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Arma 08-357 Franciscan Complex, California ARMA 08-357 FRANCISCAN COMPLEX, CALIFORNIA: PROBLEMS IN RECOGNITION OF MELANGES, AND THE GAP BETWEEN RESEARCH KNOWLEDGE AND PROFESSIONAL PRACTICE. John Wakabayashi, Department of Earth and Environmental Sciences, California State University, Fresno, CA 93740, USA Copyright 2008, ARMA, American Rock Mechanics Association This paper was prepared for presentation at San Francisco 2008, the 42nd US Rock Mechanics Symposium and 2nd U.S.-Canada Rock Mechanics Symposium, held in San Francisco, June 29-July 2, 2008. This paper was selected for presentation by an ARMA Technical Program Committee following review of information contained in an abstract submitted earlier by the author(s). Contents of the paper, as presented, have not been reviewed by ARMA and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT The Franciscan Complex comprises the largest bedrock unit of the California Coast Ranges, so it impacts many engineered projects in coastal California, but the ignorance of basics of Franciscan geology plagues many projects. The stratigraphic name given by engineers or geologists in many projects, 'Franciscan Formation', a term abandoned by research geologists nearly four decades ago, illustrates some of this misunderstanding. The "layercake" stratigraphy inferred by 'Formation' differs markedly from reality, for faults separate most Franciscan rock units, and chaotic block-in-matrix units, known as melanges, are widespread. While the layercake view results from a pre-plate tectonic understanding, others mistakenly believe that the entire Franciscan is a melange. Research shows that the Franciscan consists of both intact units ("coherent") and melanges. Franciscan geology knowledge continues to progress, but researchers have virtually no understanding of the engineering significance of these rocks, while the engineering community remains largely unaware of the current research knowledge. There is substantial potential benefit from better communication between the engineering and geologic research communities. Education is also critical, for as undergraduate field studies continue to diminish in geology programs, most existing field classes tend to focus on areas of layercake stratigraphy and excellent exposure, providing dismally poor preparation for professionals who might work in areas such as coastal California. INTRODUCTION Melange units, composed of a weak matrix melange is present or not [12]. It is here where a huge enclosing stronger blocks, make up a significant gap exists between the state-of-knowledge of the proportion of the world's bedrock in areas of former research geologic community, in which melanges subduction zones [1,2]. Recognition of melanges is have been common knowledge for at least 40 years, important for engineering geology and related and the applied geoscience community (the term I will engineering because their physical properties differ use to define engineering and environmental geology markedly from simple bedded or isotropic rock and related engineering fields) where it is still [3,4,5,6). Because of their discontinuous, rather than common to think of all geologic units as either being continuous structure and/or stratigraphy, particular homogeneous and isotropic (as an idealized granite) or care must be taken in terms of interpretation of field a layered sedimentary stack (herein termed "layercake data, be it the interpolation between or extrapolation stratigraphy" as one might expect in the Grand from boring or surface geologic information, or the Canyon). In this paper I will discuss misconceptions scaling between outcrop and boring levels of about melanges (or failure to recognize them) within observations to the scale of engineering interest the context of the evolution of geologic thought on the [6,7,8,9,10,11]. These studies [3-11] provide a good Franciscan Complex of California, one of the world's framework for engineering characterization of most famous localities for melanges and also a melanges, but a non-trivial step is to recognize that a geologic unit upon which a large population lives and builds many engineered structures. I will then offer about the state of field studies in geologic curricula some tips for identifying melanges in the field and and its impact on melange characterization, with some show how melanges have a number of characteristics recommendation for better communication between that can be recorded that may be useful in engineering the academic ("research") and applied geosciences. characterization. I will conclude with some concerns the concept became widely accepted throughout the geologic community [2], or more accurately, the MELANGES AND THE FRANCISCAN research geologic community. Front and center in COMPLEX: THE OUTCROPS DID NOT Hsü's work was the Franciscan Complex of coastal CHANGE, BUT OUR VIEWS DID California (Fig. 1). Today we know the Franciscan Complex as the world's type subduction complex, because the pioneers of the Plate Tectonic Revolution Melanges were first recognized by Greenly [13] first used the Franciscan to illustrate the connection who coined the term based on exposures in Wales, between subduction and high-pressure/low- although chaotic, non-stratiform (ie., not layercake) temperature metamorphism [15], and the tectonic rocks had been noted decades earlier by geologists shuffling of oceanic and continentally derived working in the Alpine region of mainland Europe [2]. materials [16]. The melange concept, while not In spite of Greenly's work, and research of others originally envisaged in a subduction context by (whose observations and interpretations were parallel Greenly or Hsü, now became part of the plate tectonic and independent of those of Greenly) conducted in paradigm [16], so the Franciscan became triply other parts of the world in the decades following recognized for three critical components in Greenly's work, nearly five decades elapsed before development plate tectonic theory. Hsü [14] published his historic paper on melanges and We now view the Franciscan as a product of Investigations in the late 60's and early 70's showed subduction that began about 165 million years ago and that the Franciscan consists of both melange terranes continued for over 140 million years [17]. Faults and so-called "coherent" (non melange) terranes and bound major units, and the age of incorporation into that the proportion of melange to coherent terranes the Franciscan youngs structurally downward (across varied spatially within the Franciscan [21]. Workers major faults), as a consequence of progressive such as Blake et al.[22,23] and Wakabayashi [17,18] offscraping of material from the downgoing plate extended the studies of Maxwell [21] and his students, [17,18]. This contrasts markedly with a so that the dual melange and coherent components sedimentary/stratigraphic view held prior to the were mapped throughout the Franciscan. acceptance of plate tectonic theory and the melange As pointed by Wakabayashi and Medley [12], concept. Although the spatial distribution of ages outcrops have not changed over the past 40 years, but made little sense from a stratigraphic standpoint (ie a prevailing theories have, so that the same set of sedimentary sequence that youngs progressively outcrops have been or are interpreted three ways: (1) upward), researchers resolutely tried to force fit the stratigraphic (pre-1968), (2) all melange (the next Franciscan into a layercake stratigraphic model [19]. step, post-1968), (3) mixed coherent and melange The term "Franciscan Formation", a formal (Fig.2). This points out an often unappreciated aspect stratigraphic term that most researchers discarded in of field geology: Field studies do not represent a 1968, represents the pre-melange/pre-plate tectonic purely objective, non-analytical "stamp collecting" view of the Franciscan Complex as a sedimentary exercise as wrongly believed (and even ridiculed) by sequence. many field-phobic academicians. The latter see little Following the recognition of melanges, some value in geologic mapping, because the geologic maps workers viewed the entire Franciscan as a melange exist for every part of the Earth (at least above sea [20], and indeed the informal name of the entire level). Rather, field geology has a highly interpretive complex as the "Franciscan melange" still finds aspect to it, with our drawing of contacts tied to common usage. Such a view of the Franciscan can prevailing geologic thinking. Why? We never have a have as disastrous consequences in engineering fully 3-dimensional set of field data without gaps. investigations as the mistaken view of layercake Interpolation and extrapolation comprise a critical part stratigraphy, because some default the Franciscan to of our field mapping and the prevailing theories an intractably chaotic unit from which one cannot dictate how we "connect the dots". extract any meaningful spatial field data. This summary of the evolution of thought on (consistent with the "Franciscan Formation" view), melange and the Franciscan Complex illustrates how continue
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