Gorda Ridge 

North Gorda Ridge perspective view from the south

North Gorda Ridge

The Central Gorda Ridge includes three segments named by Chadwick et al. (1998) From South to North, they are Phoenix, Central, and Jackson

South -Escanaba Trough

from http://www.volcano.si.edu/world/volcano.cfm?vnum=1301-031 -North Gorda Ridge: The northernmost of five segments of the Gorda Ridge lies immediately south of the Blanco Transform Fault that offsets the Gorda and Juan de Fuca oceanic spreading ridges. The 65-km-long North Gorda Ridge segment is located about 200 km west of the southern Oregon coast and has deep 5- 10-km-wide valleys at either ends with a shallower narrow axial valley at the center. This morphology, which in plan view resembles an hourglass, is typical of magmatically active spreading segments. A submarine lava flow was erupted in late February and early March 1996, near the center of the North Gorda Ridge segment. The eruption was initially detected through acoustic T-waves from a seismic swarm and the emission of large thermal plumes. In April submarine cameras revealed new lava flows about 100-200 m wide along a fissure that was at least 3.5 km long. A seismic swarm of uncertain origin also occurred at this location in January 1998.A fresh lava flow was erupted in 1996 along the North Gorda Ridge, the northernmost of five segments of the Gorda Ridge located immediately south of the Blanco Transform Fault, which offsets the Gorda and Juan de Fuca spreading ridges. Submarine cameras revealed new lava flows about 100-200 m wide along a fissure that was at least 3.5 km long. The 65-km-long North Gorda Ridge segment is located about 200 km west of the southern Oregon coast and has an hourglass shape in plan view, with a shallower narrow axial valley at the center

Location of Gorda Ridge and other ocean ridges (indicated by double lines) in the northeast Pacific Ocean -http://marine.usgs.gov/fact-sheets/gorda/




Knobby lava pillow draped over a rounded pillow, r Gorda Image 2002 MBAR

Animals on a hydrothermal vent chimney on Gorda Image 2000 MBARI

From MBARI -http://www.mbari.org/data/mapping/seamounts/gorda_ridge.htm

The Gorda Ridge is an oceanic spreading center located off the coast of northern California and southern Oregon. The geologic setting and tectonics of the ridge were summarized by Clague and Holmes (1987) and Fisk and Howard (1993).The ridge axis has the typical deep axial valley of slow-spreading ridges. More recently a detailed description of the ridge structure, based on comprehensive SeaBeam bathymetry, was presented by Chadwick et al. (1998). They divide the ridge into five segments named, from south to north, the Escanaba, Phoenix, Central, Jackson, and North Gorda segments, offset from one another by transform offsets. The ridge has a complex history due to spreading rate changes, intraplate deformation of the Gorda plate, and reorientation of the ridge axis (Wilson, 1986, 1989). Davis and Clague (1987, 1990), Nielsen et al. (1995), and Rubin et al. (1998) have studied the petrology of lavas erupted along the four northern segments. Disequilibrium U-Th-Ra studies have established the ages of some of these lavas (Goldstein et al., 1992; Volpe and Goldstein, 1993).

In the 1980s, NOAA discovered hydrothermal plumes in the water column above the North Gorda segment (called the GR14 site, Collier et al., 1986, Baker, 1994). Subsequently, extensive mapping and sampling was conducted in the North Gorda segment in search of the source hydrothermal vents (Rona and Clague, 1989; Clague and Rona, 1990). These vents were subsequently discovered using the SeaCliff submersible (named after the SeaCliff hydrothermal field, Rona et al., 1990; Zierenberg et al., 1995). These vents are highly unusual in that they are not located in the axial valley, but instead are about 300 meters above the valley on the east wall. Scientists from MBARI and the University of California at Davis returned to these vents in 2000.

In 1996, a seismic swarm occurred along the North Gorda segment (Fox and Dziak, 1998). Investigations in the area demonstrated that an eruption had occurred at the site (Chadwick et al., 1998). Po-Pb analyses show that young lavas collected from the proposed eruption site did in fact erupt in 1996 (Rubin et al., 1998). A special issue of Deep Sea Research II (Cowen, 1998) presents the results of a wide range of studies done to investigate the eruption.

The new high-resolution data presented here provide more detailed views of the constructional features in the axial valley and the block faults that make up the valley walls. In particular, the morphology of numerous individual vents can be seen clearly. Similarly, the valley walls commonly include partial vent structures that have been dismembered by faulting. The new high-resolution data also provide more detailed maps of the SeaCliff hydrothermal field and the 1996 eruption site.

Escanaba Trough is the southern sediment-filled portion of the Gorda mid-ocean Ridge. It extends northward from the Mendocino Fracture Zone about 130 kilometers, to the southernmost right-lateral offset on Gorda Ridge, at about 4135N. The spreading rate along this part of the Gorda Ridge is about 2.3 centimeters per year. The valley floor is 3-5 kilometers wide at the northern end and widens to about 18 kilometers near the Mendocino Fracture Zone. South of about 418N, the axial valley is filled by as much as 500 meters of sandy turbidites. These turbidites derived from the Columbia River drainage during floods produced by outbursts from glacial lakes, (including lake Missoula) during the Late Pleistocene (Zuffa et al., 2000). The sediment fill is pierced by a series of discrete volcanic centers that uplifted the sediment over sills and erupted some lava flows on the seafloor (Morton et al., 1987). The sediments are also disrupted by faulting within the trough, indicating deformation and extension continued after deposition of most of the sediment. Uplifted sediment hills above some of the sills have been used to estimate the timing and amount of vertical motion using the thickness of specific turbidite units (Normark, and Serra, 2001). The volcanic centers have been mapped in some detail using towed photographic systems and observations from submersibles (Ross et al., 1994). The lavas recovered have been studied to evaluate assimilation of sediment during shallow crustal storage and fractional crystallization (Davis et al., 1994, 1998). These uplifted hills are also the site of extensive hydrothermal mineralization, although active vents are restricted to a single region near 41N named NESCA (Morton et al., 1994). NESCA and SESCA are acronyms for Northern and Southern ESCAnaba, two regions where extensive study has been conducted. A single volume (Morton et al., 1994), published as U.S. Geological Survey Bulletin 2022 in 1994, summarizes nearly all previous work. Subsequent to these studies, the Ocean Drilling Program drilled a series of holes at the NESCA site to investigate hydrothermal circulation and massive sulfide formation (Fouquet et al., 1998). Numerous papers on the results of the drilling program can be found in the ODP Initial Reports volume 169.

The new high-resolution bathymetry shows the faults that offset the sediment fill in Escanaba Trough in great detail. In addition, it is evident that there are two types of sills present: the small deep sills that uplift sediment hills, as described by Morton et al.(1987, 1994) and a second type that are shaped like large lobate flows. These flow-shaped sills are apparently much shallower in the sediment and have uplifted large regions by 30-50 meters. Recent remotely operated vehicle (ROV) dives on the tops of several of these sills shows extensive hydrothermal deposits and outcrops around the margins of the sill where the sediment has slumped

from - http://en.wikipedia.org/wiki/Gorda_Ridge

The Gorda Ridge is a tectonic spreading center located off the coast of Oregon and northern California north of Cape Mendocino. It runs from a triple junction with the San Andreas Fault and the Mendocino Fracture Zone northward to another transform boundary, the Blanco Fracture Zone. To its east is the Gorda Plate, which together with the Juan de Fuca Plate to its north, is what remains of the once-vast Farallon Plate which has been largely subducted under the North American Plate. To its west is the Pacific Plate.

Escanaba Trough
Ridge research
Gorda Ridge

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