Magallanes-Austral Basin (MAB) fill is preserved along a >1000 km north-south trending outcrop belt in the southern Patagonia region of Argentina and Chile. Although the stratigraphic evolution of the MAB has been well documented in the Chilean sector (referred to as the Magallanes Basin), its northern terminus in southern Argentina (Austral Basin) is poorly constrained. We present new stratigraphic and geochronologic analyses of the early basin fill (Aptian-Turonian) from the Argentine sector (49-51°S) of the MAB to document spatial variability in stratigraphy and timing of deposition during the initial stages of basin evolution. The initiation of the retroarc foreland basin fill is marked by the transition from mudstone to coarse-clastic deposition, which is characterised by the consistent presence of sandstone beds > ca. 20 cm thick interpreted to represent sediment gravity flows deposited in a submarine fan system. Depositional environments within the early fill of the basin range from lower to upper deep-water fan settings as well as previously undocumented slope deposits. These facies are present as far north as El Chalten, Argentina (ca. 49°S), indicating that facies-equivalent rocks can be traced along-strike for at least 5 degrees of latitude, based on correlation with strata as far south as the Cordillera Darwin (ca. 54°S). Eight new U-Pb zircon ages from ash beds reveal an overall southward younging trend in the initiation of coarse clastic deposition. Inferred depositional ages range from ca. 115 AE 1.9 Ma in the northernmost study area to not older than 92 AE 1 Ma and 89 AE 1.5 Ma in the central and southern sectors respectively. The apparent diachronous delivery of coarse detritus into the basin may reflect (1) gradual southward progradation of a deep-water fan system from a northerly point source and/or (2) orogen-parallel variations in the timing and magnitude of thrust-belt deformation and erosion that provided more local sources for sediment delivery.Correspondence: Matthew A. Malkowski,

The Andean Cordillera is widely considered to be one of the type examples of a convergent margin setting. In the southernmost Andes, however, rifting and volcanism predated mid-Cretaceous breakup of Gondwana and formation of the South Atlantic Ocean by up to 40 m.y. and culminated in the opening of the Rocas Verdes backarc basin east of the Mesozoic Patagonian Batholith. We present new U-Pb geochronology from the Austral sector (49°S-50°S) that indicates rift volcanism occurred between 154 and 147 Ma near the northern terminus of the basin. Available data and observations from the southern Rocas Verdes Basin indicate larger-magnitude and longer-duration extension compared to the northern basin region. The Rocas Verdes Basin underwent progressive northward propagation and opening and was later backfilled concomitantly with the opening of the southern Atlantic Ocean by north-to-south deposition within a retroarc foreland setting. The influence of the inherited tectonic fabric of the Rocas Verdes backarc basin on the subsequent foreland basin explains many unique characteristics of the Patagonian Andes, such as a protracted deep basin that formed atop the previously rifted and weakened crust. Moreover, the early rift history helps account for intraplate deformation of southernmost South America during the opening of the South Atlantic Ocean.

The Farewell terrane of western Alaska is one of the more remote and understudied crustal fragments in the North American Cordillera. Although it is generally accepted that the oldest, Precambrian parts of the Farewell terrane originated along the Arctic margin (i.e., Siberia), the paleogeographic history of the Farewell terrane during much of the middle and late Paleozoic remains unknown. Here, we present new sedimentologic and provenance data from upper Paleozoic clastic strata of the Mystic subterrane, which represents the youngest part of the Farewell terrane. Sedimentary facies consist of high-and low-density sediment-gravity-flow deposits and are interpreted to represent a submarine fan depositional system. Sandstone modal composition trends show a relative abundance of lithic volcanic fragments (~65%) and subordinate occurrences of lithic sedimentary fragments (~15%) and chert (~13%). Laser-ablation-inductively coupled plasma-mass spectrometry analyses of detrital zircons reveal a bulk U-Pb age distribution of Precambrian-Paleozoic grains. U-Pb detrital zircon age spectra from Mississippian strata have a primary peak age between 400 and 325 Ma and secondary peak ages between 480 and 415 Ma and 2000 and 1800 Ma. Devonian-Mississippian zircons exhibit enriched e Hf isotopic values (-3 to-35), whereas Ordovician-Silurian zircons have both enriched (-5 to-25) and depleted (+5 to +14) e Hf values. Age spectra from Permian strata show primary peaks between 320 and 275 Ma and 460 and 415 Ma, with isolated occurrences of Precambrian-age zircons. Pennsylvanian-Permian zircons exhibit depleted e Hf values (+2 to +14). Youngest peak ages support a Mississippian-Early Permian maximum depositional age for this part of the Mystic subterrane. Overall, provenance trends reflect primary detrital contributions from arc and recycled orogen source areas, which included both enriched and primitive magmatic sources. New U-Pb and Hf isotope analyses from the Mystic assemblage match most closely with magmatic source areas of the Alexander and Wrangellia terranes. Findings are consistent with a model where the Farewell terrane was proximal to both the Alexander and Wrangellia terranes by Mississippian-Permian time.

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