The Sierra de Pie de Palo of northwest Argentina preserves middle to lower crustal metamorphic rocks that were penetratively deformed during Ordovician accretion of the Precordillera terrane to the Gondwana margin. New structural, petrologic, and geochronologic data from a 40 km structural transect reveals that the Sierra de Pie de Palo preserves a middle to lower crustal ductile thrust complex consisting of individual structural units and not an intact ophiolite and cover sequence. Top‐to‐the‐west thrusting occurred intermittently on discrete ductile shear zones from ∼515 to ∼417 Ma and generally propagated toward the foreland with progressive deformation. Ordovician crustal shortening and peak metamorphic temperatures in the central portion of the Sierra de Pie de Palo were synchronous with retro‐arc shortening and magmatic flare‐up within the Famatina arc. Accretion of the Precordillera terrane resulted in the end of arc flare‐up and the onset of synconvergent extension by ∼439 Ma. Continued synextensional to postextensional convergence was accommodated along progressively lower grade shear zones following terrane accretion and the establishment of a new plate margin west of the Precordillera terrane. The results support models of Cordilleran orogens that link voluminous arc magmatism to periods of regional shortening. The deformation, metamorphic, and magmatic history within the Sierra de Pie de Palo is consistent with models placing the region adjacent to the Famatina margin in the middle Cambrian and not as basement to the Precordillera terrane.

Pozzolanic reaction of volcanic ash with hydrated lime is thought to dominate the cementing fabric\ud and durability of 2000-year-old Roman harbor concrete. Pliny the Elder, however, in first century CE\ud emphasized rock-like cementitious processes involving volcanic ash (pulvis) “that as soon as it comes\ud into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum\ud lapidem), impregnable to the waves and every day stronger” (Naturalis Historia 35.166). Pozzolanic\ud crystallization of Al-tobermorite, a rare, hydrothermal, calcium-silicate-hydrate mineral with cation\ud exchange capabilities, has been previously recognized in relict lime clasts of the concrete. Synchrotron-based\ud X-ray microdiffraction maps of cementitious microstructures in Baianus Sinus and Portus\ud Neronis submarine breakwaters and a Portus Cosanus subaerial pier now reveal that Al-tobermorite\ud also occurs in the leached perimeters of feldspar fragments, zeolitized pumice vesicles, and in situ\ud phillipsite fabrics in relict pores. Production of alkaline pore fluids through dissolution-precipitation,\ud cation-exchange and/or carbonation reactions with Campi Flegrei ash components, similar to processes\ud in altered trachytic and basaltic tuffs, created multiple pathways to post-pozzolanic phillipsite and\ud Al-tobermorite crystallization at ambient seawater and surface temperatures. Long-term chemical\ud resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred. Raman\ud spectroscopic analyses of Baianus Sinus Al-tobermorite in diverse microstructural environments\ud indicate a cross-linked structure with Al3+ substitution for Si4+ in Q3\ud tetrahedral sites, and suggest\ud coupled [Al3++Na+\ud ] substitution and potential for cation exchange. The mineral fabrics provide a geoarchaeological\ud prototype for developing cementitious processes through low-temperature rock-fluid\ud interactions, subsequent to an initial phase of reaction with lime that defines the activity of natural\ud pozzolans. These processes have relevance to carbonation reactions in storage reservoirs for CO2 in\ud pyroclastic rocks, production of alkali-activated mineral cements in maritime concretes, and regenerative\ud cementitious resilience in waste encapsulations using natural volcanic pozzolans

Ancient Roman syntheses of Al-tobermorite in a 2000-year-old concrete block submerged in the Bay of Pozzuoli (Baianus Sinus), near Naples, have unique aluminum-rich and silica-poor compositions relative to hydrothermal geological occurrences. In relict lime clasts, the crystals have calcium contents that are similar to ideal tobermorite, 33 to 35 wt%, but the low-silica contents, 39 to 40 wt%, reflect Al 3+ substitution for Si 4+ in Q 2 (1Al), Q 3 (1Al), and Q 3 (2 Al) tetrahedral chain and branching sites. The Al-tobermorite has a double silicate chain structure with long chain lengths in the b [020] crystallographic direction, and wide interlayer spacing, 11.49 Å. Na + and K + partially balance Al 3+ substitution for Si 4+ . Poorly crystalline calcium-aluminum-silicate-hydrate (C-A-S-H) cementitious binder in the dissolved perimeter of relict lime clasts has Ca/(Si+Al) = 0.79, nearly identical to the Al-tobermorite, but nanoscale heterogeneities with aluminum in both tetrahedral and octahedral coordination. The concrete is about 45 vol% glassy zeolitic tuff and 55 vol% hydrated lime-volcanic ash mortar; lime formed <10 wt% of the mix. Trace element studies confirm that the pyroclastic rock comes from Flegrean Fields volcanic district, as described in ancient Roman texts. An adiabatic thermal model of the 10 m 2 by 5.7 m thick Baianus Sinus breakwater from heat evolved through hydration of lime and formation of C-A-S-H suggests maximum temperatures of 85 to 97 °C. Cooling to seawater temperatures occurred in two years. These elevated temperatures and the mineralizing effects of seawater and alkali-and alumina-rich volcanic ash appear to be critical to Al-tobermorite crystallization. The long-term stability of the Al-tobermorite provides a valuable context to improve future syntheses in innovative concretes with advanced properties using volcanic pozzolans.

The Franciscan subduction complex of California is considered a type example of a subduction-accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high-grade blocks within mélange. Many of the high-grade blocks record counterclockwise pressure-temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high-grade blocks show evidence for clockwise pressure-temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U-Pb ages from inclusions in garnet and Lu-Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at~176 Ma. Matrix zircon U-Pb ages and Lu-Hf estimates of final garnet growth ages record a barroisite-amphibolite assemblage overprint of eclogite at~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone.