Mendocino Triple Junction Research Articles

Comment and Reply on ‘Tectonic uplift of a middle Wisconsin marine platform near the Mendocino triple junction, California’

Research Article| October 01, 1983 Comment and Reply on ‘Tectonic uplift of a middle Wisconsin marine platform near the Mendocino triple junction, California’: COMMENT N.-A. Mörner N.-A. Mörner 1Geological Institute, Stockholm University, S-10691 Stockholm, Sweden Search for other works by this author on: GSW Google Scholar Author and Article Information N.-A. Mörner 1Geological Institute, Stockholm University, S-10691 Stockholm, Sweden Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1983) 11 (10): 621. https://doi.org/10.1130/0091-7613(1983)11<621a:CAROTU>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 N.-A. Mörner; Comment and Reply on ‘Tectonic uplift of a middle Wisconsin marine platform near the Mendocino triple junction, California’: COMMENT. Geology 1983;; 11 (10): 621. doi: https://doi.org/10.1130/0091-7613(1983)11<621a:CAROTU>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 SocietyGeology Search Advanced Search Abstract No Abstract Available. 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.

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.

Neogene Fore-Arc Basin Development in Northern California: Eel River Basin: ABSTRACT

Strata representing the youngest phase of fore-arc basin development in northern California are exposed in an unusual cross-sectional view across the basin axis. The exposure of a large part of the stratigraphic section of the Neogene Eel River (Humboldt) basin can be attributed to uplift along the northern edge of the Mendocino triple junction. The fore-arc deposits overlie the Mesozoic and Cenozoic accretionary prism and slope deposits of the Franciscan Complex. Outcrop geology of the uplifted southern flank of the basin indicates that the facies and sediment distribution patterns agree with paleobathymetric studies; a complete deep to shallow marine transition is recorded in the basin sediments. Facies studies demonstrate the time-transgressive nature of the sedimentary environments. Proximal facies are landward (east) of coeval deeper water deposits exposed along the coast. The basal contact is clearly depositional on this southern flank of the basin. Sandstones and pebbly conglomerates cut into coastal belt Franciscan accretionary prism sediments inland. A previously undescribed debris flow is conformable on similar Franciscan sediments along the coast. This debris flow is faulted against overlying faulted and fractured basin plain siltstones and fine sandstones (Miocene-Pliocene) which contain thin lenses of shell debris, pebbles, and glauconitic sand. A monotonous accumulation of organic-rich diatomaceous mudstones is capped by amalgamated channels continuing sequences of thin glauconitic sands with locally derived siltstone rip-ups, siltstone, and hemipelagic mudstones. The overlying sediments consist of fine-grained turbidites, thick bioturbated siltstones and fine sandstones, and coarser turbidites. A continental shelf sequence concludes this phase of Eel River basin development. To the south of the main basin outcrop, progressive uplift and faulting related to the migration of the triple junction have left erosional remnants of sediments coeval with Eel River basin rocks. These rocks are found up to 50 km (31 mi) south of the upturned basin edge, suggesting that the basin was at one time more extensive to the south. Shallower depositional environments in some of these basin remnants may indicate the proximity of the original southern edge of the basin. Structural complexities to the south include strike-slip faulting and possible upper Miocene and younger trusting of Franciscan melange over Neogene marine sediments. End_Page 512------------------------------ Shallow gas fields have been developed in the basin; ongoing exploration for deeper oil and gas is not yet definitive. Potentially good source rocks in the deeper parts of the basin, underlying organic-rich Franciscan sediments, and the abundance of potential reservoir rocks higher in the section make this structurally complex onshore/offshore basin an attractive exploration target. End_of_Article - Last_Page 513------------

Delineation and interpretation of seismotectonic domains in western North America

Indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western North America. The San Andreas, East‐Central California (ECC), Mendocino Triple Junction (MTJ), and Pacific Northwest (PNW) domains are found to have similar stress fields, although the style and rate of deformation in each is different. We propose that all of these domains are generated by shear between the Pacific and North American plates. In the MTJ and PNW domains this shear may be evidence for the initiation of a transform fault connection between the San Andreas and the Queen Charlotte Island fault systems. The ECC, a region including much of the Sierra Nevada, appears to result from the relatively stiff nature of the San Andreas fault system. The Big Bend and region of relatively new faulting in northern California act to make the San Andreas fault system appear stiff on a regional scale. Two other domains, the Northern Basin and Range (NBR) and the Intermountain Seismic Belt (ISB), show evidence of being controlled by more local forces. The NBR has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the Juan de Fuca‐North America plate boundary. The ISB can be explained by readjustments between the Colorado Plateau and the Basin and Range Province, which may be caused by the lowering of the Basin and Range due to extension.

Evolution and thickness of the lithosphere beneath coastal California

Research Article| July 01, 1982 Evolution and thickness of the lithosphere beneath coastal California George Zandt; George Zandt 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar Kevin P. Furlong Kevin P. Furlong 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar Author and Article Information George Zandt 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Kevin P. Furlong 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1982) 10 (7): 376–381. https://doi.org/10.1130/0091-7613(1982)10<376:EATOTL>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 George Zandt, Kevin P. Furlong; Evolution and thickness of the lithosphere beneath coastal California. Geology 1982;; 10 (7): 376–381. doi: https://doi.org/10.1130/0091-7613(1982)10<376:EATOTL>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 SocietyGeology Search Advanced Search Abstract Many of the tectonic features of the California Coast Ranges are directly related to the northward migration of the Mendocino triple junction along the western edge of North America during the past 20 to 30 m.y. At the triple junction, the western edge of the North American plate slides off the (relative) northward-moving and subducting Gorda plate, leaving the thin western edge in direct contact with asthenosphere upwelling to fill the space vacated by the underlying Gorda plate. Two-dimensional, time-dependent thermal modeling of this process, constrained by teleseismic delay studies, is used to construct a map of lithospheric thickness of coastal California. Among the implications of this map are that (1) the high heat flow in the Coast Ranges can be almost entirely accounted for by the asthenospheric upwelling associated with the migrating Mendocino triple junction; (2) the general elevation and the late Cenozoic volcanism in the California Coast Ranges are responses to a zone of unusually thin (20 to 45 km) lithosphere that extends southward behind the Mendocino triple junction; and (3) the course of some segments of the San Andreas fault (in both central and southern California) appear to deviate from a deeper, more fundamental, transform boundary separating the Pacific and North American plates. 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.

Triple-junction instability as cause for late Cenozoic extension and fragmentation of the western United States

Regional extension in the western United States began soon after 30 m.y. B.P., the time of first interaction between the Pacific and North American plates. Previous explanations for extension include lithospheric-active processes and asthenospheric-active processes; the former are favored because of coincidence of timing with lithospheric interactions, relative timing of volcanism and deformation, and coincidence with pre-existing structures. Although extension has occurred, in part, in a back-arc setting, it cannot be attributed to age of subducted crust or azimuth of subduction, because both attributes favor back-arc contraction (characteristic behavior before 40 m.y. B.P. The Mendocino triple junction has been unstable since its inception. The continental margin was relatively straight before 30 m.y. B.P., and it has become more convex westward as the triple junction has migrated northward. The continental margin and arc have been anchored to the subducting slab (Farallon-Juan de Fuca), whereas the triple junction must move parallel to the San Andreas transform. The combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the San Andreas transform relative to the coast. These effects result from the unstable geometry of the Mendocino triple junction. Lack of a subducted slab (slab window) beneath the extended lithosphere enables asthenospheric rise but does not cause extension.

Seismic images of the deep structure of the San Andreas Fault System, Central Coast Ranges, California

Three‐dimensional inversion of teleseismic P wave travel time residuals recorded at the U.S. Geological Survey central California array has resolved small‐scale (∼tens of kilometers) crustal and upper mantle heterogeneity down to depths of 90 km beneath the California coast ranges. Upper crustal lateral velocity variations of ±8% correlate closely with surface geology. Lower‐than‐average velocities are associated with thick Tertiary sedimentary fill and higher‐than‐average velocities with basement exposures. Lower crustal velocity heterogeneity of ±4% appear to reflect crustal thickness variations. A thinner crust is indicated southwest of the San Andreas fault and northwest of San Pablo Bay. A linear zone of low‐velocities (0 to −4%) subparallel to the San Andreas fault was resolved in the upper mantle. The preferred interpretation is that the low‐velocities indicate a narrow upwarp of asthenosphere to unusually shallow depths (∼45 km) beneath the coast ranges. Such an unusual upper mantle structure may have been produced by the northwestward migration along the California coast of a transiently unstable Mendocino triple junction. The inversion results also indicate the possibility of partial decoupling of the crust from the upper mantle.

San Luis Obispo Transform Fault and Middle Miocene rotation of the Western Transverse Ranges, California

A fault‐disrupted, linear belt of lower middle Miocene pyroclastic and volcanic rocks was erupted 15–17 m.y. B.P. along a trend 140 to 200 km long in the southern Coast Ranges and along the northern flank of the western Transverse Ranges of California. Offset segments of the belt are coincident with the Oceanic‐West Huasna, Santa Maria River‐Little Pine, and Lompoc‐Solvang fault zones. The volcanic rocks thin eastward and also vary in thickness along strike, the latter reflecting the undulation of ‘porpoising’ highs and lows of basins. To account for the linear distribution of over 3000 km2 of bimodal volcanic rock types, it is proposed that they were intruded, extruded, and ejected into a continental submarine margin environment along a feature here named the San Luis Obispo transform (SLOT). The fault was near the mid‐Miocene continental margin and evidently joined the early mid‐Miocene Mendocino triple junction with the crest of the East Pacific Rise. Subduction of the Farallon Plate had stopped along the adjacent continental margin before Miocene magmatism in this region. The inferred position of the East Pacific Rise crest was too far to the south, and the inferred position of the Mendocino triple junction was too far to the north between 15 and 20 m.y. B.P. to have been directly related to the Miocene volcanic event near 35°N latitude at that time. A part of the western Transverse Ranges was transported northwestward immediately following Miocene volcanism. Earlier movement is possible, but evidence is inconclusive. Microplate transport was concurrent with the development of pull‐apart structures and the ≃12‐ to ≃16‐m.y.‐old period of volcanism along the southern margin of the microplate within the Los Angeles basin. Variable paleomagnetic directions that display clockwise deflections in the Oligocene (e.g., Morro Rock‐Islay Hill Complex) and Miocene igneous rocks (e.g., western Transverse Ranges) in the vicinity of SLOT can be accounted for by extension and rotational tectonics during pull‐apart development, rotation of blocks between faults during the northwestward transport of the western Transverse Ranges along SLOT, and by rotation and possible northward translation of a part of the western Transverse Ranges.

Heat flow and energetics of the San Andreas Fault Zone

Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000‐km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m−2) throughout the 550‐km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200‐km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near‐lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of these basal tractions, the average shearing stress in the seismogenic layer would increase with distance from the main fault, and the seismogenic layer would offer substantial resistance to plate motion even though resistance on the main fault might be negligible. These speculative models have testable consequences.

Gorda plate motions from magnetic anomaly analysis

Significant variations of spreading rates and directions in space and time along the Gorda ridge can be determined by analysis of magnetic anomalies. Converted to motion vectors, they suggest the existence of at least two and possibly three Gorda plate segments. Assuming rigid plate tectonics, present motion between the northern and two southern segments should be NW-SE right-lateral at around 3 cm/yr and any fault between them should intersect the ridge near 42°N. A linear bathymetric trough may represent at least part of such a proposed fault zone, named here the Gorda fault zone. At the continental margin, the northern Gorda plate segment has probably continued to converge obliquely for at least 5 m.y. However, the southern Gorda plate segments may have recently ceased to subduct and now be moving almost parallel to the slope. This seems to be in accord with seismological and geological evidence near and north of Cape Mendocino. In the future, rapidly decreasing spreading on the southern Gorda ridge may effectively lock the southern Gorda plate segment to the Pacific plate so that the present Cape Mendocino triple junction will transfer northwards to a point on the margin near 41°30′N.

Temporal Changes in Depositional Facies in Great Valley Forearc Basin of California--Influence of Basin Evolution and Tectonics: ABSTRACT

The Great Valley forearc basin began in the Late Jurassic as a residual forearc basin on top of oceanic crust, and evolved into a composite forearc basin on top of both oceanic and continental crust in the Late Cretaceous and Paleogene. The depositional basin widened through time owing to the westward and upward growth of the subduction complex and the eastward migration of the coeval magmatic arc. Depositional facies reflect changes in shape, size, tectonic activity, and inherited characteristics of the basin. Late Jurassic depositional environments primarily consisted of slope with locally incised channels. The basin was relatively narrow and a steep slope allowed sediment movement from the shoreline on the east directly into the trench to the west. By the Early Cretaceous, a bathymetric barrier was formed by the upward- and outward-building subduction complex, thus trapping arc-derived sediment within the forearc basin. Basin-plain environments dominated in this terraced forearc. The subduction complex continued to grow, and the magmatic arc migrated eastward during the Late Cretaceous, resulting in a wider, composite basin. Complex interbedded submarine fan, slope, and basin-plain facies formed in this setting. Submarine fan systems became larger owing to the concentration of sediment gr vity flows within submarine canyons incised into the widening shelf on the east side of the basin. The basin evolved into a broad ridged forearc and, eventually, into a broad shelved forearc during the Paleogene as the subduction complex emerged above sea level and the forearc basin filled. As a result, Upper Cretaceous submarine fan complexes are overlain by slope facies, which are overlain by shelf facies. Nonmarine environments have persisted following filling of the basin and sequential termination of subduction by the northward migration of the Mendocino triple junction during the Neogene. End_of_Article - Last_Page 727------------

Intracontinental plate boundary east of Cape Mendocino, California

Two parallel strike-slip fault zones east of Cape Mendocino are the apparent northward continuation of a northwest-trending line (San Andreas fault) that bifurcates into two great en echelon, recently active right-slip fault zones. North from Hollister, the eastern fault zone–the Hayward–Lake Mountain fault system–extends past Arcata onto the continental shelf southwest of Crescent City. The Hayward–Lake Mountain fault system defines the Humboldt plate, a small northwest-elongate sliver of the North American continent bounded on the west by the San Andreas fault zone and on the north by the Gorda plate. This newly recognized plate is converging northeastward against the Gorda plate, which is being thrust beneath it. Steps between en echelon member fault zones in the Hayward–Lake Mountain fault system suggest that the line of faults is so recent that there has been insufficient time to integrate the fault zones into a single surficial break. The fault system may have resulted from a recent and still continuing realignment of the North American plate boundary near the Mendocino triple junction.

Tectonic implications of space-time patterns of Cenozoic magmatism in the western United States

Locations of 2,100 radiometrically dated igneous rocks were plotted on a series of 20 maps, each representing an interval within the period 80 m.y. B.P. to present. Derivative maps showing the distributions in space and time of dated granitic intrusive rocks, silicic lavas and domes, ash-flow tuffs, andesitic-dacitic rocks, and basalts depict well the two main petrogenetic assemblages noted previously by others: (1) mainly intermediate andesitic-dacitic suites, including associated granitic intrusive rocks, silicic extrusive rocks, and minor basaltic lavas, are interpreted as reflecting plate interactions related to subduction along the continental margin; and (2) bimodal suites, dominantly basaltic but with minor silicic extrusive rocks, are interpreted as reflecting extensional tectonics. Space-time distribution of the two assemblages suggests that magmatic arcs extended continously parallel to the continental margin from Canada to Mexico in latest Mesozoic and in Oligocene times. An early Cenozoic null in magmatism in the Great Basin may delineate the region where subduction was arrested temporarily by development of the proto-San Andreas fault as a transform in coastal California or, alternatively, may reflect complex subsurface configurations of subducted plates. The late Cenozoic transition from subduction-related magmatism to extention-related basaltic volcanism in the southern Cordillera occurred at different times in different areas in harmony with current concepts about the migration of the Mendocino triple junction as the modern San Andreas transform fault was formed. The plots also reveal the existence of several discrete magmatic loci where igneous activity of various kinds was characteristically more intense and long-lived than elsewhere.

Melanges in the Franciscan Complex, a product of triple-junction tectonics

Tectonism near Cape Mendocino is ascribed to the north-northwestward migration of the Mendocino triple junction; it is characterized as a process in which the wedge of continental plate left in the wake of the retreating subducted extension of the Juan de Fuca plate is pushed and folded back by the advancing corner of the Pacific plate. Between Late Jurassic and Paleocene time, a different type of triple junction in which the oceanic plate and its obliquely subducting continuation advanced rather than retreated, herein named the Humboldt type, transited the western side of the continental plate. The laterally advancing subducted extension of the oceanic plate uplifted and snowplowed aside the continental plate, resulting in up-heaval, disruption, and gravity sliding of deep elements of the crust, including blueschist. The melange of the Franciscan Complex is a product of successive transits of Humboldt- and Mendocino-type triple junctions along the western edge of the continental plate.

Tectonics of the Mendocino Triple Junction

Interpretation of reflection profiles and of the magnetic anomaly pattern over the Gorda Basin and Escarpment gives broad agreement with the triple junction model of McKenzie and Morgan (1969). However, the basin has undergone internal deformation, a local departure from rigid plate tectonics, and the escarpment has had a component of underthrusting by the Gorda block. Faults in the Gorda Basin which disturb young turbidites parallel the trends of magnetic anomalies, suggesting deformation of the oceanic crust along lines of primary weakness. The northeast trends of the faults give a constraint on first-motion solutions for earthquakes within the basin and suggest left-lateral slip on the faults. Analysis of the geometry and timing of the Gorda Basin deformation based on the magnetic pattern gives an average gross tectonic strain rate of 10 −14 /sec. These observations give a measure of the mechanics of deformation of oceanic lithosphere very close to a spreading rise crest.

Transitional Tectonics and Late Cenozoic Structure of the Continental Margin off Northernmost California

The continental margin north of the Cape Mendocino triple junction is a region of tectonic transition between structures related to underthrusting on the continental slope and those related to right-lateral strike-slip along the coast. Underthrusting of late Cenozoic rocks of the continental slope by the Gorda lithospheric plate is seen by the presence of magnetic anomaly 3, age 5 m.y., beneath the slope. Folds and faults mapped by reflection profiling on the continental slope are parallel to it, and the faults have dip separations predominantly with west side down. These structures are best explained as resulting from oblique underthrusting, with structural trends controlled by the slope direction. Northwest-trending faults occur along the coast and reported earthquake mechanism solutions show right-lateral first motions. These faults and related folds affect Mesozoic rocks and parallel the San Andreas fault system which lies to the south, most probably sharing its origin—that of shear interaction between the Pacific and North American lithospheric plates.