This is a quick post to summarize what I said about modern Japan being an analogue to the Taconic orogeny. For example, here’s a photo of Mt. Fuji, seen from the ocean (Fig. 1). (Imagine being in the back-arc basin during the Cambrian period.)

Figure 1. Mt. Fuji from the sea.

The Sea of Japan is more than 500 miles across at its widest point, so sediment eroding from the mountain chain that forms the backbone of Honshu is collecting along the western coast of Honshu as well as in deeper water offshore.

Here’s a schematic cross-section of the most-likely geography during the Taconic orogeny (Fig. 2). Imagine Honshu as the island arc shown offshore of the ancient North American continent (to the left in the cartoons).

Figure 2. Schematic cross-sections of North America and a hypothesized island chain during the Taconic orogeny.

Modern Honshu and the Sea of Japan are most representative of the Taconic orogeny earlier than 543 my, before subduction began on the western margin in the top panel. There is no subduction in the Sea of Japan today; in fact, spreading stopped about 20 million-years ago; details are hard to find because there are no easily accessible seismic sections of the Sea of Japan. Thus, to apply the cartoon from Fig. 2, ignore the subducting back-arc ocean crust (black layers) and focus on the deformed gray areas in the middle panel.

The lower panel is probably what will happen to Honshu in the distant future. For example, the Pacific plate is being subducted at ~10 cm/year (4 inches). We can use an average width of 1000 km (625 miles) to estimate that it will take 10 million years [1000 km/(10 cm/y)] for the lower panel of Fig. 2 to become reality.

With respect to the scale of the analogous processes occurring in the Japanese Islands and N. America (during the Taconic orogeny), we can do a simple comparison (Fig. 3).

Figure 3. The left panel shows the extent of rocks associated with the Grenville orogeny, associated with closing of a precursor to the Atlantic Ocean (Iapetus Ocean). The right panel shows the island of Honshu. The arrows show a hypothesized, similar-sized island arc (outlined in black) during the early stages of the Taconic orogeny, when sediments eroded from the earlier (Grenville) mountain belt (orange) were buried deeply beneath the back-arc basin between the Grenville Belt and the offshore island. They would have been subjected to intense heat and pressure. Note the immense scale of this geologic province, based on the similar size of Honshu and the island arc that became the rocks we’ve seen throughout eastern N. America.

This has been a very simple, hypothetical reconstruction, but I hope it helps you envision what the proto-north American continent was experiencing. The key point is that a massive mountain-building event, something like the Taconic-AcadianAlleghanian orogenies, which lasted throughout the Paleozoic era, wouldn’t have been an earth-shattering event…

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