Marco G. Malusà

Contrasting styles of (U) HP rock exhumation along the Cenozoic Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)

Since the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel-flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria-Europe plate boundary, and along three different transects (Western Alps, Calabria-Sardinia, and Corsica-Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica-Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present-day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria-Europe plate boundary.

First seismic evidence for continental subduction beneath the Western Alps

The first discovery of ultrahigh-pressure coesite in the European Alps 30 years ago led to the inference that a positively buoyant continental crust can be subducted to mantle depth; this had been considered impossible since the advent of the plate tectonics concepts. Although continental subduction is now widely accepted, there remains debate because there is little direct (geophysical) evidence of a link between exhumed coesite at the surface and subducted continental crust at depth. Here we provide the first seismic evidence for continental crust at 75 km depth that is clearly connected with the European crust exactly along the transect where coesite was found at the surface. Our data also provide evidence for a thick suture zone with downward-decreasing seismic velocities, demonstrating that the European lower crust underthrusts the Adriatic mantle. These findings, from one of the best-preserved and long-studied ultrahigh-pressure orogens worldwide, shed decisive new light on geodynamic processes along convergent continental margins.

Detrital Fingerprints of Fossil Continental-Subduction Zones (Axial Belt Provenance, European Alps)

Collision orogens such as the Alps or the Himalayas are generated by plate convergence, culminating in attempted subduction of a thinned continental margin. Massive amounts of metamorphic rocks displaying high-pressure parageneses are produced during such relatively brief tectonic events and then rapidly exhumed to form the axial backbone of the new orogen. Sediment composition provides a fundamental key to identify past events of continental subduction, although coupled detrital-geochronology techniques are needed to discriminate neometamorphic and paleometamorphic sources of detritus. Within the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments and mostly produces lithic to quartzolithic sedimentaclastic sands yielding very poor heavy mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements and sheds quartzolithic to feldspatholithoquartzose metamorphiclastic sands yielding moderately rich epidote-amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent-ocean transitions and supplies feldspatholithoquartzose/feldspathoquartzose high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely rich suites dominated by garnet, hornblende, or epidote, depending on protoliths (continental vs. oceanic) and pressure/temperature paths during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the heavy mineral concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulic-sorting effects. Rather than the pressure peak reached at depth, the metamorphic index (MI) and hornblende color index (HCI) reflect peak temperatures reached at later stages, when subduction is throttled by arrival of thicker continental crust and geothermal gradients increase, as documented in detritus derived from the Tauern window and Lepontine Dome. Experience gained from modern sediments provides fundamental help to decrypt the information stored in the sedimentary record and thus to identify and reconstruct subduction events of the past.

Continuity of the Alpine slab unraveled by high-resolution P wave tomography

The question of lateral and/or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data. The complex slab structure beneath the Alpine region is only partly resolved by available geophysical data, leaving many geological and geodynamical issues widely open. Based upon a finite-frequency kernel method, we present a new high-resolution tomography model using P wave data from 527 broadband seismic stations, both from permanent networks and temporary experiments. This model provides an improved image of the slab structure in the Alpine region and fundamental pinpoints for the analysis of Cenozoic magmatism, (U)HP metamorphism, and Alpine topography. Our results document the lateral continuity of the European slab from the Western Alps to the central Alps, and the downdip slab continuity beneath the central Alps, ruling out the hypothesis of slab break off to explain Cenozoic Alpine magmatism. A low-velocity anomaly is observed in the upper mantle beneath the core of the Western Alps, pointing to dynamic topography effects. A NE dipping Adriatic slab, consistent with Dinaric subduction, is possibly observed beneath the Eastern Alps, whereas the laterally continuous Adriatic slab of the Northern Apennines shows major gaps at the boundary with the Southern Apennines and becomes near vertical in the Alps-Apennines transition zone. Tear faults accommodating opposite-dipping subductions during Alpine convergence may represent reactivated lithospheric faults inherited from Tethyan extension. Our results suggest that the interpretations of previous tomography results that include successive slab break offs along the Alpine-Zagros-Himalaya orogenic belt might be proficiently reconsidered.

Focused erosion in the Alps constrained by fission-track ages on detrital apatites

Abstract Fission-track dating on detrital apatites from modern sands of the Po Delta is used for a provenance study of sediments in the Po River basin. Analysed samples show a fission-track grain-age distribution characterized by two prominent peaks at 7.7 Ma and 17 Ma. The youngest peak accounts for 46% of the total population of dated grains. This young component in the grain-age distribution is consistent with bedrock cooling ages observed in the Western Alps between the External Massifs and the Houiller unit, as well as in the Lepontine dome of the Central Alps and in the Miocene foredeep units of the Apennines, that overall represent only 12% of the orogenic source area. Results suggest that most of the sediment load in the last 102–105 years was supplied by focused erosion of relatively small areas that experienced short-term erosion rates one order of magnitude higher than in the rest of the belt.

Tracking Adria indentation beneath the Alps by detrital zircon U-Pb geochronology: Implications for the Oligocene–Miocene dynamics of the Adriatic microplate

The Adriatic microplate is a key player in the Western Mediterranean tectonic puzzle, but its Oligocene–Miocene dynamics are not yet fully understood. In fact, even though the timing and magnitude of Adriatic slab rollback and backarc extension in the Apennines have long been established, the timing of progressive Adria indentation beneath the Central Alps and major strike-slip motion along the Insubric fault are still poorly constrained. Here we tackle these issues by utilizing detrital zircon U-Pb geochronology on Adriatic foredeep turbidites, i.e., by comparing the geochronologic fingerprints of the exhuming tectonic domes of the Central Alps (Ticino and Toce subdomes) with those of the Oligocene–Miocene turbidites chiefly derived from their erosion. We analyzed 11 sandstone samples ranging in age from 32 to 18 Ma. The ratio between Variscan and Caledonian zircon grains (which are dominant in the Toce and Ticino subdomes, respectively) sharply increases at ca. 24–23 Ma. This major provenance change marks the westward shift of the Adriatic indenter beneath the Central Alps and the associated right-lateral activity of the Insubric fault. The coexistence of strike-slip motion at the northern boundary of the Adriatic microplate at ca. 24–23 Ma and trench retreat during scissor-type backarc opening to the west requires a near-vertical rotation axis located at the northern tip of the Ligurian-Provencal basin. We propose that the rotation axis position was controlled by the interaction between the European and the Adriatic slabs, which may have collided at depth by the end of the Oligocene, triggering the westward shift of the Adriatic indenter beneath the Central Alps.

Precollisional development and Cenozoic evolution of the Southalpine retrobelt (European Alps)

The retrobelts of doubly vergent collisional orogens are classically interpreted as late-stage postcollisional features. Here, we integrate literature data with new structural and thermochronological evidence from the European Alps in order to document the precollisional development of the retrobelt segment exposed in the central southern Alps. During the Late Cretaceous, by inversion of inherited extensional faults of Permian age, the Variscan basement of the central southern Alps was stacked southward onto the Permian–Mesozoic cover sequences of the Adria margin. These thrust systems were first deformed within regional-scale antiforms (the “Orobic anticlines”) and then cut by Eocene magmatic bodies. Our apatite fission-track data show that these units were largely structured and exhumed to shallow crustal levels before the intrusion of the Eocene magmatic rocks. Therefore, thrusting and folding in the Alpine retrobelt took place before the final closure of the Alpine Tethys and subsequent continental collision between Adria and Europe. Final exhumation and uplift in the northern part of the Southalpine retrobelt took place under a dextral transpressional regime largely coeval with the right-lateral strike-slip activity along the Insubric fault. In Neogene times, deformation propagated southward, leading to the formation of a frontal thrust belt that is largely buried beneath the Po Plain.

A Guide for Interpreting Complex Detrital Age Patterns in Stratigraphic Sequences

Thermochronologic age trends in sedimentary rocks collected through a stratigraphic sequence provide invaluable insights into the provenance and exhumation of the sediment sources. However, a correct recognition of these age trends may be hindered by the complexity of many detrital thermochronology datasets. Such a complexity is largely determined by the complexity of the thermochronology of eroded bedrock that may record, depending on the thermochronologic system under consideration, cooling during exhumation, episodes of magmatic crystallisation, metamorphic mineral growth and/or late-stage mineral alteration in single or multiple source areas. This chapter illustrates how different geologic processes produce different patterns of thermochronologic ages in detritus. These basic age patterns are variously combined in the stratigraphic record and provide a key for the geologic interpretation of complex detrital thermochronology datasets. Grain-age distributions in sedimentary rocks may include stationary age peaks and moving age peaks. Stationary age peaks provide no direct constraint on exhumation, as they relate to episodes of magmatic crystallisation, metamorphic growth or thermal relaxation in the source rocks. Moving age peaks are generally set during exhumation and can be used to investigate the long-term erosional evolution of mountain belts using the lag-time approach. Post-depositional annealing due to burial produces age peaks that become progressively younger down section. The appearance of additional older age peaks moving up section may provide evidence for a major provenance change. When interpreting detrital thermochronologic age trends, the potential bias introduced by natural processes in the source-to-sink environment and inappropriate procedures of sampling and laboratory processing should be taken into account.

Crustal Exhumation of Plutonic and Metamorphic Rocks: Constraints from Fission-Track Thermochronology

The thermal evolution of plutonic and metamorphic rocks in the upper crust may be revealed using fission-track (FT) analyses and other low-temperature thermochronologic methods. The segment of pressure–temperature–time–deformation (P-T-t-D) rock paths potentially constrained by FT data corresponds to the lower greenschist facies, prehnite–pumpellyite, and zeolite facies of metamorphic rocks and also includes regions where diagenetic alteration occurs. When plutonic and metamorphic rocks are exhumed, thermal perturbations caused by fluid alteration, and crystallisation below relevant closure/annealing temperatures at relatively shallow crustal depths, may preclude a simplistic interpretation of thermochronologic ages in terms of monotonic cooling. However, FT ages and track-length measurements provide kinetic data that allow interpretation of T-t paths, even in cases where assumptions based on bulk closure temperatures are violated. We show that geologically well-constrained sampling strategies, and application of multiple thermochronologic methods on cogenetic minerals from plutonic and metamorphic rocks, may provide the most promising means to document the timing, rates, and mechanisms of crustal processes. Case studies are presented for: (1) (ultra)high-pressure (U)HP metamorphic terranes (e.g., Papua New Guinea, Western Alps, Western Gneiss Region, Dabie–Sulu), (2) an extensional orogen (Transantarctic Mountains), (3) a compressional orogen (Pyrenees), and (4) a transpressional plate boundary zone (Alpine fault zone, New Zealand).

From Cooling to Exhumation: Setting the Reference Frame for the Interpretation of Thermochronologic Data

The reference frame for the interpretation of fission-track (FT) data is a thermal reference frame. Using thermochronology to constrain exhumation largely depends on understanding the linkage between this reference frame and Earth’s surface. The thermal frame of reference is dynamic, that is it is often neither stationary nor horizontal, as it is influenced by the shape of the topography, heat advection associated with rapid exhumation and mass redistribution across major faults. Here, we review the nomenclature and basic relationships related to cooling, uplift and exhumation and describe strategies to independently constrain the paleogeothermal gradient at the time of exhumation. In some cases, cooling may not be related to exhumation, but can be used instead to constrain the thermal evolution of the upper crust and the emplacement depth of magmatic rocks. In general terms, useful constraints on exhumation are often only directly provided by thermochronologic ages that are set during undisturbed exhumational cooling across the closure temperature isothermal surface. Thermochronologic ages from minerals crystallised at temperatures less than the closure temperature, e.g. in volcanic rocks and shallow intrusions, provide no direct constraint on exhumation.