Warren Hamilton

Mesozoic California and the Underflow of Pacific Mantle

The Mesozoic evolution of California is interpreted as dominated by the underflow of oceanic mantle beneath the continental margin. Underflow during part of Late Cretaceous time of more than 2000 km of the eastern Pacific plate seems required by the marine magnetic data. Correspondingly, varied oceanic environments—abyssal hill, island arc, trench, oceanic crust, and upper mantle, perhaps also continental rise and abyssal plain—appear to be represented in the eugeosynclinal terranes of California. The rock juxtapositions accord with the concept that these materials were scraped off against the continent as the oceanic plate slid beneath it along Mesozoic Benioff seismic zones, which are now seen as serpentine belts separating profoundly different rock assemblages. The chaotic Franciscan Formation of coastal California consists of deep-ocean Late Jurassic to Late Cretaceous sedimentary, volcanic, crustal, and mantle materials. As open-ocean abyssal oozes and the oceanic crust beneath them were swept into the Benioff-zone trench at the continental margin, they were covered by terrigenous clastic sediments, and the entire complex was carried beneath the correlative continental-shelf and continental-slope deposits (Great Valley sequence) and the older Mesozoic complexes. The other eugeosynclinal terranes of California can be interpreted, albeit with less confidence, in similar terms of underflow of Pacific mantle. In the Klamath Mountains and northern Sierra Nevada, for example, Ordovician and Silurian ocean-floor materials, overlain by or juxtaposed against an Upper Silurian to Permian island arc, were swept in first to the continent, along with a large fragment of oceanic crust and mantle and another fragment of an old orogenic belt. This debris was followed by Permian and Triassic ocean-floor deposits. Late Triassic and Jurassic volcanic products from stocks and batholiths forming in the welded complexes lapped across both landward and oceanward sides of the region. Reversal of Cenozoic extension, strike-slip faulting, and volcanic crustal growth in the western United States reveals a Cretaceous tectonic pattern strikingly like the modern pattern of the Andes, so the paleotectonic setting of North America can be inferred from the South American present. The Mesozoic batholiths of North America, like the late Cenozoic volcanic belt of the central Andes, are products of the same rapid motion of oceanic plates that carried oceanic sediments against the continent to form eugeosynclinal terranes. Magmas generated in the Benioff zones formed the batholiths and the volcanic fields which initially capped them.

Archean magmatism and deformation were not products of plate tectonics

Abstract The granite-and-greenstone terrains that dominate upper crust formed from about 3.6 to about 2.6 Ga, and record magmatic and tectonic processes very different from those of a younger time. They indicate heat loss by the Archean Earth primarily by voluminous magmatism from a mantle much hotter than that of the present. Plate-tectonic processes were not then operating. The distinctive array of petrologic, structural and stratigraphic features that characterize Phanerozoic convergent-plate systems—ophiolites, magmatic arcs, accretionary wedges, fore-arc basins, etc.—have no viable analogues in Archean terrains. Purported Archean plate-tectonic indicators consist merely of rock types that even superficially resemble actual Phanerozoic indicators only when considered in isolation from their association and structure. Archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite-type successions, they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic-volcanic rocks, and they require a mantle about 200°C hotter than now. Archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions, and they lack the setting and disruption that characterize modern accretionary wedges. The lithologic, structural and stratigraphic assemblages that typify Proterozoic and Phanerozoic rifted and reassembled margins also lack Archean analogues, and no evidence has been found for Archean rifting, rotation, and reassembly of continental plates. Conversely, characteristic Archean assemblages lack modern equivalents in any tectonic setting. Ultramafic lavas, erupted at liquidus temperatures, are voluminous. Granite-and-greenstone terrains have no modern analogues. Greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by, and sinking between, large rising diapiric, elliptical composite batholiths. The batholiths include both the products of new crustal melts and variably remobilized mid-crustal gneisses. The greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions, and are not relics of successively formed volcanic chains. Little deformation generally preceded the diapirism, and metamorphism was primarily of contact type. The regionally uniform areal density and accordant crustal level of the diapiric batholiths, their contacts primarily against the oldest strata of the synforms, their general age 10–20 million years younger than most of the flanking stratiform rocks, and considerations of high Archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating. Diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust, producing the orientations of the batholiths. Rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it, resulting in cooling of the deeper crust and subjacent mantle, and thus cratonization. The upper crust, containing the granite-and-greenstone aggregates, was decoupled from the gneissic middle crust, which underwent flattening and extension subparallel to the elongation of the shallow batholiths. This deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle. The early Earth was probably wholly molten. The surface of the Earth, like that of the Moon, must have been wholly recycled by impacts before 3.9 Ga and heavily modified by them until 3.8 Ga. Zircons as old as 4.2 Ga have been found as clastic grains in much younger Archean quartzites, and polycyclic migmatites, last partly melted and reconstituted under hydrous conditions only after 3.6 Ga, contain relict zircons as old as 4.0 Ga. The lithologies of the early Earth protoliths in which these zircons formed have not been established, but impact melts and breccias must be represented, and magma-ocean fractionates may be. The nature of the transition in tectonic style into the granite-and-greenstone mode is unknown. Plate tectonic rifting and convergence were operating by about 2.0 Ga and were in an essentially modern mode by about 0.8 Ga. The nature of the transition from the granite-and-greenstone mode at about 2.6 Ga to plate mode by about 2.0 Ga has yet to be defined. The change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense, but hydrated, delaminated Archean crust sank into it.

The Uralides and the Motion of the Russian and Siberian Platforms

The Uralides—the late Precambrian and Paleozoic orogenic terrane between the Russian and Siberian Platforms—in part are exposed in the Ural Mountains, in the central Soviet Arctic, along the west edge of the Siberian Platform, and in southern Siberia and Kazakhstan, and in part are buried beneath the fill of the West Siberian Lowlands and other basins. Paleomagnetic orientations suggest that the Russian and Siberian Platforms were far apart during the early Paleozoic, converged during the middle Paleozoic, and collided in the Permian or Triassic. The geology of the Uralides accords with the concept that the two subcontinents approached and collided as the intervening oceanic plate slid beneath them along subduction (Benioff) zones. The medial eugeosyncline of the Uralides consists largely of what may be oceanic material scraped off against the edges of the opposed subcontinents. Basalt-and-spilite belts may represent ocean-floor abyssal tholeiite, and the manganiferous cherts and other sediments upon them may be pelagic oozes. Andesite belts may have formed as island arcs within the ocean, swept subsequently against the continents. Fossil subduction zones are recorded by great faults soled by, or containing tectonic injections of, mafic and ultramafic rocks from the lower oceanic crust and upper mantle, and containing high-pressure metamorphic rocks. Granitic and silicic-volcanic rocks may have formed above the subduction zones in the accreted parts of the continental plates. Both these continental-margin magmatic rocks and the island-arc complexes display ratios of potassium to silicon that vary across strike and so indicate the directions of dip of the subduction zones. From the distribution of such indicators of various ages, a history of the continental margins can be deduced. An active subduction zone dipped beneath the Siberian Platform during at least parts of late Precambrian and early, middle, and late Paleozoic time. The late Precambrian and Cambrian history of the Russian side is unclear, but in the Ordovician and Silurian the Russian continental margin was stable, while somewhere offshore an island arc was present whose trench was on the Russian side; the last of the intervening oceanic plate vanished down the subduction zone in about the Early Devonian, and the island arc became part of the continental margin. During the remainder of the Devonian and during the Carboniferous and Early Permian, a subduction zone was present along the margin of the enlarged Russian continent and dipped beneath it. Each subcontinent grew oceanward as oceanic material was accreted against it, and the subduction zones stepped oceanward correspondingly. The continental magmatic zones migrated oceanward behind the accreting edges of the continental plates, so the tectonic and magmatic progression with time at any one place is analogous to the variations present across the entire orogenic belt at any one time. Severe right-lateral deformation of the Uralides, the Russian side having moved northward relative to the Siberian side during Mesozoic and early Cenozoic time, is inferred from structural and magnetic-anomaly patterns. The deformation was accomplished by oroclinal folding, strike-slip faulting, and tensional thinning of the crust. The Uralides may have been continuous in early Mesozoic time with the Ellesmerides of North Greenland and the Canadian Arctic islands. The Cenozoic (and late Mesozoic?) opening of the Arctic Ocean was accomplished by spreading of the Eurasia Basin, and by opening of the Canada Basin behind a counterclockwise-rotating Alaska.

Subduction systems and magmatism

Abstract Most published subduction modelling and much palaeotectonic speculation incorporate the false assumption that subducting oceanic plates slide down fixed slots. In fact, hinges roll back into oceanic plates and slabs sink more steeply than the inclinations of the Benioff zones which define transient positions of the slabs. The lower parts of overlying mantle wedges sink with the slabs, pulling away from partial-melt zones higher in the wedges. The complex behaviour of arc systems can be comprehended in terms of this mechanism of subduction. The common regime in overriding plates is extensional, and leading edges are crumpled only in collisions. Shear coupling between subducting slabs and overriding plates is limited to shallow depths and varies widely, with corresponding variations in tectonic erosion, accretion, and regurgitation of high-P subducted materials. Arcs can advance, lengthen, change curvature, festoon around obstacles, rotate while deforming, and fold and pinch shut. Two arcs can collide as an intervening oceanic plate is subducted simultaneously beneath both, or they can migrate apart as new lithosphere is formed between them. Subduction cannot occur simultaneously beneath opposite sides of a rigid plate because impossible retrograde subduction would be required beneath one of them. Histories, including inception ages, collisions, polarity reversals and stage of petrological evolution, vary greatly along continuous arc systems. Long-continuing steady-state systems are uncommon. Magmatic arcs are properly viewed as features migrating with sinking lower plates, not as fixed features of upper plates. Hot inclined zones within mantle wedges, midway between sinking slabs and overriding crust, are avenues for replenishment of mantle pulled away with subducting plates and also are sites of generation of arc protomelts as volatiles rise into them from dehydrating slabs. Back-arc basins form by spreading behind migrating arcs; strips of arcs may be abandoned in the spreading systems. An arc can migrate so rapidly that it plates out oceanic lithosphere rather than producing a welt. Exposed sections of the upper mantle and basal crust of arcs show that the Mohorovičić discontinuity is a self-perpetuating density filter and that the already-evolved basaltic and melabasaltic melt that leaves the mantle forms great basal-crust sheets of norite, gabbro and granulite. All more-evolved rock types in these sections are generated in the crust by fractionation, secondary melting and contamination (and this falsifies much petrological modelling).

Crustal extension in the Basin and Range Province, southwestern United States

Summary Cenozoic extension of areally varying ages and amounts has on average doubled the width of the Basin and Range Province. Extensional structures that formed at all depths down to 20 km, and which range in age from Oligocene to Holocene, are widely exposed and are here interpreted in terms of a model of depth-varying deformation. The middle crust is extended by discontinuous ductile shear as internally underformed lenses slide apart along gently dipping zones of mylonite. The tops of these lenses are undulating detachment faults, the composite area of which increases with time as deep lenses slide out from underneath shallower ones. Brittle blocks of upper-crust bedrock above the detachments respond first by rotating between range-front faults, the same direction of rotation being maintained across a series of lenses, and then by pulling completely apart, while basinal strata fill the gaps and are dragged directly on detachment faults. Some faults rise gently from the main detachment zones and surface as range-front faults. Most tilted-block ranges are isolated atop detachments. Detachment faults cut out crust. Beneath them are mid-crustal rocks of any age and type and above them are mostly upper-crustal rocks, including extensive syndeformational basin sediments rotated to steep or moderate dips. As attenuation proceeds and components rise, detachment faults evolve from ductile to brittle, develop splays, and are themselves broken by steep brittle structures related to new, deeper detachments. Parts of detachment faults remain active even after exposure at the surface, but slip on them is then limited to the down-dip direction. It is inferred from seismic reflection profiles and rock-mechanic considerations that the unexposed lower crust is extended by more pervasive ductile flattening.

Tectonics of Antarctica

Antarctica has long been considered to consist simply of a large Precambrian shield, flanked on one side by the circum-Pacific belt of Mesozoic and Cenozoic orogeny. The great amount of geologic information obtained during recent years, however, indicates that between shield and circum-Pacific belt are belts of Paleozoic orogeny. The continent is probably crossed by a late Precambrian and Early Cambrian miogeosyncline, whose contents were metamorphosed and intruded by granodioritic batholiths during later Cambrian or Ordovician time. This orogen crosses the Antarctic coast between 145° and 160° E., and flanks the Ross Sea, Ross Ice Shelf, Filchner Ice Shelf, and possibly Weddell Sea, as a continuous system of high mountains. Rocks of the orogen are overlain by little-deformed Devonian to Triassic sedimentary rocks, which are intruded by sills of diabase. Northeastern Victoria Land, west of the Ross Sea, is composed of metasedimentary rocks, striking generally east-southeastward, whose metamorphism and intrusion by granites probably occurred during Silurian or Devonian time. On strike across the Ross Sea, and trending toward the Weddell Sea at least to the region of 90° W., 82° S., are metasedimentary rocks intruded by petrographically and chemically distinctive quartz monzonites and allied granitic rocks; this terrane is considered to be of middle to late Paleozoic age. Palmer Peninsula is composed largely of a Cretaceous batholith of quartz diorite, intrusive into metavolcanic rocks, and clearly belongs to the circum-Pacific orogen. The tectonic belt is connected to the South American Andes by way of the Scotia arc, an island-sprinkled submarine ridge; the belt is probably connected to New Zealand by way of Thurston Peninsula and the coastal region of the Amundsen Sea, and thence by way of a submarine ridge that passes near the mouth of the Ross Sea. The Antarctic coastal region between 35° and 160° E. is characterized by charnockites, granulites, gneisses, and by varied younger crystalline rocks. As has long been recognized, this is part of a Precambrian shield, which may extend farther toward the Weddell Sea, and which presumably extends far toward the south pole in the ice-buried interior. The rocks of the shield were widely metamorphosed about 500 million years ago. This revised pattern of Antarctic tectonics is essentially that required by Du Toits reassembly of the southern hemisphere continents before post-Paleozoic continental drift.

Crustal evolution by arc magmatism

Arc magmas generated at depths near 100 km by dehydration of subducting slabs are olivine-rich melabasalts, but the magmas that reach the surface in mature continental magmatic arcs have an average composition near that of rhyodacite. Depth-varying fractionations and equilibrations profoundly modify the initial magmas. Plate tectonics has operated throughout Proterozoic and Phanerozoic time much as it does now, and so many deeply eroded terrains must expose products of arc magmatism. Most exposed middle and deep continental crust consists of igneous rocks plus older rocks equilibrated at magmatic temperatures, variably deformed and metamorphosed subsequently; each crustal level displays a typical assemblage of magmatic rock types, which are here deduced to be products mostly of arc magmatism. Pre-arc mantle consists of depleted dunite and harzburgite. Rising arc magmas precipitate much additional olivine, and add the equivalent of 10 or 15% of basalt in clinopyroxene, orthopyroxene, garnet and spinel. The seismic M. discontinuity may be controlled primarily by the shallow limit of the depth zone in which crystallization is mostly of ultramafic components, and the fractionated magmas that reach the crust are mostly of gabbroic to granodioritic compositions. The lower crust is characterized by differentiated layered complexes, which often contain mafic or ultramafic basal cumulates, medial floated-plagioclase anorthosite, and upper quartz-poor pyroxene—mesoperthite granites, and by other magmatic rocks. Magmatic rocks are more voluminous than pre-existing rocks. Supracrustal rocks are in middle granulite facies, and much granitic material has been melted from them by magmatic heat. The magmatically modified middle crust consists primarily of migmatites in lower granulite facies in the deeper part, and upper amphibolite facies in the shallower part. Much dissociation of hydrous mineral assemblages is caused by the magmatic heat, producing water-rich, aluminous magmas, assimilation and anatexis. The high water contents restrict rise of the equilibrated magmas; voluminous pegmatites are expelled into the wall rocks, and crystallization is forced. Sheets of two-mica granites characterize the upper part of the middle crust. The comparatively dry magmas that rise into the upper crust are mostly tonalite to adamellite. These magmas spread out in steep-sided batholiths above the migmatites, erupt as ash flow sheets from calderas, and produce voluminous far-travelling volcanic ash. Inverted metamorphic gradients and outward-verging structures are produced beneath the spreading batholiths. Magmatic arcs are extensional at all crustal levels.

ARCHEAN TECTONICS AND MAGMATISM

The Earth began to form ∼4.56 Ga, and probably was entirely molten about 4.45 Ga and likely also before. Felsic igneous rocks of unknown provenance were in the crust by 4.3 Ga (clastic zircons to 4.3 Ga in <3.0 Ga quartzites, and relict zircons to 4.0 Ga in migmatites stabilized <3.6 Ga). Lunar analogy requires that the entire surface of the Earth was recycled by impact melting and brecciation before 3.9 Ga, and greatly modified until 3.8 Ga, but no certain relics of this history have been identified. When the fog clears and Archean time proper begins, at ∼3.6 Ga, the oldest granite-and-greenstone terrains are beginning to form. The granite-and-greenstone terrains that dominate the upper crust, formed from ∼3.6 to 2.6 Ga, record magmatic and tectonic processes that have no close younger analogues. They indicate heat loss by the Archean Earth primarily by voluminous magmatism. The upper mantle was 200°C, perhaps 300°C, hotter than at present. Magmatism in the granite-and-greenstone terrains began with regi...

Origin of the Gulf of California

The probable cumulative Late Cretaceous and Cenozoic right-lateral strike-slip displacement along the San Andreas fault in central California is 350 miles. The San Andreas and the allied faults into which it branches southward trend longitudinally into the Gulf of California, and the seismicity of the region indicates that the fault system follows the length of the Gulf and enters the Pacific basin south of Baja California. Crustal structure of most of the Gulf is of oceanic type, so that an origin by structural depression of continental rocks is not possible. Tectonic styles north and south of Los Angeles differ greatly. To the north, the Coast Ranges expose thick Upper Cretaceous and Cenozoic sedimentary rocks that were deposited in local basins and deformed tightly and repeatedly. To the south, in the Peninsular Ranges and Baja California, correlative rocks are thin and show little compressive deformation. The California batholith of mid-Cretaceous age and allied crystalline rocks form the basement of Baja California, southwestern Arizona, and northwestern Sonora and probably extend along the coast of mainland Mexico; the Gulf apparently bisects the crystalline belt longitudinally. These features suggest that Baja California initially lay 300 miles to the southeast, against the continental-margin bulge of Jalisco. The Gulf of California may be a pull-apart feature caused by strike-slip displacement plus up to 100 miles of cross-strike separation of the continental plate, subcontinental materials having welled up into the rift gap. The strike-slip motion has a tensional component across the continental margin south of Los Angeles but a compressional component to the north.

Correlation of metamorphosed Paleozoic strata of the southeastern Mojave Desert region, California and Arizona

Isolated outcrops of deformed, regionally metamorphosed Paleozoic strata are scattered within the southeastern Mojave Desert region of California and western Arizona. These strata unconformably overlie a basement of Proterozoic crystalline rocks and are overlain in turn by metamorphosed Mesozoic sedimentary rocks. The strata can be correlated lithostratigraphically with the classic cratonal Paleozoic section of the western Grand Canyon, Arizona, and with nonmetamorphosed Paleozoic sections transitional between cratonal and miogeoclinal in the Ship, Marble, and Providence Mountains, California. The strata evidently were once continuous with Paleozoic epicontinental strata exposed throughout the southern Great Basin and Colorado Plateau. Outcrops of Paleozoic strata and of the underlying Proterozoic basement in the southeastern Mojave Desert region define a terrane that has been disrupted by Mesozoic thrust faults and by Tertiary detachment faults but that nevertheless retain a gross paleogeographic coherence. This coherent terrane extends at least as far west and southwest as the Big Maria, Palen, and Calumet Mountains, and possibly beyond to include Paleozoic exposures in the San Bernardino Mountains and near Victorville. Poorly understood tectonic boundaries separate the area of paleogeographic coherence from known or suspected allochthonous terranes in the western Mojave Desert and the eastern Transverse Ranges.