Drainage on Evolving Fold-Thrust Belts: a Study of Transverse Canyons in the Apennines W

Home , Biscubio, Burano (river), Candigliano, Foglia, Fold and thrust belt, Iesi

Basin Research (1999) 11, 267–284 Drainage on evolving fold-thrust belts: a study of transverse canyons in the Apennines W. Alvarez Department of Geology and Geophysics, University of California, Berkeley, USA, and Osservatorio Geologico di Coldigioco, 62020 Frontale di Apiro (MC), Italy

ABSTRACT Anticlinal ridges of the actively deforming Umbria–Marche Apennines fold-thrust belt are transected by deep gorges, accommodating a drainage pattern which almost completely ignores the presence of pronounced anticlinal mountains. Because the region was below sea level until the folds began to form, simple antecedence cannot explain these transverse canyons. In addition, the fold belt is too young for there to have been a flat-lying cover from which the rivers could have been superposed. In 1978, Mazzanti & Trevisan proposed an explanation for these gorges which deserves wider recognition. They suggested that the Apennine fold ridges emerged from the sea in sequence, with the erosional debris from each ridge piling up against the next incipient ridge to emerge, gradually extending the coastal plain seaward. The new coastal plain adjacent to each incipient anticline provided a flat surface on which a newly elongated river could cross the fold, positioning it to cut a gorge as the fold grew. Their mechanism is thus a combination of antecedence and superposition in which folds, overlying sedimentary cover and downstream elongations of the rivers all form at the same time. A study of Apennine drainage, using the sequence of older-to-younger transected Apennine folds as a proxy for the historical evolution of drainage cutting through a single fold, shows that transverse drainage forms when sedimentation dominates at the advancing coastline. Longitudinal drainage forms when uplift dominates, the folds first emerge as offshore islands and the Mazzanti–Trevisan mechanism is suppressed. Complicating factors include several departures from steady-state growth of the fold-thrust belt, a possible case of precursory submarine drainage, early emergence of anticlinal culminations and the location of several transverse canyons at the structurally highest point along anticlinal axes.

INTRODUCTION the advancing front of deformation. The fold-thrust belt of the Umbria–Marche Apennines is still forming, and This paper deals with the classic geological and geo- comparison of younger to older anticlines provides a morphological problem of the origin of river gorges that proxy for the evolution of the drainage cutting a single cut through topographic barriers. It focuses on the anticline and shows in detail how the mechanism of Umbria–Marche Apennines of the Italian Peninsula, Mazzanti and Trevisan has operated. where transverse gorges are being cut in a very young Nineteenth-century attempts to explain deep river fold-thrust belt. Neither antecedence nor superposition gorges transecting the broad structural uplifts of the alone can explain the cross-cutting Apennine rivers. The Colorado Plateau led to formulation of the well-known little-known work of Mazzanti & Trevisan (1978) offers concepts of (1) antecedence, in which the rivers pre-date an attractive solution to the origin of the Umbria–Marche the structural deformation and simply cut down through gorges; it invokes a combination of antecedence and the uplifts as they grow, and (2) superposition, in which superposition taking place at the advancing coastline near old uplifts are buried by younger, flat-lying sediments, and with resumed erosion, rivers wandering across the Correspondence: Walter Alvarez, Department of Geology and flat-sediment surface are let down onto the old, stable Geophysics, University of California, Berkeley, CA uplifts beneath. 94720–4767, USA. E-mail: [email protected] The Appalachian Valley and Ridge Province is the

© 1999 Blackwell Science Ltd 267 W. Alvarez classic area for the problem of rivers cutting through the sea during the late Cenozoic. Therefore, the rivers are narrow ridges of fold-thrust belts. Mechanisms proposed no older than this, which eliminates subaerial antecedence to account for transverse drainage in fold belts have been as a mechanism. There is little time available and no classified in various ways (Strahler, 1945; Oberlander, evidence to be found for the deposition of an overlying 1965, 1985; Thornbury, 1965), but the principal sug- flat sedimentary surface from which the streams could gested mechanisms are (1) antecedence, (2) superposition, have been superposed. The peculiar stratigraphic (3) headward erosion into a ridge, following weak zones sequence of the Zagros, with an easily erodable interval along transverse faults, and (4) modification of original sandwiched between resistant units, which made Oberlan- consequent drainage by captures. der’s (1965) mechanism plausible, does not occur in the In a study of spectacular river gorges in the Zagros Apennines. However, a different mechanism has been Mountains fold-thrust belt, Oberlander (1965, 1985) proposed by Mazzanti & Trevisan (1978). It is evaluated offered a new mechanism for generating transverse drain- below and appears to offer a satisfactory explanation for age. He noted (Oberlander, 1965, figs 59, 60, 64) that in the Apennine gorges. an area of gorges not explainable by standard mechanisms, two resistant limestone formations are separated by a unit of easily eroded flysch which is thicker than the Apennine geology and landscape amplitude of the folding. He proposed that a relatively flat erosion surface developed in the folded flysch interval, The Apennine Mountains, running the length of the and from this surface the streams were let down across peninsula, are composed largely of Mesozoic and Ceno- the folded limestone unit below. Oberlander’s mechanism zoic sedimentary rocks deformed by north-eastward is thus a variant on superposition. transport which began in the Oligocene and continues to Burbank et al. (1996) have studied the response of the present. Trains of folds are rare in the Central and rivers and their deposits to growing folds emerging from Southern Apennines, which were formed by the thrusting aggrading alluvial plains. Such a stream may maintain its of massive platform carbonate sequences. In the northern course across the fold, or may be defeated and diverted part of the range, and particularly in the Umbria–Marche around the fold by avulsion or capture, depending on Apennines (Fig. 2), well-bedded Mesozoic and lower the ratio of aggradation behind the fold to uplift, and Cenozoic pelagic carbonates and upper Cenozoic turbidite the ratio of stream power to erosional resistance. In the units have been deformed into a fold-thrust belt which cases studied by Burbank et al. (1996), there is no is continuing to propagate north-eastward, toward the question that these transverse streams are antecedent to foreland of the Adriatic Sea. the folds. Italian geologists have long known that deformation in the fold belt of this part of the Apennines migrated TRANSVERSE DRAINAGE IN THE progressively north-eastward (Migliorini, 1948; Merla, UMBRIA–MARCHE APENNINES 1951). This is clear from the age of the synorogenic turbidites deposited just in front, and quickly deformed The present paper calls attention to another way in which by, the advancing thrust front. These turbidites date transverse drainage may form. In the Umbria–Marche from the late Oligocene in Tuscany (Macigno Formation), Apennines of northern peninsular Italy, anticlinal ridges the early Miocene around Perugia (Cervarola Formation), of a young fold-thrust belt are transected by gorges the middle Miocene around Gubbio (Marnoso-arenacea several hundred metres deep, allowing the rivers to flow Formation) and the late Miocene east of the main ridges straight to the sea, across the structural and topographic of the fold belt (Piceno Formation, south of the study grain of the mountains (Fig. 1). area). North-eastward migration was well understood by These canyons have been important as routes of the date of the first major synthesis in English (Bortolotti communication since antiquity. One such canyon, the et al., 1970, fig. 39). Modern ideas on the tectonics of Gola del Furlo, forms a bottleneck on the Via Flaminia, fold-thrust belts were just starting to emerge at that time the road that connected the imperial capitals of Rome (Bally et al., 1966; Dahlstrom, 1969) and had not yet and Ravenna in late Roman times, and was the site of a reached Italy, where, in the early 1970s, there was still battle during the Gothic War of the 6th century AD. In debate over autochthony vs. allochthony in the Apennines that battle, the Byzantines defeated the Goths and (Abbate et al., 1970; Alvarez, 1972). reopened communications between Rome and Ravenna In a remarkable tectonic pattern, a north-eastward- by means of a geological tactic – they pushed boulders propagating extensional front is following about 100 km over the cliffs and down onto the fortifications below, behind the compressional front (Elter et al., 1975), so until the Goths surrendered in terror (Procopius, #560, that normal faults tear apart the fold-thrust edifice shortly Book VI, Ch. xi). after it is generated. At present the extensional front These gorges are difficult to explain by standard has reached Gubbio (Fig. 2). The propagating com- mechanisms. The presence of Miocene turbidites across pressional–extensional couple inferred from structural the region and Pliocene marine sediments in the north- observations is confirmed by focal mechanisms of earth- east demonstrates that the fold belt emerged from the quakes (Lavecchia et al., 1994) and by detailed studies

Fig. 1. View east into the Frasassi Gorge. This valley is representative of the transverse canyons of the Umbria–Marche Apennines, although steeper sided than some others, because it cuts through the massive Jurassic platform limestone of the Massiccio Formation.

Fig. 2. Map of northern peninsular Italy, showing the asymmetric drainage pattern and the present positions of the north- eastward-migrating compressional and extensional fronts which apparently control the asymmetric drainage. The Umbria– Marche Apennines trend NW–SE between Perugia and the Adriatic Sea. CROP-03 and Bally 2 (Barchi et al., 1998, plate 4; Bally et al., 1986, section 2) are the lines of the contrasting interpretive structural cross-sections in Fig. 7. of the CROP-03 seismic reﬂection proﬁle (Pialli et al., these stresses would not change from extensional to 1998). extensional in only 100 km. The pattern suggests that The reason for the propagating tectonic couple is not the block between the extensional and compressional certain. It clearly cannot be due to applied stresses fronts is moving north-eastward, generating compression transmitted horizontally through the lithosphere, because in front and extension behind. Yet the block is too narrow

© 1999 Blackwell Science Ltd, Basin Research, 11, 267–284 269 W. Alvarez to be driven by viscous coupling to flowing mantle at primary questions have been addressed in this literature. its base. From the broadest to the most detailed, the questions Therefore, various authors have suggested that some are: (1) Why is the drainage of this part of the Peninsula portion of the crust and/or upper mantle of the Apen- asymmetric? (2) Why is the main drainage divide located nines is delaminating, rolling back toward the north-east, in relatively low country, offset to the south-west of the and sinking into the mantle. This rollback would require highest ridge? (3) Why are many of the anticlinal ridges the overlying block to move north-eastward, as observed. cut by transverse gorges? It is not clear which entity is delaminating. Reutter et al. Italian geomorphologists working in this area have (1980) identified the sinking entity as the subcontinental been most interested in the first two questions (Bonarelli, lithospheric mantle; Castellarin et al. (1982) considered 1891; Marinelli, 1926; Castiglioni, 1934; Merla, 1938; it to be the lower continental crust; Malinverno & Ryan Giannini & Pedreschi, 1949; Sestini, 1950; Selli, 1952; (1986) showed most or all of the continental crust Ghelardoni, 1958, 1962; Gonsalvi & Papani, 1969; Cat- detaching. Despite these differences, all these authors tuto, 1976; Mazzanti & Trevisan, 1978; Dramis & Bisci, agreed on the basic geometry. Pialli & Alvarez (1997) 1986; Cencetti, 1988; Cattuto et al., 1989). The origin of argued that the delaminating entity is the lower continen- transverse gorges has received less attention. Neverthe- tal crust and lithospheric upper mantle, and proposed less, one paper (Mazzanti & Trevisan, 1978) proposed a that the sinking and rollback is driven by the density novel and important solution to this problem, which is increase that accompanies the inversion from basaltic in accord with recent concepts of the evolution of fold- composition to eclogite when the lower continental crust thrust belts and with new observations on the Apennines reaches about 50 km depth. Careful study of the results presented here. of the CROP-03 profile (Pialli et al., 1998) should allow further progress in understanding this mechanism for generation of the Apennines. THE MECHANISM OF MAZZANTI AND The rivers of the Umbria–Marche Apennines form TREVISAN the eastern half of an asymmetrical drainage pattern in In the course of a paper which deals with all three classic this part of the Italian Peninsula (Fig. 2). Drainage to problems of landscape in the Umbria–Marche Apennines the Tyrrhenian Sea, through the region behind the – drainage asymmetry across the Peninsula, displacement extensional front, is collected into a few long trunk of the drainage divide and transverse canyons – Mazzanti streams – the Tiber, the Ombrone and the Arno – in a & Trevisan (1978, fig. 5) proposed an ingenious expla- trellis pattern, with long segments following grabens nation for the transverse gorges (Fig. 6). Although they parallel to the peninsula, interrupted by steps to the did not develop this idea at length, it appears to agree south-westward. In contrast, the many short rivers that with what is seen on maps and in the field, and with drain north-eastward across the compressed but not-yet subsequent theoretical understanding of fold-thrust belts. extended area (Fig. 3) are closely spaced, parallel and The stratigraphic sequence of this part of Italy is follow relatively straight paths to the Adriatic Sea (Maz- exclusively marine. Limestone and marl dominate from zanti & Trevisan, 1978; Hovius, 1996). the Jurassic to the Lower Tertiary and are replaced The fold structure in the Umbria–Marche Apennines upward by terrigenous turbidites in the Late Tertiary is expressed in elongated anticlinal ridges (Fig. 4). The (Bortolotti et al., 1970; Cresta et al., 1989). Marine ridges culminate in peaks with elevations from 1500 m deposition continued until approximately the time of in the north to 2400 m south of the study area. Because emergence of each anticline, with the emergence in any the rivers of the Umbria–Marche Apennines pass through given place dated to approximately the nearest chrono- these anticlines in deep gorges, the network of major stratigraphic stage. rivers seems not to reflect the geological structure, even Mazzanti & Trevisan could not yet know or portray though on a smaller scale differential erosion has sculpted the deep structure of the anticlinal ridges, but they prominent strike valleys in weaker units like the middle correctly showed the ridges forming progressively toward Cretaceous Fucoid Marls. The lack of structural control the north-east (Mazzanti & Trevisan, 1978, fig. 5), and on major streams can be seen by comparing a geological they realized that as each anticline emerged from the sea, map (Fig. 5) to the drainage map (Fig. 3) of the same its erosional debris would be shed into the synclines on area. From the drainage map alone, it would be difficult either side. On the seaward side, some of the sediment to infer the presence of a belt of large parallel folds. would be trapped behind the next incipient anticline, thus extending the coastal plain seaward. As a result, the Adriatic rivers were progressively elongated in the down- Geomorphological questions stream direction. In addition, the newly formed coastal plain provided a surface from which each new segment While there is a rich literature on the evolution of the of each river could progressively cut down through the landscape of this part of the Apennines, it has not underlying anticline as it grew. received the international recognition it deserves because The Mazzanti–Trevisan mechanism is thus a combi- it has been published almost entirely in Italian. Three nation of superposition and antecedence, but quite

Fig. 3. Drainage pattern of the study area, showing prominent geographical features and the locations of the proﬁles shown in Figs 7 and 8. Note that the anticlinal ridges trending NW–SE (Figs 4 and 5) are almost unrecognizable in the drainage pattern.

ff di erent from the Colorado Plateau ‘anteposition’ of RELEVANT ASPECTS OF THE Hunt (1956, p. 65). In the Apennine case, the river is TECTONICS OF FOLD-THRUST BELTS superposed on the anticline from a surface that forms at essentially the same time as the initiation of folding, and Great strides have been made in the study of fold-thrust thus the river is antecedent to all but the very first stage belts since the 1960s, and some aspects of this new of deformation. It avoids the insuperable problems of (1) understanding are relevant to evaluating the Mazzanti– simple superposition, where there is no evidence of and Trevisan hypothesis. The major breakthrough came with no real possibility for unconformable sedimentary cover the recognition that reverse faults cutting the fronts of or higher thrust sheets from which streams could be let anticlines do not steepen with depth, but flatten into sole down across the Apennine folds, and (2) simple anteced- thrusts. Seismic data from the Canadian Rockies showed ence, where rivers could not pre-date folds in sediments the entire fold-thrust belt to have been been pushed which were below sea level even after the folding began. forward over its basement, up a gently dipping basal The heart of the mechanism is that the fold, the coastal- detachment (Bally et al., 1966). More detailed studies of plain surface and the downstream growth increment of this deeply exposed and intensely drilled mountain range the rivers were all forming at essentially the same time. made it possible to recognize that flat thrusts in weak As discussed in the next sections, the mechanism of stratigraphic units ramp up through stronger units to Mazzanti & Trevisan agrees well with the understanding flatten again in weak horizons above, with anticlines of fold-thrust belt evolution that has emerged since their forming above the ramps, a pattern now recognized as paper was written, and with detailed geological obser- characteristic of fold-thrust belts (Boyer & Elliott, 1982). vations in this part of the Apennines. Before turning to Thrust arrays propagate toward the foreland, with new an evaluation of the Mazzanti–Trevisan mechanism, it active thrusts taking over from older, inactive ones, and will be useful to describe the structures across this part thus the belt of resultant anticlines also propagates toward of the Peninsula in the light of current understanding of the foreland. This progressive appearance of new folds, the tectonics of fold-thrust belts. characteristic of fold-thrust belts, is what Apennine

Fig. 4. Topography of the study area, showing the geological/topographic belts discussed in the text. Squares, triangles, two- letter abbreviations and thin lines show towns, peaks, gorges and profile lines identified in Fig. 3. geologists recognized on the basis of biostratigraphy long driven forward. In the continuum-scale view of fold- before its explanation was understood, and it was essential thrust belts, the detailed erosional features of the surface to the Mazzanti–Trevisan hypothesis. slope are below the limit of resolution. However, these It is now clear, however, that orderly, in-sequence details – the transverse gorges of the Apennines, for formation of new thrusts during overall forward propa- example – may provide a record of historical conditions gation may be modified in some cases by out-of-sequence such as the uplift–sedimentation balance at the coastline, thrusts (Morley, 1988). There is presently an active debate or of structural conditions at depth, including the pres- over the extent of out-of-sequence thrusting in the ence of out-of-sequence thrusts. Umbria–Marche Apennines (Ghisetti et al., 1993). Because the Mazzanti–Trevisan mechanism operates right at the TRANSECT ACROSS THE DEFORMED coast, it would not be invalidated by out-of-sequence BELT thrusting. However, the orderly progression of higher anticlines with deeper gorges shown in their diagram The present study area includes 15100 000 map sheets (Fig. 6) could be interrupted by out-of-sequence thrusting. 109 ‘Pesaro’, 110 ‘Senigallia’, 116 ‘Gubbio’, 117 ‘Iesi,’ Further understanding of fold-thrust belts came by and 118 ‘Ancona’, covering the entire Adriatic drainage backing away from the detailed geometry of the structures of this part of the Italian Peninsula. within the deformed belt and considering the entire belt The subsurface structure of the Umbria–Marche as a wedge-shaped continuum bounded by a rearward- Apennines fold-thrust belt is still controversial. Figure 7 dipping basal detachment, a forward-sloping upper sur- shows two contrasting interpretive sections across the face and an advancing driver at the rear (Davis et al., Apennines in the study area. The lower section (redrawn 1983; Dahlen et al., 1984). In this view the fold-thrust from Bally et al., 1986; plate 8, fig. 55) assumes thin- belt is seen as a steady-state system in which basal sliding skinned deformation, a gently dipping basal detachment, causes internal deformation which is accompanied by and an orderly progression of thrust and anticline forma- shallow-level adjustments in order to maintain a critical tion from south-west to north-east, based on the Canadian taper between basal detachment and surface slope, so Rockies model for evolution of fold-thrust belts. Seismic that all stresses stay in balance as the fold-thrust belt is reflection data at that time were not good enough to

Fig. 5. Generalized geological map of the study area, showing the lack of correspondence between geological structure and drainage lines. Dashed line shows location of Fig. 9. Black squares and straight lines are towns and proﬁles as in Fig. 3.

image the structure or the basement in the complicated drainage pattern of the Tyrrhenian side of the Peninsula. western half of the section. The upper section (redrawn To the north-east is a hilly belt underlain by the turbidites from Barchi et al., 1998; Plate 4) is based on the recent of the middle Miocene Marnoso-arenacea Formation, CROP-03 seismic reflection profile, which provided evi- which were deposited in a synorogenic foredeep, in or dence for involvement of basement in the thrusting and around the thrust front, and quickly deformed as the a greatly reduced shortening across the fold-thrust belt. thrust front advanced (Damiani et al., 1983). Most of The differences between these sections, although criti- this hill country is drained by short streams flowing cal for understanding the origin of the Apennines, do south-west to the Tiber. not strongly affect the interpretation of landscape evolution in the present paper, for in both cases the thrusts 2. Gubbio valley and faulted half-anticline and folds propagate north-eastward across the belt of folds from the Monte Nerone – Monte Catria anticline Gubbio is built on a fault escarpment that cuts a small to the present coastline. The two viewpoints are admir- anticline, dropping the south-west half down to make a ably contrasted in a ‘civilized debate among friends who depression occupied in Pleistocene time by a lake. The disagree’ (Ghisetti et al., 1993). Gubbio half-anticline exposes Cretaceous and Lower Starting in the south-west part of the map area and Tertiary pelagic limestones. The fault that cuts the proceding to the north-east, the following major geo- anticline is the north-easternmost major extensional fault logical and drainage belts appear (Figs 3–5). and marks the present position of the advancing extensional front. The Gubbio Valley represents the newest 1. Belt of Miocene turbidites between the part of the Tyrrhenian trellis drainage. The half-anticline Tiber Valley and Gubbio is presently being dissected by short steep valleys (Con- tessa Valley and Bottaccione Gorge) which are apparently The Tiber Valley, about 10 km south-west of the map very young, dating from the initiation of the exten- area, is the easternmost well-developed part of the trellis sional faulting.

this belt, lying at an elevation of about 850 m, which is lower than the anticlinal crests which ﬂank it on both sides. This rather surprising situation has been discussed in the Italian literature, as noted above.

4. The Umbria–Marche ridge The most prominent topographic and geological feature on this transect through the Apennines is a ridge formed by closely spaced, parallel anticlinal mountains and known as the ‘Umbria–Marche Ridge’, because it follows the boundary between these two regions. In this anticlinorium, two major anticlines (Monte Nerone, Monte Montiego) or three of them (Monte Cucco, Monte Catria, Monte della Strega) may occur side by side, separated only by narrow, faulted synclines. Exposures are not deep enough to reveal what role, if any, was played by thrusts and thrust ramps, and the deep Burano well, on the crest of the main anticline, did not intersect any major thrust fault. The anticlines typically end at steeply plunging terminations, where they may be continued en echelon by the next anticline. Local axial culminations (structural highs along the trend of the fold axis) may reflect the difference between horst and basin facies in Fig. 6. The concept for the origin of the Apennine transverse the Jurassic limestones (Alvarez, 1989a,b), or may result drainage evaluated in this paper (Mazzanti & Trevisan, 1978, from the pattern of Miocene deformation. fig. 5). The original caption reads: ‘Model of mixed process of Major streams flowing to the Adriatic transect this superposition followed by antecedence. In A, a fold which is anticlinal complex via a number of deep gorges (Biscubio, beginning to form creates a gentle rise on the ocean floor; the Gorgo a Cerbara, Bosso, Burano and Sentino). The Bosso depression between the rise and the coast fills rapidly with and Burano gorges pass through a broad saddle between sediment, largely of molassic type. In B, the original depression, the peaks of Monte Nerone and Monte Acuto, but Gorgo with gradual uplift, emerges as a flat surface inclined toward the a Cerbara cuts through the culmination of the Montiego Adriatic, while new folds continue to ripple the sea floor. In C Anticline, in a remarkable pattern also seen elsewhere in and D, the process continues to propagate, extending the the study area. Subsequent tributaries and longitudinal emergent land, while the accentuation of the preceding folds and a general uplift lead to the deepening of the gorges which portions of the trunk streams flow parallel to the axial cut the anticlines’ (transl). trends for short distances, following synclinal axes or the strike valleys of weaker units (particularly the middle Cretaceous Fucoid Marls, whose prominent valley is a clear guide to the structure throughout the anticlinal belt). 3. Miocene turbidite belt and the Apennine drainage divide 5. The Internal Marche Folds North-east of the Gubbio half-anticline, the Miocene turbidites continue in a generally synclinal belt. This North-east of the major anticlinorium is a 10-km-wide area has a complicated internal structure involving thrusts belt of lower, shorter anticlines exposing Cretaceous and folds whose geometry and kinematics are still being limestones, separated by synclines occupied by Miocene debated, particularly as to whether they formed in an terrigenous units. Some of the synclines were the depos- orderly sequence progressing to the north-east (Bally itional sites of synorogenic turbidites that flowed north- et al., 1986) or in a complicated, irregular sequence eastward, across and around the growing anticlines (Cen- involving backward, out-of-the-synclines motions (de tamore et al., 1976). Feyter & Menichetti, 1986; de Feyter et al., 1990). At Continuing the pattern seen in the Umbria–Marche the north-east edge of this belt, the youngest turbidites Ridge, the rivers of this belt cut through anticlines in have been folded into the prominent Monte Vicino several places, avoiding the easier routes around these syncline which closely reflects the geometry of the last small folds. Particularly striking are the Acqualagna deep synclinal basin in this part of the advancing fold- Anticline, where two streams enter the fold, join at the thrust belt (Centamore et al., 1977). axial culmination, and flow out through the other side of The drainage divide of the Peninsula passes through the fold, and the narrow gorge of Frasassi (Fig. 1).

Fig. 7. Two contrasting interpretive cross-sections across the Umbria–Marche Apennines fold-thrust belt in the study area, showing progressively younger thrusting toward the north-east, and the advancing extensional front, after Bally et al. (1986) and Barchi et al. (1998). The sections are drawn along lines roughly 20 km apart (Figs 3–5), and the fold-thrust structure plunges north-westward from the Bally section to that of Barchi. A topographic profile has been added to each section, showing the highest ridge line within 4 km of the line of section. Note that the grey unit represents a thicker stratigraphic interval on the Bally section. The differences between the two sections are not relevant to the landscape evolution discussed in this paper, since the shallow structure of anticlines A through F is not greatly different.

6. Paired anticlines of Furlo-Cesana, and synorogenic Miocene and Pliocene terrigenous clastics. the External Marche Ridge Although there is not much topographic grain in this area (Fig. 4), the geological map (Fig. 5) shows a pattern The large anticline of Furlo, cored by Jurassic limestones, of gentle anticlines and synclines, whose intensity dimin- is flanked by the lower Cesana Anticline, which exposes ishes north-eastward. This belt is crossed by many Cretaceous limestones. The Furlo Anticline has a double streams flowing north-eastward to the Adriatic (Nanni plunge away from a sharp axial culmination, and the et al., 1986). There is an asymmetry to the pattern of Furlo Gorge cuts exactly through the culmination. The tributaries, with the larger ones commonly entering the Cesana Anticline is transected at Fossombrone, near its trunk stream from the north side. The major streams south-east plunge termination, by the river emerging crossing this belt occupy floodplains 1–2 km wide. from Furlo, which is joined by another major stream that follows the syncline between the two anticlines. 8. Incipient folds of the Adriatic coastal South-east of a broad axial depression at Pergola, the plain and offshore Furlo structure is continued by the External Marche Ridge, a long anticlinorial ridge that rises southward to Along much of its length, the Adriatic coast in this area the Sibillini Mountains. This ridge is also cut through does not show an obvious structural control. Exceptions by a number of deep gorges, including the Gola della occur between Cattolica and Pesaro, where the coast Rossa. In front of it at one place is the Cingoli Anticline, follows the outer edge of an anticline reaching an elevation in a position analogous to that of the Cesana Anticline, of about 200 m, and south of Ancona, where the anticline and cut by two transverse gorges. of Monte Conero, exposing most of the Cretaceous, is unusually high (572 m) to be this far in front of the main 7. Foothill belt of minor folds mountain front. The Adriatic Sea in this area is shallow and is occupied Between the Furlo-Cesana Anticlines and the Adriatic by as much as 5 km of Pliocene–Quaternary terrigenous Sea is a belt of lower hills exposing easily eroded clastics, deposited in a foredeep basin in front of the

© 1999 Blackwell Science Ltd, Basin Research, 11, 267–284 275 W. Alvarez advancing Umbria–Marche fold belt (Ori & Friend, shortening, rock uplift, erosion and deposition. In the 1984). Seismic data and exploration drill holes show that belt studied here there are at least five departures from the foredeep sediments are deformed into a set of gentle steady-state evolution. anticlines, some of which produce oil or gas, without 1 The older anticlinorium of the Umbria–Marche Ridge giving rise to significant relief on the shallow sea floor appears to be more complex, with multiple adjacent folds, (Fig. 7). The present deformation front is roughly in the than the younger pair of Furlo-Cesana, and the still middle of the Adriatic Sea. younger single anticlines near the coast. This may indicate that the style of folding changed through time, or that EVALUATION OF THE out-of-sequence thrusting continues to modify earlier MAZZANTI–TREVISAN EXPLANATION anticlines, or it may simply be an artefact of depth of OF CROSS-CUTTING DRAINAGE exposure. 2 There are not any higher, more eroded folds west of The purpose of the present study is to examine evidence the Umbria–Marche Ridge, so that structure marks the relevant to the Mazzanti–Trevisan hypothesis, in order west boundary of the transect that can be used as a proxy to evaluate whether it is a satisfactory explanation of the for time sequence. cross-cutting drainage in the Umbria–Marche Apennines. 3 Through at least the Tortonian, the Adriatic foreland Mazzanti & Trevisan offered their hypothesis in the form basin was a deep-sea environment in which turbidites of a single drawing and a brief description (Mazzanti & were deposited on and around the advancing thrust front. Trevisan, 1978; Fig. 6), and did not report detailed This basin was filled, probably diachronously, during the observations from the Apennines. However, a careful Messinian and Pliocene. By Quaternary time the Adriatic examination of relevant evidence shows their hypothesis had its present configuration with a shallow, sediment- is in detailed accord with what is seen on geological maps filled sea or coastal plain in front of and above the and in the field. youngest folds. 4 The Messinian salinity crisis, in which the isolated Drainage evolution reconstructed using Mediterranean water evaporated, involved a sea-level spatial sequence as a proxy for time lowering of more than 1 km (Cita & Ryan, 1978). sequence Increased erosion and transport of sediments further from their site of erosion would have interrupted the Because of the propagation of the deformational front steady state in the fold-thrust belt, and some isostatic toward the foreland in a fold-thrust belt, a transect across uplift may have occurred, depending on water depth a set of anticlines can be taken as a rough proxy for the prior to the desiccation. time sequence of deformation and erosion in a single 5 During Quaternary time, there have been repeated anticline. The Umbria–Marche fold-thrust belt provides sea-level oscillations of about 100 m. On the steep Tyrrh- an excellent opportunity for this approach, displaying a enian continental margin, this produced repeated chan- sequence of progressively younger and less eroded anti- nelling and filling of river valleys near the coast (Alvarez clines toward the north-east, in the direction of fold- et al., 1996; Karner & Marra, 1998). On the flatter thrust propagation. A landscape history reconstructed on Adriatic continental margin the channelling effects were this basis is in agreement with the Mazzanti–Trevisan probably less, but the migration of the coastline would model, but some caution needs to be kept in mind. have been greater, and the broad floodplains of the A first caution is that the approach used here is valid Adriatic rivers may reflect incision during Quaternary to the extent that the fold belt and its internal structures sea-level lows. In addition, because the maximum rate of propagate forward in an orderly sequence. In the basic sea-level change during the Quaternary glacial oscillations tectonic model, with thrusts propagating in sequence probably greatly exceeded the rate of rock uplift, sea- (Bally et al., 1986), a single anticline would be folded level history may well have been the controlling factor rapidly as the front of thrusting passed, and subsequently in the development of transverse or longitudinal drainage would retain its folded shape while being slowly uplifted in the youngest anticlines (Robert S. Anderson, written as it was passively pushed over the upward-sloping basal communication, 1998). detachment. In a more complicated model, with out-of- sequence and rearward-directed thrusts (Morley, 1988), anticlines would continue to change their profile, as well Drainage crossing the sequence of anticlines as being uplifted, after the initial wave of deformation had passed. Which of these two models is applicable to Transverse drainage is impressive and puzzling where it the Apennines is still under debate (Ghisetti et al., 1993). chops through tall anticlines in deep, narrow gorges A second caution is that spatial pattern as a proxy for (Bosso, Burano, Gorgo a Cerbara and Furlo). It is less time sequence will be valid to the extent that fold-thrust noticeable where it passes through the gentler anticlines belt evolution has been a steady-state process, with a between Fossombrone and the coast, where the folds are roughly constant structural and topographic profile shift- barely reflected in the landscape of gentle hills. It is ing toward the foreland through a balance between instructive to examine drainage–structure relations from

276 © 1999 Blackwell Science Ltd, Basin Research, 11, 267–284 Transverse canyons in the Apennines north-east to south-west, as a proxy for the evolution of also apparent that they are being let down across gentle a single anticline through time (Fig. 8). anticlines affecting the Pliocene sediments, as shown by the situation at Cattolica (Fig. 5). Here the Conca River Adriatic coast. Trunk streams flow almost straight to the flows straight into the Adriatic across the north-west sea across the belt of coastal hills, on floodplains up to plunge termination of a low-relief anticline (Casabianca 4 km wide, cut slightly below the level of the adjacent et al., 1995; De Donatis et al., 1998) that exposes Miocene hills. Within a few kilometres of the coast, the hills are sediments and controls the position of the coastline. A mainly composed of Pliocene foraminiferal marine clays, few kilometres north-west, at Riccione, on the other side with some sandstones and rare conglomerates (Castellarin of the Conca, the gentler continuation of the anticline & Stewart, 1989). Quaternary sediments are restricted to slightly warps Pliocene sediments (15100 000 map sheet the incised floodplains (Nanni & Vivalda, 1986) and to 109, section I). This anticline will probably continue to discontinuous coastal terraces and narrow beaches (Dal rise as the fold-thrust belt is driven forward and, if so, Cin & Simeoni, 1987). The coastal Pliocene is deformed the Conca will cut down through the rising fold. into several anticlines which expose lower Pliocene, upper This situation conforms very well to the model Maz- Miocene and rarely middle Miocene. zanti & Trevisan (1978, fig. 5) proposed as the initial The throughgoing rivers cut across these anticlines in stage in the evolution of the deep gorges. The older valleys with floors 100–200 m below the flanking hills. anticlines emerged first at the edge of a deep Adriatic Because they cross Pliocene sediments of marine origin, it whereas the new anticlines face a shallow Adriatic, but is clear that these rivers have lengthened in the downstream this does not change or invalidate the mechanism. At direction as the marine Pliocene sediments emerged. It is present no anticlinal islands exist offshore, a situation

Fig. 8. Topographic profiles along the axes of six anticlines cut by the Metauro River and its tributaries (for locations, see Fig. 3). These are not sections taken along a single straight line; they are projections of the ridge line onto a N50W–S50E vertical plane and thus approximate the topography as seen from a distance. From near the coast (profile F), south-westward to the highest anticline (profile A), there is a progressive increase in topographic elevation and a progressive deepening of the transverse river valleys, except for the abnormally low Acqualagna anticline. The white band (grey where eroded) is the profile of the Middle Cretaceous Fucoid Marls along the anticlinal axes; it shows the plunge pattern of the folds and the progressively higher structural uplift from the coast inland, again with the exception of the Acqualagna anticline.

© 1999 Blackwell Science Ltd, Basin Research, 11, 267–284 277 W. Alvarez that would constrain later rivers to go around them; the clines (Acqualagna, Bellisio Solfare, Gola di Frasassi) or common occurrence of transected anticlines indicates that the axial depressions that flank them. the offshore was commonly free of islands. However, deviation of rivers around former islands evidently did Umbria–Marche Ridge. The Umbria–Marche Ridge occur in the past, from time to time, as discussed below. (Fig. 8A,B), with its several deep gorges, can now be understood as the most advanced case in a sequence of Foothills belt. Ten to 20 km inland the anticlines (Fig. 8F) progressively more uplifted and eroded anticlines. The are more continuous, reach higher elevations (300–500 m) transecting drainage, which is a puzzle when viewed in and are more deeply eroded (Coward et al., 1999) – in isolation, seems well explained by the Mazzanti–Trevisan one case exposing the Eocene–Oligocene Scaglia Cinerea mechanism. West of Piobbico, the Biscubio River cuts (near Fontecorniale). Nevertheless, the major rivers tran- across the plunge-out of the Monte Nerone Anticline, sect these anticlines as well as the younger coastal providing a probable analogue, at a more advanced stage anticlines. There can be little doubt that here we are of evolution, to the plunge-crossing streams at Cattolica simply seeing a more advanced stage in the evolution of and Montelabbate. The emergence of the Umbria– the drainage. A subsequent stage in the evolution of the Marche Ridge was not complete before the end of the Cattolica situation is probably represented at Montelabb- Miocene because, as discussed above, the Messinian ate, where a NW-plunging anticline is less eroded on the turbidite basin of Montaiate was fed laterally across it north-west side of the Foglia River than on the south-east. (Centamore et al., 1976). Its emergence was probably complete by the early Pliocene, which compresses the Cesana Anticline. The same conclusion can be extended entire erosional history of the fold belt from here to the to the still higher, more deeply eroded anticlines further Adriatic into the last 5 Myr. The proxy historical inland. The Cesana Anticline, 25 km from the coast, rises sequence ends at the Umbria–Marche Ridge, because of to about 650 m, broadly exposing the Upper Cretaceous the absence of larger, more deeply eroded folds to the Scaglia Rossa, and is transected at Fossombrone by the south-west. Metauro River in a gorge cut down to the top of the Lower Cretaceous Majolica Formation (Fig. 8E). When Conclusion. These observations not only support the one travels south-westward from the coast along the Via hypothesis of Mazzanti & Trevisan, but they suggest that transecting gorges formed at chance locations along the Flaminia, following the Metauro River, Fossombrone is anticlines, wherever a newly elongated river happened to the first noticeable gorge, because the Cesana Anticline cross an incipient anticline. This disagrees with the view is the first fold to have risen high enough to expose the that the gorges are controlled by pre-existing faults erosion-resistant limestones below the weaker marls crossing the anticlines (Dramis & Bisci, 1986, p. 100). deposited from the late Eocene on. Thus the abrupt Such cross faults along the lines of transverse gorges are north-east limit of the gorges at Fossombrone is due to dubious in any case, for of 11 gorges included in the depth of erosion in a stratigraphic sequence that contains most detailed geological-map quadrangles available an abrupt change in erosional resistance; the uplift and (1550 000 sheets 290 ‘Cagli’ and 291 ‘Pergola’), only two erosion pattern simply continues what is seen closer to (Belisio Solfare and the southern cut through the Acqual- the coast. agna anticline) show even modest offsets of geological contacts and minor inferred cross faults. Furlo Anticline. The same pattern continues still further Within the study area there are no obvious examples inland. The Furlo Anticline (Fig. 8D) reaches almost of ‘wind gaps’ – palaeovalleys through which the incising 1000 m elevation, and the Furlo Gorge cuts down through river no longer flows. The Apennine rivers have evidently the Jurassic, where the massive lower Liassic Calcare been able to cut down at a rate at least as great as the Massiccio has produced a dramatic chasm. Here for the rate of surface uplift. Considering that the Umbria– first time the exposure is deep enough to cut below the Marche Ridge, which emerged late in the Miocene, characteristic stripping surface where the soft middle around 5–7 Ma, presently reaches elevations of around Cretaceous Fucoid Marls rest on the resistant Majolica 1500 m, with several hundred metres of original cover Formation. The Furlo Gorge has an inner canyon in the removed, the surface uplift rate is roughly 0.5 mm yr−1. Jurassic and the Majolica, a shoulder on the top of the By contrast, the Wheeler Ridge anticline in the southern Majolica, and an outer canyon walled by the Scaglia San Joaquin Valley of California, on which a wind gap Rossa. The impressive chasm at Furlo is due simply to developed (Burbank et al., 1996), has been uplifted at the depth of erosion in the stratigraphic sequence, and about 3–3.5 mm yr−1 (Medwedeff, 1992; Burbank et al., evidently results from a continuation of the progressive 1996) – almost an order of magnitude faster than the valley deepening whose initial stage is seen at the coast. Apennine anticlines.

Internal Marche folds. Next inland is the belt of low Longitudinal river tracts anticlines that interrupt the gradual trend of progressively higher anticlines toward the south-west (Fig. 8C). Here Although most of the major rivers ﬂow nearly straight again the rivers pass indiﬀerently across the short anti- north-east toward the Adriatic, transverse to the

278 © 1999 Blackwell Science Ltd, Basin Research, 11, 267–284 Transverse canyons in the Apennines structure, there are a few places in the study area where longitudinal tract of the Bosso between Secchiano and the trunk stream or a major tributary flows south-east Cagli follows an anticlinal crest, which at first is puzzling. for a few kilometres, parallel to the structural trends, However, the large anticline to the south-west is very before returning to the usual transverse direction. These high and steep-flanked, and the Bosso has apparently longitudinal tracts are present along the Conca, Foglia been displaced away from the tight synclinal axis toward and Metauro, and especially along the lower Musone the much smaller anticline to the north-east. The tight- (Fig. 3). ness of the synclines is of ambiguous significance. It Longitudinal drainage, parallel to the fold axes, is what could be due to original spacing of the anticlines, to would be expected if the Mazzanti–Trevisan mechanism erosion cutting deeper into a V-shaped synclinal profile, were not at work, so that an incipient fold emerged from or to late tightening of the synclines as a result of out- the sea as an island before it was covered by the advancing of-sequence thrusting. coastal plain. In this situation of diminished sediment In this interpretation of the drainage pattern, the supply or enhanced fold uplift, a longitudinal stream, distribution of longitudinal tracts and transverse gorges trapped behind the rising coastal anticline, would flow in a deeply eroded fold-thrust belt reflects the balance of parallel to the coast until it reached an axial saddle uplift and sediment supply in its early history, providing through which it could reach the sea. information of structural importance that could not other- wise be recovered. Whichever of the mechanisms for Longitudinal drainage near the coast. A situation that tightening of the synclines has acted, the result is that supports this interpretation is seen at Monte Conero, by longitudinal tracts in older parts of the fold-thrust belt far the largest of the coastal anticlines (Figs 4 and 5). flow more straight to the south-east, rigidly parallel to With a present relief of over 500 m, it is highly probable the fold axes, whereas younger longitudinal tracts closer that this anticline emerged before it was covered by the to the coast wander in a generally south-east but oblique advancing coastal plain. This would explain the strongly direction across the broader synclines. deflected longitudinal drainage south-west of Monte Conero. Both the Musone River and its near-coast tributary, the Aspio, flow south-eastward, around the COMPLICATIONS IN THE DRAINAGE obstacle of the mountain, and enter the sea where the EVOLUTION south-east plunge termination of Monte Conero elimin- Precursory submarine drainage ates that obstacle. In contrast, the major rivers directly north-west and south-east of Monte Conero, the Esino One would expect that the drainage lines in any part of and the Potenza, flow unimpeded to the sea on trans- the fold-thrust belt would come into being only when verse paths. that place emerged from the sea through uplift and If this interpretation is correct, the paths of the coastal-plain advance. In one place, however, there is streams, transverse or longitudinal, provide a sensitive evidence for submarine sediment transport that may have record of the fluctuating balance between sediment influenced the location of a later cross-cutting river. supply, anticline uplift rate and sea-level change at the On both sides of the Umbria–Marche Ridge the advancing coastline. In a general way, the prevalence of youngest marine turbidite units fill small basins whose transverse drainage indicates that usually enough sedi- present outlines and palaeocurrent patterns indicate that ment was supplied to the advancing coastal plain for the they were shaped by the growing folds (Fig. 9). Directly Mazzanti–Trevisan mechanism to dominate. The isolated south-west of the Umbria–Marche Ridge is a narrow cases of longitudinal drainage record episodes in which syncline, 38 km long and 3 km wide, filled with lower to reduced sediment supply or enhanced anticline emerg- middle Tortonian turbidites and associated deep-marine ence locally defeated that mechanism. sediments of the Monte Vicino Sandstone. The petrogra- This interpretation is supported by the fact that the phy of the sand grains (Centamore et al., 1977) indicates longitudinal drainage tracts on Map 109 ‘Pesaro’ all that these sediments were not derived from the under- follow synclinal belts (Fig. 5). The longitudinal tract lying middle Miocene turbidites of the Marnoso-arenacea along the Foglia River lies in a very broad, gentle Formation, which were transported south-east from an syncline, whereas the tract along the Metauro follows the Alpine source (Ricci Lucchi & Pialli, 1973; Ricci Lucchi narrower, tighter syncline between the Cesana and Furlo & Valmori, 1980); in particular, the abundant Alpine- Anticlines. sourced dolomite grains of the Marnoso-arenacea are absent in the Monte Vicino Sandstone. Palaeocurrent Longitudinal drainage further inland. A few longitudinal studies (Centamore et al., 1977) show that the Monte drainage tracts can be seen further inland, on Map 116 Vicino turbidites entered the basin from the south-west, ‘Gubbio’, making it possible to infer the later evolution built a deep-sea fan, and flooded the elongated basin of these tracts as uplift continues. These longitudinal floor toward the north-west and the south-east (Fig. 9). tracts include the Metauro near Urbania, which lies in a Although the present synclinal remnant is surely narrower broad syncline, as well as the Candigliano north-west of than the original basin, distal palaeocurrents are rigidly Piobbico which occupies a tight syncline. The short parallel to the synclinal axis (Centamore et al., 1977,

Fig. 9. Main part of the Umbria–Marche Ridge (for location see Fig. 5), showing palaeocurrent and sandstone-petrography evidence for north-eastward sediment transport into minor turbidite basins. These basins probably developed as the north- eastward-advancing thrust front deformed the slightly older, large-scale turbidite units with Alpine (north-west) sources – especially the Marnoso-arenacea Formation – which ﬁlled the foredeep before arrival of the thrust front. Contours on the present topography show that the lateral turbidite feed is likely to have passed through the area of the present saddle occupied by the Bosso and Burano transverse gorges. It is thus possible that the transverse drainage developed along the line of a submarine precursor. The original studies of these small basins were done by Centamore et al. (1976, 1977). plate 1), which provides evidence for an originally narrow Whether the saddle had emerged or was still below sea basin moulded into a syncline between growing anticlines. level at that time is not yet clear. Across the Umbria–Marche Ridge, to the north-east, another small synclinal basin holds the upper Messinian Drainage response to axial culminations of sandstones, in places conglomeratic, of the Monte Tur- the folds rino Formation (Centamore et al., 1976). As in the case of the slightly older Monte Vicino Sandstone, the Monte The axis of the largest anticline in the Umbria–Marche Turrino sandstone lacks dolomite and represents a tur- Ridge, extending from Monte Nerone to Monte Catria, bidite fan derived from the south-west. The sandstone has a broad saddle in its middle portion, with bedding petrography and palaeocurrents strongly suggest that the appearing nearly level in a longitudinal section (Fig. 8A). sediment transport that fed the Monte Turrino Sandstone The relatively thick Jurassic pelagic limestone sequence crossed the major anticline through the present saddle. in this saddle area was deposited in a deep-marine setting

(Colacicchi et al., 1970; Centamore et al., 1971; Alvarez, Anticlines transected at culminations 1989a). This saddle is the site of the cross-cutting gorges of the Bosso and Burano Rivers. A remarkable feature of the transected anticlines is that North-west of the saddle, the anticline rises to the in several cases the transverse gorges cut through axial broad culmination of Monte Nerone and then plunges culminations, almost as if the stream were attracted to out and disappears further north-westward. The Monte the most difficult obstacle to cross. This is seen in the Nerone culmination occurs in an area with a thin Jurassic anticlines at Furlo, Gorgo a Cerbara and Acqualagna, limestone sequence deposited on a pelagic fault-block each of which plunges out in both directions, not far high, discussed in the papers just cited, but differences from the anticlinal culmination. in Jurassic palaeobathymetry were probably flattened out By contrast, axial culminations crowning long anti- by Cretaceous deposition. The axial rise of the Middle clines are not transected by river canyons; this is seen at Cretaceous Fucoid Marls from saddle to culmination Monte Nerone, Monte Catria and Monte Cucco. As indicates that the culmination is primarily due to Apen- discussed above, the latter culminations probably formed nine structural deformation – perhaps to a sub- early and would have been avoided by the initial streams. surface thrust. South-east of the saddle, the Majolica Other culminations look instead to be late modifications rises to a second culmination, crowned by Monte Acuto of originally less steeply plunging anticlinal axes. The (a Jurassic fault-block high) and Monte Catria (mostly Candigliano River is barely deflected as it approaches the basinal Jurassic), and the same reasoning implies thrust- Furlo Gorge, arguing that this striking culmination was ramp control at depth. The doubly plunging Monte originally less extreme, if present at all. Gorgo a Cerbara Nerone – Monte Catria anticline, with a central saddle, is even more convincingly a late modification, because may reflect a pair of lateral ramps beneath the plunging the Candigliano follows a 7-km longitudinal tract before ends of the anticline, dipping toward the central saddle entering Gorgo a Cerbara (Fig. 3). As discussed above, (Boyer & Elliott, 1982, fig. 25, section B–B∞). longitudinal tracts apparently form behind obstacles while If the culminations resulted from in-sequence thrust- associated transverse tracts cross through lower saddles. ing, they would have been present since the time of the This suggests that Gorgo a Cerbara was originally a low point along the fold axis, whereas it is now at the initial rise of the anticline, and would have been the first culmination of the Montiego Anticline. parts to emerge from the coastal plain, or perhaps even Rivers cutting through anticlines have often been taken from the sea. In either case they would have been avoided as prima facie evidence for the presence of cross-cutting by the incipient rivers. If the culminations were due to faults, providing lines of weakness, but in the Apennine late, out-of-sequence thrusting, there is no reason why case there is commonly no other evidence for such faults. they could not be transected by the drainage. The At Furlo, the Jurassic stratigraphy differs north-west and fact that the modern gorges occupy the saddle, not south-east of the gorge, suggesting the presence of the culminations, thus provides weak evidence for an original fault. In other gorges (Gorgo a Cerbara, in-sequence thrusting in this anticline. Acqualagna, Bosso, Burano, Fossombrone) there is no Along the Umbria–Marche Ridge, the axial culmi- indication of fault control of the cross-cutting rivers. nations rise above a saddle which itself is 500 m above These observations suggest the possibility that the the surrounding terrain. There is a possible analogue cross-cutting river controlled the position of the axial with the same dimensions, representing an earlier stage culmination, perhaps by removing some of the weight of of development, 20 km to the north-east: the easternmost the overburden. An analogous situation occurs in Utah, major anticlinal trend has axial culminations at Furlo and where the Colorado River in south-eastern Canyonlands Arcevia (Fig. 3), separated by an axial saddle at Pergola National Park (Huntoon et al., 1982) is followed by the which, because of lesser uplift, does not yet rise above axis of a long anticline apparently reflecting flow of its surroundings or expose pre-Tertiary rocks. The underlying Pennsylvanian evaporites of the Paradox For- Cesano River and two smaller streams cross the saddle mation into the zone of reduced overburden along the in a pattern that should produce analogues to the Bosso river canyon. The transected axial culminations in the and Burano canyons after another few hundred metres Umbria–Marche Apennines may also be due to move- of uplift. By contrast to the culmination of Monte ment in evaporites. However, the underlying Triassic Nerone, the Furlo culmination is transected by a deep Burano Formation is primarily composed of anhydrite, gorge. This would argue against control of the culmi- which should be more resistant to flow than the subsur- nation by in-sequence ramp thrusting. face salt of the Paradox Formation. Monte Conero has about the same dimensions as the Alternatively, the transected culminations may rep- axial culminations just described, and may well represent resent easier uplift through motion on late, out-of- the earliest stage in the morphological expression of these sequence thrusts. This seems probable in the case of structural features. As noted above, drainage is diverted Gorgo a Cerbara, which is located directly in front of around Monte Conero in longitudinal valleys, and its the Monte Nerone culmination on the Umbria–Marche appearance in the distant future should be much like that Ridge. The combination of structural form and drainage of modern Monte Nerone. suggests that the culmination at Gorgo a Cerbara started

© 1999 Blackwell Science Ltd, Basin Research, 11, 267–284 281 W. Alvarez out low and was later raised by indentation of the Monte It is not yet clear whether the Umbria–Marche fold- Nerone Anticline. Another possibility is that more sedi- thrust belt formed through an orderly progression of ment was shed from the incipient Monte Nerone high, thrusts, followed by uniform uplift after passage of the pushing the coastal plain out over an original culmination thrust front, or whether out-of-sequence thrusting caused in the Cerbara sector of the Montiego Anticline. the fold profiles to change as they were uplifted. Careful study of the geomorphology may contribute to dis- tinguishing between these two possible histories. CONCLUSIONS This study of Apennine drainage evolution leads to a view of the landscape as a delicately balanced system in The fold-thrust belt of the Umbria–Marche Apennines which tectonic transport, generation of fold and thrust displays deep gorges cutting through anticlinal ridges, structures, uplift, erosion of a stratigraphic sequence with allowing rivers to drain across the structural grain to the internal discontinuities, progradation of the coastal plain, Adriatic Sea. These gorges are well explained by the elongation of the distal tracts of the rivers and changes mechanism of Mazzanti & Trevisan (1978), which is a in sea level all interact in complicated ways. This view combination of antecedence and superposition, initiated raises the possibility that geomorphology may be able to at the shoreline, where folds, rivers and coastal plain are help resolve difficult questions of the subsurface structure all forming simultaneously. Rivers are progressively elon- and evolution of fold-thrust belts. gated downstream as the coastal plain builds out, trapped behind but also burying incipient anticlines, and allowing ACKNOWLEDGEMENTS the new river increments to cross the anticlines while these folds still have low relief. My indebtedness to many Italian colleagues over many Mazzanti & Trevisan based their mechanism on strati- years of research in the Apennines will be obvious. Early graphic and structural evidence that the Apennine anti- drafts of this paper benefited from careful discussion and clinal ridges have emerged in sequence from south-west review by participants in a UC Berkeley graduate seminar, to north-east. Subsequent advances in understanding the especially John Stock and Daniel Karner, and from structure of fold-thrust belts show that this observation conversations with Alessandro Montanari. I thank Robert is a natural consequence of the forward advance of the Anderson, Douglas Burbank and Peter DeCelles for their basal detachment thrust, generating a sequence of thoughtful and detailed reviews for the journal. My anticlines. research in the Apennines over the years has been The sequence of anticlines across the fold-thrust belt, supported by the National Science Foundation and by from younger to older, provides a proxy for the topo- Chevron Overseas Petroleum, Inc. It has benefited from graphic history of a single anticline and shows all the the logistical support of the Osservatorio Geologico di stages in the evolution of transverse gorges inferred by Coldigioco and the hospitality of the people of Piobbico Mazzanti & Trevisan. In some cases, however, rivers and Gubbio. This study is dedicated to the memory of have longitudinal tracts, parallel to the fold axes, between Giampaolo Pialli, one of my first Italian geological friends. transecting gorges. Longitudinal drainage is presently Paolo brought modern structural geology to the Umbria– forming near the coast, behind the high anticline of Marche Apennines in the 1970s and trained many of the Monte Conero. This allows the recognition that within leaders in the present generation. He was the driving the fold-thrust belt, longitudinal drainage reflects initial force behind the complex and highly successful CROP-03 conditions dominated by rise of a new anticline, whereas crustal-profile project (Pialli et al., 1998). CROP-03 transverse drainage reflects original coastal conditions should have been simply Paolo’s next step, but instead dominated by buildout of the coastal sediments. The will be remembered as his greatest accomplishment. drainage patterns thus preserve a memory of the relative balance between sedimentation and uplift at the advanc- REFERENCES ing coastline. Small turbidite basins on either side of the largest A, E., B, V., P, P. &S, M. (1970) anticline, showing evidence of lateral sediment feed, Introduction to the geology of the Northern Apennines. suggest that some drainage lines may be descended from Sediment. Geol., 4, 207–249. submarine precursors. Early emergent anticlines, rep- A, W. (1972) Recent Italian research. Geotimes, 17, 14–17. resented in older structures by axial culminations on long A, W. (1989a) Evolution of the Monte Nerone seamount anticlines, are systematically avoided by drainage lines. in the Umbria-Marche Apennines: 1. Jurassic-Tertiary However, in anticlines that plunge out over short dis- stratigraphy. Soc. Geol. Ital. Boll., 108, 3–21. A, W. (1989b) Evolution of the Monte Nerone seamount tances in both directions, the culminations are in some in the Umbria-Marche Apennines: 2. Tectonic control of the cases precisely transected by the drainage, suggesting seamount-basin transition. Soc. Geol. Ital. Boll., 108, 23–39. that erosional removal of overburden may have allowed A, W., A, A.J., R, P.R., K, D.B., the rise of the culmination, either through evaporite T, N. &M, A. (1996) Quaternary fluvial- mobility or through easier uplift on late, out-of- volcanic stratigraphy and geochronology of the Capitoline sequence thrusts. Hill in Rome. Geology, 24, 751–754.

B, A.W., B, L., C, C. &G, R. (1986) Giurese umbro-marchigiano ed ipotesi per un suo inquadri- Balanced sections and seismic reflection profiles across the mento regionale. Soc. Geol. Ital. Mem., 9, 839–874. Central Apennines. Soc. Geol. Ital. Mem., 35, 257–310. C, M.P., D D, M., M, S., P, W. B, A.W., G, P.L. &S, G.A. (1966) Structure, &W, F.C. (1999) Frontal part of the northern Apennines seismic data, and orogenic evolution of southern Canadian fold and thrust belt in the Romagna-Marche area (Italy): Rocky Mountains. Bull. Can Petroleum Geol, 14, 337–381. Shallow and deep structural styles. Tectonics, 18, 559–574. B, M., G, F., L, G., L, O.B- C, S., M, S., P, G., B, A. &R, , R. (1988) Sezioni geologiche bilanciate attraverso il V. (1989) Stratigrafia del Mesozoico e Cenozoico nell’area sistema a pieghe umbro-marchigiano: 1. la sezione Trevi- Umbro–Marchigiana/Mesozoic–Cenozoic stratigraphy in the Valle dell’Ambro. BSGI, 107, 109–130. Umbria–Marche area [in Italian and English]. Mem. Decrittive B, G. (1891) Il territorio di Gubbio. Notizie geologiche. della Carta Geologica d’Italia, 39. Tipografia Economica, Roma, 38. D, F.A., S, J. &D, D. (1984) Mechanics of fold- B, V., P, P., S, M. &S, G. (1970) and-thrust belts and accretionary wedges; cohesive Coulomb Development of the Northern Apennines geosyncline – the theory. J. Geophys. Res., 89, 10 087–10 101. miogeosynclinal sequences. Sediment. Geol., 4, 341–444. D, C.D.A. (1969) Balanced cross sections. Can. B, S.E. &E, D. (1982) Thrust systems. Am. Ass. J. Earth Sci., 6, 743–757. Petrol. Geol. Bull., 66, 1196–1230. D C, R. &S, U. (1987) Analisi ambientale quantita- B, D., M, A. &B, N. (1996) Interactions of tiva dei litorali marchigiani fra Gabicce e Ancona. Livello growing folds and coeval depositional systems. Basin Res., del rischio naturale e del degrado, distribuzione dei sedimenti 8, 199–223. e loro possibile impiego per ripascimento artificiale. Soc. C, D., D Donatis, M., Mazzoli, S. & Wezel, F.C. Geol. Ital. Boll., 106, 377–423. , . ., , . , . (1995) The Colbordolo-Pesaro area of the Romagna-Marche D A V P L &P G (1983) Osservazioni foothill zone (Northern Apennines, Italy). Giornale Geol., geologiche nelle aree comprese fra i massicci perugini ed i 57, 277–295. rilievi di Gubbio. Giornale Geol., 45, 127–150. , ., , . , . . C, A., C, R., P, A. &C, D D S J &D F A (1983) Mechanics of fold- C. (1982) The Jurassic-Lower Pliocene history of the Anzio- and-thrust belts and accretionary wedges. J. Geophys. Res., Ancona Line (Central Italy). Soc. Geol. Ital. Mem., 24, 88, 1153–1172.  , ., , ., , ., , . 325–336. D D M I C L A M S & , . C, A. &S, K.G. (1989) Exotic clasts in a P M (1998) CROP 03: Structure of the Montecalvo in Foglia-Adriatic Sea segment. Soc. Geol. Ital. Mem., 52, Pliocene conglomerate near Pesaro have an Alpine source. 617–630. Soc. Geol. Ital. Boll., 108, 607–618. D, F. &B, C. (1986) Aspetti geomorfologici del C, B. (1934) Studi morfologici nell’Italia centrale. territorio marchigiano. In: La Geologia delle Marche (special Boll. R. Soc. Geogr. It., Ser. 6, 11, 22–30. volume of Studi Geologici Camerti) (Ed. by E. Centamore and C, C. (1976) Correlazione tra piani carsici ipogei e G. Deiana), pp. 99–103. Universita` di Camerino, Camerino. terrazzi fluviali nella valle del F. Esino (Marche). Soc. Geol.  F, A.J. &M, M. (1986) Back thrusting in Ital. Boll., 95, 313–326. forelimbs of rootless anticlines, with examples from the C, C., C, C. &G, L. (1989) Geo- Umbro-Marchean Apennines (Italy). Soc. Geol. Ital. Mem., morphology of north-east Umbria and the Marche region [in 35, 357–370. English and Italian]. Mem. Descrittive Della Carta Geol.  F, A.J., M, N., P, G., M, M. Italia, 39, 37–44. &V, F. (1990) Palaeotectonic significance of gravity , . C C (1988) Evoluzione del reticolo idrografico in displacement structures in the Miocene turbidite series of un tratto umbro-marchigiano dello spartiacque principale the M. Pollo syncline (Umbro-Marchean Apennines, Italy). dell’Appenninico. Geografia Fisica e Dinamica Quaternaria, Geol. Mijnb., 69, 69–86. 11, 11–24. E, P., G, G., T, M. &T, L. (1975) , ., , ., , ., , . C E C M D G M A Tensional and compressional areas in the recent (Tortonian , . &P U (1971) Contributo alla conoscenza del to present) evolution of the Northern Apennines. Boll. Geofis. Giurassico dell’Appennino Umbro-Marchigiano. Studi Geo- Teorica Applicata, 17, 3–18. logici Camerti (Camerino), 1, 7–89. G, R. (1958) Spostamento dello spartiacque dell’Ap- C, E., C, U. &M, A. (1977) Analisi pennino Settentrionale in conseguenza di catture idrografiche. dell’evoluzione tettonico-sedimentaria dei ‘bacini minori’ tor- Atti della Societa` Toscana di Scienze Naturali, Memorie, Ser. biditici del Miocene medio-superiore nell’Appennino A, 65, 25–38. Umbro-Marchigiano e Laziale-Abruzzese: (3) Le arenarie G, R. (1962) Evoluzione e spostamento dello sparti- di M. Vicino, un modello di conoide sottomarina acque appenninico tra il Monte Fumaiolo e Gualdo Tadino. affogata (Marche Settentrionale). Studi Geologici Camerti Atti Della Societa` Toscana Di Scienze Naturali, Memorie, Ser. (Camerino), 3, 7–56. A, 68, 57–67. C, E., C, U., R Lucchi, F. & Salvati, G, F., B, M., B, A.W., M, I. &V, L. (1976) La sedimentazione clastica del Miocene medio- L. (1993) Conflicting balanced structural sections across the superiore nel bacino marchigiano interno tra il T. Tarugo ed Central Apennines (Italy): problems and implications. In: Arcevia. Studi Geologici Camerti (Camerino), 2, 73–106. Generation, Accumulation and Production of Europe’s hydro- C, M.B. &R, W.B.F. (1978) Messinian erosional surfaces carbons III, European Association of Petroleum Geologists in the Mediterranean. Mar. Geol., 27, 193–363. Special Publication, v. 3 (Ed. by A.M. Spencer), pp. 219–231. C, R., P, L. &P, G. (1970) Nuovi dati sul Springer-Verlag, Berlin.

G, E. &P, L. (1949) Considerazioni sullo volume of Studi Geologici Camerti) (Ed. by E. Centamore and sviluppo dell’idrografia in relazione alle piu` recenti teorie G. Deiana), pp. 105–133. Camerino. sull’orogenesi appenninica. Atti della Societa` Toscana di N, T., P, E. &R, M.L. (1986) Il bacino Scienze Naturali, Memorie, Ser. A, 56, 144–147. pleistocenico marchigiano. In: Atti della riunione-escursione G, L. &P, G. (1969) Alcune idee sulla evoluzione del Gruppo di Sedimentologia del CNR sul Pleistocene marchigi- oro-idrografica dell’Appennino settentrionale. L’Ateneo Par- ano (Ed. by T. Nanni), pp. 13–43. Dipartimento di Scienze mense, Acta Naturalia, 1, 29–45. dei Materiale e della Terra, Universita` degli Studi di Ancona, H, N. (1996) Regular spacing of drainage outlets from Ancona (Italy). linear mountain belts. Basin Res., 8, 29–44. O, T.M. (1965) The Zagros streams: a new interpret- H, C.B. (1956) Cenozoic geology of the Colorado Plateau. ation of transverse drainage in an orogenic zone. Syracuse U.S. Geol. Surv. Prof. Pap., 279, 1–99. University Geographical Series, 1. H, P.W., B, G.H. J &B, W.J. (1982) O, T.M. (1985) Origin of drainage transverse to Geologic map of Canyonlands National Park and vicinity, structure in orogens. In: Tectonic geomorphology (Ed. by M. Utah (1: 62,500). Canyonlands Natural History Association, Morisawa and J.T. Hack), pp. 155–182. Allen & Unwin, Moab, Utah. Boston. K, D.B. &M, F. (1998) Correlation of fluviodeltaic O, G.G. &F, P.F. (1984) Sedimentary basins formed aggradational sections with glacial climate history: a revision and carried piggyback on active thrust sheets. Geology, of the Pleistocene stratigraphy of Rome. Geol Soc. Am. Bull., 12, 475–478. 110, 748–758. P, G. &A, W. (1997) Tectonic setting of the L, G., B, F., B, M., M, M. & Miocene Northern Apennines: the problem of contempor- K, J.V.A. (1994) Seismotectonic zoning of east-central aneous compression and extension. In: Miocene Stratigraphy: Italy deduced from an analysis of the Neogene to present an Integrated Approach (Ed. by A. Montanari, G.S. Odin and deformations and related stress fields. Geol. Soc. Am. Bull., R. Coccioni), pp. 167–185. Elsevier, Amsterdam. 106, 1107–1120. P, G., B, M. &M, G. (1998) Results of the M, A. &R, W.B.F. (1986) Extension in the CROP03 deep seismic reflection profile. Soc. Geol. Ital. Tyrrhenian Sea and shortening in the Apennines as a result Mem., 52, 657. of arc migration driven by sinking of the lithosphere. Tecton- P  C (ca. 560) History of the Wars, v. 3 (The ics, 5, 227–245. Gothic War). Harvard University Press (1919, 1968), M, O. (1926) La maggiore discordanza tra orografia Cambridge, MA. e idrografia nell’Appennino. Riv. Geograf. Ital., 33, R, K.J., G, P. &C, H. (1980) Lithospheric split 65–74. in the descending plate: observations from the Northern M, R. &T, L. (1978) Evoluzione della rete Apennines. Tectonophysics, 64, T1–T9. idrografica nell’Appennino centro-settentrionale. Geograf. R L, F. &P, G. (1973) Apporti secondari nella Fisica e Dinamica Quaternaria, 1, 55–62. Marnoso-arenacea: 1. Torbiditi di conoide e di pianura M, D.A. (1992) Geometry and kinematics of an active, sottomarina a est-nord-est di Perugia. Soc. Geol. Ital. Boll., laterally propagating wedge thrust, Wheeler Ridge, Cali- 92, 669–712. fornia. In: Structural Geology of Fold and Thrust Belts (Ed. R L, F. &V, E. (1980) Basin-wide turbidites by S. Mitra and G.W. Fisher), pp. 3–28. Johns Hopkins in a Miocene, over-supplied deep-sea plain: a geometrical University Press, Baltimore, MD. analysis. Sedimentology, 27, 241–270. M, G. (1938) Il Tevere. Monografia idrologica. v. 1, parte S, R. (1952) Il bacino del Metauro. Giornale Geol., 24, 2. Geologia e permeabilita` dei terreni del bacino. Servizio 1–268. Idrografico (Roma), Pubblicazioni, 22. S, A. (1950) Sull’origine della rete idrografica e dei bacini M, G. (1951) Geologia dell’Appennino settentrionale. Soc. intermontani nell’Appennino centro-settentrionale. Riv. Geo- Geol. Ital. Boll., 70, 95–382. graf. Ital., 57, 249–256. M, C.I. (1948) I cunei composti nell’orogenesi. Soc. S, A.N. (1945) Hypotheses of stream development in Geol. Ital. Boll., 67. the folded Appalachians of Pennsylvania. Geol. Soc. Am. M, C.K. (1988) Out-of-sequence thrusts. Tectonics, 7, Bull., 56, 45–88. 539–561. T, W.D. (1965) Regional Geomorphology of the United N, T. &V, P. (1986) Inquadramento idrogeologico States. John Wiley and Sons, New York, 609. ed influenza della tettonica sugli acquiferi di subalveo delle pianure marchigiane. In: La Geologia delle Marche (special Received 31 August 1998; revision accepted 30 July 1999.