Sovanco Fracture Zone Research Articles

Microplate Evolution in the Queen Charlotte Triple Junction & Explorer Region: New Insights From Microseismicity

AbstractThe Queen Charlotte triple junction/Explorer microplate region offshore British Columbia, Canada, is marked by poorly understood and rapidly evolving microplate tectonics. Although the region hosts abundant seismicity, it has received relatively scant attention in recent years due to its remote, offshore location. We use the Regressive ESTimator (REST) algorithm to generate a new catalog of automatically detected earthquakes from 1995 to 2021, which, when merged with the existing Geological Survey of Canada catalog, yields the most extensive seismicity data set offshore British Columbia to date. We apply double‐difference relocation to these events and perform stress inversions using moment tensors for subregions within the study area. Our results confirm and extend previous models of microplate deformation processes. We suggest the Revere‐Dellwood‐Queen Charlotte fault system has evolved as a NW‐migrating, pull‐apart system between Haida Gwaii and the Explorer ridge that obeys global length/width scaling and whose bathymetric expression is influenced by volcanism plausibly induced by interaction with the Kodiak‐Bowie hotspot. Seismicity within the Explorer microplate is dominated by prominent, northeast‐trending lineations that emanate from the Sovanco fracture zone and parallel the Nootka fault zone. Alignment of these features with spreading structures that bound the microplate suggests that its breakup is controlled primarily by a strength fabric inherited at spreading ridges. Stress inversions are dominated by near‐vertical intermediate compressive stress reflecting the dominance of strike‐slip faulting. Stress varies systematically between transpression to the north along southern Haida Gwaii and seafloor spreading to the south along the Juan de Fuca ridge.

The Northern Terminus of Cascadia Subduction

AbstractThe Juan de Fuca (JDF) plate system is pivoting and breaking apart as a result of resistance to subduction. At the northern end, the Explorer (EXP) plate moves independently of the JDF plate along the Nootka Fault Zone (NFZ), which forms an unstable triple junction with the JDF ridge and the Sovanco Fracture Zone. We trace the subsurface extension of the NFZ using tomography by combining results from a new nearshore study with a previous ocean‐bottom study and other geophysical constraints. Cross sections from the offshore NFZ through central Vancouver Island suggest a strong fold in the subducting lithosphere (∼40°). Near the deformation front, the NFZ is defined by microseismicity that extends into oceanic mantle with low VP/VS (<1.7) values. This anomaly extends to the NE below the continental shelf with a more northerly trajectory than common NFZ depictions through the seismicity concentration off Nootka Island. Tremor locations and receiver functions indicate that this more northerly trajectory persists across Vancouver Island. We propose a revised tectonic evolution for the NFZ that leads to (i) an EXP microplate confined to shallow depths below the convergent margin and which underthrusts North America no farther landward than Brooks Peninsula and (ii) a northern edge of Cascadia subduction that cuts between Brooks Peninsula and Nootka Island. The plate fold offshore Nootka Island and abundant seismicity there and at Brooks Peninsula are ascribed to stress concentrations on either side of the NFZ associated with deformation at the edges of more competent JDF, EXP, and North American plates.

Migration, Isolation, and Speciation of Hydrothermal Vent Limpets (Gastropoda; Lepetodrilidae) Across the Blanco Transform Fault

The Sovanco Fracture Zone and Blanco Transform Fault separate the Explorer, Juan de Fuca, and Gorda ridge systems of the northeastern Pacific Ocean. To test whether such offsets in the ridge axis create barriers to along-axis dispersal of the endemic hydrothermal vent animals, we examined the genetic structure of limpet populations previously identified as Lepetodrilus fucensis McLean, 1988 (Gastropoda, Lepetodrilidae). Mitochondrial DNA sequences and patterns of allozyme variation revealed no evidence that the 150-km-long Sovanco Fracture Zone impeded gene flow between the Explorer and Juan de Fuca populations. In contrast, the 450-km-long Blanco Transform Fault separates the limpets into highly divergent northern and southern lineages that we recognize as distinct species. We describe southern populations from the Gorda Ridge (Seacliff) and Escanaba Trough as Lepetodrilus gordensis new species and refer northern populations from the Explorer and Juan de Fuca ridge systems to L. fucensis sensu stricto. The species are similar morphologically, but L. gordensis lacks a sensory neck papilla and has a more tightly coiled teleconch. To assess the degree of isolation between these closely related species, we used the Isolation with Migration method to estimate the time of population splitting, effective sizes of the ancestral and derived populations, and rates of migration across the Blanco Transform Fault.

Explorer deformation zone: Evidence of a large shear zone and reorganization of the Pacific–Juan de Fuca–North American triple junction

Research Article| March 01, 2006 Explorer deformation zone: Evidence of a large shear zone and reorganization of the Pacific–Juan de Fuca–North American triple junction Robert P. Dziak Robert P. Dziak 1Oregon State University, National Oceanographic and Atmospheric Administration, HMSC, Newport, Oregon 97365, USA Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (3): 213–216. https://doi.org/10.1130/G22164.1 Article history received: 12 Aug 2005 rev-recd: 16 Nov 2005 accepted: 20 Nov 2005 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert P. Dziak; Explorer deformation zone: Evidence of a large shear zone and reorganization of the Pacific–Juan de Fuca–North American triple junction. Geology 2006;; 34 (3): 213–216. doi: https://doi.org/10.1130/G22164.1 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 SocietyGeology Search Advanced Search Abstract Recently collected multibeam bathymetry and hydroacoustic earthquake data are used to investigate the recent tectonics of the Explorer–Juan de Fuca Ridge systems, the boundaries between the Pacific and northern Juan de Fuca plates. The bathymetric and seismic evidence presented is consistent with a zone of shear extending well south of the Sovanco Fracture Zone to include the Heck and Heckle seamounts, and potentially as far south as the Springfield Seamounts and Cobb offset along the Juan de Fuca Ridge. This indicates that the triple junction between the Pacific–Juan de Fuca–North American plates may be reorganizing southward to establish at the Cobb offset. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismotectonics of the Explorer region

The Explorer region offshore western Canada is a tectonically complex area surrounded by the Pacific, North America, and Juan de Fuca plates. Existing tectonic models for the region differ fundamentally. Proposed plate configurations range from multiple independent plate fragments to an Explorer plate now fused to North America along the continental margin and cut by Pacific–North America transform faults in the west. We present new seismological data constraining the region's current tectonics. We use three‐component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4. Combined with 34 Harvard centroid moment tensor solutions, they represent the region's largest earthquake source parameter data set obtained by robust waveform modeling techniques. In addition, we perform joint epicenter determination to relocate larger earthquakes recorded since 1918. The source parameters and improved locations provide a consistent tectonic picture. Earthquake slip vector azimuths along the Pacific plate boundary change smoothly and are significantly less northerly oriented than the Pacific‐North America plate motion direction, requiring an independent Explorer plate. The present‐day Pacific‐Explorer boundary is formed by transform faults subparallel to the Revere‐Dellwood‐Wilson fault. Plate motion vectors indicate that the Winona block is part of the Explorer plate. Current Explorer motion is more northerly than indicated by magnetic anomalies prior to 2 Ma, implying a recent change, possibly coinciding with a northwestward ridge jump near Explorer plate's northern end transferring the Winona block from the Pacific to the Explorer plate. In response to these plate motion changes the region north of the western Sovanco fracture zone was assimilated into the Pacific plate. The region around the eastern Sovanco fracture zone, characterized by broadly distributed seismicity, is composed of well‐defined sets of conjugate faults bounding rotating crustal blocks. Earthquake fault strikes agree with the dominant northwest‐southeast fault sets; however, the conjugate sets must be also active to fully accommodate present‐day Explorer plate motion. The SW portion of the strike‐slip Nootka fault zone, the Explorer‐Juan de Fuca plate boundary, is well defined by focused seismicity; however, its full extent under Nootka Island remains unresolved. The Explorer–North America boundary shows sporadic low‐magnitude seismicity. Our Explorer–North America rotation pole predicts convergence varying from negligible at the boundary's northwest end to ∼2 cm/yr at the SE end. This convergence can be accommodated either by subduction or by crustal thickening extending to the North American continent. We favor subduction based on low deformation rates observed by onshore GPS sites. The present Explorer plate system configuration is a result of stepwise reorientation of the Explorer ridge system, each step successively reducing the subduction rate relative to North America.

Evidence for magma mixing and a heterogeneous mantle on the West Valley Segment of the Juan de Fuca Ridge

We studied basaltic rocks of the northern end of the Juan de Fuca Ridge to compare the volcanology, geochemistry, and petrogenesis of spatially related rocks of an active spreading center (West Valley and West Ridge), the Sovanco Fracture Zone, the Middle Ridge, and the adjacent Endeavour and Northern Symmetrical ridges. Rocks were analyzed for mineral chemistry and major, trace, and rare earth elements (REE). Crystallization temperatures and equilibrium phase compositions were calculated. The complex zoning and disequilibrium compositions of plagioclase and olivine megacrysts and phenocrysts from all areas except West Ridge, Sovanco Fracture Zone (7123‐7), and Endeavour Ridge indicate mixing in shallow level magma chambers. The rocks range from primitive (up to Mg # = 68) in the West Valley south and West Ridge areas to evolved ferrobasalts (Mg # as low as 52) at West Valley central, West Valley north, and Northern Symmetrical Ridge. The rocks from the West Valley axis span the range of Mg #. Rocks from West Valley are transitional to enriched (e.g., [La/Ce]n= 0.89–1.06), as are rocks from Endeavour Ridge (e.g., [La/Ce]n = 0.88–1.06). Rocks from West Valley wall, Middle Ridge, Sovanco Fracture Zone, and Northern Symmetrical Ridge are light REE depleted (e.g., [La/Ge]n = 0.76–0.88), and the most depleted rocks are from West Ridge (e.g., [La/Ce]n = 0.62). The chemically heterogeneous character of the lavas exists both within site and within a single dredge haul (West Valley north and Endeavour Ridge rocks) and between sites, indicating that the mantle source is heterogeneous on a variety of scales. The range in incompatible element ratios can be modelled as binary mixing between an enriched source and a depleted source. Further, trace element systematics require variations in the degree of partial melting. Lava chemistry and ridge morphology are consistent with southward propagation of spreading in West Valley to form an overlap with the Endeavour Segment. West Valley (West Valley north, central and south) is dominated by extension over volcanism and is thus thermally and magmatically depressed. Rocks from the West Valley Segment span a broader compositional range than ridge systems to the south, reflecting the heterogeneity of the source and the lack of robust volcanism to homogenize the chemistry. Zero age rocks from West Valley are similarly enriched in [La/Ce]n as zero age rocks from the Endeavour and Explorer ridges. Rocks from the West Valley wall, the Middle Ridge, and the older Sovanco Fracture Zone rocks are depleted similar to rocks of the Juan de Fuca Ridge south of the Cobb Offset. This difference suggests that enrichment of the mantle source north of the Cobb Offset has been a recent phenomena.

Tectonic evolution of the Explorer‐Northern Juan de Fuca Region from 8 Ma to the present

New magnetic anomaly data, in conjunction with Sea Beam and SeaMARC II data, has been used to schematically model the tectonic development of the Explorer‐Northern Juan de Fuca Region with a series of five ridge‐offset propagators during the last 8 m.y. The Sovanco Fracture Zone played a major role in the evolution of this area since its origin as a spreading center offset between the Explorer and Juan de Fuca Ridges approximately 7.4 Ma. Between this time and 2.5 Ma, the transform migrated southwards due to southward propagation of Explorer Ridge, and lengthened by the addition of three propagating ridge offsets as well as by asymmetric spreading to the east on Explorer Ridge. Since the independence of Explorer Plate at 4 Ma, there has been clockwise rotation of the Explorer Spreading Center relative to the Juan de Fuca Ridge. In response to forces acting at the convergent margin, the Sovanco Fracture Zone also underwent clockwise reorientation, and in the process developed into a broad zone of shear. The tectonic deformation associated with this rotation distorted the previously existing N‐S magnetic anomaly pattern. Explorer Seamount, a major geological feature in the region, is modeled as a presently extinct southern spreading segment of the Explorer Ridge system which was isolated by the northward migration of the west end of the Sovanco Fracture Zone. The recent (<1 Ma) tectonic evolution of Explorer Ridge involved both northward and southward propagation of Southern Explorer Ridge, as well as an eastward ridge jump during the Brunhes. Ih the northern portion of the study area, the axis of spreading jumped 40 km to the NW around 0.3 Ma, from the eastern Explorer Deep to the western Explorer Rift.

Bookshelf tectonics: Rotated crustal blocks within the Sovanco Fracture Zone

The 15 kilometer wide Sovanco Fracture Zone is a dextral‐slip transform fault linking the Juan de Fuca and Explorer Ridges in the N.E. Pacific. SEABEAM bathymetry shows that the Sovanco comprises several rhomb‐shaped elevated blocks of oceanic crust bounded by northeast‐ and northwest‐trending lineaments. We hypothesize that the northeast‐trending lineaments are sinistral strike‐slip faults that are reactivated ridge‐parallel normal faults. Both the sinistral block‐bounding faults and the blocks themselves have rotated clockwise about 30 degrees in response to dextral shear within the Sovanco zone.

Detailed geomorphology and neotectonics of the Endeavour Segment, Juan de Fuca Ridge: New results from Seabeam swath mapping

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Geophysical measurements in the region of the Explorer ridge off western Canada

The region of the Explorer spreading centre off Vancouver Island, British Columbia, has been studied through a marine geophysical survey. Earthquake epicentres located by three ocean bottom seismometers confirm that the boundary between the Pacific plate and the Explorer plate (the northern extension of the Juan de Fuca plate) at present lies along the Sovanco fracture zone, the Explorer ridge, and the Dellwood Knolls. The epicentres of earthquakes in this area as determined by the onshore seismic network are found to be subject to significant errors. The ocean bottom seismometers also have been used for a detailed seismic refraction line just to the north of the Explorer spreading centre employing explosives and a large airgun as sources. A preliminary analysis of the data indicates a fairly typical crustal structure but a shallow and low velocity mantle near the ridge crest, and illustrates the value of ocean bottom seismometers in oceanic refraction studies. A new geothermal heat flux probe was employed in this study that permitted repeated 'pogostick' penetrations without raising the instrument to the surface. Six profiles with a total of 112 penetrations provided valuable data on the nature of hydrothermal circulation in the oceanic crust. Eleven standard heat probe stations provided some restraints on the poorly known age of the oceanic crust along the margin. Seismic reflection profiles using a 3.5 kHz system, a high resolution pulser profiler, and a large airgun were used as aids in the interpretation of the seismic and heat flow data.

Heat flow measured over the Juan de Fuca Ridge: Evidence for widespread hydrothermal circulation in a highly heat transportive crust

One hundred four heat flow measurements were made on a 100-km-square grid of seismic reflection profiles centered over the intersection of the northern Juan de Fuca ridge and the Sovanco fracture zone. Profile spacing was roughly 10 km, and heat flow stations were located at each track line intersection. South of the fracture zone the crust is young (0–1.8 m.y.), sediment cover is incomplete, and on average, heat flow is very low in comparison with that predicted by any reasonable model for lithosphere creation. Station to station heat flow variability is high and uncorrelated with local topography or structure, and the nature and magnitude of variability cannot be accounted for by the thermal effects of either heat flow refraction or rapid sedimentation. These observations suggest that open groundwater circulation in effectively permeable basement rocks redistributes the heat and causes highly localized scatter in conductive measurements. A large portion of the total heat flow is ventilated directly to the sea. On the Explorer ridge flank, north of the fracture zone, the crust is older (3.3–6.6 m.y.) and fully sedimented. Large local variability in heat flow values still exists, but a relationship between heat flow and sediment thickness can be detected. On average, heat flow increases with decreasing sediment thickness, and although the variation is in the proper sense, the magnitude of the observed heat flow variation cannot be explained by simple conductive mechanisms, such as deposition of cold sediments. The observed relationship between heat flow and sediment thickness can be accounted for by widespread hydrothermal circulation in a highly heat transportive crust. Water temperatures at the base of the sediments are found to range between about 100° and 200°C. A similar relationship between heat flow and sediment thickness is observed for the sediment-filled axial valley on the Juan de Fuca ridge, and basement temperatures estimated there range between about 100° and 300°C. Basement temperatures estimated over the Juan de Fuca ridge flanks are on the average much lower and range between ≪25° and 200°C. This is probably a result of groundwater ventilation through basement outcrops there. On the Explorer ridge flank, open circulation is prevented by the thick and continuous sediment cover, and the heat flow average over the area (5.7 HFU, where 1 HFU = 1 μcal cm−2 s−1) should be a reliable estimate of the total conductive heat flux for this age crust (4.5 m.y. on average). This, with other reliable data points from the Pacific, supports a simple dependence of heat flow upon age of heat flow (HFU) = 12.0/age1/2 (m.y.). 1 Supplementary Appendices 1 and 2 are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J77-005; $1.00. Payment must accompany order.

A model for recent plate interactions off Canada's west coast

Detailed re-examination of existing magnetic anomaly data reveals the fine structure of variations in spreading rates and directions at the Juan de Fuca and Explorer Ridges during the last 10 million years. A geometrical model using these variations delineates the theoretical history of the interactions between the lithospheric plates involved. These interactions demonstrate the independent movement of the Juan de Fuca and Explorer plates and the development of the Sovanco Fracture Zone. The latter was apparently initiated E–W at 7 Ma, rotated clockwise to 120° and may have been the site of up to 50 km of crustal shortening. The model demonstrates that subduction rates at the Canadian continental margin declined from 5 cm/yr to a present 1.5 cm/yr and that recent relative movements are compatible with the N–S compression observed from earthquakes. It also suggests that the existence of both E–W and NE trending faults in the downgoing lithosphere beneath Vancouver Island, shows that a triple junction remained static near the northern end of Vancouver Island from 10–4 Ma, and predicts a buried northern edge of subducted material striking NE in this area.

Tectonic structures on the Juan de Fuca Ridge

The northern Juan de Fuca Ridge is an actively spreading part of the East Pacific Rise system that is flooded with young turbidite sediments from the nearby continental margin. A detailed geophysical survey was completed at the intersection of the ridge with the Sovanco Fracture Zone. North-south and east-west seismic reflection profiles, spaced 10 km apart on a grid 110 km on a side, were taken with a medium-frequency (200 Hz) electromagnetic source. Profiles give as much as 1.5 sec of penetration, yet show very detailed structure in the turbidite layering. The ridge-crest area is typified by its two axial valleys and associated normal-fault blocks rotated away from the crest. These fault blocks, made strikingly visible by the once-horizontal turbidites, range in horizontal dimension from less than 1 km to 8 km, and they are rotated as much as 8°. Normal faults are clearly visible in the sediments of the axial valley (Middle Valley) as discontinuities along acoustic reflectors. These faults dip toward the center of the valley at angles from about 45° to nearly vertical (>80°). Of the two axial valleys, only one (West Valley) is currently active. Profiles and bottom photographs show pillow basalt cropping out over most of the West Valley floor, but some rifted sediments exist as well. The second axial valley (Middle Valley) appears to be currently inactive. It is completely sediment filled (sediment thickness as much as 3.0 km), and although clearly visible normal faults extend through the full section seen in the profiles, extensional rates calculated on these faults are only about 0.2 cm/yr. Over the rest of its length, the Juan de Fuca Ridge crest has no significant axial valley. Only on its northern end near its intersection with the Sovanco Fracture Zone does it develop an axial valley. Similar observations have been made on the Explorer Ridge and on the East Pacific Rise, Gulf of California, although the mechanism for this is not yet understood. Other observations of axial valleys in general — namely, the nonisostatic relief of the valleys, their dependence on spreading rate for generation, and the formation and rotation of normal-fault blocks away from the valley floors — all seem best explained by a passive mechanism where complete crust and upper mantle accretion occurs by passive intrusion over a zone wider than the width of the initial formation of a rigid surface layer.

Geology of the Northern End of Juan de Fuca Ridge and Sea-Floor Spreading

The northern end of Juan de Fuca Ridge consists of a series of basement ridges and valleys, inundated with sediment except for the axis of most recent sea-floor spreading. This axis is associated with the western of two branches of the Brunhes magnetic anomaly. The eastern branch of the magnetic anomaly is associated with a largely sediment-covered ridge, apparently produced by spreading early in the Brunhes Epoch. The intervening negative anomaly is probably caused by reversely magnetized rocks older than 0.7 m.y. Basalts dredged from the region of the northern end of Juan de Fuca Ridge have compositions typical of low-potassium ocean ridge basalts. They differ from basalts reported from the southern part of Juan de Fuca Ridge which have higher K2O, TiO2, FeOT, and FeOT/MgO. This difference is compatible with the hypothesis that a mantle plume exists under the southern part of the ridge. Distribution of earthquake epicenters suggests that the Queen Charlotte Fault Zone presently extends south of Explorer Ridge to intersect Juan de Fuca Ridge at 49°N and that the Sovanco Fracture Zone no longer functions as a transform fault.

Late Quaternary tectonics, northern end of Juan de Fuca Ridge (northeast Pacific)

Seismic reflection profiles from the northern end of Juan de Fuca Ridge reveal three axial valleys having a basement relief of as much as 2 sec (two-way travel time). A thick sequence, presumably of turbidites, mainly less than 0.7 m.y. old, covers much of the area. The oldest turbidites form the upper part of the fill of a possible Tertiary trench between the ridge and North America. The second turbidite unit extends beyond the trench and once formed an abyssal plain over most of northern Juan de Fuca Ridge and the area west to Explorer Ridge. Following formation of the plain, vertical movements began that broadly uplifted the crest of Juan de Fuca Ridge, block-faulted its northern end, produced faulting along Sovanco Fracture Zone, and upwarped the basement north of the ridge. Younger turbidites have filled the lowlands created by the vertical movements. The present sea floor topography and seismic activity show evidence of continued movements. Our findings indicate that the basement relief of the ridge crest and fracture zone were formed after the time of creation of the magnetic anomalies associated with sea floor spreading. Although abruptly initiated, these vertical movements have continued in a moderated form to the present. The vertical movements are thought to be associated with horizontal plate motions in this complex area and with the episodic release of strain as the northwest-salient block of the Juan de Fuca Plate is possibly being severed from the remainder of the plate.