2.1 Geological characteristics of the middle and lower reaches of the Cai River
Results of previous studies (Nguyen 1986; Cat, 1996; Tong and Vu 2005; Tran and Vu 2009), show that the study is composed of a complex lithological basement of various rock types from Jurassic to Quaternary age.
Stratigraphically, the study area consists of sedimentary and volcanic rocks that were derived from different origin, which have been divided into several geological units. The oldest rocks of the La Nga Formation (J2ln) are exposed in the middle part of the Cai River course, consisting of alternating layers of claystone, siltstone, and shale. This formation is overlain by the Bao Loc Pass Formation (J2-3dbl), which is distributed in the central part of the study area and consists of interlaying conglomerate, mixed gravelstone, shale, with minor volcanic rocks including andesitodacite, andesite, dacite, ryodacite, and their tuffs. The Nha Trang Formation (Knt) is exposed in the north and southwest of Nha Trang City, which is unconformably overlain by the granodiorite of the Dinh Quan Complex (late Jurassic), with the comosition including volcanogenic sedimentary members and andesite, rhyolite, porphyry rhyolite, dacite tuff, and felsic tuff. The Dac Rium Formation (K2đr) is exposed in the northwest of the study area and consists of conglomerate, gritstone, purple-brown sandstone, and red-brown siltstone. The Don Duong Formation (K2đd) is exposed in the southwest of the study area, with the main composition consisting of conglomerate, arkosic sandstone, red-brown siltstone, and of rhyolite, porphyry rhyolite, porphyry ryodacite, or andesite, porphyry dacite, an interlayer of sandstone and tuffacious siltstone, tufite, tufaceous andesite and dacite.
Unconsolidated Quaternary sediments are widely distributed in the Cai River valley and along the coastal zone of Nha Trang which consist of the following formations: Upper Pleistocene sediments (QIII³) mainly consist of sand, silt, clay, and coral limestone; Middle Holocene sediments (QIV²) include mainly white sands that form the 'Cam Ranh white sand, distributing as a 2-6m high sea-terrace and/or 6-10m high dune; Middle-upper Holocene sediments (QIV2-3) are mixed alluvial and marine sediments that distribute widely in the middle and lower courses of the Cai River, consisting of sand, silt, clay, that contain shell fragments and humus, Upper Holocene sediments (QIV3) include alluvial products that form the river terraces and flood plains along the Cai River. Along the coastal area, the sediments of alluvial-marine and marine origin are distributed in narrow and discontinuous plains of 1-4 m high or small dunes distributed along the coast.
Intrusive magmatic rocks of different ages and origins are abundant and scattered throughout the study area, which are grouped into various complexes. The Dinh Quan Complex (J3đq) is exposed as small bodies along the northern and southern edges of the Cai River, consisting of two phases: Phase 1 includes diorite, quartz diorite, and pyroxene-containing gabrodiorite; Phase 2 consists of medium-grain biotite-hornblende granite and biotite-hornblende tonalite. The Ca Na Complex (K2cn) exposes at the northern and southern edges of the area and consists of two main phases: Phase 1 consists of alaskite granite, biotite granite with medium-large grain muscovite, sometimes with prophetic architecture, and Phase 2 of light-colored small-grained bi-mica granite and vein rocks of porphyria and pegmatite granite. The Deo Ca Complex (K1đc) is scattered throughout the study area and penetrates into the older complexes mentioned above with components grouped into three phases: Phase 1 includes biotite granodiorite and small to medium-sized quartz monzoddiorite; Phase 2 consists of granite, biotite (+horblend) granosyenite, medium to large grain or sometimes porphyric and Phase 3 includes vascular granite of porphyria, split granite, pegmatite and pegmatoid granite.
a. Regional structural features
The underlying basement of Cai River valley has experienced strong tectonic activities, which is indicated by widespread fracturing, partly dismembered, and/or displaced of all geological units as consequence of multiple regional deformation. Field mapping and structural interpretation conducted by this and recent studies (Tran Thanh Hai 2020) have identified numerous paleo- and neotectonic (Moores and Twiss 2014) structural feattures, including faults and fracture zones of differing orientations and ages (Fig. 1). Many fault systems that straddle the pre-Qaternary units apprear to be multiple reactivated and are remained active throughout the Quaternary. On the basis of the cross-cuting relationship between the tectonic deformation with geological units, it is possible to distinguish the Paleotectonic from Neotectonic structures (Moores and Twiss 2014).
Paleotectonic structures are represented by faults and fractures, which are considered to be predated 5 Ma in age (Moores and Twiss 2014), are widely documented within pre-Quaternary lithotectonic units. These comprise numerous cross-cutting birtte to brittle-ductile fault and fracture systems that variously trends northwest-southeast, northeast-southwest, longitudinally, and lattitudationally (Tran Thanh Hai 2020). They can be identified by field evidences including large zones of brecciation, slickensides, and systemtic distribution of fractures. Kinematic indicators identified from the faults and fracture zones as well as the cross-cuting relationship between different systems indicate that the movement history along the paleotectonic faults are complicated, including several overprinting phases of reverse, strike-slip or oblique-slip. Many fault systems are multiple reactivated, which are indicated by the presence of several generations of overprinting fault-generated products in a single fault zone.
Neotectonic structures are widespread in the Cai River catchment area and environs (Tran Thanh Hai 2020; Pham 2002), which comprise neotectonic fault and fracture systems postdated 5 Ma in age (Moores and Twiss 2014). Field mapping and structural interpretation conducted by this study have recorded many evidences demostrating the occurence of the neotectonic structures, including numerous active fault and facture systems. These include widespread exposure of brittle slickensides, unconsolidated fault gouge, and numerous open-spaced, systematic fractures that straddle and dismember the Quaternary deposits and/or weathering profiles/regoliths (Fig. 1). Furthermore, many tectono-geomorphological markers resulted from neotectonic and active movements such as linear distribution of triangular facets, fault scarps, uneven occurrence of drainage systems, colluvial cones and alluvial fans, or abrupt change of flow direction of flows, local zones of fault-controlled uplift or subsidence (NRC 1985; Burbank and Anderson 2011; Molar et al. 2007, Kamp and Owen 2022; and references herein) are also widely recognized within the study area (see discussion in the next section). In addition, the reactivation of paleotectonic structures during neotectonic movement are also very common, which locally create zones of multiple overprinting of constrasting slip directions in a single fault plane/zone. This points toward a unstable neotectonic regime with opposing stress-fields has occrured and shaped the morpotiectonic features along the Cai River course and its tributary during the Quaternary periods. Major features of the Neotectonic structures and their significance to the evolution of Cai River cachment will be dissussed in the following section.
b. General geophormology
Generally, the geomorphology of the study area is characterized by the presence of numerous topographic features both at large- and small-scale including variable landforms, terrain and drainage systems. The middle and lower course of the Cai River is characterized by a narrow, west-east trending, easterly tilting and steepedly sloped valley (Fig. 1). The southern and northern flanks of the valley are represented by medium to high relief moutain chains that peaked upto more than 1000 metres. The relief is rapidly reduced towards the river valley, where narrow alluvial and floodplains that infilled by alluvial, lacustine, and/or dune and subtidal marine seditmentary deposites of Quaternary in age were deposited. The rivermouth area is charactered by a rias-type shoreline with fracture-controled steep shore-clift in the north and southern portions, whereas in the central portion, the rivermouth is represented by a narrow delta, where the basement crystalized rocks are locally exposed.
Within the Cai River Valley, the main flow and its tributaries are unevenly distributed along discontinued alluvial plains that extends to the river mouth with asymetric valley profiles. As consequence of the inhomogenity of the basement rocks as well as the strong influence of tecronic deformation and neotectonic movements, the flow direction of the tributaries and river sections in the area is irregular and forms many types of flow patterns including zig-zag shape, annular, barbed, centripetal, radial, trellis and dendritic. In addition, the main current of the Cai River and its mouth has periodically migrated northwards, which led to the systemmatic abandonments of the older sections of the flows in the south of the river valley and northwardly latteral insision of the left bank of the river and consequently expanding of the valley bottom northward (Fig. 1).
On the basis of morphological mapping, tectono-morphologic and DEM analyses of the occurrence, parameters and relative position of landforms, as well as examination of the comprehensive relationship between tectno-morphological features with basement architecture, neotectonic, endogenic and subaerial exogenic processes (Hurtrez et al. 1999; Dumont and Holbrook 2000; Keller and Pinter 2002; Kamp and Owen 2022; Hartvich and Vilímek 2008; Knight et al. 2011; Smith 2011; Whitworth et al. 2011; Otto and Smith 2013; Kamp and Owen 2022; Switzer and Kennedy 2022), the morphology of the area can be divided into a number of genetic types of relief segments resulted from denudation and deposition that caused by both exogenic and neotectonic activities including erosional, erosional-structural slopes, depositional surface remnants and the valley floor infilled by the Quaternary deposits (Fig. 1). The various types of landform related to erosion commonly occur above the exposed lithologies and within the uplifted terranes and controlled either by underlying geology or structures as well as recent tectonic movements. The depositional landforms consist of valleys and plains developed along the drainage systems, which commonly create flood plains, terraces or channels that formed by the combination of faulting driven and fracturing of the basement (Fig. 1). It is common that spatial distribuiton and morphology of the flows are governed by the underlying neotectronic structures, particular large neo- and active-fault and fracture zones (Fig. 1; NRC 1986; Molnar et al. 2007; Burbank and Anderson, 2011 and references herein). Alluvial and coastal plains, fluvial and marine terraces, lagoons, and beaches occurring along the lower course and rivermouth portion of the study area (Fig. 1). The aeolian landforms occurs along the coastal zone that formed elongated sand dunes adjacent to the shoreline in the southeastern portion of the area (Fig. 1).
Using tectono-geomorphological analysis and geomorphic index of active tectonic deformation (Toudeshki and Arian 2011), as well as the observation of relationship between these parameters with the occurrence and evolution of drainage system in the area, it can be predicted that the Cai River and its tributaries are very sensitive to neotectonic movements, especially the local changes in active tectonic activities including fracturing, uplift, subsidence, and tilting. The indicators of neo- and active tectonic activities and their influence on the evolution of the Cai River are discussed in the following sections.
2.2. Indicators of neo- and active tectonics in the central-lower courses of the Cai River
Based on the results of field investigation, structural analysis, and synthesis of all geological, geomorphological, and other features, this study has been able to identify a number of structural and neotectonic factors, that directly impact the morphology and evolution of the main flow of Cai River as well as its tributaries. The results of the analysis are shown in Fig. 1.
The main active tectono-geomorphological features include landforms that were formed and controlled by young tectonic movements, including eustatic blocks, tectonic depressions and other forms of erosion and/or accumulation (Fig. 1). Neotectonic and active tectonic structural elements mainly include faults, fractures, and neotectonic uplift blocks. Neotectonic faults (including active faults) include a variety of systems with kinematic signatures that are identifiable in the field and are summarized below.
a. Strucutral Indicators
Based on direct and indirect geological evidence, including zones of fracturing and the relationship between fractures, slickenside and slickenline, and brecciation and fault gouge, as well as the relationship between faults and other geological and morphological features, it is possible to distinguish the active tectonic faulting and fracturing as well as other movements at different scales, their kinematic and dynamic nature (Fig. 1). Neotectonic and active faults in the region are common, including many systems, including northeast-southwest, northwest-southeast, sub-latitudes, and sub-meridian, in which the northeast-southwest system is most widely developed as well as being the youngest system (Fig. 1). In many places, fault systems are either cross-cut or present as conjugate sets to form complex structural grains. Many paleofault/fracture systems are either displaced or reactivated and form part of the young systems.
Mesoscopic faults are identifiable on the outcrop (Fig. 2) in which many of them cut through and displaced young sediments, changed the weathering profile, or deformed and changed the direction of currents, especially the Cai River flow, indicating that they are young and may still be active presently (Fig. 2). The strong development of large, brittle and open fracture systems has led to the destruction of rock homogeneity, creating zones of weakness that promote down-deep weathering, and erosion to create channels that gradually evolved into currents forming the river and its tribitaria as well as the shape of the shoreline that is oriented following the trend of fracture or fault zones (Fig. 2)
b. Geomorphological indicators
+ Displacement and deformation of the surface flow
In the study area, the Cai River and its tributaries have rather unusual morphology and flow patterns in which sudden changes of flow direction or meandering of the current are observed in many places. Aside from the effects of the nature of the underlying rock and basement architecture, and the relative base-level, the abrupt change in flow pattern clearly reflects the impact of active tectonics (NRC 1986; Burbank and Anderson 2011). The study area, shape of currents or shoreline often coincident with or are coicidently oriented with the occurrence and distribution of fractures and fault systems (Fig. 1). Field observations and geomorphological analytical results show that many sections of the Cai River and its tributaries abruptly change the flow direction where they cross the fault zones and the bending is commontly coincide with the displacement direction of the fault walls (Fig. 1). In places, the ridges are also dissected to form grooves and narrow valleys or chains of triangular facets oriented along the fault lines (Figs. 1, 2). The northwest-southeast, sub-longitude and northeast-southwest trending fault systems with dominant strike-slip components play a prominent role in the diversion of flow direction and abrupt change of the currents associated with the active faults (Fig. 1).
+ Triangular facets, fault scarps and traces
In places, topographic linear features comprising of elongated narrow valleys, and straight flow segments (Figs. 1, 2) associated with other indicators such as triangular facets or elongate scarp are indicative of fault traces common in the area. In addition, the occurrence of tens to hundreds of meters high surfaces that abruptly jut out of the surrounding surface along the fault traces also reflect strong tectonic movement along faults, creating horsts, facets and scarps on the uplifting blocks and plains, valleys, lacustrines or marshes on the subsidence blocks (Fig. 2). Besides, the existence of triangular facets along the fault traces is evidence for the young faults in the study area (Figs. 2a, b).
c. Active uplift and subsidence
Recent tectonic uplift and subsidence in the area are abundant. In general, regional neotectonic activities led to relative uplifting in the west and subsidence in the east, resulting in the general west-to east tilting that governed the formation of the latitudinally elongated narrow river basin as well as the east-directed flow pattern of the Cai River and the formation of coastal plains in the east. However, as described above, the regular change and complex orientation of sections as well as the migration of the flow over time are strongly controlled by local uplift and/or subsidence within the river valley (Figs. 1, 3).
Within the study area, the relative subsidence is localized with the formation of small subsided blocks, generally in the form of pull-apart basins, along strike-slip faults or along the hanging walls of extensional faults, which contributes to the re-direction of the flow, development of a centripetal or endorheic drainage configuration (Figs. 1, 3, 4). In addition, local subsidence has led to the relative rise of the base level and lateral erosion of the riverbanks, leading to the meandering and formation of oxbow lakes or swamps within the river basin (Fig. 1). Besides, subsidence or tectonic collapse also leads to the tilting of terrain that forces the flows to reorientate towards the lower ground to keep pace with the degree of subsidence (NRC, 1986; Burbank and Anderson 2011; Zhang et al. 2014 ; Tran Thanh Hai 2020, Tran et al. 2020), which also lead to the abandonnent of old sections of flows in many places.
The relative tectonic uplifting occurs commonly in the area, as indicated by the existence of numerous geomorphological signatures. The widespread occurrence of river terraces in the middle and lower courses of the Cai River and marine terraces in the coastal area (Figs. 1, 3) are firm evidence for geological uplift or lowering of the base level due to relative basement uplifting. Strong relative regional uplift has been defined to occur along the southern flank of the Cai River basin, which are named the Cam Lam and Khanh Vinh uplifts in the south (Tran Thanh Hai 2020). The high-rate of uplifting (Tran Thanh Hai 2020 and next section) has consequently produced sub-latitude compressive stress field, followed by the development of the northwest-southeast and northeast-southwest conjugate strike slip fault systems and/or secondary uplift and subsident blocks, leading to displacement and redirecting the flows parallel to the fault segments. The relative uplifting in the south has resulted in south-side-up tilting, which forces the main flow and the mouth of the Cai River to migrate north-northesterly and asymmetrically widen its valley towards the north. The northeastern migration of the Cai River has also led to the periodically abandonment of sections of rivers that previously resided along the southern portion of the river basin (Fig. 1). Localized fault-controlled uplift blocks also occurred within the river basins, which led to the drop of base level and consequently rejuvenation and straightening of the current due to river incision, development of V-shaped, steep slop valleys, and the lowering of groundwater level (Burbank and Anderson 2011).
d. Other indicators
+ Earthquake
The study area is very prone to earthquakes of different intensities. During the last 50 years, many earthquakes have been recorded along the Cai River basin, including in Nha Trang city and neighboring areas (Bui 2010; Nguyen and Pham 2015) in which an earthquake with a magnitude of of 4.1- 4.5 Ms has been recorded. In addition, large landslides and the accumulation of volumous large-sized breccia and boulder blocks along the slopes and foothills, accompanied by the systematic development of large scarp systems (Fig. 5b; Tran Thanh Hai 2020) all indicate that large-scale and abnormal landslidies often caused by ground vibrations by earthquakes have occured in the area. Although no big earthquake has ever been recorded, the high frequency of seismic activities that occurred within the Cai River basin over a long period of time is indicative of a seismic active zone, which is directly related to active tectonic movements in the study area.
+ Hot spring
Current mapping results have identified many exposures of hot springs along the Cai River basin and environs, especially in the middle and lower course portions (Fig. 1). Previous works (Cat 1996; Tran Thanh Hai 2020) revealed that those hot springs are all located in areas of strongly fractured and faulted granitoid intrusive rocks. Where exposed, the hot water leaks through open, cross-cutting fracture sets that create large-scale, locally up to several meters wide open fracture zones. Locally, several hot springs appear to scatter along major fractured zones or fault systems. The open fractures in the study area are inturn commonly caused by elastic brittle deformation that take places near the Earth surface, which is often related to latest phases of tectonic activity (NRC 1986; Burbank and Anderson 2011).
+ Riverbank erosion and landslide
Riverbank erosion and landslides occur commonly along the banks of the Cai River as well as along the slopes within the tributary valleys (Tran Thanh Hai, 2020). Field observations show that many sites of riverbank erosion and/or landslides often coincide with the positions of the fault transaction (Figs. 2d, 2e, 2f). Especially in many places, the erosion commonly occurs at the sites of irregular bends or offsets of the channels/flows (Figs. 1, 2, 3, 4, 5), which coincide with the location of recent/active faults (Figs 4, 5). see discussion above). In some locations, the systematic and long-term effects of strong subsidence also resulted in relative rise of the base level, which in turn led to lateral erosion and destruction of the riverbank, or erosion of shoreline and destruction of beaches and coastal zones as well as the landward intrusion of seawater (Figs. 5a, 5c, 5g). The uplifting of terranes along the river basins, on the other hand, also produces landslides, especially along the slopes of the river valleys and their tributaries. As discussed above, strong relative uplift commonly occurs in many places within the area, which generally lead to the drop in base level and river incision. This produced deep valleys with steep slopes or convex slopes along the river valleys, which are favorable for landslides to occur.
2.3 The rate of active tectonic movements
As described above, young (and active) tectonic movements in the middle and lower courses of the Cai River are common and can be identified by numerous pieces of evidence, in which the most significant parameters are uplift and subsidence as well as slip along fault zones that significantly modify the topography and drainages systems (Fig. 1, see above). Recent studies (Tran Thanh Hai 2020) have, for the first time, applied modern dating techniques to define the absolute age of materials such as Quarternary marker beds, fault gouges, terraces. This, in conjunction with precise geodesic measurements has been used to calculate the degree of movement between displaced geoblocks including the displacement of flows (Tran Thanh Hai 2017a, b; 2020). The calculated results show that the total lateral or vertical displacement along the faults ranges from tens of meters to several kilometers (Tran Thanh Hai 2020; Figs. 1; 2a), which is indicated by the amount of offset of geological markers and/or flows on both walls of the fault zones, or the amount of uplift and burial of the Quaternary formations (Fig. 6). For example, in the Dien Lam area, the termination and displacement of a Quarternary sedimentary sequence (Figs 5g, 6) by a normal fault system indicates that the fault was recently active. Dating of a 1.0 m thick Quaternary lacustrine-marine marker bed conducted by Tran Thanh Hai (2020) using OSL dating technique (Walker 2005) reveals that the youngest age range for the base of this layer is ca. 5.01 Ka for the base and ca. 4.54 Ka for the top of this layer (Tran Thanh Hai 2020 for detail description of the dating).
If assumed that the rate of sedimentation is constant, then based on the thickness of the dated bed as well as the overlaying bed and the measured elevation for observed beds, it is possible to extrapolate that the timing of the deposition of the sediments on the top of the youngest bed (presently) is ca. 2.04 Ka. This means that the faulting activity must post-dated the ca. 2.04 Ka. This age is considered the oldest age of the fault in this outcrop. Based on the nature of the slip identified along the fault surfaces (Fig. 6), the minimum vertical displacement rate along the faults after 2.04 Ka was estimated to be roughly 0.1 cm/year or 10 cm/100 years. In addition, the regional uplift in this area is also significant, as indicated by the presence of the terrace surface at ca. 8.5 m high. This demonstrates that the area has been uplifted by ca. 8.5 m compared to the present base-level, after ca 2.04 Ka onwards. In this case, the rate of uplifting from ca. 2.04 Ka is approximately 0.24 cm/year or 24 cm/100 years (2). Thus, if we take into account both vertical movement caused by local faulting and tectonic uplifting, then the combined uplift rate is up to 34 cm/100 years for the central horst presented in the outcrop (Figs. 3, 6).
Besides tectonic uplift, recent tectonic induced subsidence is also commonly occurring in the foothills or along the fault traces in the middle-lower courses of the Cai River basin (see discussion above). Based on the correlation of Quaternary marker beds identified along geological sections that straddled subsided structures using borehole data, it is possible to calculate the average subsidence rate of these depressed areas as high as 0.2 cm/year or 20 cm/100 years (Tran Thanh Hai 2020). Thus, if considering all accumulative regional vertical tectonic displacement factors, the total amount of local uplift in some areas within the middle–downstream portions of the Cai River are relatively high, up to several tens of centimeters per 100 years.