Seismicity and crustal structure at the Mendocino triple junction, Northern California
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A high level of seismicity at the Mendocino triple junction in Northern California reflects the complex active tectonics associated with the junction of the Pacific, North America, and Gorda plates. To investigate seismicity patterns and crustal structure, 6193 earthquakes recorded by the Northern California Seismic Network (NCSN) are relocated using a one-dimensional crustal velocity model. A near vertical truncation of the intense seismic activity offshore Cape Mendocino follows the strike of the Mattole Canyon fault and is interpreted to define the Pacific plate boundary. Seismicity along this boundary displays a double seismogenic layer that is attributed to interplate activity with the North America plate and Gorda plate. The interpretation of the shallow seismogenic zone as the North America - Pacific plate boundary implies that the Mendocino triple junction is situated offshore at present. Seismicity patterns and focal mechanisms for events located within the subducting Gorda pl ate are consistent with internal deformation on NE-SW and NW-SE trending rupture planes in response to north-south compression. Seismic sections indicate that the top of the Gorda plate locates at a depth of about 18 Km beneath Cape Mendocino and dips gently east-and southward. Earthquakes that are located in the Wadati-Benioff zone east of 236{sup o}E show a change to an extensional stress regime indicative of a slab pull force. This slab pull force and scattered seismicity within the contractional forearc region of the Cascadia subduction zone suggest that the subducting Gorda plate and the overriding North America plate are strongly coupled. The 1992 Cape Mendocino thrust earthquake is believed to have ruptured a blind thrust fault in the forearc region, suggesting that strain is accumulating that must ultimately be released in a potential M 8+ subduction earthquake.Keywords:
Triple junction
Forearc
Pacific Plate
North American Plate
Slab
Slab window
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Forearc magmatic rocks were emplaced in a semicontinuous belt from Alaska to Oregon from 62 to 11 Ma. U-Pb and 40Ar-39Ar dating indicates that the magmatism was concurrent in widely separated areas. Eight new conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from forearc intrusions on Vancouver Island (51.2 ± 0.4, 48.8 ± 0.5 Ma, 38.6 ± 0.1, 38.6 ± 0.2, 37.4 ± 0.2, 36.9 ± 0.2, 35.4 ± 0.2, and 35.3 ± 0.3 Ma), together with previous dates, indicate that southwestern British Columbia was a particularly active part of the forearc. The forearc magmatic belt has been largely attributed to ridge-trench intersection and slab window formation involving subduction of the Kula-Farallon ridge. Integration of the new and previous ages reveals shortcomings of the Kula-Farallon ridge explanation, and supports the hypothesis of two additional plates, the Resurrection plate (recently proposed) and the Eshamy plate (introduced herein) in the Pacific basin during Paleocene and Eocene time. We present a quantitative geometric plate-tectonic model that was constructed from 53 Ma to present to best account for the forearc magmatic record using ridge-trench intersection and slab window formation as the main causes of magmatism. The model is also in accord with Tertiary to present inboard magmatic and structural features.
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Abstract With a small fraction of marginal subduction zones, the driving mechanism for the North American plate motion is in debate. We construct global mantle flow models simultaneously constrained by geoid and plate motions to investigate the driving forces for the North American plate motion. By comparing the model with only near‐field subducting slabs and that with global subducting slabs, we find that the contribution to the motion of the North American plate from the near‐field Aleutian, central American, and Caribbean slabs is small. In contrast, other far‐field slabs, primarily the major segments around western Pacific subduction margins, provide the dominant large‐scale driving forces for the North American plate motion. The coupling between far‐field slabs and the North American plate suggests a new form of active plate interactions within the global self‐organizing plate tectonic system. We further evaluate the extremely slow seismic velocity anomalies associated with the shallow partial melt around the southwestern North America. Interpreting these negative seismic shear‐velocity anomalies as purely thermal origin generates considerably excessive resistance to the North American plate motion. A significantly reduced velocity‐to‐density scaling for these negative seismic shear‐velocity anomalies must be incorporated into the construction of the buoyancy field to predict the North American plate motion. We also examine the importance of lower mantle buoyancy including the ancient descending Kula‐Farallon plates and the active upwelling below the Pacific margin of the North American plate. Lower mantle buoyancy primarily affects the amplitudes, as opposed to the patterns of both North American and global plate motions.
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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.
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The evolution of the San Andreas fault system is controlled by thermal‐mechanical processes associated with the development and evolution of a narrow “slabless window” formed beneath the western edge of North America. This fault zone evolution begins after initiation of transform motion along the plate boundary with the northward migration of the Mendocino triple junction. As a consequence of initial lithospheric structure and the shallow emplacement of asthenospheric mantle, the plate boundary separating the North American and Pacific plates follows a complex three‐dimensional geometry which varies through time. Seismic velocity structure, heat flow, seismicity, surface deformation, uplift, and fault development are controlled by the evolving thermal structure in the region after triple junction passage. Thermal‐mechanical models have been used to evaluate the fault system's time‐varying three‐dimensional dynamical behavior, simulating the principal processes involved in the thermal‐mechanical evolution of the San Andreas fault system. Results from this modeling indicate that the fault system has essentially a three‐stage history. (1) In the vicinity of the Mendocino triple junction the San Andreas fault maps the eastern edge of the Pacific plate, with a broad (∼100 km) zone of asthenospheric mantle separating the Pacific and North American plates in the 25‐ to 90‐km depth range. (2) Between 37°N and ∼39°N the plate boundary (within the mantle) separating the Pacific and North American plates has developed approximately 40–60 km east of the surface trace of the San Andreas fault and lies beneath the Hayward‐Calaveras faults and associated faults. The surface trace of the plate boundary appears to be connected to the mantle segment via a lower crust subhorizontal detachment surface. This fault zone orientation produces the surface deformations observed geodetically in the region. (3) South of 37°N the surface fault again overlies the deeper plate boundary, apparently as a result of an eastward jump in the surface fault.
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Current interpretations of Cretaceous tectonic evolution of the northwest Pacific trace interactions between the Pacific plate and three other plates, the Farallon, Izanagi, and Kula plates. The Farallon plate moved generally eastward relative to the Pacific plate. The Izanagi and Kula plates moved generally northward relative to the Pacific plate, with Izanagi the name given to the northward-moving plate prior to the Cretaceous normal polarity superchron and the name Kula applied to the postsuperchron plate. In this article I suggest that these names apply to the same plate and that there was only one plate moving northward throughout the Cretaceous. I suggest that the tectonic reorganization that has previously been interpreted as formation of a new plate, the Kula plate, at the end of the superchron was actually a plate boundary reorganization that involved a 2000 km jump of the Pacific–Farallon–Kula/Izanagi triple junction. Because this jump occurred during a time of no magnetic reversals, it is not possible to map or date it precisely, but evidence suggests mid-Cretaceous timing. The Emperor Trough formed as a transform fault linking the locations of the triple junction before and after the jump. The triple junction jump can be compared with an earlier jump of the triple junction of 800 km that has been accurately mapped because it occurred during the Late Jurassic formation of the Mesozoic-sequence magnetic lineations. The northwest Pacific also contains several volcanic features, such as Hawaii, that display every characteristic of a hotspot, although whether deep mantle plumes are a necessary component of hotspot volcanism is debatable. Hawaiian volcanism today is apparently independent of plate tectonics, i.e., Hawaii is a center of anomalous volcanism not tied to any plate boundary processes. The oldest seamounts preserved in the Hawaii-Emperor chain are located on Obruchev Rise at the north end of the Emperor chain, close to the junction of the Aleutian and Kamchatka trenches. These seamounts formed in the mid-Cretaceous close to the spreading ridge abandoned by the 2000 km triple junction jump. Assuming that Obruchev Rise is the oldest volcanic edifice of the Hawaiian hotspot and thus the site of its initiation, the spatial and temporal coincidence between these events suggests that the Hawaii hotspot initiated at the spreading ridge that was abandoned by the 2000 km jump of the triple junction. This implies a tectonic origin for the hotspot. Other volcanic features in the northwest Pacific also appear to have tectonic origins. Shatsky Rise is known to have formed on the migrating Pacific-Farallon-Izanagi triple junction during the Late Jurassic–Early Cretaceous, not necessarily involving a plume-derived hotspot. Models for the formation of Hess Rise have included hotspot track and anomalous spreading ridge volcanism. The latter model is favored in this article, with Hess Rise forming on a ridge axis possibly abandoned as a result of a ridge jump during the superchron. Thus, although a hotspot like Hawaii could be associated with a deep mantle plume today, it would appear that it and other northwest Pacific volcanic features originally formed as consequences of shallow plate tectonic processes.
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Onshore and offshore geologic mapping coupled with topical investigations constrain the tectonic relations and geometry of active plate boundaries in the Mendocino triple junction region. Along the northern California coast and offshore, Gorda-North American plate convergence is reflected by youthful west- to northwest-verging thrust fault systems that extend to or near the plate interface at depth. Interplate coupling across a minimum breadth of 70--80 km is indicated by late Quaternary uplift and shortening rates, the nature and distribution of upper and lower plate seismicity, divergent trends in upper plate structures, and a history of large late Holocene earthquakes. Offshore seismic-reflection and seismicity data from the vicinity of the Mendocino fault (MF) show that the fault dips steeply to the north, and that the older, relatively rigid Pacific plate acts as a buttress against which the southern Gorda plate is being deformed. Onshore investigations show that the San Andreas fault zone (SAF) extends on land southeast of Point Delgada (at Whale Gulch), and is manifested along the north and northeast side of the King Range (KR) by north-northeast-vergent thrust faults. This thrust fault system may root into the steeply dipping offshore San Andreas fault. Faults of this system may include active, more » blind northeast-vergent thrusts that extend from a root zone beneath the King Range northward and upward into Franciscan Complex (Coastal belt) rocks along the north flank of the range. The southern Cascadia subduction zone megathrust intersects the Mendocino and San Andreas transform faults in the Mendocino triple junction. The upper crustal location of this intersection lies nearshore and/or landward along the north flank of the King Range. An area of focused rapid uplift and repeated coseismic growth (Mendocino Uplift) straddles the triple junction. « less
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We combine results from seismic tomography and plate motion history to investigate slabs of subducted lithosphere in the lower mantle beneath the Americas. Using broadband waveform cross correlation, we measured 37,000 differential P and S traveltimes, 2000 PcP ‐ P and ScS ‐ S times along a wide corridor from Alaska to South America. We invert the data simultaneously to obtain P and S wave velocity models. We interpret slab structures and unravel subduction history by comparing our V S tomographic images with reconstructed plate motion from present‐day up to 120 Myr. Convergence of the Pacific with respect to the Americas is computed using either (1) the Pacific and Indo‐Atlantic hot spot reference frames or (2) the plate circuit passing through Antarctica. Around 800 km depth, four distinctive fast anomalies can be associated with subduction of the Nazca, Cocos, and Juan de Fuca plates beneath South, Central, and North America, respectively, and of the Pacific plate beneath the Aleutian island arc. The large fast anomalies in the lowermost mantle, which are most pronounced in the S wave models, can be associated with Late Cretaceous subduction of the Farallon plate beneath the Americas. Near 2000 km depth, the images record the post‐80 Myr fragmentation of the proto‐Farallon plate into the Kula plate in the north and the Farallon plate in the northeast. Near 1000 km depth, we infer separate fast anomalies interpreted as the Kula‐Pacific, Juan de Fuca, and Farallon slabs. This interpretation is consistent with the volume and length of slabs estimated from the tomographic images and the plate history reconstruction.
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The April 25, 1992, Petrolia earthquake ( M s 7.1) occurred at the southern tip of the Cascadia subduction zone. This is the largest thrust earthquake ever recorded instrumentally in the Cascadia subduction zone. The earthquake was followed by two large strike‐slip aftershocks (both Ms 6.6). Moment release of each of the earthquakes is as follows: 4.0 × 10 19 Nm in the first 10 s for the mainshock, 0.7 × 10 19 Nm in the first 8 s for the first aftershock, and 0.9 × 10 19 Nm in the first 2 s for the second aftershock. These indicate that the mainshock and each of the aftershocks may have different tectonic backgrounds. The best depth estimates of the mainshock and the two aftershocks are 14 km, 18 km, and 24 km, respectively. The slip direction of the mainshock is between N75°E and N80°E. This slip direction is not consistent with either the relative motion of the North American and Juan de Fuca plates (N60°E) or between the North American plate and the Gorda deformation zone (N40°E). It has been suggested that the North American‐Pacific plate motion is accommodated by right‐lateral slip on both the San Andreas and Maacama‐Rodgers Creek‐Hayward fault systems; the intervening block is the Humboldt plate. If we modify the relative motion of the southernmost Gorda deformation zone to conform with the seismicity trends and allow the Humboldt‐Pacific plate motion to be about half the total North American‐Pacific motion, then the Gorda deformation zone‐Humboldt relative motion matches the direction of the Petrolia slip vector. Also, the mixture of focal mechanisms in the two distinct aftershock clusters can be explained by motion between the Gorda deformation zone and Pacific plate and the Humboldt and North American plates. The Gorda deformation zone is subducting beneath the Humboldt plate in the Cape Mendocino area, and the Petrolia earthquake ruptured the entire subduction segment between the Gorda deformation zone and Humboldt plate.
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Analysis of 9 years of data from the Humboldt Bay seismic network sheds new light on the structure and evolution of the Gorda plate and Mendocino triple junction. Significant findings include the buttressing effect of the Pacific plate which demonstrates that there is no underthrusting of the Gorda plate along the Mendocino fault, and the pattern of left‐lateral northeast‐trending faults, which demonstrates how the Gorda plate accommodates N‐S shortening without such underthrusting. Focal mechanisms are consistent with northward compression of the Gorda plate by the Pacific plate until the Gorda plate passes the triple junction, beyond which the N‐S compressive stress is effectively removed, and the focal mechanisms show a change from strike slip to normal faulting with downslab tension. Another significant feature is the shallow Benioff zone (10–40 km) which shows a double seismogenic layer. Unlike most other double seismic layers in deep subduction zones, this one appears to be due to underplating, with a new subduction zone being developed east of the former one. A corner of continental margin material has been partially subducted. We attribute this double seismic zone to reactivation of both the old and new subduction boundaries under N‐S compression produced by Pacific‐Gorda plate interaction at the triple junction. Because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the San Andreas fault has migrated eastward, we have constructed a model that accounts for both the eastward migration of the San Andreas fault and the doubling of the seismic zone in the Gorda plate. This can be done by assuming that at the time of the last eastward jump of the fault, the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the San Andreas fault. Other scenarios for the evolution of the Mendocino triple junction do not require that the subduction zone migrate eastward. However, the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place. This is consistent with other evidence for an eastward jump of the San Andreas fault.
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