The Black Sea thermal architecture: simple or pure crustal shearing?

Abstract

Extensional or compressional processes disturb lithospheric thermal structure producing positive or negative thermal anomalies expressed through heat flow measurements. When measurements contradict lithospheric or crustal architecture an exhaustive analysis of shallow and deep processes is required. In particular, this is the case of the Black Sea basin, which shows in its center a low heat flow (~30mW/m2) and an over-extended crust. The Black Sea is the largest European back-ark basin of “oceanic” bearing, related to the closure of various Tethyan branches or inversion of smaller intra-continental rifts. The basin formed in response to the northward subduction of Neotethys under Rhodope-Pontides, during Cretaceous-Eocene. Its separation in two sub-basins suggests an asynchronous opening from west to east starting Early Cretaceous (125-135Ma) to Late Cretaceous-Eocene (70-40Ma), although a single Early Cretaceous synchronous event is not excluded. This long-lasting extension suggests the formation of transitional to oceanic crust in the central parts of these sub-basins. Basin inversion occurred with Late Eocene, being related to the continental collision of Pontides-Taurides arc and equivalents. Two important questions related to the Black Sea lithosphere architecture and its kinematics are still open: the presence of a typical MORB oceanic crust in the basin center and time frame of its accretion. Our goal is to reconcile contradicting low heat flow measured in the basin center with crustal/ lithospheric structure and stretching models. A 3D finite-element model using realistic thermal parameters was constructed taking advantage of recent basin-wide 2D seismic surveys -Black Sea SPAN. This detailed reconstruction of the today’s Black Sea lithosphere thermal state conducting towards a better understanding of past basin genesis and kinematics. The modeling considers simple-shear and pure-shear rifting types and assesses the time since rifting effects on the heat flow. The simple-shear model considers a low angle detachment sensu Wernicke cutting the lithosphere with a completely attenuated crust along the fault exposing mantle part of the lithosphere in the central part of the basin. The McKenzie’s pure-shear model assumes a homogeneous lithospheric stretching with a factor of 20 that leads to continental break up and oceanic crust accretion. Basin infill thermal parameters were correlated with past transient processes, regional lithologies, and compaction trends, while for deeper lithospheric layers typical oceanic thermal parameters were considered. Simple-shear models always show low heat flow values and independent from rifting termination, the results were always in agreement with the observed value of ~30mW/m2. The pure-shear models consider several stretching periods that end with: Middle Cretaceous (~100Ma) this would generate a present-day heat flow close to ~35mW/m2, slightly higher than the observed value and on the error range margin of heat flow modelling because of parameters uncertainty; Late Cretaceous (~70Ma) the resulting model heat flow is ~40mW/m2, considerably higher than the observed heat flow; Paleogene (~40Ma) resulting in even higher heat flow of ~50-60mW/m2. The modeling accepts that both simple-shear and pure-shear processes are acceptable as genetic mechanisms. The pure-shear deformation that can generate oceanic MORB type crust is not acceptable after Late Cretaceous, requiring a heat flow above today’s measurements. The simple-shear deformation allows flexible crustal stretching times while preserving the low heat flow, but its kinematic consequence excludes any oceanic crust accretion, the current interpretations showing in fact either the lower crust or the upper mantle itself as the basement.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018