DEFINING PROTEROZOIC PROVINCE BOUNDARIES IN THE SOUTHWESTERN U.S. WITH MAGNETIC MAPPING

Northwestern Arizona includes the possible triple-junction of the Paleoproterozoic Mojave, Yavapai, and Mazatzal provinces. Boundaries between these terranes, mapped on the basis of changes in isotopic composition and structural fabrics, do not agree. The disagreement is possibly due to diffuse boundary zones, the inability of structural analysis to resolve numerous overprinting tectonic events, or isotopic mixing caused by tectonothermal events.

Magnetic mapping of the region may provide information necessary to define the crustal province boundaries more accurately. Preliminary results reveal a strong magnetic lineament that coincides with the original Nd province 2/3 boundary of Bennett and DePaolo (1987), the proposed division between the Yavapai and Mazatzal provinces. The sharp magnetic lineament is suggestive of a deep transverse structure between the two regions.

Previous research suggests an ~75 km-wide boundary zone between the Yavapai and Mojave provinces, which exhibits Yavapai structural fabrics and Mojave Pb isotopic characteristics. Nd trends do not coincide with the Pb trends in this zone. There is a sharp transition in lead isotopic values cutting across the boundary zone, suggestive of a discrete crustal feature. Preliminary magnetic results indicate a change in the wavelengths of magnetic anomalies parallel to the trend of the lead isotope transition. Although the change in wavelength may result from Cenozoic tectonics of the Basin and Range, the signature is suggestive of a fundamental difference in crust across the boundary.

Two suites of Precambrian granites heavily intrude the region. An ~1.7 Ga suite was intruded either prior to or synchronous with the suturing of the Mojave and Yavapai provinces. A second suite of ~1.4 Ga granites post-dates the accretion of the amalgamated Mojave/Yavapai/Mazatzal terranes to the North American craton. Both suites are highly magnetic, containing magnetite possibly inherited from a highly oxidized lower crust. As such, they strongly affect the total magnetic signature of the region. The magnetic signatures of the granites on both sides of the suggested boundaries and accompanying reduced magnetic maps are currently being analyzed.