplate of the Juan de Fuca system, located off the west coast of North America. The site of intense seismicity and unusually curved magnetic lineations, the Gorda has defied rigorous explanation since it was originally mapped in 1961. A variety of models for its deformation mechanism have been proposed, two of which are analyzed in depth here. The right-lateral shear and flexural-slip models are compared using constraints imposed by geometry, seismicity, stress, and basement fabric. Also examined are the questions of convergent motion between the Pacific and Gorda plates at the Mendocino Transform Fault and shortening by means of pure shear of the magnetic lineations. While both deformation models fit most of the constraints, neither fits all, but the flexural-slip model best fits the most well-known of the constraints. If, in fact, flexural-slip is the preferred deformation mechanism, lineation shortening must be ruled out in favor of convergent motion.

Late Cenozoic Basins of northern California

The late Cenozoic basins of northern California developed in response to both convergent tectonics associated with subduction of the Farallon plate (and its modern representatives, the Juan de Fuca and Gorda plates) and transform tectonics associated with northward migration of the Mendocino triple junction and formation of the San Andreas fault system. The modern Eel River basin north of the Mendocino triple junction is an active forearc basin located between the Cascade magmatic arc to the east and the trench at the foot of the continental slope of northern California and Oregon to the west. Five different types of late Cenozoic basins or fragments of basins are preserved in onshore northern California south of the Mendocino triple junction: (1) remnants of a formerly more extensive Neogene forearc basin preserved locally in downdropped blocks within the San Andreas fault system; (2) remnants of slightly older trench‐slope basins that generally developed west of but which may locally structurally underlie the younger forearc basin; (3) younger strike‐slip‐related structural basins that have developed along active right‐lateral faults of the San Andreas fault system; (4) broad shallow embayments, perhaps similar to the modern San Francisco Bay, that were connected to the deeper Pacific Ocean to the west; and (5) structurally emplaced remnants of oceanic crust and its overlying sedimentary cover (the King Range terrane). Our preliminary stratigraphic and sedimentologic studies suggest that much of northern California was covered during the Neogene by a forearc basin that may have extended as far south as the San Francisco Bay region and into the southern San Joaquin Valley. As the Mendocino triple junction migrated northward during the late Cenozoic, the southern margin of the forearc basin was uplifted, basin deposits were stripped off by erosion, and the locus of forearc sedimentation shifted progressively northward through time. Preserved but isolated fragments of the forearc basin provide a partial record of the original paleogeography and tectonic framework of northern California during the late Cenozoic.

Tectonic evolution of Gorda Ridge inferred from sidescan sonar images

Gorda Ridge is the southern segment of the Juan de Fuca Ridge complex, in the north-east Pacific. Along-strike spreading-rate variation on Gorda Ridge and deformation of Gorda Plate are evidence for compression between the Pacific and Gorda Plates. GLORIA sidescan sonographs allow the spreading fabric associated with Gorda Ridge to be mapped in detail. Between 5 and 2 Ma, a pair of propagating rifts re-orientated the northern segment of Gorda Ridge by about 10° clockwise, accommodating a clockwise shift in Pacific-Juan de Fuca plate motion that occurred around 5 Ma. Deformation of Gorda Plate, associated with southward decreasing spreading rates along southern Gorda Ridge, is accommodated by a combination of clockwise rotation of Gorda Plate crust, coupled with left-lateral motion on the original normal faults of the ocean crust. Segments of Gorda Plate which have rotated by different amounts are separated by narrow deformation zones across which sharp changes in ocean fabric trend are seen. Although minor lateral movement may occur on these NW to WNW structures, no major right-lateral movement, as predicted by previous models, is observed.

Propagation as a mechanism of reorientation of the Juan de Fuca Ridge

We present a revised model of the tectonic evolution of the Juan de Fuca ridge by propagating rifting. The new model has three different relative rotation poles, covering the time intervals 17.0–8.5 Ma, 8.5–5.0 Ma, and 5.0 Ma to the present. The rotation pole shifts at 8.5 and 5.0 Ma imply clockwise shifts in the direction of relative motion of 10° to 15°. At each of these shifts, the pattern of propagation reorganizes, and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges. The model, containing a total of seven propagation sequences, shows excellent agreement with the isochrons inferred from the magnetic anomaly data, except in areas complicated by the separate Explorer and Gorda plates. The agreement between model and data near the Explorer plate breaks down abruptly at an age of about 5 Ma, indicating that the probable cause of the rotation pole shift at that time was the separation of the Explorer plate from the Juan de Fuca plate.

Three‐dimensional geometry of the Gorda Plate beneath northern California

The isostatic residual gravity field over northern California displays a gravity gradient interpreted to reflect the south edge of the Gorda plate where it is subducted eastward beneath the North American plate. The locus of points of maximum slope defines a line trending S60°E from a coastal point approximately 20 km south of Cape Mendocino, a point where the buried plate boundary is inferred from magnetic and seismicity data. Southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the Blanco fracture zone and the present direction of relative motion between the Pacific and northern Gorda plates. Calculations from the form of the anomaly yield depth estimates that fit an east–southeast plunge of approximately 9° for the top of the Gorda south edge. The sense of the anomaly (higher gravity to the south) supports the hypothesis that a window developed in the subducted slab east of the San Andreas fault and south of the Gorda plate. South of the Gorda boundary the base of the North American plate is thus in contact with hot material from the asthenosphere that invaded the window. Because the overlying North American plate has been moving relatively south across the Gorda boundary, the North American plate beneath the Coast Ranges east of the San Andreas fault in central California may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the North American plate passed over it.

Tectonic uplift of a middle Wisconsin marine platform near the Mendocino triple junction, California

An uplifted wave-cut marine platform eroded across bedrock of the Franciscan Complex at Point Delgada, northern California, is overlain by 0.5 to 5 m of wave-worked pea gravel, which is in turn directly overlain by fluvial gravel and silt deposited as alluvial fans. Woody plant debris at the base of the fluvial deposits includes cones of Brewer spruce ( Picea breweriana ), which today are found only at higher elevations and latitudes. Fossil wood debris from this horizon yields a 14C date of 44,800 ± 1,300 yr. The 44,800 yr radiocarbon age for the base of the fluvial deposits establishes an approximate age for the immediately underlying marine gravels and wave-cut platform. We tentatively correlate this terrace with the middle Wisconsin high sea-level stand at −37 m, dated at about 45,000 yr B.P. If this age is correct, the tectonic uplift since middle Wisconsin time has been 44 m, and the average rate of uplift has been at least 1.0 m/1,000 yr. This relatively high rate is probably related to interaction among the Pacific, Gorda, and North American plates at the Mendocino triple junction.

Seismotectonics of the Cape Mendocino, California, area

Research Article| October 01, 1975 Seismotectonics of the Cape Mendocino, California, area G. W. SIMILA; G. W. SIMILA 1Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar W. A. PEPPIN; W. A. PEPPIN 1Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar T. V. McEVILLY T. V. McEVILLY 1Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar Author and Article Information G. W. SIMILA 1Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Berkeley, California 94720 W. A. PEPPIN 1Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Berkeley, California 94720 T. V. McEVILLY 1Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Berkeley, California 94720 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (10): 1399–1406. https://doi.org/10.1130/0016-7606(1975)86<1399:SOTCMC>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. W. SIMILA, W. A. PEPPIN, T. V. McEVILLY; Seismotectonics of the Cape Mendocino, California, area. GSA Bulletin 1975;; 86 (10): 1399–1406. doi: https://doi.org/10.1130/0016-7606(1975)86<1399:SOTCMC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A radio-telemetered network of high-gain seismographs allowed determination of hypocenters for 150 earthquakes with depths of 5 to 30 km in the Cape Mendocino area. Analysis of P- and S-wave arrival times indicates that the region exhibits a Poisson's ratio in excess of 0.3. The pattern of seismicity reveals a diffuse distribution. Composite fault-plane solutions indicate general north-south compression, consistent with generally inferred movements of the Pacific, North American, and Gorda plates. Near the cape, however, the principal compression is oriented roughly northwest-southeast, consistent with Gorda plate motion normal to the ridge. Right-lateral strike-slip movement appears to be the dominant faulting pattern. Thrust-fault mechanisms seen at the continental margin near the cape may reflect late Cenozoic underthrusting at the Gorda plate boundary. East-west cross sections of hypocenters show no indications of an active Benioff zone. These results are consistent with the hypothesis that the San Andreas fault joins the Mendocino fracture zone, which absorbs most of the right-lateral strike-slip motion. The Gorda plate is now deforming near the continental margin, apparently the consequence of transition from thrusting near the cape to right-lateral faulting northward as the mode of accommodation of the relative plate motions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.