The Late Permian to earliest Triassic Sonoma orogeny has long been envisioned as the result of an arc-continent collision that closed the Havallah oceanic basin, creating the Golconda allochthon, which was emplaced eastward onto the western edge of the continental margin along the Golconda thrust. Critical reevaluation of available stratigraphic, biostratigraphic, and structural data raise some fundamental issues with this scenario, including: (1) The Golconda allochthon experienced multiple phases of deformation both older and younger than the Sonoma orogeny; (2) the tectonostratigraphic successions in the Golconda allochthon record a disrupted depositional history; (3) these punctuated events and unconformities are mirrored by simultaneous punctuated tectonic disruptions of the adjacent continental margin; (4) some of the lithotectonic units within the Golconda allochthon have clear ties to a magmatic arc. These observations indicated that the Havallah basin did not originate as a simple, post-Antler orogeny rift basin, nor is the Mediterranean model for opening of a basin a solution to the initiation of this basin. Instead they imply a more complex paleogeography for the Havallah basin. The Late Permian–earliest Triassic closure of the Havallah basin did result in the development of the Golconda allochthon sensu stricto , but final emplacement of the Golconda allochthon was likely an Early–Middle Jurassic event.

Plutons of the Patagonian batholith at 48°S can be divided into two series, a calc-alkaline tonalitic (CAT) series and a calc-alkaline granodioritic (CAG) series. The CAT series is characterized by low K 2 O and other LIL element concentrations and predominately consists of tonalite and leucotonalite with little or no K-feldspar. The CAG series is characterized by higher K 2 O and LIL element concentrations and consists of quartz monzodiorite, granodiorite, and granite. Both rock series also contain gabbros. Chemical differences of these two rock series suggest that differentiation of two distinct parental magma types occurred in the formation of the Patagonian batholith. Geochemical studies from other regions in the batholith confirm the existence of these two primary magma groups, and suggest that they were derived from different mantle sources or different depths in the mantle wedge beneath the magmatic arc. The evolved granites and leucotonalites of the batholith are likely partial melts of mafic and intermediate rocks of both magma series induced by a continuing magma flux from the mantle. Sr, Nd, and Pb isotope data suggest that crustal contamination has played an important role in the petrologic and chemical evolution of the batholith at 48°S. These data suggest that the batholith is composed of mixtures of mantle-derived magmas and crustal components, and that there was a progressive decrease in crustal involvement during evolution of the batholith. Initial 87 Sr/ 86 Sr ratios range from 0.7036 to 0.7074 (ɛSr = −14 to 40) and initial 143 Nd/ 144 Nd range from 0.51279 to 0.51217 (ɛNd = 6.7 to −6). There is a good correlation between age and isotopic composition regardless of lithology. The oldest plutons have the highest initial 87 Sr/ 86 Sr ratios and the lowest ɛNd, and progressively younger plutons exhibit progressively lower initial 87 Sr/ 86 Sr ratios and higher ɛNd. The apparent decrease in contamination through time that is suggested by the isotope data can be explained by a process in which the basement accretionary wedge complex dried out and became more refractory with time. This evolution resulted in decreased potential for chemical interaction between magmas and basement material, and thus younger plutons were less contaminated.