Alex P. Clarke

University College London | UCL · Department of Earth Sciences

Non-volcanic tremor is a particularly enigmatic form of seismic activity. In its most studied subduction zone setting, tremor typically occurs within the plate interface at or near the shallow and deep edges of the interseismically locked zone. Detailed seismic observations have shown that tremor is composed of repeating small low-frequency earthqu...

Block-in-matrix rock assemblages — such as mélanges — are structurally complex units whose fabric is primarily defined, not by planar or linear features as most rock units are, but by the orientation and aspect ratio of irregularly shaped blocks. Despite this, previous attempts to quantitatively characterize mélange fabrics have predominantly focus...

The Middle America Subduction Zone is the type example of an erosive margin; i.e. material is removed from the forearc and incorporated into the subduction channel. The Osa Mélange (SW Costa Rica) occupies the forearc of this subduction zone and is currently being tectonically eroded. This mélange consists of blocks of altered basalt, chert, and ca...

Sediment volume at the trench and topographic highs on the incoming plate are two of the main factors controlling whether a forearc will undergo subduction erosion or accretion. On oceanic plates, topographic highs such as large seamount complexes are commonly associated with significant volumes of flanking volcaniclastic sediments in the form of >...

Subduction zone interface layers are often conceived to be heterogeneous, polyrheological zones analogous to exhumed mélanges. Mélanges typically contain mechanically strong blocks within a weaker matrix. However, our geomechanical study of the Osa Mélange, SW Costa Rica shows that this mélange contains blocks of altered basalt which are now weaker...

The tectonic processes of subduction erosion — where upper plate material is entrained within the subduction zone fault as it migrates upwards — and accretion — where oceanic material is offscraped or underplated to the upper plate — are traditionally considered to operate over laterally extensive portions of convergent margins, with sediment volum...

The Osa Mélange, together with the Osa Igneous Complex, comprise an accreted igneous terrane located in southwest Costa Rica that forms the forearc of the erosive Middle America Subduction Zone at the depth of seismic nucleation. The tectonic setting in which this mélange formed remains unconstrained with both the ocean trench (DiMarco et al., 1995...

The Osa Mélange, together with the Osa Igneous Complex, comprise an accreted igneous terrane located in southwest Costa Rica that forms the forearc of the erosive Middle America Subduction Zone at the depth of seismic nucleation. The tectonic setting in which this mélange formed remains unconstrained with both the ocean trench (DiMarco et al., 1995...

The Osa Mélange, together with the Osa Igneous Complex, comprise an accreted igneous terrane located in southwest Costa Rica that forms the forearc of the erosive Middle America Subduction Zone at the depth of seismic nucleation. The tectonic setting in which this mélange formed remains unconstrained with both the ocean trench (DiMarco et al., 1995...

The Middle America subduction zone is a textbook example of the subduction erosion tectonic process—accounting for half of modern subduction zones—which causes material from the forearc to be entrained into the plate boundary interface as it migrates upwards (Vannucchi et al., 2012). The Osa Peninsula, Costa Rica, lies above the seismogenic portion...

The age of the NW – SE trending mafic dykes at Llanbadrig, north Anglesey may have implications for constraining the contentious age of the Gwna mélange if they were intruded prior to or during the early Ordovician, however their highly weathered nature precludes dating by radiometric or palaeomagnetic means. These dykes are sinuous and highly disc...