William R. Dickinson

Plate Tectonics and Sandstone Compositions

Detrital framework modes of sandstone suites from different kinds of basins are a function of provenance types governed by plate tectonics. Quartzose sands from continental cratons are widespread within interior basins, platform successions, miogeoclinal wedges, and opening ocean basins. Arkosic sands from uplifted basement blocks are present locally in rift troughs and in wrench basins related to transform ruptures. Volcaniclastic lithic sands and more complex volcano-plutonic sands derived from magmatic arcs are present in trenches, forearc basins, and marginal seas. Recycled orogenic sands, rich in quartz or chert plus other lithic fragments and derived from subduction complexes, collision orogens, and foreland uplifts, are present in closing ocean basins, diverse succ ssor basins, and foreland basins. Triangular diagrams showing framework proportions of quartz, the two feldspars, polycrystalline quartzose lithics, and unstable lithics of volcanic and sedimentary parentage successfully distinguish the key provenance types. Relations between provenance and basin are important for hydrocarbon exploration because sand frameworks of contrasting detrital compositions respond differently to diagenesis, and thus display different trends of porosity reduction with depth of burial.

Provenance of North American Phanerozoic sandstones in relation to tectonic setting

Framework modes of terrigenous sandstones reflect derivation from various types of provenance terranes that depend upon plate-tectonic setting. Triangular QFL and QmFLt compositional diagrams for plotting point counts of sandstones can be subdivided into fields that are characteristic of sandstone suites derived from the different kinds of provenance terranes controlled by plate tectonics. Three main classes of provenance are termed “continental blocks,” “magmatic arcs,” and “recycled orogens.” Sandstone suites from each include three variants, of which the subfields lie within the larger subdivisions. Average modes for sandstone suites can be classified provisionally according to tectonic setting using the subdivided QFL and QmFLt plots. To test the validity of the classification, average modes for 233 Phanerozoic sandstone suites from North America were plotted on the triangular compositional diagrams and accompanying paleotectonic maps. Paired maps and ternary diagrams were prepared for eight different time slices, for each of which the tectonic setting of each major region within the continent remained relatively unchanged. Time slices are unequal in length but are controlled by the timing of major orogenic and rifting events that affected North America during the Phanerozoic. Comparison of the sandstone compositions with inferred tectonic setting through the Phanerozoic indicates that the proposed classification scheme is generally valid and yields satisfactory results when applied on a broad scale. Its application, together with other approaches, in regions of the world where over-all trends of geologic history are less well known could lead to important conclusions about the timing and nature of major tectonic events.

Interpreting Provenance Relations from Detrital Modes of Sandstones

Detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes, although various other sedimentological factors also influence sandstone compositions. Comparisons of sandstone compositions are aided by grouping diverse grain types into a few operational categories having broad genetic significance. Compositional fields associated with different provenances can then be displayed on standard triangular diagrams.

Geometry of Subducted Slabs Related to San Andreas Transform

Development of the San Andreas transform by rise-trench encounter in coastal California influenced the structural evolution of a large region within the adjacent continent. Continuation of arc magmatism and tectonism depends upon the presence of a subducted slab of lithosphere at depth beneath an arc-trench system. The lack of subduction at the transform plate boundary along the California continental margin led to the growth of a slab-free region beneath the part of the continental block adjacent to the San Andreas transform. Geometric analysis based on ideal assumptions predicts that generation of a lengthening transform by rise-trench encounter will also generate an expanding triangular hole or window in the slab of lithosphere subducted beneath the continent. One leg of the slab-window is the adjacent transform, but the orientations and lengths of the other two legs depend upon the relative motions of the three plates involved. By inference, arc volcanism and tectonism cannot persist across the no-slab area at the surface above the slab-window. The actual configuration of the slab-free region adjacent to the transform will depart from ideal predictions where adjustments to the conditions of rise-trench encounter involve changes in the motions of surface plates or subterranean ruptures in subducted slabs. The extent of the expanding slab-free region adjacent to the San Andreas transform can be reconstructed through time from detailed knowledge of oceanic plate boundaries and motions offshore. The progressive switchoff of Neogene arc volcanism conformed to expected patterns in time and space when the age of oceanic lithosphere being consumed near the coast is taken into account. The extent of the growing no-slab area at the surface above the widening slab-window at depth has been largely coextensive with the gradually expanding Basin and Range province of extensional tectonism and bimodal volcanism. Diapiric upwelling of asthenosphere through the evolving slab-window in the subducted lithosphere probably influenced magma genesis and geodynamic behavior within the slab-free region. Bulk uplift of the adjacent Sierra Nevada and the nearby Colorado Plateau, as well as opening of the Rio Grande Rift, were possibly related to the same mantle processes.

Structure and Stratigraphy of Forearc Regions

Active continental margins and the active flanks of island arcs lie in the forearc regions of arc-trench systems generated by plate consumption. Arc-trench systems are initiated by contractional activation of previously rifted continental margins, by reversal of subduction polarity following arc collisions, and as island arcs within oceanic regions. The varied configurations of shelved, sloped, terraced, and ridged forearcs arise partly from differences in initial geologic setting, but mainly from differences in structural evolution during subduction. In regions where large quantities of sediment are delivered, forearc terranes enlarge during subduction through linked tectonic and sedimentary accretion of deformed ocean-floor sediments and igneous oceanic crust, uplifted rench-floor and trench-slope sediments, and the depositional fills of subsiding forearc basins. Where sediment delivery is small, enlargement is subdued or absent, and shortening of the arc-trench gap may be possible. Trench inner slopes typically are underlain by growing subduction complexes composed of imbricate underthrust packets of ocean-basin, trench-floor, and trench-slope sediments in thrust sheets, isoclines, and melanges. The structure of subduction complexes is governed by the thickness and nature of oceanic layers rafted into the subduction zone, variable thicknesses of trench and slope sediments, and the rate and obliquity of plate convergence. Forearc basins between the magmatic arc and the trench axis include (a) intramassif basins lying within and on basement terranes of the arc massif, (b) residual basins lying on oceanic or transitional crust trapped between the arc massif and the site of initial subduction, (c) accretionary basins lying on accreted elements of the growing subduction complex, (d) constructed basins lying on the arc massif and accreted subduction complex, and (e) a composite of these basins. Strata deposited in forearc basins are typically immature clastic sediments composed of unstable clasts derived from rapid erosion of volcanic mountains or uplands of plutonic and metamorphic rocks within the arc massif. In equatorial regions reef-carbonate associations are also common. Facies patterns of turbidites, shelf sequences, and fluviodeltaic complexes within forearc basins are governed by the elevation of the basin thresholds, the rate of sediment delivery, and the rate of subsidence of the substratum. Petroleum prospects in forearc regions typically are limited by the prevalence of small, obscure structures within the subduction complex, the scarcity of good reservoirs in the forearc basin, the limited occurrence of source beds, and low geothermal gradients except within the arc massif where heat flux is commonly excessive.

Carboniferous to Cretaceous assembly and fragmentation of Mexico

The geologic framework of Mexico evolved through the Phanerozoic assembly and fragmentation of crustal elements derived from Laurentia, Gondwana, and an intra-Pacific volcanogenic terrane. In middle Paleozoic time, an inactive south-facing Laurentian continental margin of transform origin passed through northern Mexico to connect the miogeoclinal Cordilleran margin with the passive continental margin formed by Cambrian rifting in Texas. Gondwanan blocks of eastern Mexico were accreted to Laurentia by juxtaposition along the Ouachita-Marathon suture belt in earliest Permian time. Subsequent Jurassic opening of the Gulf of Mexico by seafloor spreading displaced the Yucatan-Chiapas block southward, as it rotated anticlockwise from edge-driven shear between Colombia and Florida, along a gulf-flank transform passing through the Isthmus of Tehuantepec. The continuity of a linear north-trending Permian–Triassic arc (now represented by a granite belt) in eastern Mexico precludes strike-slip slivering of Mexico during gulf opening and implies that Colombia lay east of internally coherent Gondwanan crust of southeastern Mexico, but south of Yucatan-Chiapas, prior to Pangean breakup. During subsequent intracontinental rifting, crustal elements of eastern Mexico were displaced southeastward, along a transform between basement blocks of present northeastern and east-central Mexico, in the wake of Colombia9s retreat from Laurentia. The Chortis block of nuclear Central America also lay within Pangea west of Colombia and remained attached to southern Mexico prior to its Cenozoic displacement eastward along the Cayman transform. The Caborca block of northwestern Mexico was displaced southeastward from the Cordilleran miogeocline by Permian–Triassic slip along a transform that linked the convergent Sonoma orogen with the northern end of a subduction zone in central Mexico that paralleled the Permian–Triassic magmatic arc built on Gondwanan crust of eastern Mexico. Subsequent post–Middle Triassic initiation of a west-facing continental-margin arc-trench system (i.e., with the subducting slab moving down to the east) along the structurally modified paleo–Pacific flank of Pangea produced middle Mesozoic arc assemblages extending southeastward from California and Arizona through east-central Mexico into Colombia. Beyond a compound suture belt lying outboard of the middle Mesozoic continental margin, western Mexico is underlain by Mesozoic volcanogenic crust formed beneath an east-facing intraoceanic island arc that was accreted to Laurentian and Gondwanan Mexico by arc-continent collision late in Early Cretaceous time. The accreted arc assemblage was overlapped by late Early Cretaceous–early Late Cretaceous carbonate platforms linked depositionally with comparable facies in eastern Mexico. As the offshore island arc approached the continent, progressive consumption of the intervening oceanic plate induced slab rollback beneath the Jurassic magmatic arc on the mainland, terminating arc magmatism and promoting development of associated rift troughs that extend from northeastern Mexico to southeastern California. Following arc collision and accretion, reversal of subduction polarity along the expanded western flank of Mexico created a west-facing continental-margin arc- trench system that was continuous with the Cordilleran arc and Franciscan trench of California. Subsequent subduction produced volcanic-plutonic arc assemblages on the mainland and in Baja California, which was contiguous with the mainland prior to Neogene seafloor spreading that opened the Gulf of California. A paired late Early Cretaceous subduction complex and late Mesozoic forearc basin occurred along the Pacific flank of Baja California.

U-Pb ages of detrital zircons from Permian and Jurassic eolian sandstones of the Colorado Plateau, USA: Paleogeographic implications

Detrital zircon grains (n=468) from eolian sandstones of Permian and Jurassic sand seas on the Colorado Plateau of southwest Laurentia fall into six separable age populations defined by discrete peaks on age–probability plots. The eolian sands include significant contributions from all Precambrian age belts of the Laurentian craton and all key plutonic assemblages of the Appalachian orogen marking the Laurentia–Gondwana suture within Pangaea. Nearly half the detrital zircon grains were derived ultimately from Grenvillian (1315–1000 Ma), Pan-African (750–500 Ma), and Paleozoic (500–310 Ma) bedrock sources lying within or along the flank of the Appalachian orogen. Recycled origins for Appalachian-derived grains, except for temporary residence of synorogenic detritus in the Appalachian foreland basin or in deformed Ouachita flysch and molasses along tectonic strike, are precluded by regional geology and known geochronology from other Laurentian sedimentary assemblages. We infer that transcontinental Permian and Jurassic river systems transported detritus of Appalachian provenance westward across the subdued surface of the Laurentian craton, for deposition as proximate sources for eolian systems feeding the ergs, on unconsolidated fluvial plains, deltas, and strandlines that lay up-paleowind along or near the Cordilleran paleoshoreline north and northeast of the Colorado Plateau. The postulated river systems headed in the remnant Appalachian orogen (Permian) or the incipient Atlantic rift belt (Jurassic), and additional transport of the Appalachian-derived detritus toward the Colorado Plateau was achieved by longshore drift of sediment southward along the Cordilleran paleoshoreline under the influence of prevailing trade winds in the Permian–Jurassic tropics. Only a quarter of the eolianite detrital zircons were derived or recycled from Mesoproterozoic (1470–1335 Ma) and younger Paleoproterozoic (1800–1615 Ma) basement of the Ancestral Rocky Mountains province adjacent to the Colorado Plateau. The final quarter of eolianite detrital zircons were derived from older Paleoproterozoic (2200–1800 Ma) and Archaean (3015–2580 Ma) basement of the Laurentian shield, or recycled from its sedimentary cover. Both Laurentian shield and Ancestral Rockies detritus may have entered the same transcontinental river systems (through tributary streams), or the same Cordilleran strandline system (by longshore drift), responsible for the delivery of Appalachian-derived sediment to positions near the Colorado Plateau ergs. As Colorado Plateau ergs received contributions from all the potential bedrock sources contiguous with Permian–Jurassic Laurentia and its orogenic–taphrogenic margins, detrital zircon studies of analogous ancient erg deposits elsewhere may help test reconstructions of Rodinia and other ancient paleocontinents by providing proxy records of the full age ranges of bedrock sources distributed across the surfaces of entire landmasses.

Andesitic Volcanism and Seismicity around the Pacific

Circum-Pacific andesites, with associated basalts and dacites, are erupted from linear island arcs and marginal continental ranges whose ocean-side borders coincide approximately with the continent-side boundaries of belts of shallow seismicity that parallel adjacent trenches. The lines of active volcanoes stand above elongate subcrustal regions delineated by the intersection of inclined Benioff seismic zones and the subhorizontal Gutenberg low-velocity zone. Close correlation between content of potash in erupted lavas and vertical depth to the Benioff zone suggests that andesitic volcanism has its origins in the mantle where magmas are generated by events associated with earthquakes of intermediate focal depth.

Himalayan-Bengal Model for Flysch Dispersal in the Appalachian-Ouachita System

The relation of the modern Bengal subsea fan to the Cenozoic Himalayan suture belt and the analogous relation of the Carboniferous Ouachita flysch to a presumed Paleozoic Appalachian suture belt suggest a guiding principle of synorogenic sedimentation. Most sediment shed from orogenic highlands formed by continental collisions pours longitudinally through deltaic complexes into remnant ocean basins as turbidites that are subsequently deformed and incorporated into the orogenic belts as collision sutures lengthen. India first encountered a southern Eurasian subduction zone near the end of Paleocene time. Northward movement of India since Oligocene time choked the subduction zone, stifled the associated magmatic arc, and created a suture complex of deformed Cretaceous flysch and younger Tertiary molasse. Strata derived from the resulting orogen include continental clastic wedges shed southward toward India and voluminous turbidites fed longitudinally through the Ganges-Brahmaputra Delta into the Bay of Bengal. The eastern flank of the Bengal subsea fan is being subducted now beneath the still-active eastern extension of the subduction zone. The sequential, north-to-south welding of Europe and Africa to North America formed the complex Appalachian-Caledonide-Mauritanide suture belt, from which Taconic, Acadian, and Alleghanian clastic wedges were shed toward the North American craton. Turbidites of the Carboniferous Ouachita flysch were fed longitudinally, as sediment supplied through the Alleghanian clastic wedge, into a remnant ocean basin lying south of North America. The Ouachita system was then thrust northward across the continental edge during arc-continent collision that progressed from east to west.

Potash-Depth (K-h) Relations in Continental Margin and Intra-Oceanic Magmatic Arcs

Plots of potash content against depth from volcanoes to inclined seismic zones for active magmatic arcs reveal slightly different relations for (a) continental margin arcs containing full or excessive thicknesses of continental crust overlying presumably old continental lithosphere, and (b) migratory intra-oceanic arcs that bound interarc basins of marginal seas and lack any known continental basement rocks. Detached intra-oceanic arcs containing continental basement rocks and stationary intra-oceanic arcs where back-arc spreading is absent display intermediate or equivocal relations. The significance of the difference in K- h correlation for continental margin and intra-oceanic magmatic arcs is uncertain but should be taken into account by theories for the petrogenesis of arc magmas.