Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.

Kinematics of the western Mediterranean

Relative motions of Africa, Iberia and Europe during Alpine orogeny

Slab breakoff is the buoyancy-driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paper Davies and von Blanckenburg [1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low-degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike-slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high-pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55–40 Ma, followed by uplift at 40–35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high-pressure rocks to the surface allows the identification of this process in the Earth's mountain belts.

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Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps

D\H and 18 O\ 16 O data have now been obtained on a wide variety of granitic batholiths of various ages. The primary δD values of the biotites and hornblendes are remarkably constant at about –50 to –85, identical to the values in regional metamorphic rocks, marine sediments and greeenstones, and most weathering products in temperate climates. Therefore the primary H 2 O in these igneous rocks is probably not ‘juvenile’, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducated lithosphere. Most granitic rocks have δ 18 O = + 7.0 to +10.0, probably indicating significant involvement of high - 18 O metasedimentary or altered volcanic rocks in the melting process; such an origin is required for many other granodiorites and tonaloites that have δ 18 O = + 10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ 18 O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low- 18 O magmas probably formed by melting or assimilation of hydrothermally altered roofrocks. However, the water/rockratios were typically low enough that over wide areas the only evidence for meteoric H 2 O exchange in the batholiths is given by low D/H ratios (δD as low as –180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves from through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks haves δD values lower than –120. The lowering of δD values commonly correlates

Water/rock interactions and the origin of H2O in granitic batholiths Thirtieth William Smith lecture

Ages of late Mesozoic–early Cainozoic igneous rocks in south-western North America indicate a magmatic arc initiated near the continental margin, swept over 1,000 km northeastward, then swept back, all in 110 Myr. The sweep in and return are interpreted as due to flattening of a Benioff zone to < 15° followed by rapid collapse.

Cordilleran Benioff zones

Potassium/argon dating and chemical analyses of major oxides of volcanic rocks in areas adjacent to the Gulf of California provide a stratigraphic record of tectonic and magmatic evolution that has occurred during the past 30 m.y. The important volcanic provinces are: the Pliocene-Holocene Gulf of California dacite; the Pliocene-Holocene west Baja California alkaline basalt-andesite; the Trans-Mexican Volcanic Belt; the “proto-Gulf” basalt from the coast of Nayarit; the late Miocene alkaline basalt of the Commondu Formation found in the Peninsula; the late Miocene basalt-andesite-rhyolite rocks straddling the northern half of the Gulf; the 18- to 22-m.y.-old hornblende andesite belt in the Peninsula of Baja California and the central coast of Sonora; and the Oligocene–early Miocene basalt-rhyolite belt, largely east of the Gulf. Tectonics interpretation suggests that the subduction plane moved westward between Oligocene and middle Miocene time and that active calc-alkaline volcanism continued over a broad area around the northern Gulf even after the trench west of Baja California had been annihilated.

The record of Cenozoic volcanism around the Gulf of California

Coastal Sonora between Puerto Lobos and Bahia Kino can be subdivided into four structural-petrographic subprovinces: an inland subprovince in which unmetamorphosed upper Precambrian and Cambrian strata rest on older Precambrian gneiss and three other subprovinces in which Cenozoic volcanic strata rest on metamorphosed strata of post-Precambrian age intruded by granitic rocks of Mesozoic age. One of these latter subprovinces displays basin-and-range fault blocks; a second has northwest-trending strike-slip(?) faults; and the third, Isla Tiburon, shows structure related to the Neogene dilation of the Gulf of California depression. The pregranitic rocks include upper Precambrian and Cambrian carbonate rocks, a chert-graywacke-volcaniclastic sequence of probable Carboniferous age, and volcanic-volcaniclastic rocks of Jurassic age. The granitic rocks range from gabbro to granite and have K-Ar cooling ages of from 91 to 30 m.y. Dikes of basaltic to dacitic composition and quartz porphyritic bodies of late Mesozoic or early Cenozoic age cut the granitic rocks. The lowermost Cenozoic volcanic strata (pre–22 m.y. B.P.) are predominantly composed of rhyolite and basalt. These are followed by a sequence of predominant andesite (∼20 to 18 m.y. old), a sequence of partially marine conglomerate and pyroclastic deposits, and a widespread, predominantly rhyolitic sequence (∼14 to 10 m.y.). All strata 10 m.y. old or older are involved in basin-and-range–type tilting. Volcanic strata less than 8 m.y. old are nearly flat-lying.

Reconnaissance geology of coastal Sonora between Puerto Lobos and Bahia Kino

The results of 63 new radiometric K-Ar and Rb-Sr measurements on metamorphic minerals from the internal units of the Western Alps show Hercynian, Permian, as well as three Alpine age groups. The first of the Alpine ages cover the period between 78 and 100 m.y. and refer to high pressure parageneses. The second group comprises K-Ar 39 to 50 m.y. ages; these values are affected by some inherited argon, as indicated by Rb-Sr measurements which point to 35–36±4–5 m.y., i.e. similar to the culmination of the Lepontine crystallization. The final group includes 15 to 30 m.y. ages. It is not yet clear which geologic processes have led to this isotope re-equilibration. Large amounts of inherited argon have been found in Alpine metamorphic minerals of the basement rocks.

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K-Ar and Rb-Sr Dating of Blue Amphiboles, Micas, and Associated Minerals from the Western Alps

More than 200 K-Ar apparent ages have been determined from minerals from the Peninsular Ranges batholith of southern California and northern Baja California. In general, the apparent ages show a progressive decrease from about 120 m.y. in the southwestern (coastal) pan of the batholith to less than 70 m.y. in the northeastern (desert) part. The gradients for biotite and hornblende ages can be represented by contours of equal ages. Both concordant and discordant hornblende-biotite pairs and minerals, from a variety of plutonic and metamorphic rock types, share in the apparent-age gradient. Ages for hornblende average 5 m.y. older than the ages for coexistent biotite. Isotopic U-Pb and Pb-α measurements on zircon indicate ages greater than those calculated from K-Ar ratios of hornblende or biotite. It is believed that in the Peninsular Ranges province, the U-Pb ages for zircon approximate the ages of emplacement, whereas concordant K-Ar ages may or may not approximate the ages of emplacement, depending on the depth of emplacement and the rate of uplift and denudation.

Daniel Krummenacher

Papers

The record of Cenozoic volcanism around the Gulf of California

Reconnaissance geology of coastal Sonora between Puerto Lobos and Bahia Kino

K-Ar and Rb-Sr Dating of Blue Amphiboles, Micas, and Associated Minerals from the Western Alps

K-Ar Apparent Ages, Peninsular Ranges Batholith, Southern California and Baja California

Isotopic composition of argon in modern surface volcanic rocks