The Mystery of Mélange

By Terri Zach ‘21

Mélange (mā-ˈlänj) is a fun word to say. We can thank the French for that, it’s a derivation of the word ‘mêler,’ which means ‘to mix.’ Geologists have co-opted the word and a geologic mélange is a mixture of strong and weak rocks that were mixed together during deformation, typically on or near the seafloor, at the boundary of two colliding tectonic plates. Some researchers argue there’s an ancient geologic mélange (the Shores Mélange, named for Shores which was once a tiny village by the James River) in central Virginia that delineates a fossil tectonic plate boundary at the edge of old North America (geologists know it as Laurentia).  Other researchers are less convinced that these curious rocks form the ancient suture along which plates were welded together ~450 million years ago during the Taconian orogeny.

Cartoon diagram of Taconian orogeny.

Figure 1: Simplified model of a subduction zone, accretionary prism, and the formation of the mélange during the late Ordovician Taconian orogeny. By the end of the Taconian orogeny the Chopawamsic Arc had accreted to Laurentia.

The goal of my research is to study the Shores Mélange by taking advantage of natural outcrops exposed in the James River.  By mapping these rocks at a fine-scale I hope to be able to apply that understanding on a much grander scale to determine whether it’s an ancient suture or not.  In doing so, we can provide new insight into Virginia’s geologic history over the course of millions of years.

Geologic terrane map of Virginia.

Figure 2: The location of my study area – the Shores Mélange at the Seven Islands, Virginia (noted by the star).

I started my socially distanced field research on June 12th in a canoe along the James River in the Seven Islands region (~85 km west of Richmond).  We use the canoe to access the expansive and beautiful outcrops in the channel of the James River, at low water there are many rocks to be seen. At each outcrop, we note the rocks and their structural features such as foliation, lineation, fracture sets, and fault zones.

Photo of Terri Zach taking field notes.

Figure 3: An action shot of me in the field, masked up, and taking notes of one of the many outcrops in the James River.

Some of the outcrops in the Seven Islands are spectacular and expose blocks (chunks) of metamorphosed basalt in well-foliated metasedimentary schist.  The meta-basalt blocks may have been ripped off the seafloor and mixed into the mélange as the Chopawamsic Volcanic Arc approaches the Laurentian margin (Figure 1). Other outcrops reveal complex folds and boudins – all eye candy for the structural geologist.

Two photos of rocks with different geologic features.

Figure 4: Left. An example of a faulted ‘rootless’ fold (RF). Right. A ‘block’ (B) of meta-basalt in a schist.

We’re attempting to map these rocks and structures at a far more detailed scale than earlier workers.  To do this we’re using a drone (UAV) to take low-altitude aerial photos that serve as our base map.

Two images, one of drone image, second is geologic interpretation.

Figure 5: Left. UAV aerial photo of an outcrop in the Seven Islands region (photo by Pablo Yanez). Right. A geological map with my interpretation of the outcrop: the plagio-granite (orange) intrudes the older schistose gneiss (brown) and both rocks are folded and sheared.

After paddling across the Shores Mélange, the gradient of the James River slackens as it flows over meta-volcanic rocks of the Chopawamsic terrane.  In these calm waters, it’s easy to reflect that the pandemic summer of 2020 does not allow for the typical summer research experience.  However, there’s nothing typical about my academic journey so far. I am a non-traditional student that transferred to William & Mary from Thomas Nelson Community College in 2019. My community college geology professor, Lynsey LeMay (W&M Geology ’03), reinvigorated my interest in geosciences by inviting me to a Geological Society of America meeting in 2017. It is there that I learned about the vastly different areas of earth science research and heard talks by Professor Chuck Bailey and his research students. At the meeting, I realized that I wanted to attend William & Mary, study geology, and do tectonics research. It’s still hard for me to believe that I am here at W&M and doing cool original research. Nonetheless, I am incredibly grateful for all the support from my friends, family, mentors, and educators.

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