Exploring mechanical stratigraphy's influence on deformation and seismicity: A 2D kinematic modelling study in the central External Dinarides

The overall SW-vergent and in-sequence structural architecture of the Dinarides fold and thrust belt resulted from collision of the Adriatic Microplate with Eurasia in the Late Cretaceous. Throughout Eo-Oligocene times, the deformation front extended outward, causing substantial crustal shortening within the External Dinarides. This part of the orogen is predominantly composed of Mesozoic carbonate platform rocks, originally deposited in an external passive margin setting. Analysis of fault kinematics and two balanced cross-sections suggests a Cenozoic deformation characterized by along-strike contraction. The 250 km long dextral transgressive Split-Karlovac Fault acts as a boundary, separating a southern, SW-vergent nappe stack forethrust dominated domain (as observed in the Split cross-section) from a northern western NE-vergent backthrust dominated segment (as observed in the southern Velebit cross-section). To understand the reasons for the contrasting along-strike deformation, a reevaluation of the temporal and spatial distribution of Paleo-Mesozoic lithofacies along- and across-strike on both sides of the Split-Karlovac Fault was conducted. Additionally, an assessment of the impact of mechanical stratigraphy on deformation styles in this section of the fold-thrust belt was undertaken.

The best-fit kinematic forward model for the central Velebit Mtn. portrays a 75 km wide triangle zone, which took up at least 47 km of Eo-Oligocene shortening. The triangle structure comprises a SW-vergent thick-skinned duplex system detached in the lower Paleozoic Adriatic basement and five thin-skinned backthrusts detached in the upper Paleozoic basement. These backthrusts nucleated at lateral facies boundaries, related to extensional half grabens that formed due to passive margin extension in Middle Triassic and Late Jurassic times. During Cenozoic folding and thrusting these inherited Mesozoic half graben boundary faults were selectively inverted into the NE-vergent backthrusts. This process contributed to the observed along-strike variations in the deformation style of the External Dinarides.

An analysis of instrumentally recorded earthquakes within the northwestern structural domain unveils contrasting seismic activity along the central and southern Velebit transects. In the central Velebit Mountain, the triangle structure currently predominantly undergoes strike-slip motion, with reverse faulting predominantly occurring to the east of the Split-Karlovac Fault. Conversely, seismic activity along the southern Velebit cross-section is concentrated in the structurally lowermost parts of the triangle zone and the foreland, while its structurally higher sections exhibit lower seismic activity. The prevalence of reverse faulting along this transect suggests the ongoing accommodation of shortening in this region.