Introduction: Characteristics and tectonic settings of mélanges, and their significance for societal and engineering problems
Published:August 01, 2011
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John Wakabayashi, Yildirim Dilek, 2011. "Introduction: Characteristics and tectonic settings of mélanges, and their significance for societal and engineering problems", Mélanges: Processes of Formation and Societal Significance, John Wakabayashi, Yildirim Dilek
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Mélanges occur widely in collisional and accretionary orogenic belts around the world and represent mappable geological units consisting of blocks of different ages and origin, commonly embedded in an argillitic, sandy, or serpentinite matrix showing high stratal disruption and a chaotic internal structure. Understanding the mélange-forming processes and the significance of mélanges and related units in the geological record is of first-order significance in documenting the tectonic evolution of mountain belts; therefore, these chaotic rock units have attracted much attention in field-based structural studies since the nineteenth century. The term mélange has evolved to cover tectonic, sedimentary, and/or diapiric processes (Silver and Beutner, 1980) and tectonic settings of mélange formation, since its first use in 1919 by the British geologist Edward Greenly for the “Gwna Group” of the Mona Complex in Anglesey, north Wales (Greenly, 1919). In his classic work in the Franciscan Complex, Hsü (1968) proposed to use “mélange” only for tectonic mélanges and therein started a long-lived debate on the definition of the mélange term as well as on the processes involved in mélange formation. This controversy on the definition and formation of mélanges is livelier than ever in present time and requires a more systematic approach in mélange studies and better communication among the mélange researchers.
To this end, we organized and convened a topical session on mélanges during the Geological Society of America (GSA) Annual Meeting in Denver in 2007. The session was well attended by many international scientists from North America, Europe, and the Pacific Rim countries. This session was sponsored by the GSA International and the Structural Geology and Tectonics Divisions, and it brought together earth scientists in research communities from around the world, who do not ordinarily interact at the same meetings, in order to add an interdisciplinary dimension to our discussions on mélanges. It provided an excellent forum to discuss the new advances on the mélange concept as well as the diverse mélange types and mélange-forming processes based on some case studies. This Special Paper emanated from this successful GSA topical session. The benchmark GSA Special Paper 198 on mélanges, published more than 25 years ago (Raymond, 1984a), continues to influence research on mélanges, as testified by the frequent citation of the papers in it. The papers in the current volume build upon the solid foundation provided by the papers of Special Paper 198, as well as those published before and since, and include applications of new methodologies, exploration of new subjects, and a more international focus. The geographic spread of mélange localities around the world is also broader in parallel with the larger international authorship in the current volume.
Given the three-dimensional (3-D) complexity of mélanges, it is of little surprise that field studies form the foundation of all of the research presented in this volume. Beyond this common linkage, the papers here span a broad spectrum of features and focus. We streamlined the chapters according to a relative conceptual chronology related to mélange development (or impact) and related processes that begin with formation on the abyssal ocean floor (Part I), then proceed to subduction initiation (Part II), and accretionary wedge development and orogenic belt formation (Part III). The final section concentrates on the impact of mélanges on societies by way of their engineering properties (Part IV). We provide below first a synoptic summary of the chapters in this volume, and then we conclude with some general statements about mélanges and their significance.
PART I. MÉLANGE GENERATION IN OCEANIC FRACTURE ZONES IN ABYSSAL SETTINGS
The papers by Shervais et al. (Chapter 1) and Saleeby (Chapter 2) present integrated field, petrological, and geochemical data on serpentinite-matrix mélanges of different ages in California. Both studies provide field and geochemical evidence that block-in-matrix fabrics developed in serpentinite within an abyssal fracture zone. Shervais et al. present field evidence for subduction initiation along a fracture zone based on the presence of high-pressure, high-temperature (HP-HT) garnet-amphibolite blocks from a serpentinite shear zone structurally beneath unmetamorphosed ophiolitic rock of the Jurassic Coast Range ophiolite. The serpentinite matrix mélange may represent the original subduction interface. The authors suggest that exhumation was aided not only by the lower density of the serpentinite matrix compared to the top of the lower plate (oceanic crust) and the upper plate (suboceanic mantle), but was also driven by the positive volume change associated with serpentinite in a confined zone that might have served to force the serpentinite upward.
Saleeby presents evidence for subduction initiation along an abyssal fracture zone, based on his work in the Kings-Kaweah ophiolite belt of the southern Sierra Nevada. He suggests a long duration of time (~190 m.y.) between the early Ordovician abyssal ocean crust formation and the Permo-Carboniferous mélange development in an abyssal fracture zone environment. He argues that initiation of subduction along this fracture zone was followed by the development of supra-subduction zone igneous rocks. The evidence for subduction initiation comes from a ca. 255 Ma Sm/Nd age and (HP-HT) metamorphism of garnet-amphibolite blocks in a serpentinite matrix mélange. Saleeby argues that the emplacement of garnet-amphibolite blocks in the mélange was related to serpentinite diapirism through the upper plate of a subduction system, instead of back along the subduction interface through channel flow.
PART II. MÉLANGE FORMATION ASSOCIATED WITH SUBDUCTION INITIATION
The rock record of subduction initiation is evaluated in Myhill's paper (Chapter 3) on the metamorphic sole of the Vourinos and Pindos ophiolites in the western Hellenides (Greece). This topic of subduction initiation is also covered in the papers by Shervais et al. and Saleeby (Part I), and to a lesser extent, in the papers by Mori et al. (Mineoka belt, Japan, Chapter 4), and Wakabayashi (Franciscan Complex, California, Chapter 5) in Part III. Myhill presents detailed metamorphic evidence and argues that metamorphic soles, which are the thin high-grade metamorphic sheets commonly found beneath Tethyan ophiolites, were formed at lower pressures than commonly thought, and that they were therefore not necessarily associated with subduction initiation as has been widely assumed. He demonstrates that the high-temperature metamorphism of the metamorphic sole beneath the Mesohellenic ophiolites (Pindos and Vourinos) occurred during intra-oceanic thrusting (but not subduction) near a ridge crest and soon after subduction initiation, and that slices and blocks of the sole were incorporated into a subophiolitic mélange during further thrusting associated with ophiolite emplacement.
Mori et al. (Chapter 4) present a model for the complex tectonic evolution of the Mineoka ophiolitic mélange belt in the Boso Peninsula of central Japan, based on field relations, geochronology, and petrology. The evolution of this mélange includes early HP-HT, possibly associated with a subduction initiation event, followed by considerable deformation and mixing involving triple junction interaction and evolution. An early stage of ductile deformation at deep crustal levels was associated with synsubduction exhumation of metamorphic rocks, including HP-HT rocks, followed by a brittle phase of deformation developed at much shallower levels, as rocks were incorporated into the mélange zone. Their geochronological data suggest initiation of subduction at ca. 33–39 Ma, followed by development of the ophiolitic mélange between 15 and 18 Ma. Mori et al. propose that the present Izu arc may be an analog of the Mineoka mélange belt.
PART III. MÉLANGE DEVELOPMENT IN SUBDUCTION-ACCRETION COMPLEXES AND IN COLLISIONAL SETTINGS
In his process-oriented approach to delineating the tectonic settings of mélange formation, Wakabayashi (Chapter 5) divides and examines the classic Jurassic-Eocene Franciscan Complex mélanges of coastal California into distinct structural groups. The structurally highest mélanges in the Franciscan Complex may have formed at or shortly after subduction initiation, marking the initial subduction interface, whereas the mélanges separating coherent nappe sheets may represent later-developed paleo megathrust horizons within the accretionary prism. He argues that the large displacements associated with these nappes may have been largely accommodated along the borders of the mélanges rather than within them. He presents field and petrographic evidence supporting pretectonic sedimentary mixing of mélanges (development of block-in-matrix structure and introduction of exotic blocks), including those most likely to be classified as entirely tectonic mélanges (internappe mélanges). He shows the presence of “two cycle” high-P rocks, which were subducted to blueschist-facies depths, then exhumed and re-worked as sedimentary deposits, and then resubducted again to blueschist depths and exhumed.
Dangerfield et al. (Chapter 6) present structural, geochronological, and geochemical data from the Eldivan ophiolite, which occurs as a coherent block in the Ankara Mélange in north-central Turkey. The Ankara Mélange is part of the İzmir-Ankara-Erzincan suture zone and represents a classic Tethyan colored mélange. Dangerfield and her co-authors show development of the Eldivan ophiolite in a suprasubduction zone setting, followed by its integration into the Ankara Mélange as an oceanic block. Detrital zircon U/Pb analyses from the mélange and the overlapping epiclastic sandstones show that mélange development occurred between ~143 Ma and 105 Ma, consistent with the regional geochronological data. The authors argue that although the development of the İzmir-Ankara-Erzincan suture zone involved continental collision tectonics, its overall evolution resembles the formation of mélange terrains in the southwest Pacific rather than that of a Himalayan-type continental collision.
Erickson (Chapter 7) presents field, petrographic, and geochronological data from a Cretaceous sandstone-matrix olistostrome in the Franciscan Complex in northern California that collectively provide critical constrains on the exhumation age and patterns of various blocks and the depositional age of the olistostrome. The majority of these blocks themselves are Franciscan-derived. The evolution of the olistostrome includes initial subduction burial of the blocks; their subsequent exhumation and exposure as blueschist-, eclogite-, and amphibolite-facies blocks; their deposition some time after 83 Ma; and partial re-subduction to prehnite-pumpellyite facies conditions subsequently. As shown in the paper by Wakabayashi, Erickson's work also demonstrates the sedimentary reworking of previously metamorphosed Franciscan rocks, including “high-grade” blocks formed during the earliest stages of Franciscan subduction at ca. 165 Ma.
Osozawa et al. (Chapter 8) use map and outcrop relationships from excellent coastal exposures of the Miocene Nabae complex of Japan and petrofabric studies to show that the block-in-matrix fabric observed in this mélange was a result of early sedimentary sliding rather than tectonism. They demonstrate that the amount of shear strain associated with foliation development in the mélange matrix was minimal, and that this deformation was vastly inadequate to account for the introduction of exotic blocks of chert and basalt into the shale matrix. Their data also show evidence for reworking of clasts that include penetrative fabrics developed in an older subduction complex.
Ueno et al. (Chapter 9) document complex duplex structures from accretionary complex rocks of the Jurassic-Cretaceous Chichibu Belt of Japan, and show how some of these structures have been previously (and mistakenly) interpreted as block-in-matrix features in the absence of good exposures. The superb coastal exposures allow detailed characterization of the structures and estimation of the amount of structural thickening associated with tectonic duplexing. They describe “network duplexes,” which are themselves composed of duplexes of smaller orders, and calculate thickening of a factor of ~6–13 at the greenschist facies level of this subduction-accretionary complex.
Festa (Chapter 10) presents detailed field relationships from the Piedmont Basin in northwest Italy that developed as an episutural basin after the main stage Alpine collision, and he documents structures that formed at burial depths of 2–3 km. Utilizing sedimentary structures and sedimentary contact relationships (for establishing sedimentary origins), shear-sense indicators, and progressive strain and rotation of rocks toward faults, he distinguishes between mélanges that were formed by tectonic strain, sedimentary sliding, and diapiric emplacement. He then links the development of different types of mélanges to the regional tectonics, during which faulting (along with the formation of tectonic mélanges) may have triggered gas hydrate disassociation and rise of overpressured fluids (diapiric emplacement, preferentially following fault zones), triggering gravitational collapse and development of sedimentary mélanges.
Muraoka and Ogawa (Chapter 11) present observations on mélanges, duplexes, and folds that they interpret to have formed in a trench-fill environment, the shallowest level of preservation of an accretionary prism. The evidence comes from fine coastal exposures of the Plio-Pleistocene Chikura Group on the Boso Peninsula of Japan. The Lower Chikura Group units are interpreted to have been deposited in the trench in advance of the thrust front and later incorporated into the accretionary wedge by seaward propagation of the thrust front; the Upper Chikura Group units, on the other hand, were originally deposited in a trench slope basin setting. The lower Chikura Group deposits include evidence for interaction of methane-rich fluids from associated chemosynthetic biocommunities that suggest a trench-fill environment similar to the modern Sagami Trough. Chaotic deposits or mélanges include those with diapiric (intrusive) field relationships as well as those that appear to represent submarine slides, whereas the duplexes and thrust anticlines record significant tectonic shortening in the coherent units.
Michiguchi and Ogawa (Chapter 12) examine the internal structure of the Miocene-Pliocene accretionary prism complex exposed in the Boso Peninsula, Japan. They show that dark bands found in siltstones are the products of different deformation mechanisms in an accretionary prism toe and the frontal thrust region. The host rocks include both coherent stratal and chaotic units (as in mélanges). Their map, outcrop, and microscopic analysis suggests that some of these features formed as a result of high pore fluid pressure as shear fractures, whereas others formed as tensional fractures associated with different states of stress and deformation modes. One of their dark band types represents flexural-slip faults associated with folding, another type represents sliding planes formed during submarine landslides, whereas yet another type consists of thrust faults formed during accretion.
PART IV. SIGNIFICANCE OF MÉLANGES FOR ENGINEERING AND APPLIED GEOLOGY
The paper by Medley and Zekkos (Chapter 13) focuses on the geological engineering aspects of mélanges, bringing a societal relevance and significance to mélange studies and research. This topic has been largely overlooked in purely academic studies of mélanges, although it has been a subject of many detailed investigations in engineering geology, whose results have been published in the past 16 years. Up to now, many engineers and geologists in engineering and environmental geology force-fit block-in-matrix geology into a layer-cake stratigraphic interpretation, commonly with disastrous consequences, because this sector of applied geology has not kept pace with the most recent advances in recognition of mélanges. Medley and Zekkos fully describe the engineering issues of dealing with mélanges, including both theory and case studies.
SUMMARY COMMENTS AND CONCLUSIONS
Mélange Classification: Descriptive Rather than Genetic Schemes Recommended
Most of the previous mélange studies, including those in GSA Special Paper 198, offered detailed classification or definitions of mélanges and their sub-types. It is clear that a uniform classification scheme has merit, given that the term mélange is used differently by many authors. However, caution is urged, especially when genetic significance is attached to a definition, given how difficult it may be to ascertain mélange origins from first-order field observations, particularly for mélanges that appear strongly deformed, such as those described by Osozawa et al. (Chapter 8) and Wakabayashi (Chapter 5). The classification schemes proposed by Cowan (1985) are largely descriptive and are therefore more useful than a genetic definition presented, for example, by Sengör (2003), wherein a purely tectonic origin is a requirement for the term mélange. Raymond (1984b) proposed a detailed classification scheme, but its ultimate application required some knowledge of the genesis of the mélange. Although this may seem a regressive definition, we recommend a broad definition of mélange as a bedrock unit with a matrix and variety of blocks included in it, similar to the recommendation of Silver and Beutner (1980). In fact Silver and Beutner (1980) noted that in addition to the more common block-in-matrix fabric, some mélanges have a block-on-block fabric, a structural style that appears to best fit the Mineoka ophiolite belt of Japan (Takahashi et al., 2003; also Mori et al., Chapter 4). Festa (Chapter 10) makes a similar recommendation for a descriptive, rather than genetic definition of the term mélange.
Chapter 5 by Wakabayashi proposes mélange categories based on structural-tectonic settings that are derived from 3-D field relationships. This scheme has the primary goal of connecting the mélanges to large-scale processes during evolution of active plate margins, but it does not directly aid evaluation of strain and sedimentary processes in mélange formation in the way that a scheme such as Cowan's (1985) does. Accordingly, we think that there is no single unifying classification or nomenclature scheme for mélanges, nor should there be, because different schemes serve different purposes. We recommend that authors writing about block-in-matrix units be as specific as possible about the descriptive aspects of these units, so that readers are not misled into applying their own definition of “mélange” that may differ markedly from that intended by the author. In many ways, the problem of mélange classification and nomenclature parallels that of the term ophiolite, for which numerous definitions also exist (e.g., Dilek, 2003; Dilek and Furnes, 2009).
Sedimentary versus Tectonic Mixing in Mélanges
An increasing amount of field evidence has been presented in the past few decades, illustrating the significant contributions of sedimentary mixing to even some of the most (apparently) sheared mélanges (e.g., Aalto, 1989; Osozawa et al., 2009, Chapter 8; Wakabayashi, Chapter 5). These studies support the conclusions of earlier research (Cowan and Page, 1975; Cowan, 1978). Some of the most extreme examples include the sedimentary introduction of exotic blocks into nappe-bounding mélanges in the Franciscan Complex, which may have accommodated tens of km or more of displacement (Wakabayashi, Chapter 5). The studies of Osozawa et al. (2009, Chapter 8) also show that most or all exotic blocks in the Nabae Complex of the Shimanto Belt of Shikoku, Japan, and the Yuwan accretionary complex of the Ryukyu Islands, respectively, were integrated into the mélange by pre-tectonic phases of submarine sliding. Osozawa et al. (Chapter 8) argue that the deformation that produced the matrix foliation in the mélanges that they have examined records relatively minimal shear strain, which cannot account for introduction of exotic blocks or development of block-in-matrix fabrics. Aalto (1989) and Wakabayashi (Chapter 5) document a range of textures from undeformed sedimentary breccias to strongly foliated shale mélange matrix.
Although evidence points to submarine sedimentary (gravity) sliding as a main contributor to the development of block-in-matrix fabrics in many of the most tectonized mélanges, sedimentary sliding was not a major process in the formation of block-in-matrix fabrics in all mélanges. Some mélanges clearly have a tectonic or diapiric origin. Festa (Chapter 10) summarizes effectively the criteria for distinguishing diapiric versus tectonic mélanges, and provides field examples of both. For diapiric mélanges, the diagnostic feature is opposing shear sense on opposite mélange contacts, a criterion that was first applied by Orange (1990) and subsequently used by Dela Pierre et al. (2007), as well as Muraoka and Ogawa (Chapter 11) and Festa (Chapter 10). For tectonic mélanges, an important field characteristic is an increasing degree of deformation and rotation of fabric elements as a fault or shear zone is approached (Festa, Chapter 10).
Significance of Mélanges and Mélange Types in Orogenic Belt Development
Mélanges are characteristic features of modern and ancient convergent plate boundaries, and rank with ophiolites and HP–low-temperature metamorphic rocks as critical recorders of convergent plate margin processes. Mélanges provide critical insights into sedimentary and structural evolution in the accretionary prism and forearc basin environments, including evidence for large-scale material movement (particularly in cross-sectional view) in accretionary wedges. Mélanges form as subduction of oceanic lithosphere is punctuated by a collisional process (see discussion in Dangerfield et al., Chapter 6, although they argue for a noncollisional origin for the particular mélange of their study), and/or terminated by the final stages of continental collision (Festa, Chapter 10). In addition to recording subduction- and collision-related sedimentary and tectonic processes, mélange formation may also include pre-subduction tectonics, including deformation along abyssal fracture zones (Shervais et al., Chapter 1; Saleeby, Chapter 2) and at oceanic core complexes (Saleeby, Chapter 2), as well as supra-subduction zone oceanic crust evolution (Dangerfield et al., Chapter 6; Shervais et al., Chapter 1). Mélanges also offer major insights into the most extreme vertical movements along convergent plate margins: the exhumation of high-pressure metamorphic rocks (Shervais et al., Chapter 1; Saleeby, Chapter 2; Mori et al., Chapter 4; Wakabayashi, Chapter 5; Erickson, Chapter 7).
Mélanges, by the very nature of their chaotic block-in-matrix structure, pervasive and strong internal deformation and clay-rich soil contents, are prone to landsliding as well as creating problems because of the great contrast in ease of excavation of block and matrix (Medley and Zekkos, Chapter 13). Hence, they pose major challenges for engineering projects developed on them as well as for water supplies and infrastructure. Therefore, mélange terrains cause first-order societal problems for the people in California, Japan, Italy, Scotland, Greece, Cyprus, Turkey, the Philippines, and many other countries, where ophiolites and mélanges occur abundantly. Furthermore, most engineers and engineering geologists continue to treat mélange-containing bedrock by using the basic principles of stratigraphy and by assuming a layered structure for their formation, and fail to account for the 3-D variation of many key parameters such as rock strength and ease of excavation. This ill-informed approach results in disastrous engineering problems, leading to significant property damage and casualties. It is thus highly important for the academic community and the practicing geological and civil engineers to convey their learned experience and knowledge on mélanges and mélange structures to each other through publishing in common literature and in conference proceedings in order to maximize the dissemination of their scientific and applied findings. The academic community in particular should continue to strive to remedy the knowledge gaps through interaction with the applied community as well as through implementing contemporary reforms in undergraduate education that would revive field instruction and field-based, observation-oriented earth science education. We hope that this GSA Special Paper presents an important step in this mission of closing the knowledge gap in the purely scientific and engineering aspects of mélanges and mélange-forming processes and their significance for engineering and societal issues.