International Geology Review

Jachens, R. C.; Zoback, M. L.

doi: 10.1080/00206819909465139pmid: N/A

Recently acquired high-resolution aeromagnetic data delineate offset and/or truncated magnetic rock bodies of the Franciscan Complex that define the location and structure of, and total offset across, the San Andreas fault in the San Francisco Bay region. Two distinctive magnetic anomalies caused by ultramafic rocks and metabasalts east of, and truncated at, the San Andreas fault have clear counterparts west of the fault that indicate a total right-lateral offset of only 22 km on the Peninsula segment, the active strand that ruptured in 1906. The location of the Peninsula segment is well defined magnetically on the northern peninsula where it goes offshore, and can be traced along strike an additional ˜6 km to the northwest. Just offshore from Lake Merced, the inferred fault trace steps right (northeast) 3 km onto a nearly parallel strand that can be traced magnetically northwest more than 20 km as the linear northeast edge of a magnetic block bounded by the San Andreas fault, the Pilarcitos fault, and the San Gregorio-Hosgri fault zone. This right-stepping strand, the Golden Gate segment, joins the eastern mapped trace of the San Andreas fault at Bolinas Lagoon and projects back onshore to the southeast near Lake Merced. Inversion of detailed gravity data on the San Francisco Peninsula reveals a 3 km wide basin situated between the two strands of the San Andreas fault, floored by Franciscan basement and filled with Plio-Quaternary sedimentary deposits of the Merced and Colma formations. The basin, ˜1 km deep at the coast, narrows and becomes thinner to the southeast along the fault over a distance of ˜12 km. The length, width, and location of the basin between the two strands are consistent with a pull-apart basin formed behind the right step in the right-lateral strike-slip San Andreas fault system and currently moving southeast with the North American plate. Slight nonparallelism of the two strands bounding the basin (implying a small component of convergence with continued strike-slip movement) may explain the progressive narrowing of the basin to the southeast and the puzzling recent uplift of the Merced Formation in a predominantly extensional (pull-apart basin) setting. The 1906 San Francisco earthquake may have nucleated within the step-over region, and the step-over places a strand of the San Andreas fault 3 km closer to downtown San Francisco than previously thought.

Mukherjee, Amalbikash; Jana, Prasun; Das, Subhasish

doi: 10.1080/00206819909465140pmid: N/A

Two anorthosite massifs in the Eastern Ghats share similar structural constitutions, internal differentiation histories, and overall thermal-tectonic patterns of evolution. The (1) circular to near-circular structural patterns both inside and close-to-border outside of the plutons; (2) the merging of these structures with the straight to gently flowing, essentially unidirectional structural trend of the granulite basement within short distances from the border of the plutons; (3) the increase of strain intensity near the border of the plutons; (4) the small but recognizable differences in the dip of the anorthosite flow layers; and (5) the foliation of the granulites, all are strong indications that these anorthosites were emplaced as syntectonic diapirs. In both massifs, anorthosite is by far the dominant lithology, noritic varieties being subordinate and generally formed as narrow dike-like bodies and pods, pockets, and irregular patches near the border. The striking features of the whole-rock and mineral chemistries of these massifs are increasing Fe, Ti, Mg, P, and REE, and decreasing Si and Al from the leucoanorthosites to the noritic rocks and wide Mg-Fe variation in the pyroxenes, in contrast to a relatively uniform plagioclase composition. These variations may be the result of initial differentiation in a layered complex serving as a precursor to the anorthosites, through modal sorting, rhythmic layering and cryptic changes, and a subsequent mixing of the modally sorted and cryptically enriched layers at different stages of anorthosite diapirism. Diapiraureole structural relationships, petrology, and thermobarometry suggest a moderately steep, counterclockwise cooling and exhumation path for both massifs to the P-T range: 5 to 6 kbar and 600 to 700°C. Such considerations, supplemented by a conductive cooling model for anorthosite emplacement and its subsequent evolution, lead to a branched path as the essential topology of the P-T-time history of the anorthosite-granulite associations of the Eastern Ghats. An important corollary of this inference is that a cycle of prograde and retrograde metamorphism of the aureole rocks—before and after the anorthosite invasion, respectively—is an essential consequence of the anorthosite emplacement.

Lefort, J. P.

doi: 10.1080/00206819909465141pmid: N/A

A new image of the French continental crust between Brabant (Belgium) and the Basque province of Spain is presented on the basis of considerable recent geological and geophysical information as well as the compilation and reInterprétation of previously available data. The resulting section, which shows the main basement structures to a depth of 45 km, also is the first nonspeculative image of the westernmost part of the Variscan orogen. The French Global Geoscience Transect reveals a complete picture of this orogen between its remnant root and the surface. The divergent thrusts are bounded on the north and in the south by the old Brabant and Ebro-Aquitaine cratons, respectively; these thrusts also involve two previous plate boundaries. The lower part of the orogen is limited by a layered lower crust, probably of Permian age. Near the surface the Hercynian orogen is buried—near the northern end of the transect by the Paris Basin, which can be considered an eastward extension of the English Channel, and in the south by the South Armorican continental margin, which makes a transition between the oceanic crust of the Bay of Biscay and the axis of the Variscan orogen. In this area, the deep Parentis graben is located at the site of pronounced crustal thinning, since only 7 km of Hercynian crust are now preserved.

Brocher, Thomas M.; Brink, Uri S. TEN; Abramovitz, Tanni

doi: 10.1080/00206819909465142pmid: N/A

Compilation of seismic transects across the central and northern California Coast Ranges provides evidence for the widespread tectonic emplacement beneath the margin of a slab of partially subducted oceanic lithosphere. The oceanic crust of this lithosphere can be traced landward from the former convergent margin (fossil trench), beneath the Coast Ranges, to at least as far east as the Coast Range/Great Valley boundary. Comparison of measured shear and compressional wave velocities in the middle crust beneath the Hayward fault with laboratory measurements suggests that the middle crust is a diabase (oceanic crust). Both of these observations are consistent with recent models of the high heat flow and age progression of Neogene volcanism along the Coast Ranges based on tectonic emplacement (stalling) of young, hot oceanic lithosphere beneath the margin, but appear to contradict the major predictions of the slab-gap or asthenospheric-window model. Finally, the Neogene volcanism and major strike-slip faults in the Coast Ranges occur within the thickest regions (>14 km thick) of the forearc, suggesting that the locations of Cenozoic volcanism and faulting along the margin are structurally controlled by the forearc thickness rather than being determined by the location of a broad slab gap.

Xuechenc, Yuan

doi: 10.1080/00206819909465143pmid: N/A

The geoscience transect from Altay to Taiwan has a total length of ∼4,334 km. The transect begins along the northwestern border of China and ends in the Philippine Sea basin to the east of Taiwan. Systematic seismic soundings, magnetotelluric soundings, and heat-flow, gravity, aeromagnetic, and geological studies were conducted along the transect. The main tectonic features that are suggested by geological and geophysical data collected along the transect can be understood in the context of the tectonic history of the craton. The tectonic evolution of China can be divided into three stages— (1) development of continental crust during the Archean era with separate cratonic blocks (the West China and the East Asia cratons) that subsequently were juxtaposed to form the Old China craton; (2) remobilization and breakup of the continental land mass during the Paleozoic period, forming the Northern and the Southern China plates; and (3) development of three tectonic domains (the eastern extensional domain, the Qinghai-Tibet compressional domain, and the relatively stable tectonic domain between them) of continental China during the Mesozoic to Cenozoic period, subject to the effects of Pacific and Indian plate motions. The convergence between cratonic blocks in China resulted in wedge structures. The deformation of lithosphere began with the mantle lithosphere, followed by the crust. Where cratonic lithosphere was compressed, such as in the West China craton, the mantle lithosphere folded and the crust was imbricated along thrusts forming major nappe structures along zones of weakness. Where a relatively weak accreting lithosphere, such as Tibet, was compressed, the mantle lithosphere shortened and thickened, with decoupling between the crust and the mantle lithosphere. Where cratonic lithosphere was extended, such as in the East Asian craton, tectonic thinning resulted in formation of a rhombochasm basin or rift. The inhomogeneity of mantle material has exerted an important effect on surface structure, with the stable region at the surface frequently coinciding with mantle structure. The boundaries of mantle features coincide with zones of mineralization that contain deposits with economic potential.