Rebecca J. Dorsey

Southern Italy preserves a well-studied record of deformation and uplift related to migrating oblique collision, trench retreat, and tearing of subducted ocean slabs. However, the surface expression of these processes is incompletely understood due to a lack of reliable ages for marine terraces >200 m above sea level (masl). Here we use luminescence methods to date shallow marine sands from terraces ~ 100 to 1,000 masl in southern Calabria and interpolate the results with regional surface mapping. Burial ages overlap across the full range of sampled elevations with a weighted mean of 118.8 ± 8.7 thousand years before present (ka). Calculated uplift rates vary from 12 mm yr-1 and throw rates on active normal faults are 0.8–4.2 mm yr-1. We infer that rapid post-120 ka uplift is an ongoing isostatic response to an active tear in the Ionian slab, consistent with recent geophysical observations and predictions of geodynamic models. These results have important implications for assessment of geohazards in southern Calabria and northeast Sicily, one of the highest seismic risk areas in the Mediterranean region.

Previous studies have documented genetic divergence within the last few million years between northern and southern populations of terrestrial plants and animals along the Baja California peninsula, with many studies hypothesising that the divergence is due to an ancient seaway that crossed the central peninsula and created a physical barrier to gene flow. We test this transpeninsular seaway hypothesis through the integrated sedimentology, stratigraphy, paleopedology and ichnology of Pliocene deposits exposed at Mesa San Regis in the San Ignacio trough, a low‐lying area that coincides geographically with the locus of genetic divergence and the hypothesised seaway. This analysis shows that the San Regis beds accumulated in a sandy ephemeral stream system dominated by sheetfloods and channelised flow. ⁴⁰ Ar/ ³⁹ Ar dating yields an age of 4.097 ± 0.076 Ma for an interbedded mafic tephra bed and 3.635 ± 0.013 Ma for a capping basaltic andesite lava. Measured sections reveal 21 palaeosols containing pedogenic carbonate horizons with rhizoliths formed by periods of non‐deposition, development of carbonate‐bearing aridisols and well‐established vegetation in an arid to semi‐arid environment. A trace‐fossil assemblage including Celliforma, Coprinisphaera, Daimonelix, Fictovichnus, Parawanichnus, Megaichnus and Naktodemasis is consistent with inferred climatic conditions and depositional environments. Integration of ⁴⁰ Ar/ ³⁹ Ar ages and palaeosol chronology shows that the ~32‐m‐thick San Regis beds accumulated in 900.4 ± 160.5 ky, between ca 4.6 and 3.7 Ma. After ca 3.7 Ma, the region experienced long‐term base‐level fall and slow regional‐scale incision punctuated by episodic aggradation of terrestrial alluvial deposits separated by nested unconformities. These data record Middle Pliocene deposition in a continental setting by terrestrial and freshwater processes followed by dominantly uplifting and eroding conditions throughout Late Pliocene and Quaternary. These findings preclude marine conditions for the past 10 My and refute the transpeninsular seaway hypothesis, suggesting climate‐modulated glacial refugia and/or adaptation to regional differences in rainfall and ecology as other potential drivers of genetic divergence.

The upper Miocene Boleo Formation (Baja California Sur, México) provides a record of tectonic controls on mixed evaporite-siliciclastic sedimentation in a transtensional pull-apart basin during continental break-up and onset of seafloor spreading in the central Gulf of California. The thin basal limestone, recently dated at 6.35 ± 0.21 Ma, records abrupt onset of subsidence and rapid marine transgression over a formerly emergent landscape. The siliciclastic member contains coarsening-up mud-sand-gravel parasequences that record progradation of fan deltas into a saline shelf. The laterally equivalent gypsum member consists of shallowing-up parasequences formed by evaporative drawdown in the distal evaporite depocenter. Thin, laterally extensive mudstone and breccia units, which host Cu-Co-Zn stratiform ore deposits, record short-lived pulses of marine transgression, fault activity, hydrothermal activity, and mineralization. Growth strata, internal unconformities, and paleocurrent data record NE tilting on a network of NW-striking syn-depositional normal faults during sediment transport to the northeast. Our results support a model for syn-basinal growth of a large monocline above the tip of a propagating oblique-dextral normal fault at the southwest margin of the Santa Rosalía basin. Large-scale displacement across the monocline was the primary mechanism of subsidence, which likely was enhanced by loading related to mafic intrusions and seafloor spreading in the adjacent Guaymas basin. We propose that marine flooding was geologically instantaneous within existing age uncertainties (± 0.2 m.y.) for >1000 km along the Pacific–North America plate-boundary fault system, consistent with models for rapid acceleration of transtensional strain when the plate boundary became localized in the modern Gulf of California at ca. 7–6 Ma.

Long‐term rates of crustal uplift in southern Calabria and NE Sicily are incompletely understood due to limited information about the age of marine terraces at 1.0–1.3 km above sea level (asl). This study provides a new constraint on high‐elevation terrace ages through integrated analysis of geochronology, stratigraphy, shoreline modeling, and fault‐zone morphology. ⁴⁰ Ar/ ³⁹ Ar step‐heating experiments on glass from a tuff in marine claystone of the Argille di Spadafora, NE Sicily, yield reproducible age spectra with a mean age of 0.481 ± 0.019 Ma (±2σ). The Argille di Spadafora and equivalent marl in southern Calabria are overlain by Pleistocene marine terrace deposits, indicating the terraces are younger than 0.50 Ma. Paleoshoreline modeling at Campo Piale, southern Calabria, suggests an age of 525–590 ka for a terrace at 600–630 m asl: this is an overestimate because it assumes no fault offsets, tilting or structural warping despite geomorphic evidence for these processes. We correlate the 630‐m terrace to the marine terrace at 1.0–1.3 km asl by mapping it up a gently inclined relay ramp between the Cittanova and Sant’Eufemia faults, and geomorphic evidence for fault offset of the terrace. We conclude that marine terraces up to 1.0–1.3 km asl are all 2.5 mm/y for the highest terrace, and variable throw rates up to 1.0 mm/y on normal faults that cut the terraces.

Figure S1: Chronology of Amaurolithus genus. Table S1: Results of microfossil analysis. Figure S2: Location of cross section lines, wells, and nannofossil samples.

Figure S1: Chronology of Amaurolithus genus. Table S1: Results of microfossil analysis. Figure S2: Location of cross section lines, wells, and nannofossil samples.

Long-term rates of crustal uplift in southern Calabria and NE Sicily are incompletely understood due to limited information about the age of marine terraces at 1.0–1.3 km above sea level (asl). This study provides a new constraint on high-elevation terrace ages through integrated analysis of geochronology, stratigraphy, shoreline modeling, and fault-zone morphology. 40Ar/39Ar step-heating experiments on glass from a tuff in marine claystone of the Argille di Spadafora, NE Sicily, yield reproducible age spectra with a mean age of 0.481 ± 0.019 Ma (± 2σ). The Argille di Spadafora and equivalent marl in southern Calabria are overlain by Pleistocene marine terrace deposits, indicating the terraces are younger than 0.50 Ma. Paleoshoreline modeling at Campo Piale, southern Calabria, suggests an age of 525–590 ka for a terrace at 600–630 m asl: this is an overestimate because it assumes no fault offsets, tilting or structural warping despite geomorphic evidence for these processes. We correlate the 630-m terrace to the marine terrace at 1.0–1.3 km asl based on recognition of a relay ramp between the Cittanova and Sant’Eufemia faults, and geomorphic evidence for fault offset of the terrace. We conclude that marine terraces up to 1.0–1.3 km asl are all < 0.50 Ma, roughly half the widely cited estimate of 1.0 ± 0.2 Ma. The revised age suggests an average uplift rate of > 2.5 mm/y for the highest terrace, and variable throw rates up to 1.0 mm/y on normal faults that cut the terraces.

Despite forming under different flow conditions, the geometries of tidal and fluvial channel planforms and planform transformations display significant overlap, hindering efforts to differentiate them geometrically. Although studies have demonstrated that globally, tidal and fluvial planforms are statistically distinct based on meander metrics, there are currently no machine‐learning methodologies for classifying channels as tidal or fluvial that do not focus on meander‐specific geometries. In this study, we present a methodology for classifying channel planforms as tidal or fluvial using statistical representations of channel planforms and machine‐learning algorithms. Using a dataset of 4294 tidal and fluvial channel segments (63 channel reaches), we trained three machine‐learning classifiers (Logistic Regression, Multi‐layer Perceptron, and Random Forest) across 69 trials to identify the machine‐learning algorithm and variables that perform best at classifying channel reaches. We evaluated the performance of the classifiers at three thresholds based on the percent of channel segments correctly identified in a given reach (>50%, >66% and >75%). At the >50% classification threshold, all three classifiers attained a 95% reach‐scale accuracy during individual trials. However, at higher classification thresholds, the RF classifier performed best. Feature importances from the RF classifier indicate that measures of the central tendency and minimum/maximum of the normalized radius of curvature convolved with normalized width of channel segments play a key role in differentiating between the planforms, with normalized width also contributing to the difference. This indicates that the relationship between width and radius of curvature is more important than width or measures of curvature on their own. This result likely reflects the downstream funnelling of tidal channels and the limitation on the sharpness of bends associated with increased width. These methods have potential for application in the study of channels preserved on relict geomorphic surfaces and in mixed‐energy settings.

Messina Strait is a narrow fault‐bounded marine basin that separates the Calabrian peninsula from Sicily in southern Italy. It sits in a seismically active region where normal fault scarps and raised Quaternary marine terraces record ongoing extension driven by southeastward rollback of the Calabrian subduction zone. A review of published studies and new data shows that normal faults in the Messina Strait region define a conjugate relay zone where displacement is transferred along strike from NW‐dipping normal faults in the northeast (southern Calabria) to the SE‐dipping Messina‐Taormina normal fault in the southwest (offshore eastern Sicily). The narrow marine strait is a graben undergoing active subsidence within the relay zone, where pronounced curvature of normal faults results from large strain gradients and clockwise rotations related to fault interactions. Based on regional fault geometries and published age constraints, we infer that normal faults in southern Calabria migrated northwest while normal faults in NE Sicily migrated southeast during the past ca. 2–2.5 Myr. This pattern has resulted in tectonic narrowing of the strait through time by inward migration of facing normal faults and rapid mantle‐driven uplift.