By John Wakabayashi and Edmund W.· WAKABAYASHI AND MEDLEY: GEOLOGICAL CHARACTERIZATION OF MELANGES

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Transcript of By John Wakabayashi and Edmund W.· WAKABAYASHI AND MEDLEY: GEOLOGICAL CHARACTERIZATION OF MELANGES

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    WAKABAYASHI AND MEDLEY: GEOLOGICAL CHARACTERIZATION OF MELANGES FOR PRACTITIONERS

    FELSBAU 22 (2004) NR. 5

    Geological Characterizationof Melanges for Practitioners

    By John Wakabayashi and Edmund W. Medley

    Geologische Charakterisierung vonMelangen fr den Fachmann

    Unter Melangen versteht man ungeordnete Einheiten vonFels, der aus einer Mischung aus Felsmasse mit niedriger Fes-tigkeit und harten Gesteinsblcken besteht. Obwohl Melangenweltweit anzutreffen sind und Gefgegeologen seit Jahrzehn-ten damit vertraut sind, wissen geotechnische und geologi-sche Fachleute nicht Bescheid ber die neuesten geologischenKonzepte bezglich Melangen und deren ingenieurtechni-schen Stellenwert; diese Unwissenheit resultiert in kostspieli-gen Planungsfehlern und unwillkommenen berraschungenwhrend der Baudurchfhrung. Basierend auf Erfahrungenin Franciscan Melange werden im Folgenden Identifizie-rungsmerkmale fr Melangen und Zuordnungsmerkmale frexterne und interne Details innerhalb der Melangeeinheiten

    Many geotechnical engineers and engineer-ing geologists (practitioners) believe thatsimply drawing contact lines or other features ona geologic map or cross section produces repre-sentative characterizations of the subsurface.But the results of most investigations are oftengrossly incorrect when working with melanges(from French: mlange, or mixture). Melangesare mappable but discontinuous, often chaoticrock units, composed of mixtures of often perva-sively sheared, weak matrix enclosing a variety

    vorgestellt und auch Richtlinien angeboten, die bei der Erstel-lung einer systematischen ingenieurtechnischen Charakteri-sierung von Melangen als Hilfestellung dienen.

    Melanges are chaotic bedrock units consisting of mixtures ofweak matrix and stronger blocks. Although melanges areglobally common and have been familiar to structural geolo-gists for decades, many geotechnical and geological practi-tioners are unaware of recent geological concepts of me-langes and their engineering significance: such ignoranceresults in costly design errors and unwelcome surprises dur-ing construction. Based on experience with Franciscan com-plex melanges, criteria are provided for identifying me-langes and mapping external and internal details within me-lange units, and guidelines offered for developing orderlyengineering characterizations in melanges.

    Fig. 1 Principalmappable engineering

    geology characteris-tics of a melange.Bild 1 Ingenieur-

    geologische Grund-satzmerkmale fr

    die Kartierung vonMelangen.

    of stronger blocks of different lithologies and size(Figure 1). Melanges can form as submarinelandslides (olistostromes), by tectonic processesas fault rocks, or by a combination of the twoprocesses (1, 2, 3). The origins of melanges inter-est research geologists to the point of producingseveral thousand papers, but from an engineer-ing viewpoint, the processes all produce mix-tures of weak matrix and stronger blocks.

    Despite more than 40 years of geological un-derstanding of melanges and their origins, me-

  • WAKABAYASHI AND MEDLEY: GEOLOGICAL CHARACTERIZATION OF MELANGES FOR PRACTITIONERS

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    langes are unknown or misinterpreted by manypractitioners. Costly and imprudent consequen-ces derive from practitioners errors in the mis-characterization of melange structures as layercake strata, or incorrectly describing melangesas soil containing boulders, or miscellaneoussoils, for example. To confuse matters, the wordmelange is also used by some practitioners tomean any mixture of rock and soil materials,which is inappropriate given the long-used geo-logical meaning. Furthermore, some practitio-ners declare melanges as impossible to charac-terize and recommend geotechnical design bebased on the properties of the weak matrix. Suchsimplification can lead to too-conservative andinappropriate designs and costly surprises andunsafe ground failures during construction.

    Researchers have recently developed ap-proaches to the engineering characterization ofmelanges and other bimrocks (block-in-matrixrocks) (4, 5, 6, 7, 8). Medley (9) defined bimrocksas geological mixtures of geotechnically signifi-cant blocks of rock within weaker, bonded rockmatrices. Geotechnical significance means thatthere is sufficient mechanical contrast betweenthe blocks and the matrix to force failure sur-faces to negotiate around the blocks in tortuousfashion; and that there is a sufficient size andnumbers of blocks to affect the overall mechani-cal properties of the geological mixture.

    The authors of this paper, a Structural Geolo-gist (Wakabayashi) and a Geological Engineer(Medley), consider it necessary to apply bothfirst-order geologic field observations and quan-titative engineering methods to the characteriza-tion of melange once it is identified, and to thatend guidance for the identification, mapping,and characterization of melanges by practicinggeologists and engineers is provided.

    Melanges geologic conceptsand misconcepts

    A brief history of styles of mapping of melangesof the Franciscan Complex (the Franciscan) ofcoastal California provides examples of how geo-

    Fig. 2 Hypothetical geologic maps showing how prevailing geologic theories influencehow contacts are drawn on maps. Map A: outcrops. Map B: geology interpreted asstratabound layers. Map C: entire area interpreted as melange. Map D: area composedof both melange and coherent thrust sheets.Bild 2 Theoretische geologische Karten, die zeigen, wie vorherrschende Theorienin der Geologie die Darstellung von Kontaktflchen beeinflussen. Abbildung A: Auf-schlsse. Abbildung B: Geologie interpretiert als stratigraphische Schichten. Abbil-dung C: Gesamtflche interpretiert als Melange. Abbildung D: Flche besteht ausMelangen und zusammenhngenden berschiebungsdecken.

    logic knowledge influence how contacts aredrawn on geologic maps and cross sections (Fig-ures 2 and 3). The Franciscan hosts some of theworlds most famous melanges (10,11), as wellas engineering projects that have suffered prob-lems because of their chaotic conditions.

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    WAKABAYASHI AND MEDLEY: GEOLOGICAL CHARACTERIZATION OF MELANGES FOR PRACTITIONERS

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    (Figure 2C). Some geologists even classified theentire Franciscan as one large melange bodyand neglected the internal details, a simplifica-tion that can still be encountered in some aca-demic research papers. By the mid-1970s, geo-logists such as Maxwell (14), began to discrimi-nate the Franciscan into coherent units, fault-bounded sheets of intact non-melange Francis-can geologic rock units; and discrete melangeunits. This concept was expanded in the 1980sas the terrane concept explained the complextectonic jigsaw of the North American Cordillera,with terranes being the individual puzzle pieces(15, 16). The Franciscan was then called theFranciscan Assemblage. Although the terraneconcept led to improved categorization of coher-ent units, identification of melanges regressed,as all Franciscan melange bodies were thencollected into one Central Terrane, based onan interpretation that all Franciscan melangesformed at the same time.

    Wakabayashi (17, 18, 31) expanded on Max-wells (14) concepts, by delimiting separate Fran-ciscan melanges and coherent units, and thencorrelating melange units and coherent units todiscrete structural levels within stacks of thrustnappes. Accordingly, an up-to-date structuralgeologist mapping Franciscan outcrops todaymight find and map both coherent and melangeunits, as shown schematically in Figure 2D. Thismodern approach reflects the appropriate cur-rent Complex suffix to Franciscan Complex.

    During a century of geologic mapping in theFranciscan Complex, the rocks have not chang-ed, but the geologic maps have changed dra-matically. Although geologists long ago recog-nized melanges and how to map them, manypractitioners still treat melange bedrock as bed-ded geologic units. Others, also incorrectly, con-sider entire regions to be melange. Both groupsthus fail to secure the geologic information thatcan be collected and used for engineering pur-poses.

    When mapping, geologists most commonlyencounter the erosion-resistant blocks of a me-lange (Figure 4), rather than the weak matrix,which easily erodes and seldom forms observ-able outcrops except in bare natural slopes, orartificial cut slopes. Hence, prior to the 1960smost geologists mapped areas with scatteredoutcrops of sandstone, chert, basalt, or otherrock types (Figure 2A) and then interpreted themelanges into the layer-cake continuous strati-graphic framework of the Franciscan Forma-tion (12) (Figure 2B). If ignorant of melanges,many practitioners still map this way.

    Greenly (13) first christened chaotic units inNorth Wales as Autoclastic Mlange but wide-spread recognition of melange structures did notfollow until Hs (10) formalized the melangeconcept. Melanges were then recognized as glo-bally common, particularly in ancient orogenicbelts associated with old subduction zones (2,11). Following the acceptance of Hss (10) me-lange concepts, geologists mapping in the Fran-ciscan and similar geologic confusion mappedoutcrops as blocks in the usually unseen matrix

    Fig. 4 A view of landscape underlain by serpentinite matrix and shale matrix melange;Tiburon Peninsula, San Francisco area, California.Bild 4 Landschaftsansicht mit darunterliegender Serpentinit-Matrix und SchiefersteinMatrixmelange; Tiburon Halbinsel, Bezirk San Francisco, Kalifornien.

    Fig. 3 Cross section-al diagrams showingthe difference betweenassuming stratigraphiccontinuity and assum-ing melange structurewhen interpretingborehole data.A: borehole observa-tions. B: Cross-sectionbased on interpreta-tion of strataboundgeo