Cenozoic tectonic extension in the Sonoran Desert and reconstruction of porphyry copper deposits

Recorded 9 Jan. 2024 comprising 40-minute presentation and 20-minutes of Q&A.
In the past ~30 Ma, severe tectonic extension in the Sonoran Desert region roughly doubled the land area and halved the crustal thickness. Before this time a high mountain range in this area was the source of clastic debris shed southwestward toward the Pacific continental margin and northeastward onto what is now the Colorado Plateau. Extensional demolition and lowering of this mountain range occurred during two episodes. The first episode, at~30-18 Ma, was characterized by low-angle normal faulting, metamorphic-core-complex genesis, and voluminous felsic magmatism. The second was characterized by high-angle normal faulting, subsidence of deep sedimentary basins, and basaltic magmatism. Evaluation of normal-fault offsets along transects through the Pineleño and Rincon Mountains, and in Tucson basin and the Sierrita Mountains, allow determination of approximate total extension in southeastern Arizona and conceptual restoration of this extension to reveal the late Laramide distribution of porphyry copper deposits. The current distribution of porphyry copper deposits is dominated by two belts, one extending approximately from La Caridad and Cananea (Sonora) through the Sierrita Mountains to Sacaton near Casa Grande. The other extends from Resolution and Miami-Inspiration, through Morenci to Chino in New Mexico. Restoration of extension yields a different geometry, as represented by three belts: (1) La Caridad - Cananea - Sierrita - San Manuel - Ray - Resolution - Miami-Inspiration, (2) Ajo - Silver Bell - San Manuel - Safford - Morenci, and (3) Sierrita - Rosemont - Johnson Camp - Chino - Hillsboro. Some deposits are located at the intersections of Belt 1 with belts 2 and 3. Belt 1 is approximately parallel to the Laramide continental margin. The distribution of porphyry copper deposits in this belt is almost certainly related directly to Laramide subduction geometry and tectonics. The strong alignments in belts 2 and 3, each oriented ~N80°E, are interpreted to represent lithologic and geochemical variations in the deep crust that were inherited from Paleoproterozoic crustal genesis. These belts are also aligned with the strike of generally steeply dipping foliations in Pinal Schist and related units and in lithologic layering in some Paleoproterozoic rock units. The interpretation that Paleoproterozoic lithologic variations in deep crust influenced the formation and distribution of porphyry copper deposits is consistent with previously proposed Paleoproterozoic deep crustal signatures in gold-silver ratios of produced Au-Ag ores.
Jon Spencer Bio
Jon Spencer received his Ph.D. in Geology from M.I.T. in 1981. After a year as a USGS post-doc he worked for 33 years for the Arizona Geological Survey (AZGS) as a Research Geologist and then Senior Geologist.