An official website of the United States government
Abstract How tectonic forcing, expressed as base level change, is encoded in the stratigraphic and geomorphic records of coupled source‐to‐sink systems remains uncertain. Using sedimentological, geochronological and geomorphic approaches, we describe the relationship between transient topographic change and sediment deposition for a low‐storage system forced by rapid rock uplift. We present five new luminescence ages and two terrestrial cosmogenic nuclide paleo‐erosion rates for the late Pleistocene Pagliara fan‐delta complex and we model corresponding base level fall history and erosion of the source catchment located on the Ionian flank of the Peloritani Mountains (NE‐Sicily, Italy). The Pagliara delta complex is part of the broader Messina Gravel‐and‐Sands lithostratigraphic unit that outcrops along the Peloritani coastal belt as extensional basins have been recently inverted by both normal faults and regional uplift at the Messina Straits. The deltas exposed at the mouth of the Pagliara River have constructional tops at ca. 300 m a.s.l. and onlap steeply east‐dipping bedrock at the coast to thickness between ca. 100 and 200 m. Five infrared‐stimulated luminescence (IRSL) ages collected from the delta range in age from ca. 327 to 208 ka and indicate a vertical long‐term sediment accumulation rate as rapid as ca. 2.2 cm/yr during MIS 7. Two cosmogenic10Be concentrations measured in samples of delta sediment indicate paleo‐erosion rates during MIS 8–7 near or slightly higher than the modern rates of ca. 1 mm/yr. Linear inversion of Pagliara fluvial topography indicates an unsteady base level fall history in phase with eustasy that is superimposed on a longer, tectonically driven trend that doubled in rate from ca. 0.95 to 1.8 mm/yr in the past 150 ky. The combination of footwall uplift rate and eustasy determines the accommodation space history to trap the fan‐deltas at the Peloritani coast in hanging wall basins, which are now inverted, uplifted and exposed hundreds of metres above the sea level.
more »
« less
A FAN-DELTA ARCHIVE OF EARTHQUAKES, BASIN INVERSION, AND MOUNTAIN BUILDING IN THE PELORITANI MOUNTAINS OF NORTHEASTERN SICILY
Rapid sediment accumulation rates (SAR) in a fan delta situated on the rapidly uplifting footwall of the Taormina normal fault in NE Sicily preserves a rare record of earthquakes and base level change for a tightly coupled source to sink system. We use this sedimentary archive to reconstruct the kinematics and slip history of the fault and further an understanding of how tectonic forcing across various scales are encoded in stratigraphy. A revised luminescence-based age model indicates that ~82 m of the Pagliara fan-delta foreset facies was deposited in ~11 ka at a mean SAR of ~0.74 cm/yr during MIS 7. Syn-depositional terrestrial cosmogenic nuclide (TCN) determined paleoerosion rates of 0.91±0.12 mm/yr and 1.31 ±0.61 mm/yr are similar to published modern erosion rates for the Pagliara basin of 0.97 ±0.11 mm/yr. At the stratigraphic scale, a time series of magnetic susceptibility (c) sampled at 1 m intervals in the foresets displays four ~2,800 yr / 20 m-thick cycles of growing c, bounded by sharp decreases that do not coincide with changes in sediment texture. The c of the low-grade metamorphic bedrock in the source is 20-100 times weaker than the c of rubified soils mantling the hillslopes, which is comparable to the c of the delta sediments. We propose that large, bedrock-cored landslides quasi-periodically deliver weak c sediment to the delta that dilutes a c signal otherwise dominated by the stripping of soil-mantled hillslopes. We propose that centennial-scale recurrence interval earthquakes are most capable at triggering a basin-scale landslide only after channel incision has increased relief of hillslopes to the threshold condition, which requires millennia to achieve. At the landscape scale of delta geometry and location, the Pagliara delta accumulated in a hanging wall basin that has since been inverted. We reconstruct the history of base level fall for the delta from an inversion of fluvial topography and apportion that record to its rock uplift, delta deposition, and eustatic components. We show that footwall uplift has been unsteady over the past 600 ka ranging from -1 to 3 mm/yr. The integration of our stratigraphic- and landscape scale observations furthers our understanding of the natural hazards related to normal fault earthquakes and their impact on sediment dynamics in this steep, active tectonic setting.
more »
« less
- Award ID(s):
- 1904262
- PAR ID:
- 10506361
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Memorials Geological Society of America
- ISSN:
- 0091-5041
- ISBN:
- 9780932653291
- Format(s):
- Medium: X
- Location:
- Pittsburgh, PA
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We explore time series of magnetic susceptibility (χ) and anhysteretic remanent magnetization (ARM) in settings of rapid sediment accumulation rate (SAR) with the goals of partitioning exogenic forcings from autogenic processes and to better understand how these magnetic signals are encoded in sedimentary archives. Environmental signals of periodic external forcings commonly operate at Milankovitch frequencies, but in rapid SAR settings autogenic processes including channel avulsions and delta lobe switching both shred high-frequency external forcings, or even impart their own quasi-periodic signals. We measure χ using both a hand-held KT-10 magnetic susceptibility meter and a lab-based Kappabridge KLY-3s, and ARM in the < 2 mm size fraction using a GSD-5 alternating-field and a 2G superconducting magnetometers, with all results mass normalized to SI units. We focus on 40 samples collected at 25 cm intervals from 10 m of propagating foresets in a Gilbert delta of the Provo stage of Lake Bonneville at High Creek, Utah. A luminescence-based age model in this delta establishes a mean SAR of 8 cm/yr and terrestrial cosmogenic nuclide concentrations of both delta sediment and alluvium in the source indicates modern and paleoerosion rates (E) ranging from ~60-100 m/Myr (0.006-0.01 cm/yr). Periodicities of 18 and 33 yrs in the rock magnetic time series are greater than twice the compensation time for these foresets where peaks in χ and ARM are positively correlated with fine-grained matrix. We interpret a positive correlation between E and χ as driven by stripping of soil-mantled hillslopes that harbor greater concentrations of magnetic minerals than the underlying bedrock. The encoding of the environmental signal, here interpreted as autogenic cascading of sediment on foreset surfaces, is primarily set by the SAR and depositional processes, which are decoupled from E. Nevertheless, the strength of the magnetic signal in our sedimentary archive varies with E which can be more widely explored as a E-proxy when locally calibrated. These results offer insight into how to isolate the impact of quasi-periodic tectonic forcings on stratigraphic archives at sub-Milankovitch frequencies, where autogenic processes dominate depositional processes but which also encode critical human-dimension natural hazard information.more » « less
-
null (Ed.)ABSTRACT The stratigraphic record of Cenozoic uplift and denudation of the Himalayas is distributed across its peripheral foreland basins, as well as in the sediments of the Ganges–Brahmaputra Delta (GBD) and the Bengal–Nicobar Fan (BNF). Recent interrogation of Miocene–Quaternary sediments of the GBD and BNF advance our knowledge of Himalayan sediment dispersal and its relationship to regional tectonics and climate, but these studies are limited to IODP boreholes from the BNF (IODP 354 and 362, 2015-16) and Quaternary sediment cores from the GBD (NSF-PIRE: Life on a tectonically active delta, 2010-18). We examine a complementary yet understudied stratigraphic record of the Miocene–Pliocene ancestral Brahmaputra Delta in outcrops of the Indo-Burman Ranges fold–thrust belt (IBR) of eastern India. We present detailed lithofacies assemblages of Neogene delta plain (Tipam Group) and intertidal to upper-shelf (Surma Group) deposits of the IBR based on two ∼ 500 m stratigraphic sections. New detrital-apatite fission-track (dAFT) and (U-Th)/He (dAHe) dates from the Surma Group in the IBR help to constrain maximum depositional ages (MDA), thermal histories, and sediment accumulation rates. Three fluvial facies (F1–F3) and four shallow marine to intertidal facies (M1–M4) are delineated based on analog depositional environments of the Holocene–modern GBD. Unreset dAFT and dAHe ages constrain MDA to ∼ 9–11 Ma for the Surma Group, which is bracketed by intensification of turbidite deposition on the eastern BNF (∼ 13.5–6.8 Ma). Two dAHe samples yielded younger (∼ 3 Ma) reset ages that we interpret to record cooling from denudation following burial resetting due to a thicker (∼ 2.2–3.2 km) accumulation of sediments near the depocenter. Thermal modeling of the dAFT and dAHe results using QTQt and HeFTy suggest that late Miocene marginal marine sediment accumulation rates may have ranged from ∼ 0.9 to 1.1 mm/yr near the center of the paleodelta. Thermal modeling results imply postdepositional cooling beginning at ∼ 8–6.5 Ma, interpreted to record onset of exhumation associated with the advancing IBR fold belt. The timing of post-burial exhumation of the IBR strata is consistent with previously published constraints for the avulsion of the paleo-Brahmaputra to the west and a westward shift of turbidite deposition on the BNF that started at ∼ 6.8 Ma. Our results contextualize tectonic controls on basin history, creating a pathway for future investigations into autogenic and climatic drivers of behavior of fluvial systems that can be extracted from the stratigraphic record.more » « less
-
null (Ed.)ABSTRACT The 72-km-long Teton fault in northwestern Wyoming is an ideal candidate for reconstructing the lateral extent of surface-rupturing earthquakes and testing models of normal-fault segmentation. To explore the history of earthquakes on the northern Teton fault, we hand-excavated two trenches at the Steamboat Mountain site, where the east-dipping Teton fault has vertically displaced west-sloping alluvial-fan surfaces. The trenches exposed glaciofluvial, alluvial-fan, and scarp-derived colluvial sediments and stratigraphic and structural evidence of two surface-rupturing earthquakes (SM1 and SM2). A Bayesian geochronologic model for the site includes three optically stimulated luminescence ages (∼12–17 ka) for the glaciofluvial units and 16 radiocarbon ages (∼1.2–8.6 ka) for the alluvial-fan and colluvial units and constrains SM1 and SM2 to 5.5±0.2 ka, 1σ (5.2–5.9 ka, 95%) and 9.7±0.9 ka, 1σ (8.5–11.5 ka, 95%), respectively. Structural, stratigraphic, and geomorphic relations yield vertical displacements for SM1 (2.0±0.6 m, 1σ) and SM2 (2.0±1.0 m, 1σ). The Steamboat Mountain paleoseismic chronology overlaps temporally with earthquakes interpreted from previous terrestrial and lacustrine paleoseismic data along the fault. Integrating these data, we infer that the youngest Teton fault rupture occurred at ∼5.3 ka, generated 1.7±1.0 m, 1σ of vertical displacement along 51–70 km of the fault, and had a moment magnitude (Mw) of ∼7.0–7.2. This rupture was apparently unimpeded by structural complexities along the Teton fault. The integrated chronology permits a previous full-length rupture at ∼10 ka and possible partial ruptures of the fault at ∼8–9 ka. To reconcile conflicting terrestrial and lacustrine paleoseismic data, we propose a hypothesis of alternating full- and partial-length ruptures of the Teton fault, including Mw∼6.5–7.2 earthquakes every ∼1.2 ky. Additional paleoseismic data for the northern and central sections of the fault would serve to test this bimodal rupture hypothesis.more » « less
-
Abstract The quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/-0.22 Ma to 1.06 +0.23/-0.26 Ma, and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastward from 0.60 +0.05/-0.06 Ma to 3.30 +0.30/-0.41 Ma. Our geochronology is used to calculate rapid long-term reverse fault slip rates of 8.6–12.6 mm yr–1 since ca. 1.0 Ma for the San Cayetano fault and 1.3–3.0 mm yr–1 since ca. 1.0 Ma for the Oak Ridge fault, which are both broadly consistent with contemporary reverse slip rates derived from mechanical models driven by global positioning system (GPS) data. We also calculate terrestrial cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that range from 0.05–1.14 mm yr–1 and discuss the applicability of TCN-derived, catchment-averaged erosion rates in rapidly uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly uplifting mountain belts, catchments may attain a quasi-steady-state on timescales of <105 years even if catchment-averaged erosion rates are still adjusting to tectonic forcing.more » « less
