Seven Falls
And the Oracle Granite
Seven Falls, a scenic area on Bear Canyon northeast of Tucson, Arizona USA, is underlain by mostly 1.4-billion-year-old granite (Oracle Granite) that has been metamorphosed to gneiss and injected by later granitic dikes and sills.
Much of the Oracle Granite has been stretched, pulled, and ground up as if it had been through a grist mill, so it consists of mylonite (from Greek for “a mill”). This grinding occurred about 35 million years ago as a huge part of western North America, from Montana and Idaho through Nevada and Utah to Arizona and New Mexico was being pulled apart by extension. The basin and range province represents the crustal breaking that resulted, with down-dropped basins (grabens) and high-standing ranges (horsts).
In some places, including the Santa Catalina Mountains near Tucson, uplifts were high enough that they were unstable under the pull of gravity, and huge slabs of rock slid off them. But don’t visualize a giant landslide – most of the movement happened several kilometers beneath the surface, where the rocks were warm and under high pressures. The grinding, the friction, from that huge detachment of a thick package of rock above turned the underlying rocks to mylonite, and that’s what we see in the vicinity of the Seven Falls. The gneiss here is called a metamorphic core complex, a location from which overlying rocks broke off and slid down along what we call a detachment fault a few kilometers beneath the surface.
The rocks that broke away from the flank of the high dome of the Santa Catalina Mountains now lie under the city of Tucson. The total movement on the Santa Catalina detachment is estimated at about 30 kilometers. See this page for a good description and illustration of the detachment and its consequences.
The Oracle Granite, at 1.4 billion years in age, is part of the enigmatic anorogenic (that is, not related to mountain building) magmatism (igneous activity) that may reflect the incipient, but partially failed dismemberment of the supercontinent Columbia. Or the igneous activity may represent the thermal insulating effects of the continent, forming a long-term “lid” atop the mantle, which continued to burp up molten material in diverse places.
In the photo at top, you’re looking at the metamorphic rocks of the core complex beneath the detachment. The trace of the detachment itself, buried under later sediments, is a few miles behind me in this view as we look up Bear Canyon.
Peter Coney and others, using information about the rocks in the Santa Catalina Mountains and elsewhere, came up with the concept of metamorphic core complexes and detachment faulting in the late 1970s. Today, many such features are recognized world-wide, including one in the Bitterroot Mountains of Montana (the slide block is the Sapphire Range) and a complex zone called the Anaconda detachment that extends through parts of the Pintler and Flint Creek Ranges and beyond.
Photos from February 2017.
so hard for me to visualize how what we can see on the surface now relates to what has been going on underground!