Paleomagnetic data from Central Asia show that 1700±610 km of shortening of southern Asia since Cretaceous time have been absorbed by distributed deformation between southern Tibet and the Siberia craton. This result is based on a compilation of Cretaceous poles from the Junggar, Tarim, Tibet, Indochina, South China, North China, and Mongolia blocks, complementing the recent compilation of Enkin et al. (1992a). We propose a paleogeographic reconstruction of Asia in the Cretaceous, in which the position of Siberia is derived from the synthetic apparent polar wander path of Besse and Courtillot (1991). The resulting map, which likely represents Asia as it remained throughout the Cretaceous until the collision with India began, features an “unbent” Tibet, with an east‐west trending Andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from Sichuan to Tarim through Tibet. The map allows one to estimate continental shortening and rotations between the blocks, which are attributed to the collision. Despite large uncertainties, these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of Avouac (1991). Appendix is available with entire paper on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, DC 20009. Document B93‐007; $2.50. Payment must accompany order.

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 displacem...

Upper Jurassic to lower Tertiary sites have been sampled from three localities, separated by up to 150 km, in the Sichuan Basin (∼30°N, 103°E) of the South China Block. Each locality has at least one stability test, but none alone gives a fully reliable result. When taken together, however, identical directions are found with a global average D=12.7°, I=34.2°, k=42.9°, α95=3.3° (N=44 sites) and corresponding pole 73.9°N, 234.4°E, dp=2.2°, dm=3.8°, paleolatitude λ=18.8°. There is no significant apparent polar wander (APW) with age, which is consistent with an acquisition of magnetization during the period of the stable tip of the Eurasian APW loop (∼130–70 Ma), as defined with poles from other continents transferred onto Eurasia. This age range is compatible with the available paleontological and magnetostratigraphic age constraints but is shorter than that suggested by the assigned formation ages. The paleolatitude of Sichuan is significantly lower (8.0°±4.0° at 90 Ma) than that predicted by the reference APW path, either because of northward shortening during the Tertiary or because of errors in the APW path. Cretaceous poles from other Chinese blocks are consistent with the Sichuan pole, but there is evidence that these blocks have suffered local rotations up to 10° during the Tertiary, probably as a result of the India‐Eurasia collision.

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In order to constrain better the paleogeography of the blocks that now make up China, we collected paleomagnetic samples from Uppermost Permian to Middle Triassic limestones from two localities in the Sichuan Basin (South China Block). A low inclination component which passes both the fold test and the reversal test was found in five sites (D=46.1°, I=11.9°, k=83.6, α95=8.7°). We review corresponding remanences found in other studies of Permo‐Triassic rocks from the Sichuan Basin, and show that the component is probably characteristic of the Lower Triassic. We then review paleomagnetic results from the Uppermost Permian Emeishan Basalt. Despite small local rotations, the difference between the Emeishan Basalts and the Sichuan Basin poles is interpreted to be the result of motion of the whole South China Block around the time of the Permo‐Triassic boundary rather than of strain within the block.

Multibeam echosounder (MBES) images, 3.5 kHz seismic-reflection profiles and piston cores obtained along the southern Queen Charlotte Fault Zone are used to map and date mass-wasting events at this transform margin – a seismically active boundary that separates the Pacific Plate from the North American Plate. Whereas the upper continental slope adjacent to and east (upslope) of the fault zone offshore of the Haida Gwaii is heavily gullied, few large-sized submarine landslides in this area are observed in the MBES images. However, smaller submarine seafloor slides exist locally in areas where fluid flow appears to be occurring and large seafloor slides have recently been detected at the base of the steep continental slope just above its contact with the abyssal plain on the Queen Charlotte Terrace. In addition, along the subtle slope re-entrant area offshore of the Dixon Entrance shelf bathymetric data suggest that extensive mass wasting has occurred in the vicinity of an active mud volcano venting gas. We surmise that the relative lack of submarine slides along the upper slope in close proximity to the Queen Charlotte Fault Zone may be the result of seismic strengthening (compaction and cohesion) of a sediment-starved shelf and slope through multiple seismic events.