Do swelling clays influence the behavior of the San Andreas Fault? New results from the San Andreas Fault Obervatory at Depth (SAFOD) drill hole

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Diagenetic reorientation of phyllosilicate minerals in Paleogene mudstones of the Podhale Basin, southern Poland

We used high-resolution X-ray texture goniometry to quantify changes in the mm-scale orientation of phyllosilicate minerals in a suite of Paleogene mudstones from the Podhale Basin in southern Poland. The sample set covers an estimated range of burial depths between 2.4 and 7.0 km, corresponding to a temperature range of 60-160°C. Although mechanical compaction has reduced porosities to ∼10% in the shallowest samples, the phyllosilicate fabric is only modestly aligned. Coarser-grained (>10 μm) detrital chlorite and mica appear to be more strongly aligned with (001) parallel to bedding, suggesting their deposition as single grains rather than as isotropic flocs or aggregates. From 2.4 to 4.6 km, R0 illite-smectite with 40-50% illite layers changes to R1 illite-smectite with 70-80% illite layers. At the same time kaolinite is lost and diagenetic chlorite is formed. The mineralogical changes are accompanied by a strong increase in the alignment of illite-smectite, chlorite, and detrital illite, parallel to bedding and normal to the presumed principal effective stress. We propose that the development of a more aligned I-S fabric results from the dissolution of smectite and the growth of illite with (001) normal to the maximum effective stress. Water released by illitization may act as a lubricant for the rotation of all platy minerals into nanoporosity transiently formed by the illitization reaction. At greater depths and temperatures, further illitization is inhibited through the exhaustion of K-feldspar. After the cessation of illitization, a further 2.4 km of burial only results in a small increase in phyllosilicate alignment. At such small values for porosity and pore size, increasing stress does not substantially reorient phyllosilicates in the absence of mineralogical change

Two stage tectonic history of the SW Amazon craton in the late Mesoproterozoic: identifying a cryptic suture zone

The history of the SW Amazon craton during late Mesoproterozoic times is marked by two separate tectonic events, the first related to collision with southern Laurentia and the second caused by suturing of the Paragua craton. The polycyclic basement rocks of the SW Amazon craton exposed in the Brazilian state of Rondônia were deformed at lower amphibolite conditions during early Grenville times (ca. 1.2–1.15 Ga). This deformation episode is the last of several tectonometamorphic events that affected the granitoid rocks of the Amazon basement throughout the Mesoproterozoic. The southern margin of the Amazon craton during late Mesoproterozoic times is defined by the E–W trending Nova Brasilândia metasedimentary belt, where upper amphibolite to granulite facies rocks from a younger (ca. 1.09 Ga) collisional event are preserved. Temperature–time (T–t) paths for each domain (craton and metasedimentary belt) are constructed using U–Pb, 40Ar/39Ar, and Rb–Sr data for minerals with different blocking temperatures. The T–t paths demonstrate no overlap in the timing or spatial distribution of tectonic and metamorphic activity. The separate cooling histories indicate the presence of a major tectonic boundary between the polycyclic basement rocks and the metasedimentary belt. This structure marks the suturing of the Paragua craton in the late Mesoproterozoic and is evidence that the accretionary history of the present outline of the Amazon craton was completed during the final stages of the amalgamation of Rodinia

TEM and AEM constraints on the origin of chlorite-mica stacks in slates, central Wales, U.K.

Master of ScienceGeologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/114635/1/39015028867300.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/114635/2/39015028867300.pd