Episodic intraplate deformation of stable continental margins: evidence from Late Neogene and Quaternary marine terraces, Cape Liptrap, Southeastern Australia

Abstract

The Waratah Fault is a northeast trending, high angle, reverse fault in the Late Paleozoic Lachlan Fold Belt at Cape Liptrap on the Southeastern Australian Coast. It is susceptible to reactivation in the modern intraplate stress field in Southeast Australia and exhibits Late Pliocene to Late Pleistocene reactivation. Radiocarbon, optically stimulated luminescence (OSL), and cosmogenic radionuclide (CRN) dating of marine terraces on Cape Liptrap are used to constrain rates of displacement across the reactivated Waratah Fault. Six marine terraces, numbered Qt 6–Tt 1 (youngest to oldest), are well developed at Cape Liptrap with altitudes ranging from ∼1.5 m to ∼170 m amsl, respectively. On the lowest terrace, Qt 6, barnacles in wave-cut notches ∼1.5 m amsl, yielded a radiocarbon age of 6090–5880 Cal BP, and reflect the local mid-Holocene sea level highstand. Qt 5 yielded four OSL ages from scattered locations around the cape ranging from ∼80 ka to ∼130 ka. It formed during the Last Interglacial sea level highstand (MIS 5e) at ∼125 ka. Inner edge elevations (approximate paleo high tide line) for Qt 5 occur at distinctly different elevations on opposite sides of the Waratah Fault. Offsets of the inner edges across the fault range from 1.3 m to 5.1 m with displacement rates ranging from 0.01 mm/a to 0.04 mm/a. The most extensive terrace, Tt 4, yielded four Early Pleistocene cosmogenic radionuclide (CRN) ages: two apparent burial ages of 0.858 Ma ± 0.16 Ma and 1.25 Ma ± 0.265 Ma, and two apparent exposure ages of 1.071 Ma ± 0.071 Ma ( 10Be) and 0.798 Ma ± 0.066 Ma ( 26Al). Allowing for muonic production effects from insufficient burial depths, the depth corrected CRN burial ages are 1.8 Ma ± 0.56 Ma and 2.52 Ma ± 0.88 Ma, or Late Pliocene. A Late Pliocene age is our preferred age. Offsets of Tt 4 across the Waratah Fault range from a minimum of ∼20 m for terrace surface treads to a maximum of ∼70 m for terrace bedrock straths. Calculated displacement rates for Tt 4 range from 0.01 mm/a to 0.04 mm/a (using a Late Pliocene age, ∼2 Ma), identical to the rates calculated for the Last Interglacial terrace, Qt 5. This indicates that deformation at Cape Liptrap has been ongoing at similar time-averaged rates at least since the Late Pliocene. The upper terraces in the sequence, Tt 3 (∼110 m amsl), Tt 2 (∼140 m) and Tt 1 (∼180 m) are undated, but most likely correlate to sea level highstands in the Neogene. Terraces Tt1–Tt4 show an increasing northward tilt with age. The Waratah Fault forms a prominent structural boundary in the Lachlan Fold Belt discernible from airborne magnetic and bouger gravity anomalies. Seismicity and deformation are episodic. Episodic movement on the Waratah Fault may be coincident with sea level highstands since the Late Pliocene, possibly from increased loading and elevated pore pressure within the fault zone. This suggests that intervals between major seismic events could be on the order of 100 ka.