Creation and preservation of subduction‐related inverted metamorphic gradients

Abstract

Peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost U.S. Cordillera. Inverted metamorphic gradients are preserved in the Pelona‐Orocopia Schist (southern California), the Catalina Schist (California borderland), the Central Metamorphic Belt (Klamath province), the South Fork Mountain Schist (northern California Coast Ranges), and possibly the Shuksan Suite (northern Washington). With the exception of the South Fork Mountain Schist, peak metamorphic temperatures increase structurally upward from ∼400°C to ∼650°C over 1–2 km, indicating metamorphic gradients in excess of −100°C km−1. Estimates of metamorphic gradients in some terranes, such as the Catalina Schist, depend critically on the effects of postmetamorphic faulting. The tectonic settings and high metamorphic pressures (∼800 MPa) suggest that the inverted metamorphic gradients formed in paleosubduction zones. Four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls, whereas the Pelona‐Orocopia Schist formed beneath granitic crystalline rocks. The high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones. Two separate two‐dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved. Rapid subduction (∼10 cm yr−1) beneath young oceanic lithosphere (<10 Ma) results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of −100°C km−1in the top portion of the downgoing slab. In order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall. Accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and/or greenschist facies devolatilization reactions that weaken the descending slab. Alternatively, inverted metamorphic gradients may represent the continuous accretion of material at rates of 1–10 mm yr−1under conditions of declining temperature. Continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 Ma. Continued subduction of ∼1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient. Synsubduction uplift and/or moderately fast postsubduction uplift rates of ∼1 mm yr−1are required to preserve the inverted metamorphic gradient. The models predict that (1) the high‐grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction, and (2) inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km.