Complex Bedrock Fracture Patterns: A Multipronged Approach to Resolve Their Evolution in Space and Time

Abstract

The complex fault and fracture patterns commonly observed in metamorphic terranes are the cumulative expression of repeated episodes of brittle deformation. The temporal sequence of deformation remains, however, often obscure due to the general lack of systematic overprinting relationships and absolute geochronological constraints. Here we present a multipronged approach combining remote sensing, field work, structural analysis, and paleostress inversion with mineralogical characterization and K‐Ar dating of brittle features to develop an evolutionary model for one such complex fracture pattern. We applied this methodological approach to pervasively fractured Early to Middle Ordovician intrusive rocks in the northern Bømlo islands, SW Norway. The local brittle structural record is interpreted as reflecting a sequence of six deformation stages, three ascribable to the Caledonian orogenic cycle and three to the rift evolution of the North Sea. The Caledonian structures are assigned to (1) Late Ordovician NNW‐SSE transpression, (2) Silurian ESE‐WNW compression, and (3) Devonian NW‐SE transtension. The rift‐related structures formed during (4) Permian to Middle Triassic ENE‐WSW extension (~ 290–245 Ma), (5) Late Triassic to Late Jurassic E‐W extension (~ 210–160 Ma), and (6) Early Cretaceous ESE‐WNW extension (~ 125 Ma). The reconstructed gradual rotation of the extensional stress field during the Mesozoic might reflect the continuous northward migration of the rift activity from the North Sea toward the mid‐Norwegian margin. Our multipronged approach may be applied to the unraveling of complex and commonly long brittle histories stored in exhumed metamorphic terranes elsewhere.

Unraveling Multiple Thermotectonic Events Accommodated by Crustal‐Scale Faults in Northern Iberia, Spain: Insights From K‐Ar Dating of Clay Gouges

“…The polytype of the newly formed illite reflects mostly the thermal boundary conditions, with 1M d being representative of lower‐temperature and 2M 1 of higher‐temperature conditions (e.g., Torgersen et al, ). The age of the coarsest fractions can instead be interpreted as an inherited contribution to the fault rock from the host rock, or alternatively and under certain conditions, it may also record an earlier deformation or thermal event (Scheiber & Viola, ; Torgersen et al, ; Viola et al, , , ). The coarsest fractions are thus in general predominantly characterized by mica or illite‐2M 1 derived from a previous cooling history of the rock or from an earlier faulting or fluid flow/alteration episode.…”

Kinematic Analyses and Radiometric Dating of the Large‐Scale Paleogene Two‐Phase Faulting Along the Median Tectonic Line, Southwest Japan

“…This tendency agrees with previous studies (cf. Scheiber & Viola, 2018;Viola et al, 2018), showing that the amount of authigenic clay minerals increases with decreasing grain size.…”

New Constraints on the Evolution of the Inner Northern Apennines by K‐Ar Dating of Late Miocene‐Early Pliocene Compression on the Island of Elba, Italy

“…With this study we aim to (i) date thrust faults that contributed to the formation of the Apenninic orogenic wedge and elucidate their temporal relationship to the orogenic extension of the NA and the ZF and (ii) test the reliability and further refine the applicability of K‐Ar dating of illite to Neogene deformation episodes. Indeed, the recent advances in integrated K‐Ar dating of brittle faults that have allowed direct chronological constraints to be placed on complex and long‐lived brittle strain localization histories were mostly possible through the study of Mesoproterozoic and Paleozoic structures, while generally overlooking more recent deformation histories (e.g., Scheiber & Viola, ; Torgersen et al, , ; Viola et al, , ). We show here that by means of careful characterization of the dated material, it is possible to reliably date also Miocene and Pliocene fault rocks, thus contributing to the understanding of the tectonic evolution of recent and active orogens.…”