Geophysical-Petrological Modeling of the Crust and Upper Mantle within the Central Mediterranean and Topographic Implications
Resumen Abstract Índice Conclusiones
Zhang, Wentao
2025-A
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Premio Tesis Doctoral en Geofísica
Resumen
La región del Mediterráneo central es una zona sísmica muy activa con características tectónicas únicas, formando parte del sistema orogénico Alpino-Mediterráneo, se extiende desde el sur de Iberia hasta el extremo occidental del mar Egeo. Este cinturón orogénico se originó por la convergencia de las placas africana y euroasiática desde el Cretácico superior. La rotación en sentido antihorario de la microplaca de Adria, después de la colisión, desencadenó la apertura asimétrica de la Cuenca del Tirreno y el adelgazamiento tectónico a lo largo de todos sus márgenes: los Apeninos, el cinturón de Calabria, el cinturón Dinárico-Helénico, los Alpes y el cinturón de los Cárpatos.
Utilizando un modelo geofísico-petrológico (LitMod2D_2.0), esta tesis investiga la estructura termoquímica de la litosfera y del manto sublitosférico bajo la microplaca de Adria y sus márgenes, explorando la geometría y la continuidad en profundidad de las losas litosféricas (slabs) de los márgenes de Adria. Así, se han modelado dos perfiles, hasta 400 km de profundidad, que se extienden desde la Cuenca del Tirreno hasta la región de los Cárpatos-Balcanes, en una dirección aproximadamente NE-SO. El perfil del norte, de unos 1000 km de longitud, atraviesa la Cuenca del Tirreno, los Apeninos septentrionales, el mar Adriático, los Dinárides y la Cuenca Panónica. El perfil del sur abarca unos 1250 km, atravesando la Cuenca del Tirreno, los Apeninos meridionales, el mar Adriático, los Dinárides meridionales y los Cárpatos-Balcanes. Además, en esta tesis he desarrollado un nuevo código de modelización numérica del flujo del manto para evaluar la respuesta topográfica de las dos subducciones de vergencia opuesta a lo largo de estos perfiles y discuto sus implicaciones geodinámicas en la evolución de la región.
En el perfil norte, los resultados muestran que la microplaca de Adria tiene una estructura más compleja y una densidad media ligeramente superior en comparación con la microplaca de Tisza. Bajo la Cuenca del Tirreno y los Apeninos occidentales, la Moho se encuentra a una profundidad inferior a 25 km, mientras que en los Apeninos orientales llega hasta los 55 km. El límite litosfera-astenosfera (LAB) bajo las cuencas del Tirreno y Panónica es plano y se encuentra a ∼75 y 90 km, respectivamente. Bajo los Apeninos externos y los Dinárides, el LAB (Límite Litosfera-Astenosfera) alcanza los 150 km, mientras que es menos profundo hacia la cuenca adriática llegando a los 125 km de profundidad. Mis resultados son consistentes con la presencia de dos cuñas de manto astenosférico, resultado del retroceso de las losas oceánicas del Ligúrico-Tetis al SO y Vardar-NeoTetis al NE, seguido de la delaminación del manto continental en los márgenes distales oriental y occidental de Adria. Estas dos losas de vergencia opuesta, localizadas bajo los Apeninos y los Dinárides, se han modelado como dos anomalías térmicas sublitosféricas de −200°C.
En el perfil sur, los resultados muestran la presencia de dos cuñas de manto astenosférico alineadas con el retroceso de las losas del manto continental apenínico y dinárico, junto con anomalías térmicas (−200 °C) sublitosféricas bajo los márgenes NE y SO de Adria. Al norte de Adria, se observa adelgazamiento de la litosfera en el dominio del Tirreno y engrosamiento hacia el frente de los Apeninos septentrionales. Este hecho está asociado con el retroceso hacia el NE de la losa del SO de Adria, que provoca la subsecuente delaminación del manto continental. Al sur de Adria, la estructura que se observa es consecuencia de que la orientación de las losas litosféricas es variable y de un cambio, casi de 90 grados, de las direcciones tectónicas entre las de los Apeninos meridionales y las del arco calabrés. En el margen SO de Adria, bajo los Apeninos septentrionales, la anomalía térmica sublitosférica está conectada a la litosfera más superficial, mientras que en los Apeninos meridionales la anomalía térmica está desconectada. Este hecho puede estar asociada a una ruptura horizontal reciente de la losa (tearing). A lo largo del margen NE de Adria, la anomalía térmica penetra hasta profundidades de unos 200 km en los Dinárides septentrionales y hasta 280 km en los Dinárides meridionales. En ambos casos alcanzan una profundidad inferior a la de las anomalías que se observan en el margen occidental de Adria, que se extienden hasta al menos 400 km de profundidad.
La mayor parte de la elevación a lo largo de los perfiles se explica por isostasia térmica. La elevación actual de los Apeninos externos, Dinárides y la zona de sutura de Sava se puede explicar por isostasia regional con un grosor elástico de la litosfera de 20-30 km. Los modelización numérica de la topografía dinámica muestra que cuando las losas están conectadas a la litosfera, al ser más densas, desencadenan flujos de manto descendentes y una topografía dinámica negativa. La losa bajo los Apeninos septentrionales, al alcanzar una mayor profundidad, produce un flujo del manto mayor, y, por lo tanto, resulta en una mayor amplitud de la topografía dinámica, −400 m. La topografía residual en los Apeninos del Norte se puede explicar por la anomalía térmica fría asociada a la losa que subduce en esta zona. El resto de las anomalías del manto producen una topografía dinámica insignificante, ya sea por estar desenganchadas en el caso de los Apeninos del Sur, o por su tamaño reducido bajo los Alpes Dináricos.
The Central Mediterranean region is a highly active seismic zone with unique tectonic characteristics, forming part of the Alpine-Mediterranean belt that extends from southern Iberia to the western Aegean Sea. This belt originated from the convergence of the African and Eurasian plates since the Late Cretaceous. The post-collision counterclockwise rotation of the Adria microplate triggered the opening of the asymmetric Tyrrhenian Basin and synchronous tectonic tightening along all its margins: the Apennines, the Calabria belt, the Dinarides-Hellenic belt, the Alps, and the Carpathian belt.
Applying an integrated geophysical-petrological modeling (LitMod2D_2.0), this thesis investigates the thermochemical structure of the lithosphere and sublithospheric mantle beneath the Adria microplate and its margins and explores the geometry and depth continuity of the Adria slabs. Two transects have been modeled down to 400 km, spanning from the Tyrrhenian Basin to the Carpathians-Balkanides region, in a roughly SW-NE direction. The northern transect, approximately 1000 km long, crosses the Tyrrhenian Sea, the northern Apennines, the Adriatic Sea, the Dinarides and the Pannonian Basin. The southern transect spans approximately 1250 km, crossing the southern Tyrrhenian Basin, the southern Apennines, the Adriatic Sea, the southern Dinarides, and the Carpathians-Balkanides. In addition, I developed new numerical modeling codes of mantle flow to evaluate the topographic response of opposed subductions along these transects and discuss their implications in the evolution of the region.
In the northern transect, results show a more complex structure and slightly higher average crustal density of Adria compared to Tisza microplate. Below the Tyrrhenian Sea and Western Apennines, Moho lays at <25 km depth while along the Eastern Apennines it is as deep as 55 km. The modeled lithosphere-asthenosphere boundary (LAB) below the Tyrrhenian Sea and Pannonian Basin is flat lying at ∼75 and 90 km, respectively. Below the External Apennines and Dinarides the LAB deepens to 150 km, slightly shallowing toward the Adriatic foreland basin at 125 km depth. My results are consistent with the presence of two mantle wedges, resulting from the rollback of the Ligurian-Tethys and Vardar-NeoTethys oceanic slabs followed by continental mantle delamination of the eastern and western distal margins of Adria. These two opposed slabs beneath the Apennines and Dinarides are modeled as two thermal sublithospheric anomalies of −200 °C.
In the southern transect, the modeling shows the presence of two asthenospheric mantle wedges aligning with the Apenninic and Dinaric continental mantle slab rollback, along with cold (−200 °C) sublithospheric anomalies beneath Adria’s NE and SW margins. In the northern Adria region, the lithosphere undergoes synchronous thinning in the Tyrrhenian domain and thickening toward the forefront of the northern Apennines. This is associated with the northeastward rollback of the SW Adriatic slab, leading to subsequent delamination of the continental mantle. In the southern Adria region, the complex deep structure results from the variably oriented lithospheric slabs, and nearly 90-degree shift of the tectonic grain between the southern Apennines and the Calabrian Arc. At the SW Adria margin, beneath the northern Apennines, the thermal sublithospheric anomaly is attached to the shallower lithosphere, while a slab gap is modeled in the southern Apennines. One possibility is that the gap is due to a recent horizontal slab tear. Along the NE margin of Adria, the thermal anomaly penetrates to depths of about 200 km in the northern Dinarides and 280 km in the southern Dinarides, shallower than the SW Adria anomaly, which extends to at least 400 km depth.
Most elevation along the profiles is attributed to thermal isostasy. Elevation in the External Apennines, Dinarides, and Sava Suture Zone can be explained by regional isostasy with an elastic thickness of 20-30 km. The numerical modeling of dynamic topography permitted the identification of the dynamic deflection caused by deep mantle flow and buoyancy forces related to density contrasts. This modeling indicates that the denser attached slabs trigger downwelling mantle flow and negative dynamic topography. A longer slab beneath the northern Apennines leads to higher mantle flow and larger amplitudes of dynamic topography, -400 m. Residual topography in the northern Apennines can be explained by the cold thermal anomaly associated with the northern Apenninic slab. The rest of mantle anomalies produce a negligible dynamic topography, either because of being detached in the case of the southern Apennines, or because of their reduced size below the Dinarides.
Table of contents
ABSTRACT …………………………………………………………………………………….. i
RESUM ………………………………………………………………………………………… iii
RESUMEN …………………………………………………………………………………….. v
CHAPTER 1 General Introduction………………………..1
1.1. Motivation and overview………………………..3
1.2. Tectonic setting………………………..8
1.2.1. Tyrrhenian Basin………………………..12
1.2.2. Apennines fold-thrust belt………………………..13
1.2.3. Dinarides fold-thrust belt………………………..14
1.2.4. Pannonian Basin………………………..15
1.3. Objectives………………………..16
1.4. Outline………………………..17
CHAPTER 2 Fundamentals and Methodology………………………..19
2.1. Fundamental concepts………………………..21
2.1.1. Internal structure of the Earth………………………..21
2.1.2. Composition of the upper mantle………………………..26
2.1.3. Topography………………………..28
2.2. Methodology………………………..30
2.2.1. General workflow………………………..31
2.2.2. Thermal modeling and boundary conditions………………………..32
2.2.3. Potential field: Gravity and Geoid Height Anomalies………………………..33
2.2.4. Mantle composition………………………..34
2.2.5. Densities distribution………………………..36
2.2.6. Mantle seismic velocities………………………..37
2.2.7. Elevation………………………..38
2.3. Geophysical data………………………..43
2.3.1. Elevation………………………..43
2.3.2. Gravity anomaly………………………..44
2.3.3. Geoid………………………..45
2.3.4. Surface heat flow………………………..46
2.3.5. Mantle seismic velocities………………………..47
CHAPTER 3 Geophysical-Petrological Model of the Crust and Upper Mantle Beneath the Northern Apennines and Dinarides Orogenic Systems ………………………………………………………………………………………. 51
3.1. Crustal structure from previous studies………………………..54
3.2. Mantle characterisation from previous studies………………………..58
3.3. Results………………………..60
3.3.1. Crustal structure………………………..60
3.3.2. The LAB and upper mantle physical properties and composition………………………..66
3.3.3. Isostatic and dynamic topography………………………..68
3.4. Discussion………………………..71
3.4.1. Crustal and lithospheric structure………………………..71
3.4.2. Mantle composition and sublithospheric anomalies………………………..72
3.4.3. Geodynamic implications………………………..73
3.5. Chapter conclusions………………………..76
CHAPTER 4 Geophysical-Petrological Model of the Crust and Upper Mantle Beneath the Southern Apennines and Dinarides Orogenic Systems ………………………………………………………………………………………. 79
4.1. Model parameters and constraints………………………..82
4.2. Results………………………..86
4.2.1. Crustal structure………………………..86
4.2.2. The LAB and upper mantle physical properties and composition………………………..92
4.2.3. Isostatic and dynamic topography………………………..96
4.3. Discussion………………………..100
4.3.1. Crustal and lithospheric structure………………………..100
4.3.2. Mantle composition and sublithospheric anomalies………………………..101
4.3.3. Geodynamic implications………………………..104
4.4. Chapter conclusions………………………..107
CHAPTER 5 General Discussion ………………………..109
5.1. Present-day crustal and lithospheric structure………………………..111
5.2. Mantle composition and anomalies………………………..114
5.3. Dynamic topography………………………..118
5.4. Geodynamic implications………………………..123
CHAPTER 6 Conclusions, limitations and future work ………………………..129
List of Figures and Tables………………………..137
List of Figures………………………..139
List of Tables………………………..147
References ………………………………………………………………………………… 149
Annex ……………………………………………………………………………………….. 189
This thesis presents the findings of a geophysical-petrological model of the lithosphere and uppermost sublithospheric mantle along two transects spanning from the Tyrrhenian Basin to the Carpathians-Balkanides region, in a roughly SW-NE direction. The northern profile is approximately 1000 km, running from the Tyrrhenian Sea, the northern Apennines, the Adriatic Sea, the Dinarides and the Pannonian Basin. The southern profile is about 1250 km long and spans from the southern Tyrrhenian Basin, trough the southern Apennines, the Adriatic Sea, to the Carpathian-Balkanides region. Modeling these geotransects provides an integrated view of the deep structure of the Adria and Dacia microplates, as well as of the slabs located along the western and eastern Adria margins.
In particular, the geophysical-petrological model, further constrained by additional geological and seismological data, has allowed me to present the present-day crust and upper mantle structure (down to a depth of 400 km) of the Adria microplate and its margins. Furthermore, the model has allowed for the distinction of different mantle domains, the mapping of the geometry and extent of the slabs at depth, and the evaluation of the implications of deep mantle anomalies on surface dynamic topography.
Based on the results of the thesis I draw the following conclusions:
1) Crustal and Lithospheric structure
The crustal structure of the Adria microplate is more complex than that of the Tisza microplate, particularly near collisional zones. This complexity suggests that subduction and delamination primarily affect the Adria domain. These differences are also evident at the Moho level, with variations observed beneath the Internal and External zones of the Apennines and Dinarides. The complexity of the external zones is highlighted in seismic data, where discrepancies arise between the Moho depths obtained using different seismic methods, such as Receiver Functions (RF) and Deep Seismic Soundings (DSS). The modeling resolves these discrepancies, indicating that the Moho lies at depths of less than 25 km and 35 km along the Internal Apennines and Dinarides, respectively, while it is found at depths greater than 50 km along the external zones of both orogens. In the internal Dinarides (Adria microplate) and the Carpathian-Balkanides (Dacia microplate), the crust has a relatively constant thickness of 30-35 km.
The Adria and Tisza microplates have different densities. The Adria crust has an average density of 2830 kg/m³, while the Tisza crust is between 2790 and 2800 kg/m³. The lower average density of the Tisza plate is partly because the lower crust of the Tisza plate is thinner than that of Adria.
The density distribution and crustal structure of the northern Tyrrhenian Basin, corresponds to a thin continental crust with widespread volcanism, which extends to the Tuscany Magmatic Province. In contrast, in the southern Tyrrhenian Basin, the crust is oceanic, with the presence of serpentinized mantle at crustal depths.
The Tyrrhenian Sea and the Internal Apennines are characterized by the presence of elevated temperatures at shallow crustal levels, which is consistent with well-documented magmatic intrusions and volcanism.
The LAB shows significant lateral variations, reflecting different tectonic evolutions since the Mesozoic. Beneath the Tyrrhenian Sea, the LAB is flat and shallow at approximately 70-75 km, deepening slightly to the east, below the Internal Apennines. A significant thickening is observed beneath the External Apennines and Dinarides, reaching depths around 125 km in the Adriatic foreland basin. From the Internal Dinarides to the Carpathian-Balkanides region, the LAB is relatively flat at around 120 km.
2) Mantle Composition and Anomalies:
The thermo-geochemical model shows two different mantle compositions: a slightly depleted mantle with an average Mg# of 89.5 for the Adria microplate, and a more fertile mantle below the southern Tyrrhenian and Apennines (DMM‐6%) and Dinarides (DMM‐3%). This is consistent with the presence of two sublithospheric mantle wedges, attributed to the rollback of the Apennines and Dinaric slabs.
The model also indicates two lithospheric mantle compositions: a re-enriched basalt layer beneath Adria and a fertile mantle below the Tisza microplate. The lithospheric mantle composition below the Apennines and Dinarides is more fertile, which aligns with the presence of sublithospheric mantle wedges caused by the delamination of the Adria lithospheric mantle.
I also find that along the western margin of Adria, a cold thermal sublithospheric anomaly extends down to depths of at least 400 km, while in the eastern margin, the anomaly is much shallower. In the northern Dinarides, the anomaly penetrates to ~200 km, and in the southern Dinarides, it extends around 280 km. Beneath the northern Apennines, the sublithospheric anomaly is connected to the shallower lithosphere, whereas in the southern Apennines, a small slab gap is observed.
3) Geodynamic Implications
Most elevation along the transects is due to thermal isostasy. Elevations in the External Apennines, Dinarides, and Sava Suture Zone can be explained by regional isostasy with an elastic thickness of 20-30 km. Residual topography in the northern Apennines can be explained by the flow created by the cold thermal anomaly associated with the Apennines slab. The rest of mantle anomalies produce a negligible dynamic topography, either because their reduced size below the Dinarides, or because of being detached from the surface beneath the southern Apennines.
In the northern Adria region, thinning of the lithosphere in the Tyrrhenian domain and thickening toward the forefront of the northern Apennines fold belt can be attributed to the northeastward rollback of the SW Adria slab and the resulting continental delamination. In the southern Adria region, variably oriented lithospheric slabs determine a more complex deep structure, primarily due to the nearly 90‐degree shift in tectonic directions between the southern Apennines, the Calabrian Arc, and their respective slabs. The small slab gap observed in the southern Apennines, it is likely the result of a slab horizontal tearing.