Howard Day - Research Interests
Petrology and Tectonics

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    The major focus of my multidisciplinary research is to understandthe origin and controls on metamorphism in light of our knowledge of the thickening and exhumation histories of mountain belts. In addition, I am collaborating with a colleague in Epidemiology and Preventive Medicine to investigate possible hazards posed by naturally occurring amphibole asbestos in California. My students and I engage in a wide variety of field, experimental, thermodynamic, geochemical and geochronological studies that are illustrated in the attached lists of publications and theses supervised. In addition to thesis projects that I am supervising, I have three studies under way : (1) Low-grade metamorphism of mafic rocks, Sierra Nevada, California; (2) Implications of low-grade blueschists for exhumation and uplift of the Franciscan Complex, California; (3) Origin and controls on the occurrence of tremolite asbestos, Sierra Nevada foothills, California

The Sierra Nevada, California. The observed metamorphism of the rocks is one of the few constraints on their inferred thermal history, but almost no information exists about whether or not the metamorphism in volcanic and plutonic arcs at collisional margins supports the thermal evolution implied by numerical models (Oxburgh and Turcotte, 1970). I have previously proposed (Day et al.,1988; Day, 1992) that much of the metamorphism recorded in the rocks of the northern Sierra Nevada may be inherited from their pre-orogenic history as volcanic arcs. If so, the metamorphism may owe much to the active constructional phases of arc volcanism as well as to tectonic and thermal effects of subsequent accretionary or orogenic processes. We are studying the metamorphism of greenschist and sub-greenschist facies metavolcanic rocks in a Jurassic volcanic arc that extends for about 300 km along the western margin of the Sierra Nevada foothills in order to determine the thermal structure, to constrain the thermal history, and to evaluate this hypothesis (Springer et al., 1992; Beiersdorfer and Day, 1995; Fagan and Day, 1997; Springer and Day, 2002; Day and Springer, in preparation).

Franciscan Complex, California. The metamorphic grade of the Franciscan subduction complex increases eastward across the Coastal, Central and Eastern Franciscan Belts from zeolite, to prehnite-pumpellyite to glaucophane-lawsonite facies. Although exotic blocks of high grade blueschists have attracted the attention of numerous workers, very little detailed work constrains metamorphic conditions in the metagraywackes that dominate the complex. Our work in the Eastern Franciscan Belt (Bröcker and Day, 1995) suggests that mineral assemblages are consistent with a small range of P-T conditions (240 - 280 °C, 6.5 - 7.5 kbar) across its entire 50 km width and raises the possibility that rocks in the eastern part of the belt with no pumpellyite and coarse lawsonite may have experienced larger fluxes of water. We plan to continue studying the metamorphic history of these low-grade blueschists in an effort to constrain models for their exhumation.

Amphibole Asbestos in the Sierra Nevada Foothills. Asbestos is common in California and the geological factors that control the occurrence of the most common, chrysotile form are well-known. Recent discoveries of relatively uncommon tremolite asbestos in rapidly developing areas of El Dorado County have created significant public concern about the potential for environmental exposure, especially because amphibole asbestos is widely considered to be more toxic than serpentine varieties. In El Dorado County, our preliminary work suggests that tremolite asbestos occurs primarily as slip- and cross-fibers where amphibolites are deformed in a major fault zone and may also occur in serpentinites. These occurrences suggest the hypothesis that faulted greenstones and amphibolites may indicate an enhanced potential for finding tremolite asbestos. Because amphibolites and related rocks are common in the Sierra Nevada, the factors that govern the occurrence of tremolite asbestos are important environmental concerns. Because the occurrence of tremolite asbestos is poorly documented, the main goal of our current studies is to identify more clearly, the rocks, structures and alteration features with which tremolite asbestos is commonly associated. Having done case studies, it will be possible to identify those geological features that serve best as the proxies for potential exposure to amphibole asbestos.

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