Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

442 PATA Days 2024 New observations We collected observations along several sections of the coast of Guam that argue for Holocene uplift in the form of raised notches and uplifted fossil reefs (Figures 3 and 4). Fossil corals at 5-6 m elevation collected in the uplifted reef complex at Agua Cove (Fig. 4) returned a Holocene 14 C age of 3495+20, which calibrates to 3.2-3.5 ka. The uplifted reef preserves fresh-appearing corals that lack recrystallization that are found growing on older completely recrystallized coral of presumed Pleistocene age (Fig. 5). Coral collected from the 12 m terrace at another site yielded a 14 C age of 29,600+180, which may date the higher uplifted reef to MIS 3 or 5a. Long term sustained uplift is clearly evident in a sequence of raised coral reef tracts (Figure 6) that rise from shoreline elevations of ~12 m (~30 ka 14 C date) to 160 meters for the capping fringe reef that rings the north end of Guam Island. This region of the Pacific experienced a middle Holocene (4-3 ka) relative sea level (RSL) highstand of 0.3 m to 1.5-1.8 m (measured at Yap and in the Marshall Islands)(Kayanne et al., 1993), which is older Fig. 3: Evidence for Holocene coastal uplift on Guam. The lowest shoreline notch correlates to modern sea-level whereas the raised notches represent uplift. Coral in the lowest raise notch is interpreted to represent inter-seismic subsidence of a raised notch, thereby allowing for coral growth below low tide. than the age of the uplifted coral reef that we dated, which suggests that the mid-Holocene RSL notch may be preserved higher than the coral reef shown in Figure 4. The elevated notches locally have preserved fossil coral that grew in the notch (Fig. 7); similar such coral in elevated notches have been dated elsewhere in the región at 2.3-4 ka and have been interpreted to represent inundation during the high RSL during the middle Holocene, as well as uplift (Kayanne et al. 1993). However, along the NW coast of Guam where we sampled the 3.2-3.5 ka uplifted coral reef at Agua Cove at 5-6m elevation, there are locally at least two well- preserved broad notches, with coral preserved at the base of the notch (Fig. 7). We suggest a simple model where uplift occurs during a mega-thrust event, preserving an uplifted notch, which is then further trimmed during inter-seismic subsidence. Once the base of the notch subsided below low tide, coral growth can occur in the notch until the next uplift event, at which time the coral die and record the timing of uplift. The coral preserved in the notches at Agua Cove have yet to be dated. Fig. 4: Uplifted Holocene coral reef on Guam near Agua Cove. 14 C dating of coral dates the reef to 3.2-3.5 ka.