I: Normal Faulting on the Austroalpine ‘Overthrust’ Constrained by Thermochronometry and Kinematic Analysis, Central Alps, Graubünden Region, Switzerland. II: Clumped Isotope Thermometry of Carbonate Phases Associated with the Copper Deposits of Kennecott, Alaska

Citation

Price, Jason Brian (2017) I: Normal Faulting on the Austroalpine ‘Overthrust’ Constrained by Thermochronometry and Kinematic Analysis, Central Alps, Graubünden Region, Switzerland. II: Clumped Isotope Thermometry of Carbonate Phases Associated with the Copper Deposits of Kennecott, Alaska. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9JM27NJ. https://resolver.caltech.edu/CaltechTHESIS:05142017-011958758

Abstract

I. A compilation of 362 cooling ages, including 52 newly reported in this study, from nine thermochronometric systems, ⁴⁰K/³⁹Ar amphibole, ⁴⁰K/³⁹Ar white mica, ⁸⁷Rb/⁸⁶Sr white mica, ⁴⁰K/³⁹Ar biotite, ⁸⁷Rb/⁸⁶Sr biotite, zircon and apatite fission track, zircon and apatite (U-Th)/He, indicate that the base of the Austroalpine allochthonous ‘orogenic lid’ was not in full thermal equilibrium with its Penninic substrate until at least the middle Oligocene, approximately 29-28 Ma, to allowably as late as the early Miocene, ca. 18 Ma. There is about a factor-of-five difference in cooling rates between the hanging wall (ca. 4°C/m.y.) and footwall (ca. 20°C/m.y.) during this period. In addition, there are demonstrably higher metamorphic grades, including blueschist- and eclogite-facies, in the Pennine footwall compared to lower greenschist-facies in the Austroalpine hanging wall. Together these two facts demonstrate that hot, high-pressure Penninic nappes were forced upward against the cold, low-pressure overriding Austroalpine plate in a very short time window of approximately 7-10 m.y. between the time of peak metamorphism during the Eocene and the time of thermal equilibration with the overriding plate during the Oligo-Miocene. The most likely mechanism to produce such a cold-on-hot juxtaposition is a normal fault, and therefore, we conclude that an important period of nappe emplacement in the Central Swiss Alps occurred concurrently with orogen-perpendicular normal fault motion at the base of the Austroalpine allochthon persisting well into the Oligocene and possibly into the early Miocene, post-dating the 32-30 Ma age of the Bergell intrusion.

Mesoscopic structural measurements made at the top and bottom of the Pennine zone in eastern Switzerland indicate multiple, spatially heterogeneous directions of movement. At the top, in the Oberhalbstein Valley, movement directions vary from dominantly top-east to top-south-southeast a very minor top-north component within Pennine rocks of the Martegnas shear zone and no preferred movement direction within the Austroalpine hanging wall. Near Piz Toissa, a minimum of two kilometers of nearby structural section in the Err and Platta nappes have been faulted out. At the bottom of the Pennine zone in Val Lumnezia and the Chur Rhein Valley at Trimmis, we observe top-northwest, top-north, and top-northeast movements. In Val Lumnezia, the Sub-Penninic Scopi zone (Gotthard cover rocks) shows movement in a top-northwest direction; the superjacent Peidener imbricate fault zone, a relatively thin (ca. 50 to 100 m thick) structural zone consisting of Scopi zone lithologies, shows movement in a northeasterly direction; above that, the basal Penninic Bündnerschiefer shows no dominant movement direction. To the east, in the Chur Rhine Valley, movement is well defined as exclusively top-north. Therefore, movement directions in the lower Bündnerschiefer are broadly top-north but heterogeneous in direction along strike between Val Lumnezia and Chur Rhein Valley, and, as first suggested by Weh and Frotizheim (2001), it may be erroneous to regard the basal Pennine thrust as a simple through-going structure. In Val Lumnezia, the Scopi-Peidener-Pennine nappes resemble a “jelly sandwich” in which the thick Pennine mass utilized the Peidener zone to move in an oblique sinistral-normal slip sense past the southeast-dipping allochthonous Scopi zone and its east-dipping Gotthard “massif” substrate. If the Peidener zone continues northeastward beneath alluvial cover of the Chur Rhein Valley, it may serve as a late, NE-directed shear zone that separates the Pennine nappes from European units. If so, it would explain the apparent truncation and progressive omission of allochthonous elements of European affinity along the zone from southwest to northeast beneath alluvium of the Chur Rhein Valley. We therefore infer that the direct juxtaposition of Penninic units to the east with the Helvetic autochthon to the west at the latitude of Trimmis records an episode of top-northeast, orogen-parallel strike-slip and extensional movement.

Zircon (U-Th)/He (ZHe) cooling ages from the Oberhalbstein Valley indicate that the Austroalpine-Pennine contact was still active at ca. 27 Ma, and that the Martegnas shear zone was active, in part, between ca. 27 and 24 Ma. It is likely that the Piz Toissa klippe formed around this time during the late Oligocene. The pattern of much younger ZHe ages at the bottom of the Pennine zone is independent of any nappe boundaries, including the Peidener imbricate fault zone, but is consistent with the rise of the Aar massif during the Miocene. Tectonic movements, as recorded by the mesostructure in the Austroalpine, Penninic, and Sub-Penninic domains, and local ZHe cooling ages generally support the conclusion drawn strictly from cooling ages that the Pennine zone was emplaced en masse as a coherent ‘piston’ or ‘mega-pip’ during Oligocene to early Miocene time (approximately 29 to 18 Ma), well after juxtaposition of Apulia with cratonic Europe (continent-continent collision) and during the development of Alpine topography and the peripheral basins (viz. Molasse and Lombardi). Additional top-north movement and late uplift and flexure of the nappe stack, along with the Aar massif, occurred primarily in middle to upper Miocene time, following the post-collisional structural interposition of the Pennine zone between Europe and Apulia.

II. Nine carbonate phases at Kennecott, Alaska were measured for their clumped isotope (∆₄₇) equilibration temperatures. The total range for carbonate temperatures spans 38-164°C. Premineral phases are relatively cool (43-71°C); synmineral phases are relatively warm (89-157°C); late postmineral phases are the most cool (38-59°C) but overlap some premineral phases. Zebra dolomite precipitated in the range 130-163°C. Dedolomite, a hallmark alteration feature of the mineralizing fluids, falls into a narrow range of 98-109°C, consistent with the stability field for the low-temperature chalcocite polymorph. Except for one sample, none of the synmineral calcites crystallized within the stability field of djurleite, a volumetrically significant component of the main-stage ore, which suggests that intergrown djurleite may have been a somewhat later recrystallization product of chalcocite rather than a coeval phase.

Calculated compositions for δ¹⁸O_water vary from -4.2 to +11.0‰. The most depleted water precipitated hydrothermal baroque dolomite, whereas the most enriched water was associated with recrystallized limestone wallrock on the periphery of the orebody. Waters that precipitated calcite+copper vary from -1.1 to +9.3‰.

Intriguingly, rhythmic layering in zebra dolomite can be resolved in ∆₄₇ space, and preliminary data indicate that the coarser-grained baroque dolomite bands precipitated at temperatures 5-10°C cooler than the surrounding, finer-grained dolomite wall rock bands.

The calculated values of δ¹⁸O_water support a genetic model that invokes redox changes associated with fluid mixing as the likely mechanism responsible for copper deposition. In this model a sulfidic, basinal fluid having δ¹⁸O similar to seawater mixes with a cuprous fluid having heavier δ¹⁸O (5 to 8‰) which was derived from the Nikolai Greenstone during prehnite-pumpellyite-facies metamorphism.

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