Figure 4
From: The Guanacaste Volcanic Arc Sliver of Northwestern Costa Rica
Field photographs. Locations as lower-case bold letters in Figs 2 and 3. (a) Fault trace where it juxtaposes fresh ignimbrite (ignim) against clay gouge (cl). White fieldbook is 14 cm in width. (b) Looking down on ignimbrite with thin, ~vertical clay seams that strike ~305°. White arrows show three seams. Pocket transit 7 cm wide. (c) Cutbank exposure of fault gouge containing anastomosing vertical subsidiary shear planes, slip shown by magenta half arrow. (d) Drainage deflected ~5.5 m dextrally within a small pull-apart basin. Dashed white lines highlight the curvature of the margins of the stream. (e) Thrust scarp in the Upala fault segment. (f) Planar, steeply dipping fault trace in deeply weathered exposure on the road to Tenorio Volcano. Detail at bottom shows steeply raking, subtle lineations (~parallel to pencil) on a polished fault plane preserved in deeply weathered, clay rich fault rock. (g) Fault exposed in an ephemeral outcrop that extended >120 m west from this location along the shores of Lake Arenal in 2008. The poles to fault planes observed in this outcrop are shown in the stereonet in Fig. 2. Photographs annotated using Adobe Illustrator CC 2015 release.
