BREAKUP OF RODINIA

Rodinia began to break apart somewhat before 550 mya in the latest Precambrian, slightly before the origin of modern animal groups in the Cambrian period (543-489 mya). Basalts (volcanic rocks rich in iron-magnesium silicates) of 550 mya flowed from spreading centers into marine sediments in southern Quebec and northern Vermont. These rocks and similar basalts and sedimentary rocks north of Grafton Lakes State Park in Rensselaer Co. record continent breakup and the origin of the Iapetus Ocean east of New York State.

fig 28
Continued Riftin and Volcanism

fig 31
Continued Opening of Iapetus Ocean

Features expected of continent breakup, such as broken edges having as zigzag outline with the fractures meeting at about 120o angle, are recorded in the New York State region. One fracture zone trends southwest, approximately parallel to the northern margin of the Appalachians and the St. Lawrence River in Quebec. This “active arm” is replaced at a 120o angle by a second arm that runs south from Montreal and somewhat east of the Lake Champlain-Hudson Lowlands to New York City. The less active “failed arm” of this rift system runs northwest of Montreal to Ottawa. Another 120o angle in the fracture system in the New York City region made the new continent margin angle west into Pennsylvania.

This configuration of rifts formed in the New York Promontory at the edge of the new continent of Laurentia (ancestral North America). Promontories tend to remain as high areas along the edge of contents, which means that the thickness of Paleozoic marine rocks laid down across New York State is thinner than that in the Pennsylvania Reentrant. Continent breakup is not along a single fracture, and many smaller fractures parallel the major rifts. For example, smaller fractures roughly parallel to the Lake Champlain-Hudson Lowlands define the north-northeast trend of many eastern Adirondack features such as Long, Indian, Schroon, and Brant Lakes; Lake George; upper West and East Branches of the Ausable River; and stretches of the Northway where it runs along valleys. Old fractures contain shattered rock, and erosion by the last continental glaciation scoured them to form the modern Adirondack topography. Fractures are zones of weakness, and the abundance of small earthquakes on a line from Ottawa, Ont (on the failed arm), along the eastern Adirondacks and Hudson Valley to New York City likely reflects these ca. 550 mya fractures.

fig 33
Development of Passive Margin on Proto North America


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