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The Skagit
Crystalline Core with its high pressure garnets makes up the eastern half of
the North Cascades. To the west, there
are a number indicators of deep thrust (reverse) faulting associated with the
plate convergence that accompanied the arrival and docking of Wrangellia, 100 million years ago. One indicator of extremely deep faulting (reverse faulting) is the appearance of the Twin Sisters Dunite in the North Cascades. These are very deep MANTLE rocks that have been thrust back toward the surface by deep faults. This dunite is an ultramafic mantle rock that is more than 90% olivine. West of the Straight Creek fault, the geology differs dramatically from the Skagit Crystalline Core. The rocks to the West are still ocean-floor rocks, but they are stacked in massive thrust-sheet layers. This can be seen most clearly in the "Mount Baker Window" where deeper rocks are exposed by river-cutting. In the Mount Baker window, we find a series of rocks (all oceanic in origin) that date from the Triassic to the Jurassic (THE CULTUS ROCKS -- marine sandstones -- 225 million - 190 million years ago); from the Jurassic (THE WELLS CREEK FORMATION -- Mt. Rainier-type volcanic rocks from volcanic islands -- about 190 – 150 million years ago); and from the Jurassic to the Cretaceous (THE NOOKSACK FORMATION -- marine sandstones derived from the erosion of volcanic islands -- 150 - 110 million years ago). These rocks are in the "right order" with respect to the principle of Superposition -- oldest on the bottom and progressively younger toward the top. But, above the Nooksack rocks, most easily seen on the slopes of Mount Shuksan, are the CHILLIWACK GROUP of rocks -- an ophiolite that dates from Devonian to Permian time (400 million -- 250 million years ago). And above the Chilliwack group of rocks are the SHUKSAN GROUP of rocks, which are pre-Devonian (older than 400 million). Thus, two batches of older rocks sit above the younger rocks, signifying two major thrust sheets – a thrust sheet made up of the Chilliwack rocks and the overlying Shuksan thrust with the Shuksan Blueschist. The deeper thrust layer -- the Shuksan thrust -- also brought the Twin Sisters Dunite (the mantle rock) up to the surface. The blueschist in the Shuksan thrust contains an important mineral -- GLAUCOPHANE. This mineral forms under HIGH PRESSURE...BUT LOW TEMPERATURE...the conditions geologists associate with a subduction trench. The Shuksan Blueschist provides geologists with insight into the timing of collisional events. The blueschist formed at a depth of roughly 30 km (20 miles) at 120 million years ago. Thus, the ocean-floor rocks were down in a trench and the upper portion of the subduction zone. Then Wrangellia hit about 100 million years ago and the great compressive force thrusted the Shuksan Blueschist back to the surface. |
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