Tectonic control on southern Sierra Nevada topography, California

In this study we integrate the apatite (U-Th)/He thermochronometric technique with geomorphic, structural, and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late Cenozoic faulting in the southern Sierra Nevada. The geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low-relief landscape, little affected by internal deformation, to a more complex landscape affected by numerous faults. Regionally, the relict landscape surface is readily resolved by age-elevation relationships of apatite He ages coupled to geomorphology. These relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting. On the basis of 63 new apatite He ages and stratigraphic data from proximal parts of the San Joaquin basin we resolve two sets of normal faults oriented approximately N–S and approximately NW. Quaternary west-side-up normal faulting along the N–S Breckenridge–Kern Canyon zone has resulted in a southwest step over from the Owens Valley system in the controlling structure on the regional west tilt of Sierran basement. This zone has also served as a transfer structure partitioning Neogene-Quaternary extension resulting from normal displacements on the NW fault set. This fault system for the most part nucleated along Late Cretaceous structures with late Cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region

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

Late Cenozoic structure and tectonics of the southern Sierra Nevada–San Joaquin Basin transition, California

This paper presents a new synthesis for the late Cenozoic tectonic, paleogeographic, and geomorphologic evolution of the southern Sierra Nevada and adjacent eastern San Joaquin Basin. The southern Sierra Nevada and San Joaquin Basin contrast sharply, with the former constituting high-relief basement exposures and the latter constituting a Neogene marine basin with superposed low-relief uplifts actively forming along its margins. Nevertheless, we show that Neogene basinal conditions extended continuously eastward across much of the southern Sierra Nevada, and that during late Neogene–Quaternary time, the intra-Sierran basinal deposits were uplifted and fluvially reworked into the San Joaquin Basin. Early Neogene normal-sense growth faulting was widespread and instrumental in forming sediment accommodation spaces across the entire basinal system. Upon erosion of the intra-Sierran basinal deposits, structural relief that formed on the basement surface by the growth faults emerged as topographic relief. Such “weathered out” fossil fault scarps control much of the modern southern Sierra landscape. This Neogene high-angle fault system followed major Late Cretaceous basement structures that penetrated the crust and that formed in conjunction with partial loss of the region’s underlying mantle lithosphere. This left the region highly prone to surface faulting, volcanism, and surface uplift and/or subsidence transients during subsequent tectonic regimes. The effects of the early Neogene passage of the Mendocino Triple Junction were amplified as a result of the disrupted state of the region’s basement. This entailed widespread high-angle normal faulting, convecting mantle-sourced volcanism, and epeirogenic transients that were instrumental in sediment dispersal, deposition, and reworking patterns. Subsequent phases of epeirogenic deformation forced additional sediment reworking episodes across the southern Sierra Nevada–eastern San Joaquin Basin region during the late Miocene break-off and west tilt of the Sierra Nevada microplate and the Pliocene–Quaternary loss of the region’s residual mantle lithosphere that was left intact from the Late Cretaceous tectonic regime. These late Cenozoic events have left the high local-relief southern Sierra basement denuded of its Neogene basinal cover and emergent immediately adjacent to the eastern San Joaquin Basin and its eastern marginal uplift zone

Fracture zone tectonics, continental margin fragmentation, and emplacement of the Kings-Kaweah ophiolite belt, Southwest Sierra Nevada, California

The Sierra Nevada foothill met amorphic belt is a 450 km long assemblage of remnant continent-derived epiclastics, arc volcanics, pelagic-hemipelagic sediments, and ophiolite slices. The various lithologic units range in age from Ordovician to Jurassic. Litho logic units are lenticular at scales ranging up to 150 km and strike about N. 30°W. parallel to the trend of the metamorphic belt (Fig. 1). Many units are penetratively deformed with a variety of near vertical foliation surfaces. The lithologic units are generally bounded by steep dipping fault and melange zones, but locally depositional contacts can be recognized. From at least latitude 38°30'N southward, latest Paleozoic to possibly early Mesozoic disrupted ophiolite occurs as remnant oceanic basement beneath Triassic to Jurassic arc volcanics and interstratified continent-derived epiclastics. Along the northern part of this segment of the metamorphic belt the ophiolitic rocks occur as scattered basement exposures surrounded by the younger volcanic and epiclastic rocks (Morgan and Stern, 1977; Behrman, 1978; Saleeby, unpub. field data). Further south in the Kings-Kaweah terrane deeper structural levels of the foothill metamorphic belt are exposed. Here a nearly continuous 125 km long ophiolite belt occurs with scattered remnants of early Mesozoic arc volcanic and epiclastic rocks depositionally above It. The ophiolite belt is named informally the Kings-Kaweah ophiolite belt after the Kings and Kaweah Rivers which transect it. This ophiolite belt constitutes part of the same oceanic basement terrane that is locally exposed further north amidst the arc volcanics and epiclastics

Recognition and significance of boundary transforms and rift edges within island arc terranes of the western Sierra Nevada

Fragmentation of island arcs and related dispersion of the resulting arc segments is common in modern continental margin environments. Inasmuch as a majority of Cordilleran accreted terranes show island arc affinities one would expect to find remnants of arc-related transforms and rifts within such terranes. Jurassic arc and ophiolitic terranes of the western Sierra Nevada and Klamath Mountains have regional age and structural patterns which suggest multiple rifting and transform faulting episodes, and some areas offer remnants of the actual rift or transform complexes

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

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

Accreted island arcs and cross-cutting batholithic belts of the North American Cordillera

The basement framework of the western Cordillera consists in large part of tectonically accreted island arc terranes and cross-cutting batholithic belts. The arc terranes are diverse in terms of magmatic history, tectonic disruption and basement relations, and represent several distinct systems. Terranes of the two oldest systems occur in inner and outer belt positions. The inner belt runs from central Alaska to the northern Sierra Nevada. It was in its major developmental phases by the Devonian, and was constructed on imbricated North American continental rise strata outboard of a passive margin. The outer belt includes the Alexander Terrane (AT) of SE Alaska and younger amalgamated arc terranes of the Alaska Peninsula and Queen Charlotte-Vancouver Islands

The Coast Range Ophiolite (CRO) debate is fraught with complimentarities and indeterminacy - a few examples

Because we work primarily with rocks, we perhaps assume that our science is immune to complimentarity and indeterminacy. But these information- based limitations plague tectonic analysis as they do for any branch of science. An educated and unbiased (if possible) observer of the CRO debate (Dickinson et al., 1996) should have little trouble recognizing these limitations in the three views (V1, 2 & 3) expressed. Each view is dependent on missing geology: V1 requires the near total destruction of two opposing subduction complex-forearc systems along a Nevadan suture by subduction and/or erosion. V2 requires an unobservable subduction zone along the axis of the Great Valley. V3 appeals to total removal or burial of screens of older Sierran lithosphere from the outer edge or within the CRO. Each view has a different pre-Cretaceous origin for the source of the Great Valley geophysical anomaly, yet basement cores from the area of the anomaly are primarily Early Cretaceous mafic batholithic rocks. V3 is predicated on forearc magmatism. This view is dismissed in V1 on the basis of forearcs being cold and amagmatic, yet V1 requires a regional belt of Middle Jurassic (pre-Nevadan) plutons to have intruded the remnants of the juxtaposed subduction complexes. V1, 2 & 3 are all dependent on subducted slab-related geochemical tracers within the CRO, and on absolute age relations within the igneous sequences. A global survey of Neogene mafic magma systems reveals large uncertainties in the geochemical tracers, particularly when taking into account hydrothermal alteration and mantle metasomatism. Furthermore, Neogene arc systems show reorganization in magmatic loci and microplates at time scales comparable to typical uncertainties in absolute age determinations

A Method for the Characterization and Selection of Compliant Joints

A compliant joint is a connection between two bodies that derives its movement from the deflection of flexible members rather than rigid connections, like traditional joints. Compliant joints have potential advantages that include longer part life, reduction of parts in assemblies, and reduced wear. Traditional compliant mechanism design methodologies have limitations involving the burden of necessary knowledge required to satisfactorily use them. The method presented in this thesis was developed to provide compliant joint design solutions independent of the traditional methods of compliant joint design by allowing the selection of compliant joints from a repository. The repository is populated by a set of twenty compliant joint models which are characterized by their geometric characteristics and parametric equations. A Finite Element Analysis (FEA) simulation is used to validate each of the individual models. The selection algorithm solves the models systematically using the design requirements set by the user. Results are presented to the user in the form of a list of compliant joints that fulfill the user requirements, and Pareto curves that represent the potential range of stiffness and deflection of compliant joints across the set of geometric characteristics in the design space. Ten test cases were applied to the selection algorithm to validate the output results