Continental Margin Assemblage Research Articles

Detrital zircon geochronology from the Bighorn Basin, Wyoming, USA: Implications for tectonostratigraphic evolution and paleogeography

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, subsidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra. Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America. The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1–TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, first-order patterns within the detrital zircon age distributions. TSA1 represents a Paleozoic–Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600–1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic–Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0–1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, reflecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7–1.8 Ga, which probably reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts. Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10–100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy.

Petrology of the İspendere (Malatya) ophiolite from the Southeast Anatolia: implications for the Late Mesozoic evolution of the southern Neotethyan Ocean

Abstract The İspendere ophiolite forms part of the Tauride active continental margin assemblage in SE Anatolia. The ophiolite exhibits an intact oceanic lithosphere section and is intruded by Late Cretaceous calc-alkaline granites. The ophiolite comprises mantle tectonites, ultramafic to mafic cumulates, isotropic gabbros, isolated diabase dykes, a sheeted dyke complex, plagiogranite and volcanic rocks. The volcanics and the sheeted dyke complex exhibit (1) similar rare earth element patterns, with flat to light rare earth element depletion (La–Yb) N =0.71–1.14 and 0.65–1.22, (2) negative Nb anomalies and (3) flat-lying high field strength element trends. These features differ from a typical Normal--Mid Ocean Ridge Basalt fractionation trend and could have resulted from c. 15% partial melting of a previously depleted mantle source. The whole-rock chemistry and the mineral chemistry of the ultramafic to mafic cumulates [high Ca plagioclases (An 89–81 ), magnesian olivines (Fo 88–81 ) and clinopyroxenes (Mg# 90–83 )] show that the primary magma of the plutonic suite is compositionally similar to modern island arc tholeiites. The available evidence suggests that the İspendere ophiolite formed at a northerly supra-subduction zone spreading centre of the Southern Neotethys, between the Taurides and the Bitlis–Pütürge metamorphic units, during the Late Cretaceous. Comparison with the adjacent Göksun, Kömürhan and Guleman ophiolites suggests that the İspendere ophiolite represents part of a single regional-scale sheet of oceanic lithosphere that was accreted to the base of Tauride active continental margin where it was cut by arc-type magmatic rocks.

The timing and provenance record of the Late Permian Klondike orogeny in northwestern Canada and arc‐continent collision along western North America

The northern Canadian Cordillera exhibits coeval accreted arc, subduction zone, ocean basin, and continental margin assemblages that make the region an exceptional place to understand tectonic processes involved in arc‐continent collision. In this study, we use U‐Pb zircon and monazite geochronology to define the timing and provenance record of Late Permian collisional orogeny related to the accretion of the Yukon‐Tanana terrane onto the ancestral North American continental margin of northwestern Canada. New U‐Pb crystallization ages of Permian intrusive rocks in the Klondike District of western Yukon bracket the timing of collision‐related ductile deformation and greenschist‐ to amphibolite‐facies metamorphism on the Yukon‐Tanana terrane between 260 and 252.5 Ma. This tectonothermal event is herein named the Klondike orogeny. Detrital zircon U‐Pb geochronology of Triassic strata provides the sedimentary record of arc‐continent collision and crustal reworking along the Cordilleran margin. Arc‐derived detrital zircons in Early to Middle Triassic (251–235 Ma) strata overlying the ancestral North American continental margin in Yukon suggest that a foreland‐style basin developed adjacent to the Klondike orogen. Regionally extensive Late Triassic (235–200 Ma) strata containing primarily North American detrital zircons form an overlap assemblage that covered the accreted terranes and western North America. The timing of the Klondike orogeny is roughly synchronous with other contractional events along the ∼5000 km strike length of the Cordillera, including the Late Permian‐Early Triassic Sonoman orogeny in Nevada. Global plate reorganization linked to assembly of Pangaea may have been the tectonic engine for late Paleozoic‐early Mesozoic development of the North American Cordillera.

Tectonostratigraphic evolution of Cenozoic marginal basin and continental margin successions in the Bone Mountains, Southwest Sulawesi, Indonesia

The Bone Mountains, located in Southwest Sulawesi along the SE margin of Sundaland, are composed of Oligocene to possibly lower Miocene marginal basin successions (Bone Group) that are juxtaposed against continental margin assemblages of Eocene–Miocene age (Salokalupang Group). Three distinct units make up the latter: (i) Middle–Upper Eocene volcaniclastic sediments with volcanic and limestone intercalations in the upper part (Matajang Formation), reflecting a period of arc volcanism and carbonate development along the Sundaland margin; (ii) a well-bedded series of Oligocene calc-arenites (Karopa Formation), deposited in a passive margin environment following cessation of volcanic activity, and (iii) a series of Lower–Middle Miocene sedimentary rocks, in part turbiditic, which interfinger in the upper part with volcaniclastic and volcanic rocks of potassic affinity (Baco Formation), formed in an extensional regime without subduction. The Bone Group consists of MORB-like volcanics, showing weak to moderate subduction signatures (Kalamiseng Formation), and a series of interbedded hemipelagic mudstones and volcanics (Deko Formation). The Deko volcanics are in part subduction-related and in part formed from melting of a basaltic precursor in the overriding crust. We postulate that the Bone Group rocks formed in a transtensional marginal basin bordered by a transform passive margin to the west (Sundaland) and by a newly initiated westerly-dipping subduction zone on its eastern side. Around 14–13 Ma an extensional tectonic event began in SW Sulawesi, characterized by widespread block-faulting and the onset of potassic volcanism. It reached its peak about 1 Ma year later with the juxtaposition of the Bone Group against the Salokalupang Group along a major strike-slip fault (Walanae Fault Zone). The latter group was sliced up in variously-sized fragments, tilted and locally folded. Potassic volcanism continued up to the end of the Pliocene, and locally into the Quaternary.

From the Early Paleozoic Platforms of Baltica and Laurentia to the Caledonide Orogen of Scandinavia and Greenland

The Caledonide Orogen in the Nordic countries is exposed in Norway, western Sweden, westernmost Finland, on Svalbard and in northeast Greenland. In the mountains of western Scandinavia, the structure is dominated by E-vergent thrusts with allochthons derived from the Baltoscandian platform and margin, from outboard oceanic (Iapetus) terranes and with the highest thrust sheets having Laurentian affinities. The other side of this bivergent orogen is well exposed in northeastern Greenland, where W-vergent thrust sheets emplace Laurentian continental margin assemblages onto the platform. Svalbard's Caledonides are disrupted by late Caledonian faults, but have close affinity with the Laurentian margin in Northeast Greenland. Only Svalbard's Southwestern terrane is foreign to this margin, showing affinity to the Pearya terrane of northern Ellesmere Island in arctic Canada. Between the margins of western Scandinavia and eastern Greenland, the wide continental shelves, now covered by late Paleozoic and younger successions, are inferred to be underlain by the Caledonide hinterland, probably incorporating substantial Grenville-age basement. In northernmost Norway, the NE-trending Caledonian thrust front truncates the NW-trending Neoproterozoic Timanide orogen of northwest Russia. Much of the central and eastern parts of the Barents Shelf are thought to be underlain by Caledonian-deformed Timanide basement. Caledonian orogeny in Norden resulted from the closure of the Iapetus Ocean and Scandian collision of continent Baltica with Laurentia. Partial subduction of the Baltoscandian margin beneath Laurentia in the midlate Silurian was followed by rapid exhumation of the highly metamorphosed hinterland in the early Devonian, and deposition of Old Red Sandstones in intramontane basins. Late Scandian collapse of the orogen occurred on major extensional detachments, with deformation persisting into the late Devonian.

Southern continuation of the Coast shear zone and Paleocene strain partitioning in British Columbia–southeast Alaska

This paper documents the newly recognized southern continuation of the early Tertiary Coast shear zone, extending its known length by ∼350 km to more than 1200 km. Three sites along the shear zone in British Columbia, Douglas Channel, Bella Coola, and Machmell River, have similar histories during the period ca. 65–55 Ma. The shear zone is 2–11 km thick and is defined by well-developed mylonite zones that strike northwest and dip steeply northeast. Motion on the shear zone was predominantly reverse, with the northeast side up. Synkinematic plutons are common in the shear zone. Lower plate rocks, high-grade gneiss derived from an ancient continental margin assemblage, show little evidence of the extensive deformation and plutonism in the shear zone. North of Bella Coola, high- grade gneiss forms the upper plate, but to the south weakly metamorphosed rocks of Stikinia compose the upper plate. Geochronologic data show that the shear zone was active between ca. 60 and 55 Ma at Douglas Channel, ca. 62 and 56 Ma at Bella Coola, and after 66 and before 56 Ma near the Machmell River. These features match those of the Coast shear zone in southeast Alaska and adjacent British Columbia. Together the shear zones formed a continental-scale reverse ductile fault in Paleocene time. Plate reconstructions show an oblique component to the dominantly dextral transcurrent Paleocene margin. Partitioning of this motion across the continental margin produced a regional system of strike-slip faults and contraction on the Coast shear zone.

Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America

Abstract The peneplaned Archean craton in the Lake Superior region was the platform upon which a continental margin assemblage was deposited. Extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform. Seas transgressed onto the continent several times and an ocean basin opened south of the present-day Lake Superior. Island arcs that formed during subduction collided with the craton margin as the ocean basin closed; oceanic crust is poorly preserved as a dismembered ophiolite sequence. The arc volcanics are preserved as the Wisconsin magmatic terranes. The collision resulted in a fold-and-thrust belt known as the Penokean orogen. To the north of the fold-and-thrust belt, a northward-migrating foreland basin — the Animikie basin — developed. Thick turbidite successions were deposited along the basin axis, and terrigenous clastics and Lake Superior-type iron-formation were deposited on the shelf along the northern margin of the basin. The primary paleoclimatic indicators are: (1) glaciogenic rocks at the base of the Paleoproterozoic succession in Michigan indicating ice-house conditions; (2) remnants of a paleosol on the glaciogenic rocks indicative of deep weathering, probably under subtropical conditions and therefore of greenhouse conditions; and (3) carbonate minerals after gypsum, halite, and anhydrite in stromatolitic dolomite, indicative of aridity. Three second-order depositional sequences are bounded by major unconformities, and can be correlated throughout the Lake Superior region.

Basin evolution of the Paleoproterozoic Karelian Supergroup of the Fennoscandian (Baltic) Shield

Abstract The peneplaned Archean craton of the Fennoscandian Shield served as a platform upon which a continental margin assemblage, the Karelian Supergroup, was deposited between ∼2.45 and ∼1.9 Ga. Major subaerial unconformities separate five sedimentary-volcanic groups of the supergroup — the Sumian, Sariolian, Jatulian (Lower and Upper), Ludicovian, and Kalevian. Second-order depositional sequences are implied. Early extension (∼2.45 Ga) resulted in localized rifts that were likely areas of later subsidence as well; they received thicker accumulations of sediments and volcanic rocks than did the adjacent platform. It is in these rifts and perhaps other downwarped areas that the sediments that were once more widespread were preserved, leading to interpretations of separate depositional basins by some workers. Seas transgressed onto the craton at least three times — in Sariolian time as evidenced by interpreted glaciomarine deposits, in Jatulian time as evidenced by widespread orthoquartzites (including tidalites) and stromatolitic carbonates, and in Ludicovian time as evidenced by organic-rich shales and turbidites. The tectonic-magmatic history is complex. Three episodes of mafic volcanism were widespread at 2.45, 2.2, and 2.1 Ga. Island arcs formed to the south of the craton and collided at ∼1.9–1.85 Ga (the Svecofennian orogeny). This collision resulted in northeastward thrusting (e.g. the Outokumpu nappe) and folding and metamorphism of the Karelian Supergroup. The primary paleoclimatic indicators are (1) glaciogenic rocks near the base of the Paleoproterozoic succession indicating ice-house conditions; (2) remnants of a major paleosol on the glaciogenic rocks, indicative of deep weathering under greenhouse conditions (subtropical or tropical?); and (3) carbonate pseudomorphs after evaporite minerals in stromatolitic dolomites, perhaps indicative of aridity. Similarities in magmatism, tectonics, sedimentary rock types and sequences, and paleoclimatic indicators have led to hypotheses that the Fennoscandian Shield and North America may have been part of the same supercontinent during Neoarchean and Paleoproterozoic time.

Paleozoic and Mesozoic rocks of Stikinia exposed in northwestern British Columbia: Implications for correlations in the northern Cordillera

Stikinia is a tectonostratigraphic terrane in the Canadian Cordillera that formed in a volcanic arc environment during Paleozoic and Mesozoic time. Until now, the Paleozoic rocks that form a semicontinuous belt along its western margin (Stikine assemblage) were only recognized in a restricted area in northern British Columbia, between the Stikine and Taku river areas. In contrast, Mesozoic Stikinia rocks form an almost continuous belt that extends much farther to the north, leading some authors to question the nature of the unexposed Paleozoic basement north of the Taku River area. The following correlations have significant implications for tectonic reconstructions of the northern Cordillera because they suggest that Stikinia9s Paleozoic volcanic-sedimentary basement is more widespread than previously thought. On the basis of similar rock types and lithologic associations, six new U-Pb zircon dates, and the common intrusive relationship with 184–195 Ma plutons, the Stikine assemblage is correlated with the Boundary Ranges suite, a metamorphosed Paleozoic volcanic assemblage exposed in the Tagish Lake area, north of the Taku River and south of the Yukon–British Columbia border. The recognition of the Boundary Ranges suite and the Jurassic plutons that intruded it (Tagish Lake suite) as part of Stikinia has implications for the age and character of the Stikinia–Tracy Arm terrane boundary because the Boundary Ranges and Tagish Lake suites form the footwall of a major Middle Jurassic shear zone that carried the continental margin–like rocks of the Tracy Arm terrane in its hanging wall. This correlation also implies that the late Paleozoic basement to the Mesozoic Stikinia arc is not a continental margin assemblage, at least as far north as the British Columbia–Yukon border, and possibly farther. The Boundary Ranges suite, and therefore the Stikine assemblage, are also tentatively correlated with parts of the Yukon-Tanana terrane in Yukon (Aishihik Lake area), parts of the Taku terrane in southeast Alaska, and undivided metamorphic rocks in west-central British Columbia. Differences in the isotopic signatures of these rocks may reflect along-strike changes in the character of the basement rocks of the late Paleozoic Stikinia volcanic arc.

Allostratigraphic relationships of early Proterozoic iron-formations in the Lake Superior region

Early Proterozoic sequences in the Lake Superior region, which are sedimentologically related and contain major units of iron-formation, are divisible into allostratigraphic sequences--packages of rock bounded by unconformities. Analysis that emphasizes the hounding unconformities therefore emphasizes processes external to the depositional system, which initiate and terminate deposition of the sedimentologically related successions of rock types. An analysis of this type shows that the region9s iron-bearing strata were deposited throughout the progressive growth and ultimate destruction of a rifted continental margin. The extensional phase of this continuum is marked by sedimentary units having an Archean provenance to the north and iron-formation having many descriptive attributes associated with Algoma-type iron-formation. Unconformities developed during extension resemble the rift-onset and the breakup unconformities recognized in younger continental margin assemblages. The compressional phase deposited in a northward-migrating foredeep basin is marked by sedimentary units having an Early Proterozoic provenance to the south and iron-formation having Lake Superior-type attributes. The boundary between the two is time-transgressive, migrating to the north as the basin migrated in that direction. This stratigraphic reconstruction departs significantly from those of the last 50 years which were based on the assumption of a single correlatable Early Proterozoic iron-formation in all the Lake Superior iron ranges. Sedimentation, although episodic, started around 2200 Ma and was essentially over by 1850 Ma. Thus units of iron-formation were deposited over some 350 m.y. of geologic time, an interval approximately 60 percent as long as that attributed to all of the Phanerozoic eon. Thus precipitation of iron-formation in the Lake Superior region is not a unique product of a single set of factors restricted to a short interval of geologic time. Our challenge will involve identifying underlying factors on a range-by-range basis.

U–Pb geochronology of detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska

Metamorphic rocks within and west of the northern Coast Mountains in southeastern Alaska consist of an Upper Proterozoic(?) to upper Paleozoic continental margin assemblage that we interpret to belong to the Yukon-Tanana terrane. U–Pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of ~495 Ma, ~750 Ma, 1.05–1.40 Ga, 1.75–2.00 Ga, ~2.3 Ga, 2.5–2.7 Ga, and ~3.0 Ga. Multigrain fractions from two samples yield upper intercepts between 2.0 and 2.3 Ga, but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of < 2.0 and > 2.3 Ga grains. Zircons in these rocks generally overlap in age with (i) detrital zircons in metasedimentary rocks of the Yukon–Tanana terrane in eastern Alaska and Yukon, (ii) detrital zircons in strata of the Cordilleran miogeocline, and (iii) plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata. In addition, the pre-1.7 Ga grains overlap in age with dated crystalline rocks of the western Canadian Shield. These similarities raise the possibility that metaclastic rocks in the northern Coast Mountains accumulated in proximity to western North America. The younger zircon populations were likely shed from mid-Proterozoic to early Paleozoic igneous rocks that now occur locally (but may have been widespread) along the Cordilleran margin. Recognition of a continental margin assemblage of possible North American affinity in the Coast Mountains raises the possibility that some arc-type and oceanic terranes inboard of the Coast Mountains may be large klippen that have been thrust over the North American margin.

Stratigraphic and isotopic link between the northern Stikine terrane and an ancient continental margin assemblage, Canadian Cordillera

Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle model ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390, 660, and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910, 1770, and 2450 Ma and highly radiogenic 87 Sr/ 86 Sr ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin.

Ancient continental margin assemblage in the northern Coast Mountains, southeast Alaska and northwest Canada

Geologic relations indicate that quartz-rich metasedimentary rocks in the northern Coast Mountains separate strata to the east that belong to the Stikine terrane from strata to the west of the Alexander, Wrangellia, and Taku terranes. The quartz-rich rocks structurally overlie western terranes along a mid-Cretaceous thrust fault and are overlain structurally (originally stratigraphically ) by strata of the Stikine terrace. These rocks are interpreted to be a continental margin assemblage that belongs to the Yukon Crystalline terrane. U-Pb and Nd isotopic data indicate that the metasedimentary rocks were shed from a source terrane consisting at least in part of Proterozoic rocks.

Effect of Oceanographic and Depositional Events on Neogene Benthic Foraminiferal Biostratigraphy of California: ABSTRACT

The relationships between Neogene environmental events and associated faunal distributional patterns of continental margin benthic foraminiferal faunas are affected or controlled by both local and regional events. These events are related to the complex evolution of the oceanic environment and the California continental margin. Evidence from Deep Sea Drilling Project sites drilled in the Pacific Ocean indicates that major changes in the oceanic environment have produced major faunal turnovers, migrations, and reorganizations of deep-sea benthic foraminiferal assemblages during the Neogene. DSDP Sites 467, 468, and 469, and the Naples Bluff section of southern California, the distributional patterns of continental margin benthic foraminiferal assemblages are strongly related to Neogene climatic-oceanic events. The faunal changes and responses described from the continental margin assemblages are similar in composition and timing to changes documented in the deep sea. The response of the benthic foraminiferal faunas is almost simultaneous across the continental margin in regard to the regional oceanic-climatic events. Several of these events were associated with changes in surface productivity and expansion of the oxygen minima. Local events involved the change in sedimentation and tectonic activity. The oceanic reorganization that occurred 15-13 Ma is responsible for a major change in the faunal patterns acrossmore » the continental margin. Early Neogene faunas (the Saucesian, Relizian, and Luisian assemblages) were replaced by the Mohnian assemblage. The climatic-oceanic event at 9 to 8 Ma involved an increase in Antarctic glaciation and a resultant increase in upwelling and surface productivity. The associated faunal changes consist of a shift in the character of the Mohnian assemblage. At approximately 5.2 Ma, the Repettian assemblage increased considerably probably in relation to the Messinian event.« less

Paleozoic and Lower Mesozoic Volcanism and Continental Growth in the Western United States

Compositions of Paleozoic and Lower Mesozoic volcanic rocks of western Nevada and adjoining areas reflect the tectonic condition of their time and place of eruption. Despite structural complication and metamorphic overprinting, a general change can be observed from continental, cratonic, volcanic assemblages in the east to oceanic assemblages in the west. In Paleozoic (pre-Sonorria) time, an ensimatic basin was present west of the craton margin and east of correlative island-arc assemblages. In early Mesozoic time, continental assemblages passed westward and northwestward into continental margin assemblages in far western Nevada and into island-arc assemblages farther west. These arc assemblages may have formed on a crust that was compositionally intermediate between oceanic and continental crust. The North American continent grew westward by accretion of one or more island arcs in late Paleozoic–early Mesozoic (Sonoma) time and by general segregation of sial along the margin. Crustal thickening occurred during and probably following the Sonoma orogeny. The sialic material must have been newly formed from the mantle and represented significant westward continental growth in the past 300 m.y.