Myrl E. Beck

ON THE MECHANISM OF CRUSTAL BLOCK ROTATIONS IN THE CENTRAL ANDES

Abstract The results of 62 paleomagnetic studies of Jurassic and younger rock units from the Andes of Argentina, Bolivia, Chile and Peru are examined for indications of the timing and mechanism of crustal block rotations. Rotations and their 95% confidence intervals are recalculated in pole space, using a new South American reference APW path. As observed by many investigators, the Arica deflection (the change in topographic and tectonic trends located near Arica, northernmost Chile) divides the data into a counterclockwise-rotated set north of the deflection and a clockwise-rotated set to the south. Rotations range from 75° counterclockwise to nearly 50° clockwise, and show substantial differences between neighboring crustal blocks. South of Arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin, but no such correlation is observed north of Arica. Rotation increases with age both north and south of Arica for rock units of ≤90 Ma. For older rock units no correlation between age and amount of rotation exists. Unlike the western margin of North America, no significant margin-parallel displacements accompany the rotations. Several models for rotation are examined. It is concluded that a combination of late Cenozoic oroclinal bending and post-mid-Cretaceous local block rotations in response to shear-driven sub-crustal ductile flow can explain most of the paleomagnetic and geologic observations.

A tale of two continents: Some tectonic contrasts between the central Andes and the North American Cordillera, as illustrated by their paleomagnetic signatures

Comparison of patterns of paleomagnetic poles from orogenic belts with appropriate reference poles for the craton can help to delineate important large-scale tectonic processes. Comparison of the paleomagnetic signatures of the western Cordillera of North America and the central Andes shows that the western edges of these belts have had profoundly different Mesozoic and Cenozoic histories. Specifically, the North American Cordilleran pattern shows strong evidence of post-middle Cretaceous relative northward displacement of outboard crustal blocks, but there is almost no comparable evidence of margin-parallel displacement in the Andes. We speculate that this may largely be a consequence of a simple difference in shape: the convex-westward western margin of North America facilitates margin-parallel displacement as a response to oblique subduction, whereas the concave-westward margin of South America inhibits it. The patterns of block rotations found along the western edge of the two orogens also are quite different. Nearly everywhere within the western North American Cordillera crustal blocks have rotated clockwise since mid-Cretaceous time, reflecting a pervading state of dextral shear. Within the western Andes of Peru and northern Chile, however, Mesozoic and Cenozoic rocks in Peru (and northernmost Chile) are rotated strongly counterclockwise, whereas rocks of the same age in the remainder of Chile (to about latitude 48°S) are rotated clockwise. A model combining oroclinal bending and block rotations driven by oblique subduction can account for the paleomagnetic observations.

Escape tectonics and the extrusion of Alaska: Past, present, and future

The North Pacific Rim is a tectonically active plate boundary zone, parts of which may be characterized as a laterally moving orogenic stream. Crustal blocks are transported along large-magnitude strike-slip faults in western Canada and central Alaska toward the Aleutian–Bering Sea subduction zones. Throughout much of the Cenozoic, at and west of its Alaskan nexus, the North Pacific Rim orogenic Stream (NPRS) has undergone tectonic escape. During transport, relatively rigid blocks acquired paleomagnetic rotations and fault-juxtaposed boundaries while flowing differentially through the system, from their original point of accretion and entrainment toward the free face defined by the Aleutian–Bering Sea subduction zones. Built upon classical terrane tectonics, the NPRS model provides a new framework with which to view the mobilistic nature of the western North American plate boundary zone.

Case for northward transport of Baja and coastal southern California: Paleomagnetic data, analysis, and alternatives

This paper, which accompanies the preceding article by Gastil, reviews Cretaceous paleomagnetic evidence for Baja and coastal southern California. Like Cretaceous paleomagnetic data for the Cordillera as a whole, results from Baja and southern California require northward transport of 1000km or more, relative to interior North America. Agreement between independent results, for different rock types and experimental techniques, is overwhelming. Dextral shear driven by right-oblique subduction readily accounts for the paleomagnetic results. Alternative explanations such as experimental errors, rock-magnetic effects, anomalous geomagnetic behavior, errors in dating, or local structure are inadequate, improbable, or both. Some geologists have difficulty reconciling the findings of paleomagnetism with their own results; hence, as emphasized by Gastil, a conflict and enigma exist. To resolve them will require a willingness on both sides to honestly scrutinize basic assumptions.

Paleomagnetism and tectonic significance of the Goble Volcanic Series, southwestern Washington

The mean direction of remanent magnetism of the upper Eocene-Oligocene Goble Volcanic Series, located on the western flank of the Cascades in southwestern Washington, points about 25° east of the expected middle Tertiary geomagnetic field direction for the area. This deflection is in the same sense as discrepant paleomagnetic directions reported for Tertiary rocks from the Oregon Coast Range, but seems to be somewhat less in magnitude. From their major-element chemistry and their structural location, it appears that the Goble flows may be an early arc assemblage that was ancestral to the present Cascade magmatic arc, although origin as an oceanic basalt assemblage akin to the Yachats basalts and the Siletz River and Tillamook Volcanic Series of the Oregon Coast Range cannot be ruled out. Whatever its origin, the Goble block has been rotated about a pivot located somewhere nearby, either as part of the Coast Range or as an independent microcontinental block.

Pillow metabasalts in a mid-Tertiary extensional basin adjacent to the Liquiñe-Ofqui fault zone: the Isla Magdalena area, Aysén, Chile

Pillow metabasalts and interbedded slates adjacent to the Liquine-Ofqui fault zone (LOFZ) of the southern Chilean Andes have been studied using electron-microprobe mineral analysis, major and trace element whole-rock geochemistry and RbSr geochronology. The rocks show previously unrecognized mineralogical evidence of two metamorphic events in a low-pressure environment: an early greenschist-facies metamorphism, possibly during emplacement on the sea-floor, and a patchy amphibolite-facies overprint, which could represent either continuing sea-floor metamorphism or contact metamorphism associated with Miocene plutons. The meta-igneous rocks are considered to have formed from mid-Tertiary basaltic magmas with mixed within-plate/volcanic arc characteristics as seen in present-day volcanoes in the area, but enriched in immobile elements such as Ti, Zr, And Y. They were probably emplaced between 45 and 20 Ma, into a contemporaneous marine pull-apart or extensional basin with thin continental or oceanic floor. Many of the petrological and geochemical features of an ophiolite suite may be recognized in the immediate area.A chain of such basins developed immediately west of the LOFZ strike slip system, probably during an Eocene- Early Miocene period of oblique subduction-convergence in southern Chile, and closed when the approach angle became more orthogonal 25 Ma. Magmatic arc plutonism was resumed at about 20 Ma and continued into the latest Cenozoic.

Testing terrane transport: An inclusive approach to the Baja B.C. controversy

The Baja British Columbia hypothesis holds that a large segment of the western edge of northern North America (Baja B.C.) was situated alongside California and northern Mexico in middle Cretaceous time, was displaced northward in the Late Cretaceous and Paleocene by north-oblique convergence of the Kula plate with North America, and arrived near its present location by the early Eocene. A consistent body of paleomagnetic data supports this hypothesis. However, doubt persists, and various crucial tests of a geologic nature have been proposed. One such test concerns the provenance of zircons in Cretaceous sedimentary basins of Baja B.C. In this paper we use both paleomagnetic data and zircon occurrences to reconfirm the Baja B.C. hypothesis. We first argue that the only truly crucial tests yet performed have been paleomagnetic, and that all such tests have been positive. Second, we show that detrital-zircon data from the Upper Cretaceous Nanaimo Group, although not a crucial test, provide valuable paleogeographic information. Available data demonstrate a change in detrital-zircon provenance in the Nanaimo Group that closely matches the position of these rocks predicted by the Baja B.C. hypothesis. Detrital zircons in the Nanaimo Group suggest a change from a southwestern North American source rich in Grenville and 1.4–1.5 Ga rocks to an increasing contribution from older parts of the craton such as the Wyoming province. Together, detrital zircons and paleomagnetic inclinations allow us to assemble a detailed schedule of northward tectonic transport of the Baja B.C. terranes.

Tectonic rotations in the Cascade Range of southern Washington

Paleomagnetic directions from 34 widely distributed sampling sites in presumed Oligocene volcanic rocks from the Cascade Mountains of southern Washington are well grouped with a mean and confidence interval of declination = 27°, inclination = 64°, and α 95 = 4.5°. When compared to an expected direction, computed from the Oligocene reference pole for stable North America, a clockwise rotation of 33.5° ± 14° has apparently occurred. This result is not significantly different from several other studies from neighboring parts of the Cascades and Coast Range of southern Washington and suggests that the two ranges have rotated as a single unit during post-Eocene time. Comparison with the results from rocks of similar age in Oregon suggests that at least two crustal blocks are involved in Pacific Northwest tectonics. Inclinations in most Coast Range and Cascade Paleogene rocks indicate that these provinces have been translated slightly northward, as well as rotated.

Paleomagnetism and tectonic significance of Eocene basalts from the Black Hills, Washington Coast Range

The Black Hills, located about 15 km west of Olympia, constitute one of several large basement uplifts in the Washington Coast Range. The oldest rocks exposed in this west-dipping homocline consist of early to middle Eocene basalt flows and breccias with minor sedimentary interbeds, which are correlated with the Crescent Formation in the northern Olympic Peninsula. K/Ar geochronology of five basalt samples yields an average age of 53.1 ± 2.0 m.y. B.P., or early Eocene. A total of 263 paleomagnetic cores were collected from 37 sites. All sites were cleaned using AF demagnetization, and 35 sites were corrected for measured and inferred tilt. The mean direction of these sites was virtually unchanged by applying tectonic corrections, although the precision was slightly improved. No flattening or steepening of inclination is apparent, although the mean declination of the 35 sites is easterly discordant with respect to the expected early Eocene declination for “stable” North America by 28.7° ± 15.4°. This discordance indicates that the Black Hills have rotated clockwise about 29° since early Eocene time. The Black Hills basalts show significantly less rotation than Eocene rocks in the Oregon Coast Range, but they show nearly the same amount as analogous basalts in the southwestern Washington Coast Range and latest Eocene to late Oligocene volcanic rocks in the Oregon-Washington Cascade Range. Thus, the rotations of coastal Oregon and Washington, while identical in direction, differ significantly in amount, suggesting that the entire Coast Range block, from the Olympic Peninsula to north of the Klamath Mountains, has not been a coherent terrane since early Eocene time. Several tectonic models are discussed that involve accretion and independent clockwise rotation of two or more Coast Range blocks, or “microplates,” in response to episodic periods of underthrusting and extensive right-lateral shear along the Farallon-North America plate boundary during early and middle Tertiary time. The easterly discordances of declination that are observed in almost all Tertiary rocks sampled in the Oregon-Washington Coast and Cascade Ranges reflect a composite of differential rotation during accretion of the Coast Range seamount terrane, followed soon afterward by large-scale rotation of the western margin of the Pacific Northwest in response to oblique subduction and consequent distributed shear.

Tectonic rotations on the leading edge of South America: The Bolivian orocline revisited

Cretaceous paleomagnetic poles for South America form three distinct groups, one each for the Andes north and south of Arica (northernmost Chile) and one for the craton. Relative to the cratonal group, the northern Andean group is displaced in a counterclockwise sense and the southern Andean group in a clockwise sense. These data have been used to support a model of orodinal bending of the Andes, but distributed shear caused by oblique subduction is a more likely mechanism.