Accretionary Tectonics of the North American Cordillera

Continental geology stands on the threshold of a change that is likely to be as fundamental as plate-tectonic theory was for marine geology. Ongoing seismic-reflection investigations into the deep crustal structure of North America are verifying that orogenic zones are underlain by low-angle faults of regional extent (Brown et al 1981). The growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments. Paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture-bounded fragments, particularly within the North American Cordillera. Such interdisciplinary studies are leading to a consensus that the Cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in Mesozoic and early Cenozoic time. Such crustal growth is referred to as accretionary tectonics. In this paper, we review some of the important concepts in accretionary tectonics, discuss the nature of the materials accreted between central Alaska and southern California in Jurassic and Cretaceous time, and consider the general relations between Cordilleran accretion and the movement of lithospheric plates. The concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest. With the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen, along with mechanisms for crustal attrition. Accretion mechanisms include the growth of imbricated sedimentary prisms along inner-trench walls, slicing off of submarine topographic irregularities within subducting plates, and collision of continents and volcanic arcs by ocean-basin closure. Tectonic attrition mechanisms include rifting, transform faulting, and strike-slip or underthrust removal of inner-trench wall materials coincident with or in place of accretionary prism growth. Growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments. An important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins. Since attrition by nature leaves little material evidence of having operated, one of the major problems confronting Cordilleran geologists lies in the recognition of such attrition within the ancient record, particularly when interspersed with accretionary events. The spectrum of accretion and attrition mechanisms viewed at cm yr^-1 plate-transport rates over time scales of 100 m.y. leads one to suspect a highly mobile history for continental-margin orogens. The serial arrangement of subducting, transform, and rifting links along the modern Cordillera plate-juncture system and both serial and parallel arrangements in the western Pacific systems show the complex interplay of such mechanisms through space. Similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the Cordillera, which presently resemble a collage of crustal fragments (Davis et al 1978). Recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust, and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and Earth history. The first problem to be considered is the recognition of native North American crust from exotic fragments that have been accreted to its edge

Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development

The assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea-floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology. However, this assumption has only been verified in a few locations by an integrated geochronological and structural-stratigraphic approach. Many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind. Such reconstructions are prone to error without adequate age control on each of the reconstructed fragments. This is a significant problem in structurally complex regions where more than one generation of ophiolite may be present. In this paper new Pb/U zircon ages are presented for key locations along a 375 km segment of the western Sierra Nevada ophiolite belt. These age data are combined with structural-stratigraphic observations and published ages, and significant tectonic implications for the ophiolite belt emerge. Three different ophiolitic assemblages are recognized with igneous ages of about 300, 200 and 160 m.y. B.P. Rocks of the 300 m.y. assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite-matrix melange. Fragments of upper Paleozoic seamounts occur in association with the ophiolitic melange, and together these assemblages constitute the basement framework for the western Sierra. Pb/U and K/Ar isotopic systematics are complex within this framework and indicate a polymetamorphic history. Systematics in the 200 and 160 m.y. assemblages are less complex and give tighter igneous age constraints. Rocks of the 200 m.y. assemblage are in a semi-intact state with only local tectonite and melange zones. Rocks of the 160 m.y. assemblage are intact, but nevertheless deformed. Both the 200 and 160 m.y. assemblages have equivalent age basinal volcanic-sedimentary sequences that lie unconformably above the ophiolitic melange basement. In each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age-equivalent ophiolite sections. These relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement. The Sierran ophiolite belt is considered to have formed by a multistage process initiated by the early Mesozoic tectonic accretion of upper Paleozoic sea-floor in general proximity to the ancient continental margin. Regional metamorphism and ophiolitic melange resulted. This accretionary nucleus became the basement of Jurassic-age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m.y. ophiolites. The rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic-sedimentary sequences. Non-volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes. Contact zones between pre-existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles, protoclastic deformation zones and cross-cutting dikes. Such edge-zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges. Both the 200 and 160 m.y. basins were destroyed by compressional orogenic episodes shortly after their formational episodes. Destruction of young ophiolite floored basins may be a common course of events when small oceanic-type plates are generated along continental margin environments. Such tectonic settings are ideal for the emplacement of young ophiolite sheets

Guadalupe pluton–Mariposa Formation age relationships in the southern Sierran Foothills: Onset of Mesozoic subduction in northern California?

We report a new 153 ± 2 Ma SIMS U-Pb date for zircons from the hypabyssal Guadalupe pluton which crosscuts and contact metamorphoses upper crustal Mariposa slates in the southern Sierra. A ~950 m thick section of dark metashales lies below sandstones from which clastic zircons were analyzed at 152 ± 2 Ma. Assuming a compacted depositional rate of ~120 m/Myr, accumulation of Mariposa volcanogenic sediments, which overlie previously stranded Middle Jurassic and older ophiolite + chert-argillite belts in the Sierran Foothills, began no later than ~160 Ma. Correlative Oxfordian-Kimmeridgian strata of the Galice Formation occupy a similar position in the Klamath Mountains. We speculate that the Late Jurassic was a time of transition from (1) a mid-Paleozoic–Middle Jurassic interval of mainly but not exclusively strike-slip and episodic docking of oceanic terranes; (2) to transpressive plate underflow, producing calcalkaline igneous arc rocks ± outboard blueschists at ~170–150 Ma, whose erosion promoted accumulation of the Mariposa-Galice overlap strata; (3) continued transpressive underflow attending ~200 km left-lateral displacement of the Klamath salient relative to the Sierran arc at ~150–140 Ma and development of the apparent polar wander path cusps for North and South America; and (4) then nearly orthogonal mid and Late Cretaceous convergence commencing at ~125–120 Ma, during reversal in tangential motion of the Pacific plate. After ~120 Ma, nearly head-on subduction involving minor dextral transpression gave rise to voluminous continent-building juvenile and recycled magmas of the Sierran arc, providing the erosional debris to the Great Valley fore arc and Franciscan trench

Paleogeographic and tectonic setting of axial and western metamorphic framework rocks of the southern Sierra Nevada, California

This paper represents an update of our 1978 S.E.P.M. Mesozoic Paleogeography synthesis for the southern Sierra Nevada. We originally postulated that much of the southern Sierra Nevada pre-batholithic metamorphic framework consisted of lower Mesozoic siliciclastic, carbonate and pelitic strata with variable arc volcanic admixtures (Kings sequence). Recent syntheses, however, have attempted to minimize the importance of early Mesozoic strata in the region and to extend coherent Paleozoic terranes into the framework as the predominant protoliths. Neither lithologic correlations nor structural analysis can substantiate such a view, however, and the proposed configuration of the Paleozoic terranes is in conflict with the petrochemical zonation pattern of the Cretaceous batholith. We present stratigraphic relations for the relatively well-preserved lower Mesozoic stratified rocks of the southern Sierra which in general supports our 1978 synthesis. As pointed out by more recent syntheses, however, we now recognize the likelihood of Paleozoic basement rocks occurring in some or many of the Kings sequence pendants. Such rocks are disparate fragments of a highly dismembered polygenetic basement composed of Paleozoic ophiolitic, Shoo Fly, miogeoclinal and possibly Antler belt rocks rather than coherent terranes or crustal blocks. The lower stratal levels of the lower Mesozoic Kings sequence appears to have formed part of a regional post-Sonoman (Triassic) marine overlap sequence above this basement complex. Dismemberment and accretion of the basement complex involved transform truncation of the southwest Cordillera and Foothills ophiolite belt emplacement prior to and coincident with Sonoman thrust tectonics. Following the establishment of a Carnian-Norian carbonate platform as part of the overlap sequence, the region subsided and became part of a regional Early Jurassic forearc to intra-arc extensional basin system with the deposition of Kings sequence turbidites and olistostromes. The basin system was destroyed by Middle and Late Jurassic thrusting. The assertion that much of the Kings sequence is Paleozoic in age is based on the discovery of probable Eocambrian-Cambrian miogeoclinal strata in the Snow Lake pendant of the east-central Sierra Nevada (Lahren and others, 1991). These authors offer a reconstruction of the displacement of these strata as part of a large crustal block from the western Mojave region through the axial Sierra Nevada along a now cryptic fault. The bounds of the hypothetical crustal block, however, are at odds with batholithic petrochemical patterns. We propose a more conservative offset history for the Snow Lake pendant rocks which considers a broader uncertainty in the bounds of the possible source area for the rocks, and satisfies offsets of both batholithic petrochemical patterns and igneous-metamorphic assemblages of the Sierran batholithic complex

Tectonic history of the eastern edge of the Alexander Terrane, southeast Alaska

Rocks exposed west of the Coast Plutonic Complex in southern southeast Alaska form an imbricate thrust belt that overprints the tectonic boundary between two of the largest allochthonous crustal fragments in the North American Cordillera, the Insular and Intermontane composite terranes. In the Alexander terrane (Insular composite terrane), lower Paleozoic metavolcanic and metasedimentary rocks (Descon Formation) and dioritic plutons are unconformably overlain by Lower Devonian clastic strata (Karheen Formation). These rocks are overlain locally by Upper Triassic basalt, rhyolite and marine clastic strata (Hyd Group). Upper Jurassic and Lower Cretaceous metavolcanic and metasedimentary strata of the Gravina sequence unconformably overlie the Alexander terrane. The Gravina sequence forms a structural package over 15 km thick and records intermittent arc volcanism along the eastern edge of the Alexander terrane. The Gravina sequence is structurally overlain by upper Paleozoic and lower Mesozoic metamorphosed basaltic strata, marble, and argillite (Alava sequence), and locally by lower Paleozoic supracrustal rocks and orthogneiss (Kah Shakes sequence). Together, these constitute the Taku terrane which we correlate with the Intermontane composite terrane. Local unconformity of Gravina sequence strata over the Alava sequence demonstrates that the Gravina sequence overlapped an earlier structural boundary between the Intermontane and Insular composite terranes. The rocks were deformed in the mid-Cretaceous by west-vergent thrusting that was was broadly coeval with arc magmatism. Deformation involved emplacement of west-directed thrust nappes over the structurally intact and relatively unmetamorphosed Alexander terrane basement. Mid-Cretaceous tonalite, granodiorite, and quartz diorite intrude rocks of the thrust belt and are locally affected by the deformation. Mid-Cretaceous deformation occurred during two episodes that were contemporaneous with the emplacement of large sill-like plutons. Older structures record ductile southwest-vergent folding and faulting, regional metamorphism, and development of axial-planar foliation. The second-generation structures developed during the later stages of southwest-directed thrust faulting, which juxtaposed rocks of contrasting metamorphic pressures and temperatures. Structural, stratigraphic, and geochronologic data indicate that the two phases of regional thrusting in southeast Alaska occurred between 113 Ma and 89 Ma. Rocks in the western part of the thrust belt were uplifted regionally by 70 Ma. Deformation involved the collapse of a marginal basin(s) and a magmatic arc, and overprinted the older tectonic boundary between the Insular composite terrane and the late Mesozoic western margin of North America (at that time the Intermontane composite terrane). Contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism, and thus records intra-arc tectonism. Late Paleocene to early Eocene igneous activity and extensional (?) deformation subsequently affected the thrust belt

Geologic framework, tectonic evolution, and displacement history of the Alexander Terrane

The Alexander terrane consists of upper Proterozoic(?)-Cambrian through Middle(?) Jurassic rocks that underlie much of southeastern (SE) Alaska and parts of eastern Alaska, western British Columbia, and southwestern Yukon Territory. A variety of geologic, paleomagnetic, and paleontologic evidence indicates that these rocks have been displaced considerable distances from their sites of origin and were not accreted to western North America until Late Cretaceous-early Tertiary time. Our geologic and U-Pb geochronologic studies in southern SE Alaska and the work of others to the north indicate that the terrane evolved through three distinct tectonic phases. During the initial phase, from late Proterozoic(?)-Cambrian through Early Devonian time, the terrane probably evolved along a convergent plate margin. Arc-type(?) volcanism and plutonism occurred during late Proterozoic(?)-Cambrian and Ordovician-Early Silurian time, with orogenic events during the Middle Cambrian-Early Ordovician (Wales orogeny) and the middle Silurian-earliest Devonian (Klakas orogeny). The second phase is marked by Middle Devonian through Lower Permian strata which accumulated in tectonically stable marine environments. Devonian and Lower Permian volcanic rocks and upper Pennsylvanian-Lower Permian syenitic to dioritic intrusive bodies occur locally but do not appear to represent major magmatic systems. The third phase is marked by Triassic volcanic and sedimentary rocks which are interpreted to have formed in a rift environment. Previous syntheses of the displacement history of the terrane emphasized apparent similarities with rocks in the Sierra-Klamath region and suggested that the Alexander terrane evolved in proximity to the California continental margin during Paleozoic time. Our studies indicate, however, that the geologic record of the Alexander terrane is quite different from that in the Sierra-Klamath region, and we conclude that the two regions were not closely associated during Paleozoic time. The available geologic, paleomagnetic, and paleontologic data are more consistent with a scenario involving (1) early Paleozoic origin and evolution of the Alexander terrane along the paleo-Pacific margin of Gondwana, (2) rifting from this margin during Devonian time, (3) late Paleozoic migration across the paleo-Pacific basin in low southerly paleolatitudes, (4) residence in proximity to the paleo-Pacific margin of South America during latest Paleozoic(?)-Triassic time, and (5) Late Permian(?)-Triassic rifting followed by northward displacement along the eastern margin of the Pacific basin

Structural history of continental volcanic arc rocks, eastern Sierra Nevada, California: A case for extensional tectonics

Mesozoic metavolcanic rocks forming part of the continental volcanic arc along the eastern Sierra Nevada near Mt. Goddard and in the Ritter Range show a complex history related to extensional tectonics. The rocks comprise a thick section of tuffs, breccias, lava flows, sills, and ash-flow tuffs deposited in a subaerial to subaqueous environment, with some subvolcanic sill-like plutons. Pb/U ages of the rocks in the Mt, Goddard area range from ca. 130–160 Ma, while rocks in the Ritter Range have a somewhat wider age range as reported previously. Repetition of the section occurs by faulting, and with the exception of parts of the mid-Cretaceous Minarets Caldera, all the volcanic rocks show a regional slaty cleavage which was subsequently crenulated and/or folded locally. The first cleavage has well-developed stretching lineations, and does not appear to have been associated with significant folding. Finite strain measurements show considerable variation both in magnitude and symmetry. The Mt. Goddard rocks, however, tend to show slightly higher overall strain magnitude and greater constrictional component than the Ritter Range for rocks of comparable age. Calculations based on the strain data suggest the Mt. Goddard section has been thinned by about 50% normal to bedding, much as that documented previously for rocks in the Ritter Range. Deformation within this part of the continental arc was originally thought to have formed by regional compression during the late Jurassic (Nevadan) orogeny. However, our study indicates that (1) parts of the deformed volcanic section are younger than late Jurassic, (2) Nevadan-age breaks in deposition are not present, (3) large-scale folds expected during a regional compression event are not common, and (4) the beds were tilted to a high dip prior to internal deformation. An extensional model is proposed in which beds were rotated to high tilts early in the deformation as a result of listric normal faulting. This normal faulting is thought to have occurred above a regional tumescence related to voluminous magmatism at depth, with preservation of the steeply tilted Goddard and Ritter sections being facilitated by their downward transport along the margins of rising plutons. Flattening and steeply plunging constrictional fabrics superimposed on the tilted sections are related to strain induced by high-level inflation of magma chambers and downward return flow of the keellike pendants. The main tectonic fabric shown by the continental volcanic arc rocks in the eastern Sierra Nevada is largely of Cretaceous age, rather than Jurassic (Nevadan) as originally supposed. In addition, the deformation, both rotation of beds and subsequent tectonite fabric, appears to be genetically related to the dynamic evolution of the magmatic arc, and not the result of an externally imposed tectonic event

Upper Jurassic-Lower Cretaceous basinal strata along the Cordilleran Margin: Implications for the accretionary history of the Alexander-Wrangellia-Peninsular Terrane

Upper Jurassic and Lower Cretaceous basinal strata are preserved in a discontinuous belt along the inboard margin of the Alexander-Wrangellia-Peninsular terrane (AWP) in Alaska and western Canada, on the outboard margin of terranes in the Canadian Cordillera accreted to North America prior to Late Jurassic time, and along the Cordilleran margin from southern Oregon to southern California. Nearly all of the basinal assemblages contain turbiditic strata deposited between Oxfordian and Albian time. Arc-type volcanic rocks and abundant volcanic detritus in many of the assemblages suggest deposition within or adjacent to a coeval arc complex. On the basis of the general similarities between the basinal sequences, we propose that they record involvement of the AWP in the Late Jurassic-Early Cretaceous evolution of the Cordilleran margin. A geologically reasonable scenario for the accretion of the AWP includes (1) Middle Jurassic accretion to the Cordilleran margin, in particular the Stikine and Yukon-Tanana terranes, in a dextral transpressional regime, (2) Late Jurassic-Early Cretaceous overall northward translation of the AWP and evolution of a series of transtensional basins within a complex dextral strike-slip system along the Cordilleran margin, and (3) mid-Cretaceous structural imbrication of the AWP and inboard terranes that either terminated or resulted in a change in the character of deposition in the marginal basins. Mid-Cretaceous deformation along the inboard margin of the AWP was broadly synchronous with contractional deformation throughout the Cordillera and most likely due to changes in subduction zone parameters along the Cordilleran margin, outboard of the AWP, rather than collision of the AWP