Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large-scale transport (original) (raw)
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The paleomagnetic effects of reheating the Ecstall pluton, British Columbia
Earth and Planetary Science Letters, 2004
Paleomagnetic data of plutons from the western Canadian Cordillera are currently interpreted to indicate thousands of kilometers of latitudinal transport, in situ tilting, or post-intrusion folding. For the Ecstall pluton of British Columbia, the inclination of magnetic remanence directions steepens to the east from 16 ‡ to 81 ‡ [Butler et al., J. Geophys. Res. 107 (2002)] between 24 and 12 km west of a thermal boundary with the Coast Mountains batholith (CMB). The CMB was at 700^800 ‡C between 60 and 52 Ma. The stable remanent magnetization is in ilmenohematite grains with exsolved lamellae of ferrian ilmenite. It has the characteristic properties of lamellar magnetism [Robinson et al., Nature 418 (2002) 517^520] which is acquired with exsolution in ilmenohematite below 390 ‡C [Ghiorso, Phys. Chem. Minerals 25 (1997) 28^38]; thus, it is thermal chemical remanent magnetization (TCRM). Our heat flow calculations show that the thermal effects of Eocene CMB on the adjacent 91 Ma Ecstall pluton were enough to reset the remanent magnetization directions. Reheating to the temperatures necessary for TCRM is supported by K/Ar and Ar/Ar cooling dates on hornblende and biotite, which young from west to east towards the CMB in concert with the change in inclinations. Thus, the progressive steepening of magnetization inclinations is attributed to reheating during the Eocene of the 91 Ma Ecstall pluton by the CMB. This reheating followed post-solidification northward latitudinal displacement of the pluton. The amount of translational displacement of the coastal terranes of British Columbia prior to 60 Ma hinges on the interpretation of discordant magnetizations from plutons that may have cooled slowly after emplacement in the mid to lower crust, as was the case for the Ecstall pluton. Many of these plutons contain ilmenohematite as an accessory magnetic phase. Our interpretation for the low temperature acquisition of TCRM in the Ecstall pluton may have far-reaching implications for understanding enigmatic discordant paleomagnetic directions reported from plutons of the western Canadian Cordillera. ß
Canadian Journal of Earth Sciences, 1995
The Porteau Pluton is a variably foliated quartz diorite to granodiorite intrusion in the southern Coast Belt of the Canadian Cordillera (49.6"N, 123.2"W). 40Ar/39Ar ages are 95 f 5 Ma from biotite and 101.5 + 0.7 Ma from hornblende, which, together with an earlier U-Pb zircon age of 100 f 2 Ma, indicate that the body was emplaced, uplifted, and cooled rapidly in mid-Cretaceous time. The rocks contain high coercive force (hard) remanent magnetizations with unblocking temperatures between 500 and 600°C, close to those of Ar in hornblende, indicating that remanence was acquired at or close to the hornblende plateau age. The hard remanence directions have an elongate distribution, in agreement with the predictions of M.E. Beck regarding magnetization acquired during tilting, uplift, and cooling of plutons.
Journal of Geophysical Research, 1995
The Spences Bridge Group is a mid-Cretaceous (104 Ma) volcanic succession in the southern Intermontane Belt of the Canadian Cordillera (50.5øN,121øW). It comprises the Pimainus Formation (mafic to felsic lava, volcaniclastic and interbedded epiclastic rocks) and the overlying Spius Formation (andesitic lava flows). Including previous work, we have 55 sites distributed among 15 localities representing most of the > 3000 m thickness. Forty-seven sites (286 oriented cores, 457 specimens), mainly andesites, yielded acceptable data. The beds are gently to moderately tilted, partly due to synvolcanic deformation. Samples taken from a sequence of flows at any one locality have, with one exception, well-grouped magnetization directions. Polarities are all normal, as expected for rocks laid down in the Cretaceous Normal Superchron. Declinations always are clockwise of that expected of cratonic North America, indicating 60 ø rotation of the Spences Bridge Group as a whole. However, declinations differ from locality to locality, implying relative interlocality rotations about vertical axes. Hence inclination-only analysis has been used to estimate dispersion, mean inclination, and paleolatitude. Minimum dispersion for the Spius Formation (27 sites) was achieved after 80% untilting, but the changes between 80 and 100% untilting are insignificant, indicating that magnetization was acquired predominantly before tilting. By contrast, minimum dispersion for the Pimainus Formation (20 sites) was achieved after 50% untilting, indicating that magnetization was acquired after synvolcanic tilting when buried beneath the overlying Spius Formation. Polished thin section studies show that magnetite in the Pimainus Formation has undergone extensive low temperature hydrothermal alteration, whereas magnetite in the Spius Formation shows little alteration. Throughout the range of tilt correction, from 0% to 100%, the mean inclinations of both formations were less than expected from observations obtained from mid-Cretaceous rocks of cratonic North America. The best estimate of paleolatitude (from the Spius Formation) is 50.8 ø q-5.0 ø (P=0.05), which is 9.5ø-t-5.7 ø less than would be expected had the rocks had been rigidly attached to North America. This corresponds to displacement from the south of 1100 q-600 km. Displacement between the northern Intermontane Belt and craton probably was accommodated along major strike-slip faults (Northern Rocky Mountain Trench, Finlay, Pinchi etc.). In the south, our results require a major dextral fault (the Intra-Quesnellia fault) to be situated during Late Cretaceous or Paleocene time within or marginal to the Omineca Belt, along which about 1000 km of dextral motion occurred. This could be a southern extension of the Pinchi Fault whose trace is now obscured by Eocene extension and tectonic denudation. The results also indicate that the largest tectonic discontinuity in the Canadian Cordillera occurs not to the east of the Intermontane Belt, as commonly assumed, but to the west, because the displacements relative to cratonic North America observed from the Intermontane Belt are only about one third of those observed from the Coast Belt. Introduction The Canadian Cordillera is divisible into five morphotectonic belts [Wheeler et al., 1991]. The Foreland Belt comprises mainly Precambrian to Mesozoic miogeoclinal strata which were folded and thrust toward the continent in
Geosphere, 2016
Contrasting magnetic fabrics in five successively emplaced syntectonic plutons reveal temporal and spatial variations in tectonic strain in the oceanic terranes of the Blue Mountains province, northeastern Oregon, during the Late Jurassic to Early Cretaceous. The inferred strain regimes changed from: (1) thrusting and sinistral shearing at ca. 160 Ma, to (2) horizontal stretching at ca. 147 Ma (in the forearc-accretionary wedge Baker terrane), to (3) dextral transpression that started from ca. 140 Ma onward and was associated with progressive anticlockwise rotation of the principal horizontal shortening direction from ca. 130 Ma to ca. 126 Ma (in the Wallowa oceanic arc terrane). These progressive strain reorientations are interpreted in terms of an outboard Wallowa-Baker terrane collision, lateral extrusion, docking of the amalgamated Blue Mountains superterrane into a continental-margin reentrant, and onset of oroclinal bending, respectively. The changes in crustal strains are then interpreted as recording a progressive change in relative motions between the Pacific Ocean basin and North America and suggest a transition from Late Jurassic sinistral deformation to Early Cretaceous dextral terrane translations along the paleo-Pacific margin. We speculate that these events may have been linked along large portions of the North American Cordillera, from central California to Blue Mountains, and may have culminated in the onset of accretion in the Franciscan complex and voluminous plutonism in the Sierra Nevada magmatic arc. A similar plate-kinematic change is inferred to have occurred in British Columbia several tens of millions of years later (at ca. 100 Ma), implying that these kinematic transitions may have varied in space and time along the length of the Cordilleran orogen.
Paleomagnetism of Eocene plutonic rocks, Matanuska Valley, Alaska
Geology, 1988
Paleomagnetic analysis of 88 samples from 14 sites in four Eocene sills in the Matanuska Valley, south-central Alaska, yields a paleomagnetic pole at lat 72.9°N, long 281.1°E, precision parameter (k) = 15.4. The paleolatitude of this result suggests that this part of the Alaskan crust was at or near its present position relative to North America in the middle Tertiary. This pole also indicates that the region has been rotated clockwise by approximately 50°. These results can be explained by local block rotations of fault-bounded blocks in response to regional right-lateral shear along both the Castle Mountain and Border Ranges faults.
Tectonics, 1999
The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from-110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between -101 and-85
Tectonophysics, 2000
The White Pass mafic dikes are located in the Coast Belt, inboard of the Coast Shear Zone-Denali Fault System, east of Skagway, Alaska, astride the Alaska-British Columbia border. The dikes intrude Eocene felsic plutons in the Coast Plutonic Complex. Three K-Ar analyses date the dikes at 51.9±0.8 Ma (biotite), 50.1±1.0 Ma and 45.0±0.8 Ma (both whole rock). Structural analysis on these vertical dikes indicates that the study area has not been tilted significantly since dike emplacement. Conventional alternating field and thermal step-demagnetization and saturationremanence paleomagnetic methods were used to analyse 407 specimens from 27 dike and five host rock sites. Three contact tests show that the specimens retain a primary thermoremanent magnetization (TRM) carried in pseudosingle to multidomain magnetite, although some magnetization in a few dikes is carried by pyrrhotite. The dual-polarity TRM is mostly reversed and the 24 accepted dike sites give a pole position of 245.7°E, 76.3°N (d p =4.2°, d m =4.4°), indicating a paleolatitude discordance of 8±4°south and a clockwise rotation of 40±9°with respect to the North American craton. These data, along with other paleomagnetic data on Eocene rocks, indicate that the allochthonous eastern Coast and Intermontane Belt terranes had halted their northward translation by about 51 Ma but continued to be rotated clockwise by an average of 24±10°thereafter, probably on underlying sole faults.
Geochemistry, Geophysics, Geosystems, 2011
1] Paleomagnetic studies of the 91 Ma Ecstall pluton and other Cretaceous plutons of British Columbia imply large northward tectonic movements (>2000 km) may have occurred during the tectonic evolution of western North America. However, more recent studies have shown that the eastern edge of the Ecstall pluton experienced considerable mineralogical changes as younger Eocene plutons, such as the ∼58 Ma Quottoon Pluton, were emplaced along its margins. We investigated changes in the rock magnetic properties associated with this reheating event by examining isolated grains of intergrown ilmenite and hematite, the primary paleomagnetic recorder in the Ecstall pluton. Measurements of hysteresis properties, low-temperature remanence, and room temperature isothermal remanent magnetization acquisition and observations from magnetic force microscopy and off-axis electron holography indicate that samples fall into three groups. The groups are defined by the presence of mineral microstructures that are related to distance from the Quotoon plutonic complex. The two groups closest to the Quottoon Pluton contain magnetite within hematite and ilmenite lamellae. Reheating of the Ecstall pluton led to an increase in coercivity and magnetization, as well as to development of mixed phase hysteresis. These results indicate that shallow paleomagnetic directions from the western Ecstall pluton are not affected by reheating and are therefore likely to record original field conditions at the time of pluton emplacement. In the absence of structural deformation, these shallow inclinations are consistent with large-scale northward translation suggested by the Baja-British Columbia hypothesis.