Nicholas W. Walker

Neoproterozoic-Cambrian basement-involved orogenesis within the Antarctic margin of Gondwana

High-grade metamorphic tectonites of the Nimrod Group in the central Transantarctic Mountains compose a major ductile shear zone that formed within the paleo-Pacific margin of Gondwana. Despite demonstrated Precambrian protoliths, the timing of metamorphism and tectonite development has been poorly constrained. Igneous rocks of diverse compositions intrude the Nimrod tectonites. Four intrusive units with incipient to well-developed ductile fabrics yield U-Pb zircon ages of 541-521 Ma, and a nondeformed pegmatite has a U-Pb zircon age of ∼515 Ma. These data show that early Paleozoic Ross magmatism was compositionally, texturally, and temporally more heterogeneous than previously recognized. Fabrics in the igneous rocks are concordant with those in their host tectonites, indicating that Nimrod tectonism was in part synchronous with plutonism. U-Pb ages of 525-522 Ma for metamorphic monazite from two pelitic tectonites support this interpretation. Thus, ductile deformation was in its peak to waning stages between about 540 and 520 Ma. This timing provides compelling evidence for transcurrent basement involvement in oblique plate convergence along the Neoproterozoic to Early Cambrian Antarctic margin of Gondwana.

A magma-loading model for Barrovian metamorphism in the southeast Coast Plutonic Complex,British Columbia and Washington

Barrovian metamorphism linked to crustal thickening in the southeast Coast Plutonic Complex of British Columbia and Washington is proposed in this paper to be the product of magmatic loading. Relevant observations and arguments are the following. (1) Isotopic ages coupled with structural and fabric relations document that throughout the region Barrovian metamorphism and plutonism are broadly coeval. (2) Baric patterns in country rock indicate steep-sided crustal loads originating within the high-grade parts of the orogen.(3) Patterns of subsidence and uplift are localized and diachronous, precluding a single-event regional thickening mechanism. (4) Country-rock fabrics are dominated by steep, orogen-parallel foliations and shallow, strike-parallel lineations, features not easily reconciled with tectonic thickening mechanisms. (5) Abroad zone of schists flanking the large (25 x 100 km) Scuzzy batholith in British Columbia bears metamorphic isograds and foliations that track the border of the pluton across regional structural trends, implicating pluton emplacement as the cause of metamorphism. (6) The crustal-loading event in the vicinity of the Scuzzy pluton is bracketed by mineral and textural features as being coeval with emplacement of the pluton, which occurred over a period of ∼7 m.y. (7) The regionally developed Chiwaukum Schist in Washington has petrologic and structural features and a metamorphic age that suggest it was formed by, and represents the floor of, an eroded extension of the Scuzzy pluton. The proposed magma-loading model invokes the diapir and ballooning concepts of previous workers in which rising plutons displace country rock downward. This mechanism results in a type of convective crustal overturn, and at least partially solves the space problem related to batholithic emplacement. As a result of this process, relatively broad tracts of country rock acquire Barrovian mineralogy. Following plutonism, magmatically thickened crust is eroded and isostatically up-lifted to expose the plutonic roots and high-grade country-rock basement.

Middle Proterozoic geologic evolution of Llano uplift, Texas: Evidence from U-Pb zircon geochronometry

The Llano uplift is a gentle structural dome that exposes Middle Proterozoic crystalline rocks of Grenville affinity. Zircon U-Pb geochronometry of polydeformed metaigneous rocks and post-tectonic plutons in the uplift defines a more protracted and complex geologic evolution than previously documented. Results indicate that the uplift harbors crust at least 1,303 m.y. old, demonstrate that igneous protoliths of some metamorphic units were emplaced between 1,252 and 1,232 m.y. ago, and constrain major Grenville deformational processes to have taken place between 1,232 and 1,116 m.y. ago. This metamorphosed and deformed crust was then perforated by a suite of coarse-grained granitic plutons the emplacement ages of which range from 1116 to 1070 Ma. When considered in conjunction with the present geometric organization of metamorphic units within the uplift, the U-Pb data suggest that contacts between some of the metamorphic units are tectonic. As in the case of Grenville tracts elsewhere, it appears that the Llano crust is structurally imbricated. The Middle Proterozoic temporal and geologic evolution of the uplift is similar to that described from other Grenville-age terranes in North America, thus verifying the synchroneity of orogenic processes along most of the Middle Proterozoic oceanward margin of Laurentia.

Kinematic evolution of the Miller Range Shear Zone, Central Transantarctic Mountains, Antarctica, and implications for Neoproterozoic to Early Paleozoic tectonics of the East Antarctic Margin of Gondwana

High-grade ductile tectonites of the Precambrian Nimrod Group in the central Transantarctic Mountains form the Miller Range shear zone (MRSZ). With no exposed boundaries, this zone has a minimum structural thickness of 12–15 km. Shear-sense indicators record consistent top-to-the-SE, or left-lateral, shear within the NW striking, moderately SW dipping zone. Cylindrical folds with axes normal to elongation lineation (Le) are kinematically consistent with other shear indicators. They may represent early stages in the development of subordinate noncylindrical sheath folds, which indicate locally high bulk ductile strain and a moderate strain gradient. Pervasive, open to tight cylindrical folds with axes parallel to Le formed during shear and may reflect a component of constrictional strain. Quartz c axis fabrics from micaceous quartzites show asymmetric single girdles evident of dominantly rhombohedral slip, with limited basal-plane slip, affirming both the consistency of shear sense and high-grade syn-kinematic conditions. Deformation did not persist during subamphibolite facies cooling, as shown by (1) a lack of basal-plane slip in ductilely deformed quartz, (2) a lack of quartz subgrains and grain shape-preferred orientation, and (3) the presence of oriented muscovite “fish” included within polygonal quartz grains, which show that quartz grain boundaries migrated and annealed under static conditions following ductile shear. From the uniform Le orientation and consistent shear sense, we interpret that ductile deformation resulted from a single, kinematically simple, left-lateral (top-to-the-SE) shear event. Together, the scale, high total strains (γ ≥ 5), fabric uniformity, and the widespread presence of asymmetric microstructures formed at high temperatures, all indicate that strain rates within the MRSZ were high and that it represents a major crustal structure. Orogen-parallel displacements within this zone during the latest Neoproterozoic to Early Cambrian were at a high angle to penecontemporaneous orogen-normal contraction in outboard supracrustal rocks, suggesting that the Neoproterozoic to early Paleozoic plate margin of Antarctica was characterized by left-oblique convergence in which strain within the orogen was partitioned into deep-level strike slip and shallow-level contraction.

Provenance, detrital zircon U-Pb geochronometry, and tectonic significance of Permian to Lower Triassic sandstone in southeastern Quesnellia, British Columbia and Washington

U-Pb ages of Permian detrital zircons within eastern Quesnellia are similar to ages of western and southwestern Laurentian basement provinces, suggesting sedimentologic ties between the Quesnellian arc terrane and the Laurentian continental margin. The Permian to Lower Triassic Mount Roberts Formation forms the easternmost exposures of the Harper Ranch subterrane of Quesnellia. The formation consists of interbedded lithic graywacke, argillite, limestone, and volcanic rocks deposited within an ensimatic arc environment. Petrology of Mount Roberts Formation sandstones indicate derivation from lithologically diverse sources that consisted predominantly of volcanic rocks but also included significant amounts of sedimentary, metamorphic, and plutonic rocks. U-Pb ages of 27 individual detrital zircons from a single Permian to lower Triassic sandstone of the Mount Roberts Formation indicate that detritus was derived from source terranes with ages of approximately 0.37–0.40 Ga, 1.0–1.3 Ga, 1.50 Ga, 1.61–1.66 Ga, 1.8–2.1 Ga, 2.36 Ga, 2.48 Ga, and 2.5–2.7 Ga. Devonian zircons are interpreted to represent first-cycle detritus derived from an igneous source terrane that may make up local basement to the Mount Roberts Formation. Most Precambrian zircons are well rounded by sedimentary abrasion and thus were probably derived from older sedimentary rocks. The ultimate sources of 1.0–1.66 Ga detrital zircons are unknown, but it is unlikely that they were derived from the presently adjacent craton. Zircons 1.0–1.66 Ga may represent detritus ultimately derived from southwestern Laurentia. Zircons 1.8–2.7 Ga are similar in age to geologic provinces within western Laurentia, suggesting presently adjacent provinces within the Laurentian craton as ultimate sources of some Precambrian detritus within the Mount Roberts Formation. Although Laurentia may have been the ultimate source of Precambrian detritus within the Mount Roberts Formation, sedimentologic and petrologic data and regional geologic considerations strongly suggest that most of this detritus was recycled from craton-derived sedimentary and metasedimentary rocks that composed part of Quesnellia.

Tectonic setting of the Slide Mountain terrane, southern British Columbia

The Slide Mountain terrane (SMT) in southern British Columbia consists of rocks of continental and oceanic affinity that are juxtaposed with parautochthonous rocks of the North American continental margin. In southern British Columbia, SMT consists dominantly of fine-grained quartzose clastic rocks, limestone and lesser amounts of conglomerate and volcanic rocks of the Carboniferous McHardy assemblage, and predominantly mafic volcanic rocks of the Permian Kaslo Group. U-Pb ages of individual detrital zircons from the McHardy assemblage are 1.7 Ga to 3.1 Ga and are similar to published ages of zircons from sedimentary rocks of the adjacent Kootenay terrane and the North American continental margin. These data and the petrology of McHardy assemblage sandstones and conglomerate suggest Kootenay terrane and the North American miogeocline as sources for McHardy assemblage detritus. U-Pb zircon ages of granitoid clasts within McHardy assemblage conglomerate indicate that Silurian granitic rocks also provided detritus to the SMT. Mafic volcanic, ultramafic, and sedimentary rocks of the Kaslo Group conformably overlie the McHardy assemblage. New geochemical data demonstrate that the Kaslo Group consists of light rare earth element depleted basalts. On the basis of geochemical and geologic data, we suggest that Kaslo Group volcanics were erupted within an ocean ridge proximal to the North American continental margin and probably represent the eastern (continental) margin of a Permian marginal basin. Lithologie, stratigraphie, and U-Pb geochronologic data suggest that the SMT was deposited on autochthonous, distal miogeoclinal rocks of the adjacent western North American craton and in close proximity to an early Paleozoic arc terrane. We infer that correlative late Paleozoic basinal terranes in western North American were deposited in a similar tectonic setting.

Is the southeast Coast Plutonic Complex the consequence of accretion of the Insular superterrane? Evidence from U-Pb zircon geochronometry in the northern Washington Cascades

Zircon U-Pb geochronometry of orthogneisses and plutons in the southwestern crystalline core of the North Cascades, coupled with fabric and textural studies of the orthogneisses, plutons, and their metamorphic host rocks, indicates extensive synmetamorphic plutonism at 89-96 Ma. Metamorphic mineral assemblages define a culmination composed of an axial kyanite-sillimanite zone rimmed by lower grade zones. High-grade index minerals are typically syntectonic to posttectonic. Metamorphic fabrics are characterized by an orogen-parallel, northwest-striking, steep foliation that contains a subhorizontal stretching and mineral lineation interpreted to be the product of ductile strike-slip deformation. This fabric is crosscut by 96-92 Ma plutons yet is imprinted on 92-89 Ma orthogneisses, suggesting spatially diachronous fabric development during orogeny. Documentation of the spatial and temporal coincidence of magmatism with the peak of orogeny, together with the kinematic significance of the metamorphic fabric, precludes generation of the metamorphic fabric and plutons in response to thrust loading. The authors suggest that this part of the Coast Plutonic Complex evolved as a transpressional magmatic arc.

Late Cretaceous juxtaposition of metamorphic terranes in the southeastern San Gabriel Mountains, California

New structural, petrological, and geochronological data from the southeastern San Gabriel Mountains define the amalgamation history of suspect terranes in this region. Four metamorphic terranes constitute the area—the Cucamonga, San Antonio, San Gabriel, and Baldy terranes—and were juxtaposed in two discrete episodes during the Late Cretaceous. The Cucamonga, San Antonio, and San Gabriel terranes were juxtaposed along subparallel, east-west-trending, left-lateral mylonitic fault zones during the first episode. The Cucamonga terrane consists of Lower Cretaceous granulite-facies gneissic rocks. These rocks were intruded in the Late Cretaceous by granitic rocks as the Cucamonga terrane was faulted ductilely against the southern margin of the San Antonio terrane. Mylonitized units were subjected concurrently to retrograde amphibolite-facies metamorphism. Metasedimentary pendants in the San Antonio terrane were metamorphosed under upper-amphibolite-facies conditions as plutonic rocks were emplaced syntectonically. Along its northern boundary, the San Antonio terrane was juxtaposed concurrently against the San Gabriel terrane, a terrane consisting of Precambrian gneiss and Precambrian and Mesozoic intrusive rocks. The boundary is represented by Middle Fork Complex, a ductile zone intermingling rocks of the two terranes. After palinspastic restoration of local Cenozoic fault displacements, the left-lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the San Gabriel Mountains. Displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left-lateral shear system. This system is modeled as a lateral ramp or tear fault to a west-directed, synplutonic, ductile thrust system underlying the San Gabriel terrane. Crosscutting syn- and post-tectonic intrusives indicate that most of the displacement occurred between ∼88 and 78 m.y. ago. No lithologic, isotopic, or structural data support previous assertions that these terranes were accreted to North America during the Tertiary. The Late Cretaceous synplutonic juxtaposition of the Cucamonga, San Antonio, and San Gabriel terranes was followed by underthrusting of oceanic rocks of the Baldy terrane (that is, Pelona Schist) along the Vincent thrust. Precise timing of this episode is as yet uncertain.

A new structural and tectonic interpretation of the western part of the Shuksan blueschist terrane, northwestern Washington

The Shuksan Metamorphic Suite (SMS) is a blueschist terrane exposed in the northwest Cascades of Washington. Along its eastern flank, the SMS is known from previous studies to consist of meta-MORB and pelitic and metalliferous metasedimentary rocks that resemble ocean-floor deposits. In the western part of the SMS, however, the metasedimentary rocks lack metalliferous enrichment and contain interbeds of mafic to felsic lithic: tuff and volcanic-derived sandstone. Relatively small, isolated bodies of meta-igneous rock in this area comprise a variable suite of arc-related mafic to felsic plutonic and volcanic rocks. These have been assigned previously to units other than the SMS, postmetamorphic faulting being invoked to explain their emplacement. In the South Chuckanut, Lyman, and Bowman Mountain areas, however, the SMS metasediments contain clasts derived from the nearby meta-igneous units, metamorphic structures have a similar orientation in both the metasedimentary and metaigneous rocks, and the metamorphic facies in the metasedimentary and meta-igneous units are the same. These observations require (1) reassignment of the meta-igneous units to the Shuksan Suite, (2) rejection of previous interpretations of the contact between the metasedimentary and meta-igneous rocks as being an expression of either the Shuksan thrust or Haystack thrust, and (3) broadening of the tectonic setting of deposition of Shuksan protolith rocks to include an arc component. A maximum protolith age for the western part of the SMS of 163 ± 2 m.y. is established by a U/Pb zircon age of metaquartz diorite from Bowman Mountain. We infer deposition of the SMS protolith in a Late Jurassic marginal basin behind a west-facing arc.

Origin and tectonic significance of the Aldrich Mountains serpentinite matrix melange, northeastern Oregon

The Permian - Triassic Aldrich Mountains serpentinite melange consists of a matrix of massive and foliated serpentinite (lizardite + chrysotile) that encases a variety of metamorphic, igneous, and sedimentary block types. Serpentinite matrix foliation trends NE-SW, dips steeply (> 75°), and is parallel to long dimensions of blocks. At block-matrix contacts, the foliation traces block margins. The volumetrically most abundant block types are greenstone, chert, and metagabbro; subordinate block types are amphibolite tectonites, epiclastic sediments (breccia, conglomerate, sandstone, and argillite), silicic volcanic rocks, silicic plutonic rocks, and ultramafic rocks. Three types of metamorphism are evident from textures and mineral assemblages of blocks and matrix: (1) serpentinization and related hydrous alteration confined to matrix, ultramafic blocks, and margins of mafic blocks; (2) lower to middle greenschist and prehnite-actinolite facies metamorphism characterized by static, non penetrative fabrics of some blocks, and; (3) dynamothermal epidote amphibolite to upper amphibolite/lower granulite metamorphism exhibited by strongly schistose amphibolite tectonites. Mineral assemblages and mineral chemistry indicate amphibolite tectonites formed at moderate to high temperatures (550°–780°C) and low to moderate pressures (3–5 kbar). Most blocks were affected by variable degrees of post metamorphic brittle deformation prior to immersion in serpentinite matrix. The melange is interpreted to have formed within a Permian to Late Triassic sediment-starved, tectonically active forearc. Fragments of abyssal oceanic crust and Early Permian ensimatic arc crust were mixed by tectonic and sedimentary processes within the forearc. Highly mobile serpentinite, generated beneath the forearc, was protrusively emplaced into the tectonized forearc and mixed with the blocks, forming the melange. Melange generation probably began in the late Early Permian and ceased by the Late Triassic.