John Wakabayashi

Stream Incision, Tectonics, Uplift, and Evolution of Topography of the Sierra Nevada, California

Stream incision, faulting, thermochronologic, and geobarometric data suggest that Sierra Nevada topography is a consequence of two periods of uplift. Stream incision of up to 1 km has occurred since ∼5 Ma. Maximum Eocene‐Miocene incision was 150 m. Uplift of the Sierra Nevada, westward tilting, stream incision, and east‐down normal and dextral faulting along the present eastern escarpment of the range began at ∼5 Ma. Estimates of Late Cenozoic crestal rock uplift for different areas in the Sierra Nevada range from 1440 to 2150 m. Low summit erosion rates suggest that the rock uplift approximates the surface uplift of crestal summits. Tertiary stream gradients were lower than modern ones, suggesting that the bottoms of the canyons have been uplifted in the Late Cenozoic and that the mean elevation of the Sierra Nevada has increased. The elevation of pre‐Cenozoic basement rocks above the base of Tertiary paleochannels ranges from <200 m in the northern part of the range to >1000 m in the south, and shows that significant relief predates Late Cenozoic incision. Elevations at ∼5 Ma (before Late Cenozoic uplift) may have been <900 m in the northern Sierra and >2500 m in the southern Sierra. Minimal Eocene‐Miocene stream incision suggests that paleorelief and paleoelevations are relics of pre‐Eocene uplift. Reduction of elevation and relief following pre‐Eocene uplift may have coincided with eclogitic recrystallization of the mafic root of Sierran batholith. This eclogitic keel may have foundered in the Late Cenozoic, triggering uplift.

What constitutes ‘emplacement’ of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles

Abstract Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.

Counterclockwise P-T-t Paths from Amphibolites, Franciscan Complex, California: Relics from the Early Stages of Subduction Zone Metamorphism

Overprinting mineral relationships in amphibolites from the Franciscan Complex, California suggest metamorphic evolution from high-temperature amphibolite facies to the blueschist facies with increasing P/T ratio, a counterclockwise P-T-t path. Early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are, in turn, cut or rimmed by sodic amphiboles. Some amphibolites are overprinted successively by eclogite and blueschist facies assemblages. Clinopyroxenes are zoned with increasing jadeite component from core to rim. Multiple generations of phengites are present in the same sample and are more Si-rich with each successive generation of growth. Garnets in eclogite-overprinted amphibolites are zoned with an initial MgO increase away from the core to a maximum MgO zone followed by MgO decrease toward the rim. Garnets in other amphibolites show continually increasing MgO from core to rim. Several samples, representative of the variety of Franciscan amphibolites, are described in detail. The best-constrained sample exhibits a calculated P-T trajectory from 626-664°C at 9.2-10 kb for early amphibolite metamorphism to 496-537°C at 11.2-11.8 kb eclogite metamorphism to 300-350°C at >6.5-7 kb for final blueschist overprint. Overprinting mineral assemblages in other samples, as well as most Franciscan amphibolites described by other workers, are qualitatively, and in some cases quantitatively, suggestive of the same type of P-T trajectory. Geochronologic, textural, and petrologic data indicate that the overprinted amphibolites are probably the product of a single metamorphic event rather than a consequence of separate metamorphic episodes, but additional age dating is desirable for better confirmation. Amphibolite to blueschist facies metamorphic evolution may have taken place within 5 Ma. The following model is presented for the evolution of Franciscan amphibolites: (1) The amphibolites were metamorphosed as a dynamothermal aureole underneath the hanging wall of a subduction zone at the inception of subduction. (2) The amphibolites were then accreted to the upper plate. (3) Subsequent subduction and underplating of cold material insulated the hanging wall and allowed the upper plate and amphibolite to cool at depth, with the development of metamorphic assemblages of increasing P/T ratio. (4) Subduction beneath the accreted amphibolite may have broken up the amphibolite sheet, dragging blocks deeper down the subduction zone, leading to the pressure increase with cooling that is recorded by some of the samples. Deeper burial by thrusting within the upper plate after amphibolite metamorphism may also explain the pressure increase. The P-T trajectory of these rocks is consistent with P-T paths derived from published thermal models for similar tectonic settings.

Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California

This paper presents a new synthesis of Franciscan Complex tectonics, with the emphasis on the pre-San Andreas fault history of these rocks. Field relations suggest that the Franciscan is characterized by nappe structures that formed during sequential accretion at the trench. The presence of these structures along with other field relations, including the lack of evidence for large offset of conglomerate suites, indicates that strike-slip fault systems of large displacement (>500 km) did not cut the Franciscan Complex during subduction. Regional geology and comparisons to modern arc-trench systems suggest that strike-slip faulting associated with oblique subduction took place inboard (east) of the Franciscan in the vicinity of the magmatic arc. The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, seamounts, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of erosion produced the present-day surface distribution of high P/T metamorphism. Franciscan Complex tectonic history can be summarized as follows: (1) East-dipping Francistan subduction initiated beneath, and shortly after formation of the Coast Range ophiolite. (2) High-temperature precursors to Franciscan high-grade tectonic blocks formed as a dynamothermal aureole during subduction initiation beneath the hot hanging wall and were underplated to the upper plate. (3) Subduction continued, the high-grade metamorphic rocks were overprinted with assemblages of increasing P/T ratio as the hanging wall cooled, and the aureole was dismembered into blocks. (4) As subduction progressed, more material was underplated and metamorphosed as coherent blueschist. Peak metamorphic temperatures of successively subducted units decreased with time as hanging wall heat continued to dissipate. (5) Trench sediments with parts of seamounts, oceanic rises, and other small masses on the downgoing plate were underplated or offscraped during approximately 140 m.y. of continuous subduction, forming stacks of nappes. (6) The Mendocino Triple Junction migrated northward, and the subduction zone was replaced by a transform plate boundary associated with the San Andreas fault. Deformation and faulting related to the Neogene transform tectonic regime obscured many subduction-related structures.

Suprasubduction-zone ophiolite generation, emplacement, and initiation of subduction: A perspective from geochemistry, metamorphism, geochronology, and regional geology

Ophiolites are on-land remnants of oceanic lithosphere, and most of the more extensive ophiolites apparently formed above a subduction zone, a tectonic setting known as a suprasubduction-zone setting. Thin sheets of high-grade metamorphic rocks, known as metamorphic soles, crop out structurally beneath many suprasubduction-zone ophiolites. Such rocks may have formed during the inception of subduction beneath young and hot oceanic lithosphere. Disagreement exists as to whether suprasubduction-zone ophiolites are emplaced over the same subduction zone over which they once formed or over a later one. High-grade metamorphic rocks (blocks-in-melange and coherent sheets) from the Franciscan Complex may represent a metamorphic sole beneath the suprasubduction-zone Coast Range ophiolite. Trace-element and isotopic data indicate that the Franciscan high-grade metamorphic rocks formed in a suprasubduction-zone envi ronment, requiring the existence of a pre-Franciscan sub duction zone, whereas later-subducted, lower-grade oceanic rocks are of mid-ocean-ridge or oceanic-island basalt affi ni ties. The Coast Range ophiolite and Franciscan high-grade rock protoliths formed over a pre-Franciscan subduction zone that may have dipped westward. The high-grade Franciscan rocks were metamorphosed at the inception of east-dipping subduction beneath the Coast Range ophiolite, and the ophiolite was subsequently emplaced over this later subduction zone. Suprasubduction-zone protolith signatures have been obtained for other metamorphic soles beneath suprasubduction-zone ophiolites, suggesting that our proposed model of suprasubduction-zone ophiolite generation over one subduction zone and emplacement over a second one may be globally applicable. Regional geology suggests that this dual subduction-zone model may also apply to suprasubduction-zone ophiolites with midocean-ridge and/or oceanic-island basalt soles.

Spatial and temporal relationships between ophiolites and their metamorphic soles: A test of models of forearc ophiolite genesis

Thin sheets of high-grade metamorphic rocks, called metamorphic soles, are found structurally beneath many ophiolites, apparently formed at the initiation of intraoceanic subduction, and may be useful in determining the tectonic setting of ophiolite genesis. Many ophiolites are interpreted to have formed in a supra-subduction zone setting largely on the basis of their petrology and geochemistry. Of these supra-subduction zone ophiolites, most have been interpreted to have formed in a forearc setting. Generation of ophiolites in a forearc setting indicates that such ophiolites must postdate the initiation of subduction beneath them and must therefore be younger than their metamorphic soles. However, the examples of pairs of ophiolites and their metamorphic soles reviewed herein show that the ophiolites are not younger than their soles. The spatial and temporal relationships reviewed herein may be incompatible with the generation of ophiolites in forearcs, although volumetrically subordinate igneous rocks that postdate the formation of the soles have been found in some of the examples. The apparent forearc setting of the ophiolites may be an artifact of their emplacement tectonics, not their igneous environment of formation. Although we know of no geochronologic confirmation of an older subduction zone structurally beneath purported forearc ophiolites, detailed geochronology has yet to be conducted on most ophiolite and metamorphic sole pairs. For the scenario in which an ophiolite is formed above one subduction zone but emplaced over a different one, either backarc formation or intra-arc formation of the ophiolite can be compatible with its regional geochronologic and structural relationships.

Distribution of displacement on and evolution of a young transform fault system: The northern San Andreas fault system, California

This paper presents a working model for the spatial and temporal distribution of dextral slip on the northern San Andreas fault system of coastal California, based primarily on field relations in the San Francisco Bay area, and offers insight into the evolution of a young transform fault system. A fundamental difference between this and previous models of slip distribution is that this model assigns negligible slip to the Pilarcitos fault, which has been suggested to have 120 to 250 +km of post early-Miocene dextral slip, in previous models. Because separation on the San Francisco Peninsula reach of the San Andreas fault is about 25 km, and displacement on the San Andreas fault in central California is 310 to 320 km, more than 250 km of late Cenozoic dextral slip must be accommodated east of the San Francisco Peninsula. The distribution of this dextral slip is constrained by offset late Cenozoic and basement units. Slip distribution, combined with ages of offset features and plate boundary kinematic constraints, show that the distribution of slip rates on groups of faults along the transform boundary has changed in an irregular fashion through time, in contrast to existing models that propose progressive eastward migration of active faulting in the San Andreas system. In addition to the shifting patterns of displacement, a migrating transition zone connecting the eastern faults of the strike-slip system to the Mendocino triple junction may have resulted in distributed dextral faulting and shortening in the northernmost and youngest part of the transform fault system.

Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales

The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises mélange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Mélanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimentary origins as submarine mass transport deposits, whereas mélanges formed by tectonism comprise disrupted ocean plate stratigraphy and lack exotic blocks. The former are interbedded with and grade into coherent siliciclastic units. Palaeomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow zones of <100 m thickness. Exhumation of Franciscan units, both coherent and mélange, was accommodated by significant extension of the hanging wall and cross-sectional extrusion. The amount of total exhumation, as well as exhumation since subduction termination, needs to be considered when comparing Franciscan architecture to modern and ancient subduction complexes. Equal dextral separation of folded Franciscan nappes and late Cenozoic (post-subduction) units across strands of the (post-subduction) San Andreas fault system shows that the folding of nappes took place prior to subduction termination. Dextral separation of similar clastic sedimentary suites in the Franciscan and the coeval Great Valley Group forearc basin is approximately that of the San Andreas fault system, precluding major syn-subduction strike-slip displacement within the Franciscan.

Subduction and the rock record: Concepts developed in the Franciscan Complex, California

Ernst’s (1970) paper was chosen as the classic paper for the Franciscan Complex because it related high-pressure, low-temperature (high P-T) metamorphism to subduction. Perhaps most significantly, the paper explained the association of low geothermal gradients and the metamorphism. The paper also pointed out the difficult tectonic problem of the exhumation of the high P-T rocks, a problem still vigorously debated today, and proposed a tectonic model explaining the exhumation of the deeply buried rocks. In addition, the paper explained the tectonic contact of the Great Valley forearc over the Franciscan subduction complex in the context of plate tectonics theory (Hamilton, 1969, gave a similar explanation; see following). Ernst’s paper was one of the key advances in the plate tectonics revolution. Variations of Figure 3 of Ernst (1970) have become the textbook model of subductionzone metamorphism. Geologists now regard high P-T (including blueschist facies) metamorphism as the strongest evidence of an exhumed subduction complex. Evaluation of thermal gradients associated with subduction and their connection to metamorphic assemblages, introduced by Ernst (1970), has become an important concept in understanding the evolution of orogenic belts (e.g., Ernst, 1975, 1988). An example of this type of analysis is the premise that Franciscan subduction was continuous, from its inception in the late Mesozoic to conversion to a transform plate boundary in the late Cenozoic, because Franciscan high P-T rocks lack thermal overprints (such as late greenschist facies assemblages) which should have resulted from any cessation of subduction (Cloos and Dumitru, 1987; Ernst, 1988). Ernst’s (1970) paper was one of several key papers that related the Franciscan Complex to subduction processes and established the Franciscan as the type subduction complex. Hamilton (1969) equated the Franciscan to a subduction complex, related subduction to arc volcanism in the Sierra Nevada, and pointed out the far-traveled nature of some Franciscan Complex rocks. Hsu (1968, 1971) formalized the concept and principles of melange (Bailey et al., 1964, had recognized the shear-zone character of what were later called melanges). Dickinson (1970) placed the Franciscan in the context of an arc-trench system, with the Franciscan, Great Valley Group, and Sierra Nevada as the subduction complex, forearc basin, magmatic arc (and main terrigenous sediment source), respectively. Bailey et al. (1964) set the stage for these papers by compiling and evaluating an enormous amount of data and presenting ideas that forecast the plate tectonic interpretation of the Franciscan; this is still a useful reference on the Franciscan. Many major conclusions of these landmark papers have not been significantly challenged since their publication. Subsequent research has continued to provide insight into fundamental processes in subduction zones. Some developments in Franciscan geology since 1970, as well as major controversies, are discussed in the following. The general geology of the Franciscan Complex is shown in Figure 1.

Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California

We present new U/Pb ages for detrital zircons separated from six quartzose metagraywackes collected from different Franciscan Complex imbricate nappes around San Francisco Bay. All six rocks contain a broad spread of Late Jurassic–Cretaceous grains originating from the Klamath–Sierra Nevada volcanic-plutonic arc. Units young structurally downward, consistent with models of progressive underplating and offscraping within a subduction complex. The youngest specimen is from the structurally lowest San Bruno Mountain sheet; at 52 Ma, it evidently was deposited during the Eocene. None of the other metagraywackes yielded zircon ages younger than 83 Ma. Zircons from both El Cerrito units are dominated by ca. 100–160 Ma grains; the upper El Cerrito also contains several grains in the 1200–1800 Ma interval. These samples are nearly identical to 97 Ma metasedimentary rock from the Hunters Point shear zone. Zircon ages from this melange block exhibit a broad distribution, ranging from 97 to 200 Ma, with only a single pre-Mesozoic age. The Albany Hill specimen has a distribution of pre-Mesozoic grains from 1300 to 1800 Ma, generally similar to that of the upper El Cerrito sheet; however, it contains zircons as young as 83 Ma, suggesting that it is significantly younger than the upper El Cerrito unit. The Skaggs Spring Schist is the oldest studied unit; its youngest analyzed grains were ca. 144 Ma, and it is the only investigated specimen to display a significant Paleozoic detrital component. Sedimentation and subduction-accretion of this tract of the trench complex took place along the continental margin during Early to early–Late Cretaceous time, and perhaps into Eocene time. Franciscan and Great Valley deposition attests to erosion of an Andean arc that was active over the entire span from ca. 145 to 80 Ma, with an associated accretionary prism built by progressive underthrusting. We use these new data to demonstrate that the eastern Franciscan Complex in the northern and central Coast Ranges is a classic accretionary prism, where younger, structurally lower allochthons are exposed on the west, and older, structurally higher allochthons occur to the east, in the heavily studied San Francisco Bay area.