Importance of local uplift rates for the creation of marine platforms and terraces built by repeated sea-level occupations

Abstract:

Marine terraces are pivotal landforms for tectonic and climatic studies. They serve as passive markers to record uplift rates along the coast at a timescale of several tens to hundreds of kyr integrating repeated earthquake cycles. Marine terraces also record parts of past sea-level evolution. By knowing the age and elevation of a marine terrace and one out of two critical parameters — local uplift rate or local sea-level curve — the other can be constrained. Here we evaluate the possibility that 1) terraces do not always record the elevation of the high-stand they are associated with, and that 2) marine terraces may not record all uplift rates with equal likelihood.

Marine terraces result from the creation shallow marine platforms that are then emerged and preserved on land. The marine platforms are carved by wave erosion. The more time waves can do work at a given bedrock elevation datum, the larger is the bevelled platform. After relative sea level fall and the emergence of previously platform, the less erosion is done by the same wave process at the elevation datum immediately below, the larger is the preserved terrace above.

Individual marine terraces are typically associated with individual sea level high-stands due to the rapid sea level fall that follows (abandonment and preservation) and due to the period of equal rates of sea level rise and rock uplift before the high stand peak. However, the period of time available for erosion during Interglacial high-stands is very limited compared to the length of the Glacial “medium-” and low-stands. The total sea level occupation at different bedrock elevations integrated over several Glacial-Interglacial cycles vary depending on the rock uplift rate. The bedrock elevations most often occupied by sea level (with reoccupation) without significant occupation immediately below, and thereby most likely to host wide terraces, are unevenly distributed and do not necessarily correspond to sea level high-stands.

A global database of marine terraces shows a potential overrepresentation of sites uplifting at 0.7–1.2 mm/yr. In this uplift rate window, the narrow range of bedrock elevations that is occupied by sea-level for over 20 kyr during MIS 6d-e will be reoccupied during the brief MIS 5e high-stand (whilst not necessarily recording the effective sea-level at MIS 5e).