11 Future Voices in Geoscience- Undergraduate Research Posters

This session is for two-year college and four-year college and university students presenting research posters in any subdiscipline of geosciences. Posters and presentations will be judged for awards.

Monday, 18 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Seismic Velocities of Potential Exhumed Slow Slip and Tremor Source: A Case Study from the Cycladic Blueschist Unit, Ios, Greece

Paper number: 11-14
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Enigmatic slow slip and tremor (SST) events occur along the plate interface of many modern subduction zones near the brittle-ductile transition and are collocated with anomalous seismic properties (Low Velocity Zone, LVZ), including high VP/VS and low velocities. Exhumed rocks suggest that SST may result from coeval brittle-ductile deformation within an interface shear zone characterized by a ductile matrix that accommodates strain during aseismic intervals and brittle blocks that are tremor sources. The Cycladic Blueschist Unit (CBU) exposed on Ios, Greece, is a fossil subduction complex exhumed from SST depths (500–550°C, 1.8–2.0 GPa). Heterogeneous lithologies (e.g., marbles, metabasites, and mica schists) record complex rheology potentially consistent with SST. As a preliminary test, we focused on a cryptic thrust-sense shear zone containing a ~20 m outcrop of metabasite with a block-in-matrix structure. We estimated seismic velocities during early retrogression and underplating and compared to the range of modern LVZs. 

 

We estimated mineral modal abundances for the block and matrix lithologies and calculated seismic velocities using the Matlab Seismic Anisotropy Toolbox (MSAT; Walker & Wookey, 2012). We averaged seismic velocities for a 20° cone centered on foliation-perpendicular incidence angles to account for slab dip and fabric variations and varied block/matrix relative abundances to test sensitivity. The matrix has significantly higher chlorite (35%) than the blocks (15%) and less quartz (15% compared to 20%). Chlorite is aligned within the foliation plane, resulting in significant seismic anisotropy, and increasing the proportion of blocks increases VP (6.61 to 6.83 km/s), VS1 (3.88 to 3.98 km/s), and VP/VS (1.84 to 1.92), and decreases VS2 (3.33 to 3.14 km/s).  

 

We attribute the rheological contrast between the blocks and the matrix to their mineralogy, specifically the matrix’s elevated chlorite content. VP/VS for modern LVZs vary widely (1.8-3.0) but are dominantly higher than our estimates. Minor sensitivity to assumed block/matrix proportions indicates that accurate characterization of the percentages is crucial. Our future calculations will account for other lithologies within the CBU on Ios and fractures. Low seismic velocities, high VP/VS, and rheological contrasts between the matrix and blocks support a potential interpretation of these rocks as SST sources preserved in the rock record, but further work is needed to fully characterize the seismic velocities. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410645

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