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Problems in Melange Recognition and Research-Practice Gap

This article explores the challenges in recognizing melanges in applied geologic projects, leading to costly mistakes. It discusses the importance of identifying melanges and their field relations, as well as the gap between research knowledge and professional practice. It highlights the need for improved understanding and collaboration between the research and applied communities.

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Problems in Melange Recognition and Research-Practice Gap

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  1. Problems in melange recognition and the gap between research knowledge and professional practice(the fault lies in BOTH research and applied communities) John Wakabayashi, California State University, Fresno

  2. The Melange Concept and Its Application;It Aint New Folks! • Hsü wakes up the academic world to melanges in 1968 • Medley, Lindquist, Goodman, and others introduce the engineering geology community to melanges starting in 1994. But… many (most?) still fail to recognize melanges in applied geologic projects, leading to costly mistakes.

  3. Is this really a major problem? Yes! Melanges are common in California and throughout the world and many engineered structures are built on them. The reduction in field studies in many undergraduate programs threatens to make this problem worse.

  4. Common Melange Misdiagnoses • Melange mapped as layer cake stratigraphy • Melange mapped as soil with boulders (you’ll have to dig a lot to get to ‘bedrock’!) • All Franciscan considered melange Does THIS look like layercake stratigraphy?

  5. Beware of the Layer Cake

  6. If we recognize and characterize a melange… We can use appropriate techniques for engineering analysis.

  7. Before we jump to engineering… We need to be able to identify melanges, and their basic field relations first.

  8. Where Melanges Lurk…(N. CA)

  9. Engineering vs Geologic Melange “Engineering melange”: Contrast in strength between weak matrix and stronger blocks Some geologic melange units are not “engineering melange” because of recrystallized matrix. This commonly depends on metamorphic grade. Greenschist and higher grade metamorphism will tend to reduce or eliminate matrix-block strength contrasts, but recrystalized matrix can occur at lower grades.

  10. Engineering vs Geologic Melange pt 2 Geologic: We tend to care about origin: sedimentary (olistostrome), tectonic, diapiric, etc. Engineering: Origin not important; Various 3D properties of rock unit are.

  11. Some geologic melanges aren’t engineering melanges

  12. Outcrops didn’t change… But interpretations did!

  13. Interpretations Change for Boreholes, too...

  14. Contrary to popular belief, we CAN map them…

  15. Field Technique or Mindset Layer cake--Contact mapping. Find a few points on contact and connect the dots Melanges--Saturation mapping: Record every outcrop internally (few learn this technique in today’s field classes). Contact mapping may apply to melange boundaries

  16. Review of Academic Literature is Commonly Helpful But beware of generalizations. For example: The Franciscan is nowhere near 100% melange.

  17. Franciscan ‘nappe columns’: melange and coherent (Only the thickest melange zones are depicted with pink shade)

  18. Example: El Cerrito Quarry (melange and coherent)

  19. El Cerrito Quarry: view to north

  20. But how do you know you’re in a melange or not? (especially if you don’t have the 100 percent exposure provided by a quarry, sea cliff, or roadcut)

  21. Partial Field Checklist… • Geomorphology: “plum pudding” topography; melanges generally cannot hold steep slopes • Occurrence of “exotic” lithologies. (a) isolated outcrops of chert, basalt, serpentinite, blueschist, etc. in otherwise sandstone and shale (b) Blocks of something other than gabbro within serpentinite (c) Blocks of material with different metamorphic grade than most of outcrops in area (say blueschist, eclogite, or amphibolite in otherwise prehnite-pumpellyite grade terrane) (d) (rarest) actual field exposure(s) of block-in-matrix structure

  22. Melange:Lumpy, bumpy and not very steep Coherent: Holds steep slopes

  23. But… Pseudomelange topography All that’s lumpy is not melange (Coyote Hills)

  24. Without the quarry: another clue

  25. Best preparation for dealing with melange geology? Map it!: Mapping in Franciscan melange (Fresno State advanced field methods class “part 2” field area).

  26. Academic-Professional Practice Gap Professional problem: •Obsolete (pre-1968) concepts and lack of motivation to keep up (catch up) with current geologic knowledge: poor scholarship Academic problems: •Lack understanding of professional needs and practice (few academic geologists have a clue about engineering and environmental geology); worse yet…. •Many academicians DON’T CARE what goes on in professional community (beneath their dignity?) •Decline in field programs in undergraduate geology and virtually no field programs that expose students to mapping and identifying melanges (ie the problem is getting worse).

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