Magmatic and tectonic continuous casting in the circum-Pacific region

Continuous casting is an industrial process for producing metal in continuous linear form. Cooling of the cast medium occurs across a slip surface in this process. The same process was active in geologic settings in which lava extrusion was accompanied by formation of striations on the surface of the extrusion. Such grooves resulted from irregularities on colder and harder rock that formed the confining orifice from which the grooved lava was extruded. In a variant of this process, lavas that emerged from beneath deep-sea lava-lake crusts were striated on their upper surfaces as they emerged. In the case of the grooved extrusion at Sacsayhuamán, Perú, lateral displacement of rock overlying a very shallow intrusion may have caused striations to form on the underlying intrusion as it was exhumed. Deep-sea metamorphic core complexes are known primarily by their corrugated upper surfaces as identified with multibeam sonar. Recovered samples, including from drilling, typically include gabbro and basalt, indicating that magmatism was a major process in construction of the corrugated normal-fault footwall. The 125-km-long Godzilla Mullion in the Parece Vela Basin of the Philippine Sea, which is the largest known metamorphic core complex on Earth, includes a single antiformal groove 60 km long and 5 km wide. As with all deep-sea core complexes, grooves are parallel to nearby transform faults and to plate spreading direction. Tectonic continuous casting beneath a corrugated fault surface is the only known process that could produce such long grooves. Abrupt terminations and changes in groove form were likely due to incision and excision in which fault slivers were transferred from one side of the fault to the other, thereby changing the shape of the hanging-wall mold and cast footwall. One of the major insights in identifying processes of continental lithospheric extension is that the deep crust, or “fluid crustal layer,” is sufficiently mobile because of quartz crystal plasticity that it gradually flows in response to lateral pressure gradients. Large-displacement normal faults known as detachment faults have uncovered this fluid crustal layer in the mylonitic footwalls of continental metamorphic core complexes. Some core complexes are grooved over tens of kilometers, with grooves parallel to extension direction. While these grooves were likely produced by tectonic continuous casting, it has not yet been demonstrated that molding actually occurred. Examples of corrugated normal faults, or of exceptionally long or well developed grooves that were plausibly produced by tectonic continuous casting beneath corrugated normal faults, include the Wasatch front in Utah, Rincon Mountains and Harcuvar-Buckskin Mountains in Arizona, Dayman dome in Papua New Guinea, and the Pompangeo Mountains of central Sulawesi in Indonesia. These five areas are described to illustrate the evolution from corrugated range-front normal fault to corrugated metamorphic core complex. e-mail: Spencer: jon.spencer@azgs.az. gov; Ohara: yasuhiko.ohara@gmail.com (also at: ohara@jodc.go.jp) Spencer, J.E. , and Ohara, Yasuhiko, 2008, Magmatic and tectonic continuous casting in the circum-Pacific region, in Spencer, J.E. , and Titley, S.R. , eds., Ores and orogenesis: Circum-Pacific tectonics, geologic evolution, and ore deposits: Arizona Geological Society Digest 22, p. 31-53. J.E. Spencer and Y. Ohara 32