Chronology of deformation of a Franciscan melange near San Francisco (California, USA)

Authors
Citation
P. Jeanbourquin, Chronology of deformation of a Franciscan melange near San Francisco (California, USA), ECLOG GEOL, 93(3), 2000, pp. 363-378
Citations number
35
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
ECLOGAE GEOLOGICAE HELVETIAE
ISSN journal
0012-9402 → ACNP
Volume
93
Issue
3
Year of publication
2000
Pages
363 - 378
Database
ISI
SICI code
0012-9402(2000)93:3<363:CODOAF>2.0.ZU;2-1
Abstract
The Permanente Terrane at pacifica (a part of the Franciscan Complex SW of San Francisco. California, USA) is a melange with big slices and blocks bou nded by a matrix of sheared polymictic breccia. This matrix results from a multistage evolution made of several episodes of fracturing, brecciation, s hear and fluid flow. The microstructures and the cement stratigraphy allow at least four stages of deformation to be separated: 1) a first group of pr e-melange structures lacking in silica/calcite cements testifying to brecci ation and sill-dike injection (syn-depositional to early accreation?), IIa and IIb) two important groups of accretionnary deformations comprising a wi de range of structures and several episodes of cement (IIA with K-feldspar) , and III) a last group of post-melange deformation structures clearly over printing older structures (late strike-slip motion of the San Andrea Fault system). The observations indicate that hydrothermal activity was associate d with melange development. This activity may be related to the late Cenozo ic hydrothermal activity in the region or an earlier hydrothermal history, possibly as old as the beginning accretion of the Franciscan Complex. In ad dition, the data presented indicate that the fluid pressure was high, influ encing the mechanisms of deformation (hydrofracture and particulate now) an d they demonstrate the tectonic origin for the melange breccia although its texture is that of a pebblymudstone.