Evolution of the Rodgers Creek-Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California
Robert J. McLaughlinAndrei M. Sarna‐WojcickiDavid L. WagnerRobert J. FleckV. E. LangenheimR. C. JachensKevin B. ClahanJames Allen
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Abstract:
The Rodgers Creek–Maacama fault system in the northern California Coast Ranges (United States) takes up substantial right-lateral motion within the wide transform boundary between the Pacific and North American plates, over a slab window that has opened northward beneath the Coast Ranges. The fault system evolved in several right steps and splays preceded and accompanied by extension, volcanism, and strike-slip basin development. Fault and basin geometries have changed with time, in places with younger basins and faults overprinting older structures. Along-strike and successional changes in fault and basin geometry at the southern end of the fault system probably are adjustments to frequent fault zone reorganizations in response to Mendocino Triple Junction migration and northward transit of a major releasing bend in the northern San Andreas fault.Keywords:
Triple junction
Overprinting
Transform fault
Neotectonics
Pacific Plate
Triple junction
Lineation
Pacific Plate
Seafloor Spreading
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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.
Triple junction
Pacific Plate
North American Plate
Transform fault
Slab window
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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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Hotspot (geology)
North American Plate
Slab window
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Understanding the evolution of the San Andreas transform is a key not only to broad aspects of regional geology but also to the development of specific structural provinces. Though stable in the regional sense, the paired triple junctions at the ends of the transform have had transient unstable configurations wherever the trends of the prior trench and the newly developing transform were locally not collinear. The resulting instabilities were mainly of kinds inferred to induce extensional tectonics within a nearby region. Passage of the Mendocino fault‐fault‐trench triple junction northward along the central California coast coincided well with pulses of initial subsidence in Neogene sedimentary basins near the continental margin and with eruptions at local volcanic centers in the Coast Ranges. Passage of the Rivera ridge‐trench‐fault triple junction southward was associated with the rifting events that formed the California Continental Borderland and the Gulf of California.
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Neogene
Continental Margin
Echelon formation
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In the Central Pacific Basin, an abandoned, NE trending spreading ridge is evident on the satellite gravity map and is confirmed in bathymetric and magnetic data. Our identification of magnetic anomaly lineations flanking the abandoned ridge suggests that between chrons M21 (147 Ma) and M14 (135 Ma) it was the plate boundary between the Pacific plate and a microplate, which we here name the Trinidad microplate. The configuration of the Pacific‐Farallon‐Phoenix triple junction until chron M22 (149 Ma) was fault‐fault‐ridge (FFR). After chron M22, the Pacific‐Farallon‐Phoenix triple junction changed its configuration. The reorganization of the triple junction led to the birth of the Trinidad microplate in its vicinity. Around chron M15 (136 Ma), the spreading ridge between the Pacific plate and the Trinidad microplate was abandoned and the Magellan microplate was born and remained active till chron M9 (128.5 Ma). After welding of the Magellan microplate to the Pacific plate, the triple junction had a ridge‐ridge‐ridge (RRR) configuration until the major reorganization around chron M0 (120 Ma).
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Lineation
Pacific Plate
Seafloor Spreading
Slab window
Transform fault
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Triple junction
Transform fault
Pacific Plate
Rift zone
Shield volcano
Echelon formation
Crest
Rift valley
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ABSTRACT The triple junctions predicted to be ridge–ridge–fault (RRF) types on the basis of large‐scale plate motions are the Azores triple junction between the Gloria Fault and the Mid‐Atlantic Ridge, the Juan Fernandez triple junction between the Chile Transform and the East Pacific Rise and the Aden–Owen–Carlsberg (AOC) triple junction between the Owen fracture zone (OFZ) and the Carlsberg and Sheba ridges. In the first two cases, the expected RRF triple junction does not exist because the transform fault arm of the triple junction has evolved into a divergent boundary before connecting to the ridges. Here, we report the results of a marine geophysical survey of the AOC triple junction, which took place in 2006 aboard the R/V Beautemps‐Beaupré . We show that a rift basin currently forms at the southern end of the OFZ, indicating that a divergent plate boundary between Arabia and India is developing at the triple junction. The connection of this boundary with the Carlsberg and Sheba ridges is not clearly delineated and the triple junction presently corresponds to a widespread zone of distributed deformation. The AOC triple junction appears to be in a transient stage between a former triple junction of the ridge–fault–fault type and a future triple junction of the ridge–ridge–ridge (RRR) type. Consequently, the known three examples of potential RRF triple junctions are actually of the RRR type, and RRF triple junctions do not presently exist on Earth.
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Abstract The Pacific, Antarctic, and Macquarie lithospheric plates diverge from the Macquarie Triple Junction (MTJ) in the southwestern Pacific Ocean, south of Macquarie Island. Morphobathymetric, magnetic, and gravity data have been used to understand the evolution of the three accretionary/transform boundaries that meet at the MTJ. Plate velocities, estimated near the MTJ and averaged over the past 3 m.y., indicate an unstable ridge–fault–fault triple junction. The long life (>6 m.y.) of this configuration can be attributed to a rapid increase in spreading asymmetry along the Southeast Indian Ridge segment as it approaches the MTJ, and to transtension along the southernmost strand of the Macquarie–Pacific transform boundary. A major change in plate motion triggered the development of the Macquarie plate at ca. 6 Ma and makes clear the recent evolution of the MTJ, including (1) shortening of the Southeast Indian Ridge segment; (2) formation of the westernmost Pacific-Antarctic Ridge, which increased its length over time; and (3) lengthening of the two transform boundaries converging in the MTJ. The clockwise change of the Pacific-Antarctic motion (ca. 12–10 Ma) led to complex geodynamic evolution of the plate boundary to the east of the triple junction, with fragmentation of the long-offset Emerald transform fault and its replacement over a short time interval (1–2 m.y.) with closely spaced, highly variable transform offsets that were joined by short ridge segments with time-varying asymmetries in the spreading rates.
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Transform fault
Pacific Plate
Convergent boundary
Clockwise
Transtension
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The Rodgers Creek–Maacama fault system in the northern California Coast Ranges (United States) takes up substantial right-lateral motion within the wide transform boundary between the Pacific and North American plates, over a slab window that has opened northward beneath the Coast Ranges. The fault system evolved in several right steps and splays preceded and accompanied by extension, volcanism, and strike-slip basin development. Fault and basin geometries have changed with time, in places with younger basins and faults overprinting older structures. Along-strike and successional changes in fault and basin geometry at the southern end of the fault system probably are adjustments to frequent fault zone reorganizations in response to Mendocino Triple Junction migration and northward transit of a major releasing bend in the northern San Andreas fault.
Triple junction
Overprinting
Transform fault
Neotectonics
Pacific 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.
Triple junction
Pacific Plate
North American Plate
Slab window
Convergent boundary
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