The Geology of Our Mountains
By Jackie Craig
D.M.C.G. Executive Committee
The mountains that surround the Tucson Basin have a fascinating history. Shallow seas covered much of Arizona (and all of North America) for many millions of years. Then about 165 million years ago (mya), the Farallon Plate started diving beneath the North American Plate, causing folding, faulting and uplift of the crust throughout the West. Arizona underwent significant lifting and volcanism due to the diving plate between 80 to 50 mya.
Then, about 35 mya, another plate hooked the North American plate to the large Pacific Plate and began moving it west. This stretched the North American crust which began to crack and collapse. The collapsed places became valleys and the higher areas remained as mountains. The massive affected area from SE Oregon to Sonora Mexico is called the Basin and Range Province.
In our area, the stretching caused a major part of the Catalina Mountains to slide off to the west. After 15 million years of slow creep, they ended up in their current position, the Tucson Mountains. Free from the massive weight of that block, the granite that had been under it rose to the surface and above, becoming the Catalinas and Rincons that we know today. The uplift then caused large blocks to slide off and fault further, forming Oro Valley, and separating the forerange from the main mountain mass, and as a wonderful by-product, creating Sabino and Romero Canyons. The Tortolita Mountains are considered to be a part of the same metamorphic core complex as the Catalinas, separated only by faulting and valley sediment fill.
