Constriction fracture flow; a mechanism for fault and vein formation in the Coeur d'Alene District, Idaho

Transection cleavage, faults, and veins outside the Terror fault in the Middle Proterozoic sedimentary rocks of the Coeur d'Alene district, Idaho, contain steep stretching fabrics. These structures formed during metamorphism in the lower greenschist facies. Cleavage formed in sequential plane strain and constriction flow. Late metamorphic faulting and veining followed, and local mylonite overprinted cleavage synchronously with mineralization. High-angle reverse and normal faults and veins constitute constrictionlike arrays with widely spaced slip planes which formed during non-Andersonian strain. Bulk strain consisted of plastic slip parallel to the line of fracture intersection, along spaced anastomotic shears. The proposed deformation type is termed "constriction fracture flow." Inhomogeneous shear or cross trend shortening may account for synchronous reverse and normal slip in different parts of the rock.