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18.4: Structure of greenstone belts

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    If you examine a map view of the Pilbara Craton in Western Australia, you’ll see a distinctive pattern of shapes... Ignore the roads and towns, and focus on the colors and shapes in the unvegetated landscape instead:

    Notice there how there are broad oval shaped light colored features, separated by more linear dark colored rocks? This is a fairly typical arrangement of rocks in Archean crust. The big light colored features are ‘granite’ (really: tonalite, trondjehmite, and granodiorite) and the dark colored lines separating them are steeply dipping layers of lightly metamorphosed volcanic and sedimentary rock. In the third dimension (depth), we think the metavolcanic and metasedimentary cover sequence pinches out, making a thin septum between blob-like granite domes. We call this geometry “dome and keel,” and it’s thought to represent a kind of vertical tectonics that dominated during the Archean. Here’s a diagram showing the relative motion of the plutonic domes and the surface strata folded into the thin keels:

    A block diagram showing the structure of a typical Archean craton, such as the Pilbara. Broad round granite "domes" are shown rising vertically, while thin metavolcanic and metasedimentary "keels" sink downward between the domes. In map view, the surface outcrop pattern is round blobs of granite separated by a branching cuspate belt of metavolcanic and metasedimentary rocks.
    Figure \(\PageIndex{1}\): Block diagram showing the structure of a typical Archean craton, such as the Pilbara. Arrows show relative motion of the granite “domes” relative to the metavolcanic & metasedimentary “keels.” (Callan Bentley cartoon)

    The idea here is that the crust hadn’t yet organized itself into thick, laterally-continuous slabs like plates. Instead, on a much smaller scale, the primordial crust was forming through both plutonic processes (pushing up from below) and surface processes (eruption of lava and production and deposition of sediment). The rising domes of granite moved vertically, and to accommodate this relative motion, the surface rocks sagged downward, perhaps even foundering (akin to a sinking ship) and “dripping” into the dynamic mantle.

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    Figure \(\PageIndex{2}\): Dome and keel structure seen in a Canadian greenstone belt. (Geological Survey of Canada map)

    Though Archean cratons and greenstone belts tend to be in relatively placid corners of the modern Earth, where we can’t get a significant view into their three-dimensional structure, airborne magnetometry and gravitotometry can produce some insights into their structure, as this magnetic map of a greenstone belt in Manitoba, Canada, shows.

    Overall, the dome-and-keel structure suggests that during the Archean, vertically-oriented, small-scale tectonics dominated, forming stable cratons of continental crust that have fortunately persisted until the present day, where they form the nuclei of most continents.

    Did I Get It? - Quiz

    Exercise \(\PageIndex{1}\)

    What is the large-scale structure of greenstone belts?

    a. Greenstone belts are found wrapping around dome-shaped bodies of "granite" in Archean cratons, thinning downward into blade-like "keels."

    b. Broad rift-like features that show normal faulting and evaporite sedimentation.

    c. Parallel metamorphic belts, indicating subduction and parallel magmatic arcs.

    Answer

    a. Greenstone belts are found wrapping around dome-shaped bodies of "granite" in Archean cratons, thinning downward into blade-like "keels."

    This page titled 18.4: Structure of greenstone belts is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts (VIVA, the Virginia Library Consortium) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.