The Closing of Iapetus: Island Arcs and Metamorphism in Rock Creek Park, Washington D.C.

I’ve spoken before about the Iapetus Ocean that separated North America from Europe. It is more than conjecture because there are pieces of the sediments and volcanic rocks underlying it to be found along the east coast of N. America. These ophiolites are proof of the existence of ocean crust and upper mantle pushed onto what later became Maryland (albeit at great depths beneath the surface) during the closing of an ocean basin between approximately 500 and 300 Ma.

Figure 1. Map of Washington DC area, showing the study area. The photos below were taken at the marker within the circled area in the inset map.

The green area in the inset map of Fig. 1 has been classified as the Sykesville Formation. These are sedimentary rocks, formed from a melange that collected behind an island arc. They contain a crazy mixture of sediments eroded from volcanic islands and exhumed rocks when these islands were smeared onto a continent at geological time scales (e.g. Japan and the Philippines today). This is a very slow process. However, this entire episode was nothing more than the prelude to closing of the Iapetus Ocean when two continental land masses collided, neither of which could be subducted beneath the other. Thus, these marine sediments were buried and subjected to incredible pressure and heat, producing the metasedimentary rocks of the Sykesville Formation.

Figure 2. Exposure of Sykesville Formation rocks along the Rock Creek Parkway (see inset of Fig. 1). The photo is approx. six feet across.

The original sedimentary texture (layers of sediments deposited horizontally) has been overprinted by foliation, a metamorphic texture caused by compression deep beneath the surface, usually at high temperature. The nearly vertical, yellow line in Fig. 2 indicates this metamorphic foliation, which is probably close to the orientation of original bedding. The circled areas show striations on a foliation surface (lower right) and perpendicular (upper left).

After being metamorphosed at great depth these rocks were folded and faulted, so that nearly horizontal bedding and foliation were tilted to nearly vertical. Figure 2 is looking towards the NW (field estimate) and is consistent with similar deformed rocks observed at Great Falls. In other words, these metasedimentary rocks have been folded and faulted (there are several faults within the area) when two continental land masses collided.

A closer look reveals a hint at what occurred more than 300 million years ago.

Figure 3. Close up from the outcrop in Fig. 2, showing quartz lens in a dark background The field of view is about two inches.

The highlighter in Fig. 3 indicates a bleb of quartz (the gray mineral) with a distinctive pinching (note the lower and upper parts of the circled area). Although this does indicate very high temperatures and pressure, the rock has not melted and there is no indication of veining or other evidence of contact metamorphism.

Figure 4. Close up of Fig. 2 looking along the beds (i.e. downward into the ground). Note the rectangular, white segment, and the irregular mineralogy and (sedimentary?) relationship of the darker rock to the lower right of the image.

Textures like those seen in Figs. 3 and 4 are associated with depositional/tectonic environments where clumps of rock are either being eroded by surface processes (e.g. rain and erosion) or plucked from a thick layer of overlying rock as it slides over a given sequence (in this case the Sykesville Formation) when it is buried deeply enough for the rock to be ductile. This poorly understood process produces allochthons like ophiolites. The textures of these rocks have been interpreted (using radiometric and textural data) as resulting from the latter process.

Take a long look at Fig. 4 and imagine sediments deposited in the Sea of Japan being jammed into Korea, buried deeply and sliding over each other for millions of years.

The earth is relentless…

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