Geology of Ammouliani Island (Northern Greece) – implications for the tectono-magmatic evolution of the Serbo-Macedonian Massif

<p>Ammouliani Island is located in Northern Greece with its exposed rocks belonging to the Serbo-Macedonian Massif of the Hellenic hinterland. Its geology is of great importance because it lies in an area where a striking change in trend of the orogenic fabric from NW-SE to ENE-WSW occurs. For this purpose, a geological map at a scale of 1:10,000 is presented based upon detailed fieldwork and the interpretation of satellite imagery. In addition, special interest has been paid to the tectonic elements of the map units. This detailed mapping results in the definition of a new unit, the Ammouliani Unit, which is placed between the underlying Kerdilion and the overlying Vertiskos Unit of the Serbo-Macedonian Massif. The main features of the Ammouliani Unit are the predominance of leucosomes and the intense migmatization and strong shearing. Three main folding events (F1, F2 and F3) have been defined in the exposed rocks of the island with the first related to intense migmatization and shearing, whereas the last two are progressive to each other and are related to Mesozoic orogenic processes. In addition, NE-ENE striking shear zones were observed with the Ammouliani granite placed parallel to these. The bending of the orogenic fabric from NW-SE to ENE-WSW is younger than the F2 and F3 folding events and is related to Tertiary orogenic (tectono-magmatic) processes due to the convergence between Apulia and Eurasia and the ongoing retreat of the Hellenic subduction zone.</p

Insights into the Paleostress Analysis of Heterogeneous Fault-Slip Data by Comparing Different Methodologies: The Case of the Voltri Massif in the Ligurian Alps (NW Italy)

One of the most critical stages in fault-slip data stress analysis is separating the fault data into homogeneous subsets and selecting a suitable analysis method for each subset. A basic assumption in stress tensor computations is that fault activations occur simultaneously under a homogeneous stress regime. With that rationale, this work aims to attain improvements in the paleostress reconstruction from the polyphase deformed region of Voltri Massif in the Ligurian Alps by using already published heterogeneous fault-slip data inverted using best-fit stress inversion methods and in the absence of any tectonostratigraphic and overprinting criteria. The fault-slip data are re-examined and analyzed with a best-fit stress inversion method and the Tensor Ratio Method (TRM) in the absence of any tectonostratigraphic and overprinting criteria. This analysis defines crucial differences in the paleostress history of the Voltri Massif in the Ligurian Alps, and gives insight into the analysis and results of different stress inversion methodologies. Best-fit site stress tensors have substantial diversity in stress orientations and ratios, implying possible stress perturbations in the region. The reason for these diversities is that the Misfit Angle (MA) minimization criterion taken into account in the best-fit stress inversion methods allows for acceptable fault-slip data combinations, which under the additional geological compatibility criteria used by the TRM, are found to be incompatible. The TRM application on this already published and analyzed data defines similar site and bulk stress tensors with fewer diversities in stress orientations and ratios defined from fault-slip data whose orientations always satisfy the same additional geological compatibility criteria induced by the TRM, and not only from the MA minimization criterion. Thus, TRM seems to define stress tensors that are not as sensitive to the input of fault-slip data, compared to the best-fit stress tensors that appear to suffer from the &lsquo;overfitting&rsquo; modeling error. Five distinct TRM bulk paleostress tensors provide a more constrained paleostress history for the Voltri Massif and the Ligurian Alps, which after the restoration of the ~50&deg; CCW rotation, comprise: (a) a transpression&ndash;strike-slip stress regime (T1) with NNE-SSW contraction in Late Eocene, (b) an Oligocene NW-SE extensional regime (T2), which fits with the NW-SE extension documented for the broader area north of Corsica due to a significant change in subduction dynamics, (c) a transient, local, or ephemeral NE-SW transtension (T3) which might be considered a local mutual permutation of the T2 stresses, and (d) a Miocene transpression with a contraction that progressively shifted from ENE-WSW (T4) to NNE-SSW (T5), reflecting the stress reorganization in the Ligurian Alps due to a decrease in the retreating rate of the northern Apennines slab. Therefore, paleostress reconstruction can be fairly described by enhanced Andersonian bulk stress tensors, and requires additional geological compatibility criteria than the criteria and sophisticated tools used by the best-fit stress inversion methods for separating the fault-slip data to different faulting events

Insights into the Paleostress Analysis of Heterogeneous Fault-Slip Data by Comparing Different Methodologies: The Case of the Voltri Massif in the Ligurian Alps (NW Italy)

One of the most critical stages in fault-slip data stress analysis is separating the fault data into homogeneous subsets and selecting a suitable analysis method for each subset. A basic assumption in stress tensor computations is that fault activations occur simultaneously under a homogeneous stress regime. With that rationale, this work aims to attain improvements in the paleostress reconstruction from the polyphase deformed region of Voltri Massif in the Ligurian Alps by using already published heterogeneous fault-slip data inverted using best-fit stress inversion methods and in the absence of any tectonostratigraphic and overprinting criteria. The fault-slip data are re-examined and analyzed with a best-fit stress inversion method and the Tensor Ratio Method (TRM) in the absence of any tectonostratigraphic and overprinting criteria. This analysis defines crucial differences in the paleostress history of the Voltri Massif in the Ligurian Alps, and gives insight into the analysis and results of different stress inversion methodologies. Best-fit site stress tensors have substantial diversity in stress orientations and ratios, implying possible stress perturbations in the region. The reason for these diversities is that the Misfit Angle (MA) minimization criterion taken into account in the best-fit stress inversion methods allows for acceptable fault-slip data combinations, which under the additional geological compatibility criteria used by the TRM, are found to be incompatible. The TRM application on this already published and analyzed data defines similar site and bulk stress tensors with fewer diversities in stress orientations and ratios defined from fault-slip data whose orientations always satisfy the same additional geological compatibility criteria induced by the TRM, and not only from the MA minimization criterion. Thus, TRM seems to define stress tensors that are not as sensitive to the input of fault-slip data, compared to the best-fit stress tensors that appear to suffer from the ‘overfitting’ modeling error. Five distinct TRM bulk paleostress tensors provide a more constrained paleostress history for the Voltri Massif and the Ligurian Alps, which after the restoration of the ~50° CCW rotation, comprise: (a) a transpression–strike-slip stress regime (T1) with NNE-SSW contraction in Late Eocene, (b) an Oligocene NW-SE extensional regime (T2), which fits with the NW-SE extension documented for the broader area north of Corsica due to a significant change in subduction dynamics, (c) a transient, local, or ephemeral NE-SW transtension (T3) which might be considered a local mutual permutation of the T2 stresses, and (d) a Miocene transpression with a contraction that progressively shifted from ENE-WSW (T4) to NNE-SSW (T5), reflecting the stress reorganization in the Ligurian Alps due to a decrease in the retreating rate of the northern Apennines slab. Therefore, paleostress reconstruction can be fairly described by enhanced Andersonian bulk stress tensors, and requires additional geological compatibility criteria than the criteria and sophisticated tools used by the best-fit stress inversion methods for separating the fault-slip data to different faulting events

Structural controls on granitoid-hosted gold mineralization and paleostress history of the Edikan gold deposits, Kumasi Basin, southwestern Ghana

The >9 Moz total aggregate gold endowment at the Edikan mine, Kumasi Basin, Ghana, is contained within a cluster of orogenic gold deposits located along the Akropong fault zone. The granitoid-hosted orebodies at Edikan (e.g., AG2, AG3, Fobinso, Esuajah), essentially an interconnected mesh of gold-bearing quartz veins, formed during deformation event D-3Edk, which postdates the penetrative regional D-2Edk deformation. The gold-bearing quartz veins developed in, and adjacent to, N-S- and NW-SE-trending, low-angle thrust faults that crosscut lithological contacts and earlier formed, steeply dipping D-2Edk faults. Our paleostress analysis shows that the D-3Edk deformation, during which the mineralized fault system developed, was characterized by a WNW-ESE "hybrid" compression that evolved to a strike-slip regime. This progressive deformation is best described with the following stress regimes: WNW-ESE transpression-pure compression (T1) associated with low-angle thrusting, subsequent transpression-strike-slip (T2), and later strike-slip-transtension (T3) associated with steeply dipping strike-slip faulting. The bulk of the granitoid-hosted gold mineralization at Edikan is associated with two principal sets of gold-bearing quartz veins, including low-angle fault-fill veins controlled by thrusts and shallow dipping oblique-extension veins that developed during T1. The activation of the reverse and sinistral strike-slip faults led to the development of restraining jogs characterized by abundant shallow and steeply dipping gold-quartz veins with moderately NE-plunging ore shoots. The geometry of the mineralized fault-fracture meshes is consistent with fault-valve behavior in a horizontal compressive stress regime under sustained conditions of supralithostatic fluid pressures at low differential stress