Abstracts for Virtual Thermo2020

March 23, 2021, Session 6

Talk 1

Hanging our interpretations on the footwall? Thermochronology of the hanging wall of a metamorphic core complex

Mark Coleman1*, David Schneider2, Konstantinos Soukis2, Bernhard Grasemann3

1 University of Ottawa
2 National and Kapodistrian University of Athens
3 University of Vienna

* colemanmarco@gmail.com

Within the Attic-Cycladic Complex of Greece, low-angle detachments have accommodated significant extension since the Oligo-Miocene. We present zircon (U-Th)/He thermochronology from the footwall and hanging wall of the northernmost metamorphic core complex within the Cyclades. Bedrock mapping of Mt. Hymittos identified a paired ductile-then-brittle detachment system with top-to-SSW kinematics. The detachments are ~500 metres apart and divide the local tectonostratigraphy into three packages: prehnite-pumpellyite facies phyllites and marbles in the uppermost package (hanging wall; Upper Unit), and high-pressure greenschist-facies schists and marbles in the lower two packages (footwalls; Middle & Lower units). Ductile mylonites in the footwalls of both detachments grade into brittle-ductile mylonites and finally into cataclastic fault cores indicating cooling during extension. Geochronology on samples from the Middle & Lower units produce late Oligocene to early Miocene white mica 40Ar/39Ar ages and mid-to-late Miocene zircon (U-Th)/He ages, suggesting synchroneity of the detachments. Dates from the Upper Unit are more dispersed: white mica 40Ar/39Ar ages range from the Early to Late Cretaceous, whereas zircon (U-Th)/He ages span the Early Cretaceous through the early Oligocene with eU values of 53 to 1084 ppm. Since the footwalls exhumed at relatively high rates, cooling was rapid and residence within the partial retention zone was short. In contrast, forward and inverse modelling of dispersed ages in the Upper Unit can produce significant insight. Forward modelling confirms that some hanging wall samples preserve a detrital population of zircon (U-Th)/He ages, but an overall positive then negative age-eU correlation within the data suggests a discernible thermal signature. Inverse modelling indicates that the majority of the hanging wall experienced a greenschist facies metamorphic event that partially to fully reset most samples, correlated to a regionally recognized Late Cretaceous greenschist to amphibolite facies metamorphic event elsewhere within this tectonic unit. The tectonic unit subsequently became the upper plate of the closure of an oceanic basin during a c. 55 Ma high-pressure metamorphic event and the Upper Unit remained sufficiently cool such that the high-pressure event was not recorded in the zircon (U-Th)/He data. Finally, unmetamorphosed sedimentary samples from late Miocene to Pliocene basins within the hanging wall, overlying the Upper Unit, exhibit similar zircon (U-Th)/He ages and models, supporting the conclusion that sedimentation into these basins was locally derived and occurred as a result of uplift and denudation of the Upper Unit perhaps driven by upward flexure of the metamorphic dome in the late Miocene.

Talk 2

High geothermal gradient in the Alps-Apennine transition zone induced by mantle upwelling

C. Amadori1*, M. Maino1,2,M. Marini3, L. Casini 4, A. Langone2, S. Reguzzi3, B. Carrapa5 and A. Di Giulio1

1 University of Pavia, Department of Earth and Environmental Sciences, Pavia, Italy
2 IGG-CNR U.O.S. – Pavia, Italy
3 University of Milan, Department of Earth Science ‘A. Desio’, Milan, Italy
4 University of Sassari, Department of Chemistry and Pharmacy, Sassari, Italy
5 University of Arizona, Geosciences Department, AZ, USA

* chiara.amadori@unipv.it

The Tertiary Piedmont Basin (TPB) is an episutural basin developed since the Late Eocene at the Alps–Apennines transition zone. During Late Eocene-Miocene time the basin was filled by clastic deposits in response to erosion related to the late stages of Alpine collision and progressive NE-migration of the Apenninic orogenic front. The opening of the Liguro-Provençal Basin driving the anticlockwise drift of the Corsica-Sardinia block marginally affected the basin. Here, we present the results of combined detrital apatite low-T thermochronology (fission-track and (U-Th-Sm)/He dating), zircon U-Pb dating, burial history and 2D numerical modeling in order to better constrain the tectonic-thermal history of the region.

Results show that detrital apatite in Late Priabonian deposits (~34 Ma) experienced fission-track thermal reset at ~25 Ma, similarly to the Ligurian Alps. Conversely, apatite grains in Early Rupelian strata (~32) Ma are only partially annealed, and Late Rupelian-early Miocene sediments (28-16 Ma) have no traces of annealing.

These observations show that the bottom of the TPB sequence and its Alpine basement experienced >110 °C heating before the Miocene. In the eastern sector of the basin, the restored maximum sedimentary thickness of less than 3 km indicates an elevated geothermal gradient of about 45°C/Km to account for this result.

Different mechanisms are tested to explain this high heat flow, anomalous for a thickened orogenic lithosphere, and mantle upwelling (linked to Apennine slab rollback and/or Alpine slab break-off), even coupled with exhumation, is the scenario that better explains the data.

Talk 3

Hematite (U-Th)/He thermochronometry and microstructural analysis constrain past slow slip events adjacent to the southern San Andreas Fault, California

Alexandra A. DiMonte1*, Alexis K. Ault1, Kelly K. Bradbury1, Greg Hirth2

1 Dept. of Geosciences, Utah State University, Logan, UT
2 Dept. of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI

* alex.dimonte@usu.edu

Shallow, transient slow slip events are observed via geodesy along major fault segments worldwide. Slow slip events are an important part of the earthquake cycle, but the influence of mineralogy on the mechanics of shallow slow slip is poorly constrained. Outcrop to nanoscale structures and mineral textures observed in exhumed faults record evidence of varying slip rates in the past and may inform ongoing fault deformation mechanisms at depth. Hematite, a common mineral that precipitates in shallow faults, exhibits textures potentially diagnostic of prior slip rate and is amenable to (U-Th)/He thermochronometry. We report field and scanning electron microscopy (SEM) observations of structures and textures with hematite (U-Th)/He dates from minor fault surfaces in the exhumed basement damage zone of the Painted Canyon fault zone (PCF) in Mecca Hills, CA, adjacent and parallel to the southern San Andreas fault, to evaluate the rock record of slow slip events. Hematite slip surfaces are pervasive within the PCF damage zone in association with ‘green schist’ or minor clay-rich faults. SEM reveals multiple hematite morphologies including ubiquitous high-aspect ratio plates with rounded or serrated grain boundaries, as well as stubby and euhedral, hexagonal plates. The absence of recrystallization textures likely precludes high temperatures generated along faults due to frictional heating during earthquakes. Hematite-filled injection veins formed during fluid overpressure events; interlayered hematite, calcite, and phyllosilicate; S-C fabric; and reworked clasts of hematite imply multiple periods of precipitation and deformation at subseismic velocities. New hematite (U-Th)/He dates (n = 37) expand previously published apatite and hematite (U-Th)/He thermochronometry datasets (Moser et al., 2017; Spotila et al., 2020). Data document PCF exhumation in the upper 2 km from ~2.2 to 0.8 Ma and superimposed fluid flow and hematite mineralization events between ~0.8 to 0.5 Ma. Together, these observations support repeated episodes of hematite precipitation and subsequent deformation during shallow slow slip events