Seismic imaging of the western Hellenic subduction zone : the relationship between slab composition, retreat rate, and overriding lithosphere genesis

Abstract

In this dissertation, I investigate the structure and dynamics of the Western Hellenic Subduction Zone (WHSZ) by using two complementary seismic imaging methods and interpreting the resulting images with models that describe the dynamics of retreating subduction. First, I produce high-resolution seismic images across northern and southern Greece using a two-dimensional teleseismic migration method. These images show subducted oceanic crust beneath southern Greece and subducted continental crust beneath northern Greece, with the relative position of the two crusts indicating ~70 km of additional slab retreat in the south relative to the north, a result consistent with the predicted relationship between slab buoyancy and retreat rates in recent geodynamic models. Second, I develop a three-dimensional receiver function imaging method, test it with synthetic data, and use it to constrain along strike variations in lithospheric structure. I find a continuous slab Moho across northern and southern Greece between ~40 and 80 km depth, with a gentle, trench-parallel component of dip accommodating the observed differential slab retreat. The overriding Moho is deepest beneath the northern Hellenides (35-40 km) and shallowest beneath the Aegean Sea (25-30 km). It also exhibits several characteristics consistent with a retreating subduction model: (1) it is asymmetric when viewed perpendicular to the trench, not symmetric as has been found in previous studies, (2) the location of its leading edge closely tracks the 70 km depth contour of the slab Moho, (3) a well-developed Moho is not observed below the peak topography of the Hellenides, and (4) it exhibits Moho depth fluctuations that are much larger than those predicted assuming surface topography is locally compensated by Airy- Heiskanen isostasy (>+/-4 km). Finally, I combine the seismic-based constraints with those from geologic data and geodynamic models to better understand how the overriding lithosphere is built and deformed during slab retreat. In northern Greece, the overriding crust is found to be predominately built by accretion of slab-derived continental blocks, while in southern Greece the present-day subduction of an oceanic slab domain has caused previously accreted continental blocks to rapidly extend, yielding an asymmetric, valley-shaped pattern in the top of the crystalline basement.