THE EOCENE GOLPAYGAN METAMORPHIC CORE COMPLEX, CENTRAL IRAN: A CASE HISTORY OF OROGEN-PARALLEL FOREARC RIFTING ALONG AN ANDEAN-TYPE CONTINENTAL MARGIN

The NW-trending Sanandaj-Sirjan zone (SSZ) is the metamorphic core of the Zagros orogen in the Arabia-Eurasia collision zone. After the formation of the Neotethys Ocean in Permian time, the SSZ developed as a magmatic arc (with associated metamorphism) along the southern margin of Central Iran, via northward subduction in the Mesozoic. Precambrian-Paleozoic basement rocks are exposed in two parallel, roughly ENE- trending, elongate extensional fault complexes, one at Golpaygan-Muteh, and another 35 km to the NNW, near Mahallat, in the central SSZ. In the Golpaygan-Muteh area, the footwall of the core complex includes Late Proterozoic-Early Cambrian granitoid orthogneisses that intrude low- to medium-grade metasedimentary rocks, most likely metamorphosed in the Jurassic. The hanging wall contains unmetamorphosed Paleozoic and younger sedimentary rocks. Previous work shows that trends of subhorizontal mylonitic lineation are complex. They trend NNW in the SSW part of the complex (sub-parallel to the arc), but trend NE in the NNE part. We have determined a U-Pb age of 63 ± 2 on a mylonitized pluton in the SSW. Previous Ar-Ar mica ages on mylonites from this pluton are ca. 55 Ma, suggesting deformation in early- to middle Eocene time. Along the N margin of the complex, hanging wall rocks include a thick successions (>1 km) of Eocene clastic sediments filling half grabens, which include numerous thick beds of W-derived monolithologic breccias; a succession to the N contains nummulitic limestones. We have determined a U/Pb age of 46 ± 1 Ma on zircon from a felsic tuff within Eocene basin fill. Mica Ar through AHe ages in the NNE are all Eocene, but AHe ages along a N-S traverse in the SSW part of the core indicates northward younging from 25 to 18 Ma. These ages suggest incomplete exhumation in Eocene time, and S to SE tilting of the footwall in early Neogene time, perhaps as a result of syn-collisional deformation. The Mahallat structure exposes mainly subgreenschist Paleozoic strata in the footwall that dip NW. The hanging wall is largely composed of an Eocene supradetachment basin formed along its SSE boundary. These structures in the forearc of the Tertiary Urumieh-Dokhtar magmatic arc show that trench parallel-rifting during Eocene subduction affected the overriding plate, in addition to rifting in the intra- and back-arc regions.