William C. McClelland

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite-in, staurolite-in, garnet-in, and chloritoid-in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E-W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N-S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post-D1 and pre- to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end-member metamorphic core complex-type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

U‐Pb dating of zircon by LA‐ICP‐MS

In this study we used LA-ICP-MS (laser ablation–inductively coupled plasma–mass spectrometry) to determine U-Pb ages of 5 zircon samples of known age (∼1800 Ma to ∼50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double-focusing ICP-MS coupled with a New Wave Research UP-213 laser system. The laser ablation pit sizes ranged from 30 to 40 μm in diameter. Laser-induced time-dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA-ICP-MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP-MS, can be used for the U-Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.

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.

Pennsylvanian pluton stitching of Wrangellia and the Alexander terrane, Wrangell Mountains, Alaska

A quartz monzonite-syenite-alkali granite plutonic complex in eastern Alaska crosscuts the contact of the Alexander terrane and Wrangellia and intrudes the basement rocks of both terranes. Zircon U-Pb data indicate an intrusion age of 309 {plus minus} 5 Ma (Middle Pennsylvanian) for the pluton, and {sup 40}K-{sup 40}Ar age for hornblende separates indicate cooling to about 450 C during Middle Pennsylvanian-Early Permian time. The new field relations and age data demonstrate the Wrangellia and the Alexander terrane were contiguous during the Middle Pennsylvanian. This conclusion provides an important new constraint on paleogeographic reconstructions of the northwest Cordillera, and necessitates reassessment of stratigraphic and paleomagnetic data that were cited as evidence that the terranes evolved separately until the late Mesozoic.

Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large-scale displacement?

Stratigraphic, petrologic, and isotopic data indicate that parts of the Coast Plutonic Complex and the North Cascade Range have been tilted northeast-side-up by angles of {approximately}30{degree} about north-northwest-trending axes. These tilts can account for discordant paleomagnetic directions observed in mid-Cretaceous plutons from these regions without large-scale displacement relative to North America.

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.

Two-phase evolution of accretionary margins: examples from the North American Cordillera

Abstract The Coast Mountains orogen, southeastern Alaska, and the Salmon River suture zone, western Idaho, record the accretion of oceanic crustal fragments to the North American Cordilleran margin. Structural elements associated with these boundaries include a western accretion-related thrust belt and steeply dipping post-accretionary shear zone. The shear zones, the Coast shear zone, southeastern Alaska, and western Idaho shear zone, Idaho, are steeply dipping crustal-scale structures characterized by steep isotopic and geophysical gradients. The presence of syndeformational tabular calc-alkaline tonalitic plutons concordant with shear zone fabrics, locally preserved dextral strike-slip indicators, and pervasive down-dip mineral and elongation lineations are consistent with an intra-arc transpressional origin for both structures. In addition to strike-slip translation, the structures accommodated exhumation of high pressure–moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure–high temperature batholithic rocks to the east. Similarities between the structures in Alaska and Idaho suggest that the observed two-stage history of underthrusting of accreting crustal fragments followed by modification by intra-arc transpressional shear zones is a common tectonic process. Our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post-accretion deformation and provides insight into lower-crustal processes presently operative under modern strike-slip–accretionary boundaries. In addition, the intra-arc transpressional shear zones (e.g. the Coast and western Idaho shear zones) are structures that could have accommodated paleomagnetically determined large-magnitude transcurrent displacements.

Structural and geochronologic relations along the western flank of the coast mountains batholith: Stikine river to Cape Fanshaw, central southeastern Alaska

Abstract Geologic and U-Pb geochronologic studies in central southeastern Alaska provide constraints on the mid-Cretaceous to mid-Tertiary deformation west of the Coast Mountains batholith. The NE-dipping Sumdum and Fanshaw faults record the W-directed emplacement of the Ruth assemblage (Yukon-Tanana terrane) and Taku terrane over the subjacent Gravina belt and Alexander terrane. Ductile fabrics of the Sumdum-Fanshaw fault system truncate early-formed foliation and thrust faults in Albian and older Gravina belt strata. U-Pb age data from syn- and post-tectonic plutons suggest that deformation was ongoing at 92.9 ± 3.0 Ma but had mostly ceased by 90 Ma. The Sumdum-Fanshaw fault system marks the tectonic boundary between the Alexander-Wrangellia terrane and inboard Yukon-Tanana and Stikine terranes and lies within a thrust belt that extends from southern Alaska to northern Washington. Mid-Cretaceous structures are truncated to the east by NE-dipping ductile fabrics of the Coast shear zone. Sheet-like Paleocene tonalites were emplaced into and deformed within the shear zone. Undeformed mid-Eocene dikes cross-cut the tonalites. Kinematic relations in the Coast shear zone suggest a complex displacement history that includes both east-side-up (reverse) and west-side-up (normal) shear. The shear zone probably accommodated the collapse of overthickened crust developed during mid-Cretaceous shortening.

Nd isotopic characterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian Cordillera: Ancient crust bounded by juvenile terranes

Nd isotopic data are reported for 52 samples from the crustal region between the Alexander-Wrangellia terrane and the Stikine terrane of the Alaskan and Canadian Cordillera. This region is composed of the Gravina belt, a Jurassic-Cretaceous assemblage of volcanic and clastic sedimentary rocks, the Taku terrane, a terrane of probable Early Permian to Late Triassic age, and four assemblages of metamorphic rocks that occur to the west of and within the Coast Mountains batholith. The Gravina belt has eNd(T) values that range from −1.1 to +8.3, similar to values of the underlying Alexander terrane, and consistent with the interpretation that it is a juvenile belt that formed in a back-arc or intra-arc basin within the Alexander terrane. Mid-Cretaceous plutons that were emplaced into the Gravina belt have eNd(T) values of +4.4 to +5.7 and were probably produced by mantle-derived melts that incorporated some Alexander terrane crust. The Taku terrane has eNd(0) values that range from −5.5 to +3.3, with corresponding depleted-mantle model (TDM) ages of 440 to 1430 Ma. A mid-Cretaceous pluton intruding the Taku terrane has an eNd(T) value of +5.1, a value indistinguishable from those determined for Cretaceous plutons intruding the Gravina belt. Metamorphic rocks east of and structurally overlying the Taku terrane are divided into the Tracy Arm assemblage, eNd(0)=−26 to 0, TDM=800–2450 Ma; the Endicott Arm assemblage, eNd(0)=−10 to −1.3, TDM=950–1500 Ma; the Port Houghton assemblage, eNd(0)=−9.4 to +1.1, TDM = 550–1500 Ma; and the Ruth assemblage, eNd(0) = −9.4 to +2.0, TDM=650–1300 Ma. These isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from Precambrian continental crust. The metamorphic rocks from these assemblages are lithologically very similar to rocks of the Yukon-Tanana (YTT) terrane of eastern Alaska and Yukon Territory and have such similar U-Pb detrital zircon ages and Nd isotopic compositions to YTT rocks that they are considered part of that terrane. Possible tectonic scenarios that can explain the present geometry of the YTT with respect to the Alexander-Wrangellia and Stikine terranes include: (1) The YTT is the upturned stratigraphic basement of the Stikine terrane, (2) part of the YTT was structurally emplaced beside the Stikine terrane in a transpressive tectonic regime, (3) the Stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the YTT before the final accretion of the Alexander-Wrangellia terrane.

Pluton emplacement by sheeting and vertical ballooning in part of the southeast Coast Plutonic Complex, British Columbia

Batholiths of the Coast Plutonic Complex, in the area of Harrison Lake, British Columbia, are interpreted to have formed by horizontal sheeting and vertical inflation. Zoned ovoid batholiths exhibit sheeted margins as thick as 3 km. Shallow floors are indicated by moderately to gently inward dipping magmatic foliations, the observed trace of floor contacts across topography, and a published deep seismic reflection transect. Mineralogic aureoles under the floors preserve a paragenetic record of increasing pressure during aureole crystallization. Initial pressures of