Seismic Reflection Characteristics and Evolution of ...

Journal of Earth Science, Vol. 27, No. 4, p. 642–653, August 2016 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-016-0708-2

Zhao, Y. H., Tong, D. J., Song, Y., et al., 2016. Seismic Reflection Characteristics and Evolution of Intrusions in the Qiongdongnan Basin: Implications for the Rifting of the South China Sea. Journal of Earth Science, 27(4): 642–653. doi:10.1007/s12583-016-0708-2. http://en.earth-science.net

Seismic Reflection Characteristics and Evolution of Intrusions in the Qiongdongnan Basin: Implications for the Rifting of the

South China Sea

Yanghui Zhao1, Dianjun Tong1, Ying Song*2, Linlong Yang1, Chao Huang1 1. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China

2. Department of Geology, China University of Petroleum (East China), Qingdao 266580, China

ABSTRACT: The Qiongdongnan Basin (QDNB) is situated in the extensional zone at the vertex of the V-shaped northwest sub-basin, non-volcanic northern margin of the South China Sea (SCS). From north to south, the thickness of the continental lithosphere decreases from 22 km on the northern continental shelf to 17 km at the deepest area of the central depression. A sharp change on the crustal structure is of importance to hydrocarbon exploration yet the dynamic causes remain unknown. A comprehensive study including (1) interpretation of seismic profiles, (2) P-wave velocity data modeling, and (3) magnetic anomalies analysis reveals that there are some high-density intru-sions along the lithospheric thinning belt. Chaotic reflections can be found in the southwest of the QDNB, with a low velocity (<3.4 km/s), while in the center and the east, the intensively deformed strata passing towards the diapir flanks and their high velocities (>6 km/s) suggest the existence of igneous diapirs. Diapirism differentiation are primarily achieved through analysis of the contact re-lationship and the thickness variations in the surrounding strata. The first phase of diapirism along the Songnan low uplift occurred in the Late Mesozoic, and the second phase of diapirism in a form of subsequent gas movement remained active until the Late Quaternary. The distribution and the evolution of the diapirs would have major implications for post-rift emplacement. KEY WORDS: Qiongdongnan Basin, intrusion, seismic reflection anomaly, diapirism, rifting of the South China Sea.

0 INTRODUCTION In the past decades, major improvements in the acquisi-

tion and processing of seismic reflection methodology led to significant advances in their geological interpretation (Bacon et al., 2007; Yilmaz, 2001; Sheriff and Geldart, 1995). Based on clearer subsurface reflection characteristics such as continuity, amplitude and frequency, the seismic data can be used to gain a sufficient understanding of the structure, filling and evolution of sedimentary basins at a range of scales (Song et al., 2014; A-L Jackson and Kane, 2011). In this paper, seismic reflection anomalies are utilized to investigate the intrusive and extrusive origin structures in the Qiongdongnan Basin (QDNB), typical rifting structures in the northern South China Sea (NSCS).

The seismic reflection anomalies in this study include high-amplitude reflection anomalies, named after high ampli-tude discordant seismic reflections by Hansen et al. (2008), localized folds, diapirs and extrusions. Diapirs have been found in the northern margin of the SCS (South China Sea), *Corresponding author: [email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2016 Manuscript received November 26, 2015. Manuscript accepted December 18, 2015.

including a small-scale punctate distribution of volcanism in the Pearl River Mouth Basin (PRMB) and mud diapirs in the Yinggehai Basin (YGHB) (Pei et al., 2011; Shi et al., 2009; Xie et al., 2006; Yan et al., 2006). Numerous researches of volcanic continental margins, such as the South Atlantic mar-gin, West North American margin and the Indian margin, sug-gested that magmatism during the continental breakup process marked the original location of the syn-rift (White and McKenzie, 1989). While evidence for volcanism is minimal on non-volcanic rift margins like the Iberia-Newfoundland margin, there is some evidence for magma activity in the adjacent areas of the exhumed sub-continental mantle (Russell and Whitmarsh, 2003). Hence, the distribution of volcanos and volcanism are closely related to the lithospheric breakup, which is discussed in this study.

Multi-channel seismic profiles across the central PRMB reveal volcanos that are characterized by high amplitude seis-mic reflections and high magnetic anomalies close to a zone of dramatically thinned continental crust (Wang et al., 2006; Yan et al., 2006). The mantle upwelling during the Baiyun Move-ment (23.8 Ma) is believed to have resulted in rapid regional subsidence and a high heat flow in the adjacent areas, sup-ported by a feature of ductile deformation behavior found in seismic profiles, and high geothermal gradients (Zhao et al., 2014; Pang et al., 2007). This event could supply sufficient

Seismic Reflection Characteristics and Evolution of Intrusions in the Qiongdongnan Basin

643

magmas for the volcanism nearby. Further northeast, seismic reflection and ocean bottom seismic (OBS) data have proved that highly extended continental crust was overprinted with post-rift magmatism in the distal margin (Lester et al., 2014).

On the other hand, mud diapirism has been linked to ex-tensional settings on the continental margins offshore the Niger Delta (Damuth, 1994) and the western Black Sea (Ivanov et al., 1996). In the YGHB, evidence of gas and fluid injection in-cludes diapiric faults, paleo-craters and chaotic seismic reflec-tion patterns (Lei et al., 2011; He et al., 2010). Radial faults and concentric faults formed multi-scaled crestal grabens above the diapir, ranging from several centimeters to tens of meters in diameter (Zhao et al., 2014; Lei et al., 2011). More-over, the central depression belt has the highest content of mud,

highest temperature and the strongest overpressure, which resulted in intensive mud diapir development (Li et al., 2005; Hao et al., 2000).

In the QDNB, 2D seismic data interpretation, high tem-perature and overpressure analysis suggest the existence of a fluid flow system deep in the Changchang and Ledong sags (Sun et al., 2014; Wu and Qin, 2009; Li et al., 2005). Never-theless, studies based on high-resolution 3D seismic data in the QDNB relating to volcanism and diapirism have received little attention, in spite of the existence and distribution of the intru-sions. Moreover, the origin of the magmas is poorly under-stood, Late Miocene magmatism was synchronized with and may have been related to the formation of the volcanic band in the NSCS in some way (Fig. 1).

Figure 1. The distribution of the plutons in the northern SCS and the mud diapirs in the YGHB (a), compiled from Savva et al. (2014), Sun et al. (2014), Yan et

al. (2006), Zou et al. (1995); and depth-contours map of Moho of the QDNB (b) after Ren et al. (2014a). PRMB. Pearl River Mouth Basin; YGHB. Yinggehai

Basin; QDNB. Qiongdongnan Basin; LD. Ledong sag; LS. Lingshui sag; BJ. Beijiao sag; SN. Songnan sag; BD. Baodao sag; CC. Changchang sag; LNLU.

Lingnan low uplift; LSLU. Lingshui low uplift; SNLU. Songnan low uplift.

Yanghui Zhao, Dianjun Tong, Ying Song, Linlong Yang and Chao Huang

644

In this study, the amplitude and structure of seismic re-flection anomalies in the under-explored central depression of the QDNB are discussed (Fig. 1). The non-sedimentary reflec-tions have been differentiated by the interpretation of the P-wave velocity data across the anomalies. Among them, some saucer-shaped structures and the localized folds are strikingly similar to the magma intrusions at the northeastern and central Atlantic Ocean, which have been thought to be generated by an interaction of a low-viscosity fluid-filled hydraulic fracture and the overburden deformations (Galland et al., 2009; Hansen et al., 2008; Polteau et al., 2008; Rohrman, 2007). The magnetic contrast supports the distribution of the anomalies attributed to the movement of magmas.

1 GEOLOGICAL SETTING AND MAJOR MAGMA-TISM IN NSCS

Magmatism in the SCS was active primarily in the Meso-zoic and Cenozoic. The subduction of the West Pacific Plate during the Late Mesozoic triggered the extension of the An-dean volcanic arc belt and the movement of igneous rocks in the SCS (Ren et al., 2002; Yan et al., 2001; Zhou and Li, 2000; Jahn et al., 1976). This magmatism has been manifested by granites, granodiorites and rhyolites, dated from 180 to 90 Ma on the southeast margin of the South China Block (Song et al., 2015; Yan et al., 2006; Li, 2000; Zhou and Li, 2000). The dis-tribution of the Cenozoic igneous rocks relative to the spatially varying Moho depth within the NSCS has been determined by geophysical and geological investigations (Fig. 1). More than 20% of wells in the PRMB penetrated Cenozoic igneous rocks. These igneous rocks have been dated during the Paleocene– Eocene (57–49 Ma), Early Miocene (23–16 Ma) and Late Miocene to recent (Yan et al., 2006; Li and Rao, 1994). Erup-tions occurring after the cessation of the sea floor spreading (after 15 Ma) formed a volcanic band on the continental mar-gin of NSCS, extending westward to the Xisha trough (Fig. 1) (Yan et al., 2006).

The Qiongdongnan Basin (QDNB) is located at the exten-sional zone at the vertex of the V-shaped northwest sub-basin, and they are connected by the Xisha trough, which represents a failed rift of the SCS between 32 and 17 Ma (Qiu et al., 2001). Between the eastern YGHB and the western PRMB, the QDNB was formed in the Cenozoic following crustal exten-sion and thermal subsidence associated with the breakup of the SCS (Ren et al., 2015; Lei et al., 2011; Tong et al., 2009). Large displacement listric faults, i.e., No. 2 fault system, which might cross-cut the upper continental crust, were generated during the Cenozoic syn-rift phase in response to the litho-spheric thinning process of the SCS (Ren et al., 2015; Xie X N et al., 2015). In the west of central depression, a suite of Mio-cene volcanos has been interpreted in seismic reflection data (Xie et al., 2009), and fluid-flow modelling predicts the pres-ence of gases in the deep areas (Zhu et al., 2009). The discov-ery of the Lingshui 17-2 gas field with over a hundred billion cubic meters supports the existence of the gas layer.

2 DATA AND METHODS

Two-D seismic data have a cumulative line length of over ca. 10 000 km, including 22 long-span seismic lines collected

in 2011 that imaged the subsurface to a depth of 12-second two-way travel time (TWT). In this article, we interpreted these seismic data and built a tectostratigraphic framework of the QDNB (Fig. 2). Specially, we sub-classified the abnormal seismic reflections from the sedimentary strata based on the uppermost and lowermost boundaries of a non-sedimentary body generating discrete reflections due to high contrast in lithological density.

The method for determination interval velocity V is to convert from stacking velocity VR using the Dix equation (Dix, 1955). The interval velocity in the nth layer is as follows

2 2

, 0, , 1 0, 1

0, 0, 1

R n n R n n

n

n n

V t V tV

t t

(1)

where VR,n and t0,n are the stacking velocity and vertical two-way travel time, respectively, for the top of the nth layer, and VR,n–1 and t0,n–1 are for the n-1th layer.

Magnetic anomaly data and other geological results col-lected from previous publications were used to validate the distribution of the seismic reflection anomalies. This study focuses on the geometrical and structural features in the seis-mic sections, as well as their temporal and spatial relationship. 3 CLASSIFICATION OF SEISMIC REFLECTION ANOMALIES 3.1 Seismic Profile Interpretation 3.1.1 High-amplitude reflection anomalies

Four high-amplitude seismic reflection structures have been mapped in the QDNB with a peak-trough-peak reflection feature (Figs. 1 and 3). Individual reflection anomalies show elliptical or near round shapes with diameters ranging from 1 to 6 km in the plan view (Figs. 3g and 3h). The ratios between the maximum and minimum lateral dimension are less than 3 : 1, showing no inner connection with other high-amplitude reflection anomalies from the seismic sections.

Anomaly ① is located in the Lingshui sag, southwest to Well LS22-1 (Fig. 1). It exhibits a saucer-shaped geometry with a maximum vertical height differences of 150 ms (TWT) in the center (Fig. 3a). In the 5.4 s time slice, the anomaly has an ellip-tical shape with its long axis oriented NE-SW, and measures approximately 2.8 km×3.7 km, spanning an area of about 10 km2 (Fig. 3g). In the seismic section towards northeast, the edge of the anomaly crosscuts layered strata reflections (Fig. 3b). These observations suggest that the saucer morphology formed after the deposition of the sediments. More saucer-shaped anomalies, e.g., anomaly ② and anomaly ③ have been found in the northern Songnan sag (Figs. 3c and 3d). They are about 30 km away from each other and very close to the diapir to their north (Fig. 1). A lack of 3D seismic data covering anomaly ② and anomaly ③ makes it difficult to analyze the spatial relationship between each other precisely. The clustered distribution demon-strates that they may be correlated in both the lateral and vertical directions. Anomaly ④ is a sub-horizontal plank structure with 5 km length at the depth of 4 s (TWT) (Fig. 3f). An obvious diapir has been found in the intersecting section (Fig. 3e). Ana-logues found on the Atlantic margin have been interpreted as either magma or sand intrusion (Galland et al., 2009; Polteau et


645

Figure 2. Stratigraphic column of the QDNB (modified after Yuan et al., 2009 and Lei et al., 2011).

al., 2008). Since there is no evidence of nearby sand sources, it is more reasonable to identify these anomalies as magma intrusions. The diapir close to the anomaly may supply intrusive rocks, such as anomaly ② and anomaly ④ (Figs. 3c, 3e and 3f). 3.1.2 Localized folds and minor faults

The development of intrusions is often accompanied by the presence of monoclinal and domal folds, and minor faults (Hansen et al., 2008). In the Lingshui sag, there is an approxi-mately 2 km wide, maximum structural relief of ~100 ms (TWT) at the Horizon A above the diapir (Fig. 3e). The strati-graphy near the Horizon A is concordantly deformed, showing almost the same lateral extent of the diapir below. It can be inferred that the deformations are coeval with diapirism. In addition, some high-angled small-scaled faults can be observed in the stratigraphy (green faults in Fig. 3e). Judging from the subtle displacements around them, these minor faults neither have relevance to the growth fault on the northeast (the red fault in Fig. 3e), nor have controlled basin structural subsi-dence. A possible explanation is that these small-scaled faults in this section could be formed as a result of sediments con-traction and rupture during the cooling phase followed diapir-ism, which is inspired by formation mechanism of collapse structures in the YGHB (Lei et al., 2011; Xie et al., 2006) and

minor normal faults in the PRMB (Zhao et al., 2014). 3.1.3 Diapirs

In the western QDNB, uplifts found in 3D seismic sec-tions are interpreted as hydrothermal liquid diapirs. Clouds, chimney and pipe structures above them could have been formed by overpressured hydrothermal fluids penetrating the upper layer and moving rapidly to shallower depths (Fig. 4). In addition, a nearby turtle back structure (Fig. 4f) could be addi-tional evidence of the residual hydrothermal liquids. Similar features have been found in the YGHB, where the gas layer has recently been drilled and put into production (e.g., DF13 gas field) (Xie Y H et al., 2015).

Significant differences exist between uplifts since diapirism and the structural highs consist of old basement rocks. Take Songnan low uplift as an example. First, in seismic section, above the southeastern branch of the Songnan low uplift (uplift B in Fig. 5), high frequency and short (~4 km) reflections can be found in weak, low-amplitude reflections (at a depth of 2.5 s, TWT). In the southeast section across 2 branches of the Songnan low uplift, low-amplitude reflections with a 2 km wide, 1 s (TWT) height from a depth of 5 to 6 s (TWT) as well. These weak-layered reflections indicate that hydrothermal liquid diapirs developed along or close to the structural highs. However,


646

in the old structural highs, chaotic and discontinuous reflections are within the uplifts A and B. Second, small-scaled synformal down-bending of strata between Horizon T70 and T60 to the southeast of the uplift B suggest diapirism occurred after the sediments deposited (Fig. 5). While parallel reflections or thick-ening of reflections are close to the structural highs. Third, low velocity anomaly can be found in the thermal liquid diapirs (see

Section 3.2 in details). These thermal liquids movement had been active until Late Quaternary, as some high amplitude re-flections (i.e., bright spots) appear above Horizon T20 indicating a gas accumulation. The relationship between the seismic reflec-tion features with the surrounding strata and vents suggests that the numerous intrusive phases can be differentiated (see Section 4.1 in detail).

Figure 3. Seismic sections illustrating characteristics of the high-amplitude, peak-trough-peak reflection anomalies in the QDNB. Figures 3a and 3b, Figs. 3e

and 3f are two intersecting 3D seismic sections across the same anomaly. Figures 3g and 3h are the time slices of amplitude at 5.4 and 4 s (TWT), respectively.

Anomalies ①, ② and ③ are saucer-shaped reflection, and anomaly ④ is plank-shaped conformity reflection close to the diapir.


647

Figure 4. Seismic sections showing tall, narrow, pipe-like features within the chaotic seismic facies in the west of the QDNB. Figures 4b, 4d and 4f are the

interpretation of Figs. 4a, 4c and 4e. In Figs. 4c and 4d, the rim syncline on the right displays synformal geometries indicative of active or passive diapirism.

Figures 4e and 4f do not show intensive synformal reflections, which suggests that the chaotic chimney/pipe structure in the middle represents gas and/or vapor

intrusions.

Figure 5. A seismic section across the Songnan low uplift (a) and its structural interpretation (b) (see Fig. 1 for location). Even thickness of the blue stratigra-

phy on the right of the diapir eliminates the possibility of fault causing strata tilting. Similar strata thickness (blue and green stratigraphy) of both sides of the

diapir suggest the uplifts are not bounded by faults.

3.1.4 Extrusions

The biggest extrusion in the QDNB occurred between the Baodao sag and the Changchang sag (Fig. 6, see Fig. 1 for loca-tion). The exposed volcanic plug has a southeastern width of 3–5 km and a height of 0.5–1.2 s (TWT) above the seafloor. The top of the volcano shows a continuous reflection which changes to a weaker and less continuous reflection as the lithology changes

downward. Near the volcano, a cluster of chaotic, low-amplitude reflections in Miocene strata has had acoustic properties homoge-nized by hydrothermal activity. Dips of the strata above the vol-canic boundary gradually decrease with distance from the vol-canic plug. The high-amplitude seismic reflections can be found in Oligocene and Early–Mid Miocene strata with similar geomet-ric features of those in Section 3.1.1. They could also be dense,


648

Figure 6. A seismic section across the volcano in the Baodao sag (see Fig. 1 for location). Several low-amplitude reflections and high-amplitude reflections

near the volcano represent hydrothermal liquids and magmas separately.

interconnected network of minor intrusions, such as dykes and inclined sills. 3.2 Interval Velocity Model Building

Ten interval velocity profiles were imaged using Eq. 1 in Section 2; specially, typical ones with interval velocity anoma-lies were described in this article. In the southern Ledong sag, near the Lingnan low uplift, a more than 5 s (TWT) thick low-velocity zone (3–3.5 km/s) in Lingshui Formation and Sanya Formation is interpreted as a partially gas-saturated zone (Figs. 7a and 7b). This interpretation agrees with the overpres-sure situation (Li et al., 2005), as well as the high-amplitude reflections (bright spots) in Fig. 7b.

Along the Songnan low uplift, passing towards the diapir flank, the strata are intensively deformed, and the velocities within diapirs (4.8–5.8 km/s) are much higher than the sedi-mentary strata (~ 2 to 4 km/s) at the same depth. For instance, at the depth of 4.8 s (TWT), velocity of uplift B’s (~4.4 km/s) at the CDP=9500 changes dramatically to 2.8 km/s of the strata at CDP=8060 (Fig. 7c). It indicates that high-density bodies develop within or close to the low uplift. Considering both the ductility and mobilization of the diapir suggested by the seis-mic reflection section (in Section 3.1.3), the diapir may consist of volcanic rocks coming from the crust and hydrothermal liquids. In a vertical direction, interval velocities generally increase with depth. However, at CDP=11580, this gradual increase is interrupted by a velocity anomaly at the depth of 4.2 s (TWT) (the red box in Fig. 7c, Vp=4.625 km/s). Similar anomalies (the red boxes in Fig. 7e) have been found along the No. 2 fault in the Songnan sag, which are interpreted as intru-

sions related to the combined movement of magma and related hydrothermal liquid (Figs. 7e and 7f). 3.3 Magnetic Data Analysis

Magnetic data were used to increase the reliability of the seismic interpretation of the magma intrusions. Note that only large volume of magma accumulation can show obvious high magnetic anomaly (Fig. 6), thus, provides a conservative esti-mate of the volcanic rocks distribution. Two high magnetic anomalies are observed in the Baodao sag and Changchang sag, which coincides with the presence of the volcanos (Fig. 6) and the volcanic band (Fig. 1), separately. The magnetic high in the southern Ledong sag suggests that the low velocity diapirs may consist of hydrothermal liquids generating from the magmatic rocks at depth. In contrast, in the Songnan low uplift, the lack of high magnetic anomalies indicates that the diapir has no obvious magnetic properties. Therefore, the diapir could be a complex of magma and hydrothermal liquid diapirs developing along the paleo-uplifts. The magnetic anomaly in the center Lingshui sag (Fig. 8) has a roughly 10 km offset to the diapir shown on the seismic section (Fig. 1).

In general, areas with the highest density of seismic re-flection anomalies in the QDNB correspond to the distribution of high magnetic anomalies. In the western QDNB, we infer that the chaotic seismic reflection data with lower seismic ve-locities are hydrothermal liquid diapirs. In the center and east-ern areas, high-amplitude reflections with strongly localized deformation of strata represent igneous intrusions and extru-sions, apart from relatively homogeneous and structurally complexes.


649

Figure 7. P-wave velocity models (Figs. 7a, 7c and 7e, m/s) and corresponding interpretation of the seismic sections (Figs. 7b, 7d and 7f, see Fig. 1 for loca-

tions). (a) In the western area, a chaotic reflection zone with diagonal lines is characterized by low P-wave velocity (<3.4 km/s). (c) and (e) Some high velocity

anomalies (from 4.1 to 4.8 km/s) interrupt the sedimentary feature velocities. Red boxes show the high seismic velocity anomalies interrupting the gradual

increase of velocities downwards.

Figure 8. Contour map of the magnetic transform T (Wang et al., 2010 unpublished data). High magnetic anomalies shown in the southern Ledong sag, central

Lingshui sag, Baodao sag, and eastern Changchang sag generally correspond to the interpreted seismic reflection anomalies.

4 DISCUSSION 4.1 Identification of the Phases for the Magmatic Diapir-ism

Individual stages of volcano and diapir growth can be classified into reactive, active and passive stages (Magee et al., 2013; Morley, 2003; Cohen and McClay, 1996; Vendeville and

Jackson, 1992). Each stage shows distinct geometric and structural features in observed seismic sections (Fig. 9a). First, contact relationship between strata may provide the informa-tion that when a diapirism happened. For example, in the Songnan low uplift, seismic reflectors are truncated below Horizon T50 and the seismic reflectors above T50 progressively


650

terminated against the horizon. Noted that this angular uncon-formity between Sanya Formation and Meishan Formation only happens close to the uplifts. That is to say, diapirism time cannot be defined solely on the contact relationship of strata, since some basin-wide unconformities like T60 and T70 can not be regarded as a marker horizon synchronous to a diapirism.

Moreover, a wide variation in external morphology near a diapir, such as a change in thickness of the overlying strata and/or strata on the flanks, could also suggest a spectrum of phases is preserved. In the seismic section of the Songnan low uplift, the strata between Horizon T50 and T40 have even thicknesses at all distances from the uplifts (Fig. 5). In contrast, Sanya Formation is much thinner or completely absent close to the diapirs when compared to the neighboring strata. The uneven-thicknesses of Sanya Formation can be attributed to unevenly distributed sedi-ments during diapirism (Fig. 9c). When the diapirs were inactive, sediments deposited horizontally or sub-horizontally (Fig. 9d). This observation indicates that the first phase of diapirism ceased after the formation of Horizon T50.

Afterwards, the second phase of diapirism was active in the form of a subsequent gas movement (Fig. 9f). This phase would neither predate the formation of Horizon T50 nor be later than the formation of Horizon T20, by showing low-amplitude reflections above uplift B (see detail description in Section 3.1.3, Fig. 5). During that time, a wide regional erosion oc-curred, and the central canyon developed (Fig. 9e). Therefore, the first phase of diapirism in the Songnan low uplift is during

the Early Miocene, and the second phase of hydrothermal liq-uid diapirism is after the Late Pliocene, probably until present. The evolution of the diapirs along the Songnan low uplift has been illustrated by the schematic sections (Fig. 9).

4.2 Implications for the Opening of the SCS

Previous research suggests that regional extension thins the fluid overburden and source layers, making it possible that extending and unstable diapirs develop (Jackson and Vendeville, 1994; Koyi, 1988). In a hyper extensional envi-ronment accommodated by detachment faulting, such as on the west Iberia-Newfoundland margins, magmatism occurred after the breakup of continental crust, generally close to the conti-nental oceanic transition (COT) (Whitmarsh et al., 2001). In the SCS, the time of seafloor spreading in the central oceanic basin has been widely accepted as 32 to 15.5 Ma (Briais et al., 1993; Taylor and Hayes, 1980). In our study, magmatic diapir-ism along the Songnan low uplift occurred during Mid- Miocene, which is right after the seafloor spreading stopped. It supports the existence of post-rift magma intrusion in the magma-poor continental margins.

On the other hand, hydrothermal liquid diapirs have been found in the western QDNB, and magma diapirs are in the central and eastern areas. The eastern QDNB is located at the tip of the northwest sub-basin (NWSB), which is very close to the COT. The extremely thin crust in the NWSB suggests the eastern QDNB may also have experienced a hyper-extension

Figure 9. Diapirs with associated responses of adjacent sedimentary section to diapir growth, inferred to indicate reactive, passive growth. (a) A sketch of the

progressive phases of the evolution of igneous intrusion, modified after Huang et al. (2002). (b)–(f) A model of the diapir evolution along the Songnan low

uplift. The sequence of the feature formation is as follows: (1) deposition of truncated strata prior to the initiation of diapirism; (2) deposition of onlap strata

after the initiation of diapirism; and (3) formation of parallel contact strata after the cessation of diapirism (Fig. 9a). (e) Modified after Xie et al. (2012). See Fig.

5 for seismic section.


651

before the NWSB breakup, providing optimal conditions for development of diapirs. Combined with the distribution of the igneous band across the PRMB, the presence of intrusions is linked to location where the Moho had been uplifted a lot (Ren et al., 2015, 2014b; Yan et al., 2006).

In addition, the No. 2 fault system along the northern mar-gin of the central depression in the QDNB has been suggested as a detachment fault system cut through the continental crust (Ren et al., 2015, 2014b). The distribution of the magma intrusions in the north of Songnan sag and Baodao sag is in accordance with the major fault trend, which suggests the No. 2 fault system may have thinned the continental lithosphere, and serve as a pathway for magma movement. This study agrees with limited magmas located in the lithospheric thin-ning areas (Lester et al., 2014), and the spatial affinity between the magma intrusions and deep major faults may reflect the initial location of the continental lithospheric thinning on non-volcanic margins. 5 CONCLUSIONS

(1) Regional seismic mapping in the QDNB off NSCS has documented intrusions, extrusions and related structures in-cluding diapiric folds and faults. Chimney and pipe structures have been identified in the western QDNB. With a low- amplitude reflection feature, they were formed mainly by gases and liquids. Different intrusion geometries, like saucer-shaped and plank shaped are found in the Lingshui, Songnan and Baodao sags, close to diapirs. The 3D seismic imaging shows the intrusions in the central and eastern basin are related in temporal and spatial dimensions.

(2) Relatively low P-wave velocities of 3–3.4 km/s sug-gest the anomalies are hydrothermal liquids, while anomalies with high velocities (>4.1 km/s) and high magnetic fields rep-resent the geological bodies contain magmas. Combined with the magnetic anomaly distribution, hydrothermal liquid diapirs concentrate in the west, and igneous intrusions and extrusions are in the center and east of the QDNB.

(3) In the Songnan low uplift, 2 phases of diapirism can be differentiated according to the localized strata deformations. Based on analysis of the geometric relationship and contact between the sedimentary strata, the first diapirism occurred in the Early Miocene, and the second phase in a form of subse-quent hydrothermal liquids movement remains active until Late Quaternary. The diapirism is coeval with the major mag-matism in the NSCS, and generated in an extensional setting. Development of intrusions may provide information of the structural deformation and magma movement during the litho-sphere thinning, breakup and seafloor spreading. ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (No. 41272121), the Major National Sci-ence and Technology Programs in the “Twelfth Five-Year” Plan of China (No. 2011ZX05025-002-02-02), and the Fundamental Research Funds for the Central Universities (No. 16CX02038A). This study utilizes time-migrated 2D seismic reflection sections, 3D seismic projects and P-wave velocity sections to identify the seismic reflection anomalies (Fig. 1), provided by Zhanjiang

Branch of CNOOC. We thank the Research Institute of Zhan- jiang Branch of CNOOC for providing the seismic data, sup-porting this study and granting the permission to publish the geological data. We also thank Profs. Joann Stock and Bryan Riel from California Institute of Technology (Pasadena, USA) for very useful scientific suggestions as well as the English cor-rection and improvement. The final publication is available at Springer via http://dx.doi.org/10.1007/s12583-016-0708-2.

REFERENCES CITED A-L Jackson, C., Kane, K. E., 2011. 3D Seismic Interpretation

Techniques: Applications to Basin Analysis. In: Busby, C. J., Azor, A., eds., Tectonics of Sedimentary Basins: Re-cent Advances. John Wiley & Sons, Ltd.. 95–110

Bacon, M., Simm, R., Redshaw, T., 2007. 3-D Seismic Inter-pretation. Cambridge University Press, Cambridge

Briais, A., Patriat, P., Tapponnier, P., 1993. Updated Interpre-tation of Magnetic Anomalies and Seafloor Spreading Stages in the South China Sea: Implications for the Terti-ary Tectonics of Southeast Asia. Journal of Geophysical Research, 98(B4): 6299. doi:10.1029/92jb02280

Cohen, H. A., McClay, K., 1996. Sedimentation and Shale Tectonics of the Northwestern Niger Delta Front. Marine and Petroleum Geology, 13(3): 313–328. doi:10.1016/0264-8172(95)00067-4

Damuth, J., 1994. Neogene Evolution of the Niger Delta. Ma-rine Petroleum Geology, 11: 320–345

Dix, C. H., 1955. Seismic Velocities from Surface Measurements. Geophysics, 20(1): 68–86. doi:10.1190/1.1438126

Galland, O., Planke, S., Neumann, E. R., et al., 2009. Experi-mental Modelling of Shallow Magma Emplacement: Ap-plication to Saucer-Shaped Intrusions. Earth and Plane-tary Science Letters, 277(3/4): 373–383. doi:10.1016/j.epsl.2008.11.003

Hansen, D. M., Redfern, J., Federici, F., et al., 2008. Miocene Igneous Activity in the Northern Subbasin, Offshore Senegal, NW Africa. Marine and Petroleum Geology, 25(1): 1–15. doi:10.1016/j.marpetgeo.2007.04.007

Hao, F., Li, S. T., Gong, Z., et al., 2000. Thermal Regime, Interreservoir Compositional Heterogeneities, and Reser-voir-Filling History of the Dongfang Gas Field, Yingge-hai Basin, South China Sea: Evidence for Episodic Fluid Injections in Overpressured Basins? AAPG Bulletin, 84(5): 607–626

He, J. X., Zhu, Y. H., Weng, J. N., 2010. Characters of North-West Mud Diapirs Volcanoes in South China Sea and Relationship between Them and Accumulation and Migration of Oil and Gas. Earth Science—Journal of China University of Geosciences, 35(1): 75–86 (in Chi-nese with English Abstract)

Huang, C., Chen, K., Li, S., 2002. Periodicities of Diapiric Rise in the Yinggehai Basin. Petroleum Exploration and Development, 29(4): 44–46 (in Chinese with English Ab-stract)

Ivanov, M. K., Limonov, A. F., van Weering, T. C. E. V., 1996. Comparative Characteristics of the Black Sea and Medi-terranean Ridge Mud Volcanoes. Marine Geology, 132(1–4): 253–271. doi:10.1016/0025-3227(96)00165-x


652

Jackson, M. P. A., Vendeville, B. C., 1994. Regional Extension as a Geologic Trigger for Diapirism. Geological Society of America Bulletin, 106(1): 57–73. doi:10.1130/0016-7606(1994)106<0057:reaagt>2.3.co;2

Jahn, B. M., Chen, P. Y., Yen, T. P., 1976. Rb-Sr Ages of Granitic Rocks in Southeastern China and Their Tectonic Significance. Geological Society of America Bulletin, 87(5): 763–776

Koyi, H., 1988. Experimental Modeling of Role of Gravity and Lateral Shortening in Zagros Mountain Belt. AAPG Bulle-tin, 72(11): 1381–1394

Lei, C., Ren, J. Y., Clift, P. D., et al., 2011. The Structure and Formation of Diapirs in the Yinggehai-Song Hong Basin, South China Sea. Marine and Petroleum Geology, 28(5): 980–991. doi:10.1016/j.marpetgeo.2011.01.001

Lester, R., van Avendonk, H. J. A. V., McIntosh, K., et al., 2014. Rifting and Magmatism in the Northeastern South China Sea from Wide-Angle Tomography and Seismic Reflection Im-aging. Journal of Geophysical Research: Solid Earth, 119(3): 2305–2323. doi:10.1002/2013jb010639

Li, P. L., Rao, C. T., 1994. Tectonic Characteristics and Evolution History of the Pearl River Mouth Basin. Tectonophysics, 235(1/2): 13–25. doi:10.1016/0040-1951(94)90014-0

Li, X. H., 2000. Cretaceous Magmatism and Lithospheric Exten-sion in Southeast China. Journal of Asian Earth Sciences, 18(3): 293–305. doi:10.1016/s1367-9120(99)00060-7

Li, X., Ou, B., Li, Q., et al., 2005. 3D Geopressure Field and Hydrocarbon Migration in Yinggehai and Qiongdongnan Basins. Geological Science and Technology Information, 24(3): 70–74 (in Chinese with English Abstract)

Magee, C., Hunt-Stewart, E., Jackson, C. A. L., 2013. Volcano Growth Mechanisms and the Role of Sub-Volcanic Intru-sions: Insights from 2D Seismic Reflection Data. Earth and Planetary Science Letters, 373: 41–53. doi:10.1016/j.epsl.2013.04.041

Morley, C., 2003. Mobile Shale Related Deformation in Large Deltas Developed on Passive and Active Margins. In: van Rensbergen, P., Hillis, R., Maltman, A., et al., eds., Sub-surface Sediment Mobilization. Geological Society, Lon-don, Special Publications, 216: 335–357. doi: 10.1144/GSL.SP.2003.216.01.22

Pang, X., Chen, C., Shao, L., et al., 2007. Baiyun Movement, a Great Tectonic Event on the Oligocene–Miocene Bound-ary in the Northern South China Sea and Its Implications. Geological Review, 53(2): 145–151 (in Chinese with Eng-lish Abstract)

Pei, J., Yu, J., Wang, L., 2011. Key Challenges and Strategies for the Success of Natural Gas Exploration in Mid‐Deep Strata of the Yinggehai Basin. Acta Petrolei Sinica, 32(4): 573–579 (in Chinese with English Abstract)

Polteau, S., Mazzini, A., Galland, O., et al., 2008. Sau-cer-Shaped Intrusions: Occurrences, Emplacement and Implications. Earth and Planetary Science Letters, 266(1/2): 195–204. doi:10.1016/j.epsl.2007.11.015

Qiu, X. L., Ye, S. Y., Wu, S. M., et al., 2001. Crustal Structure Across the Xisha Trough, Northwestern South China Sea. Tectonophysics, 341(1–4): 179–193. doi:10.1016/s0040-1951(01)00222-0

Ren, J., Pang, X., Lei, C., et al., 2015. Ocean and Continent Transition in Passive Continental Margins and Analysis of Lithospheric Extension and Breakup Process: Implication for Research of the Deepwater Basins in the Continental Margins of South China Sea. Earth Science Frontiers, 22(1): 102–114 (in Chinese with English Abstract)

Ren, J. Y., Tamaki, K., Li, S. T., et al., 2002. Late Mesozoic and Cenozoic Rifting and Its Dynamic Setting in Eastern China and Adjacent Areas. Tectonophysics, 344(3–4): 175–205. doi:10.1016/s0040-1951(01)00271-2

Ren, J. Y., Tong, D. J., Li, T., et al., 2014a. Tectonic Structure and Evolution of the Deepwater Area in Qiongdongnan Basin. National Science and Technology Major Project Report, Wuhan (in Chinese)

Ren, J. Y., Zhang, D. J., Tong, D. J., et al., 2014b. Character-ising the Nature, Evolution and Origin of Detachment Fault in Central Depression Belt, Qiongdongnan Basin of South China Sea: Evidence from Seismic Reflection Data. Acta Oceanologica Sinica, 33(12): 118–126. doi:10.1007/s13131-014-0581-8

Rohrman, M., 2007. Prospectivity of Volcanic Basins: Trap Delineation and Acreage De-Risking. AAPG Bulletin, 91(6): 915–939. doi:10.1306/12150606017

Russell, S. M., Whitmarsh, R. B., 2003. Magmatism at the West Iberia Non-Volcanic Rifted Continental Margin: Evidence from Analyses of Magnetic Anomalies. Geo-physical Journal International, 154(3): 706–730. doi:10.1046/j.1365-246x.2003.01999.x

Savva, D., Pubellier, M., Franke, D., et al., 2014. Different Expressions of Rifting on the South China Sea Margins. Marine and Petroleum Geology, 58: 579–598. doi:10.1016/j.marpetgeo.2014.05.023

Sheriff, R. E., Geldart, L. P., 1995. Exploration Seismology 2nd Edition. Cambridge University Press, Cambridge

Shi, W., Song, Z., Wang, X., et al., 2009. Diapir Structure and Its Origin in the Baiyun Depression. Pearl River Mouth Basin China. Journal of Earth Science, 34(5): 778–784

Song, Y., Ren, J. Y., Stepashko, A. A., et al., 2014. Post-Rift Geodynamics of the Songliao Basin, NE China: Origin and Significance of T11 (Coniacian) Unconformity. Tec-tonophysics, 634: 1–18. doi:10.1016/J.Tecto.2014.07.023

Song, Y., Stepashko, A. A., Ren, J. Y., 2015. The Cretaceous Climax of Compression in Eastern Asia: Age 87–89 Ma (Late Turonian/Coniacian), Pacific Cause, Continental Consequences. Cretaceous Research, 55: 262–284. doi:10.1016/J.Cretres.2015.01.002

Sun, Q., Wu, S., Chen, D., et al., 2014. Focused Fluid Flow Systems and Their Implications for Hydrocarbon and Gas Hydrate Accumulations in the Deep-Water Basins of the Northern South China Sea. Chinese J. Geophysics, 57(12): 4052–4062 (in Chinese with English Abstract)

Taylor, B., Hayes, D. E., 1980. The Tectonic Evolution of the South China Basin. In: Hayes, D. E., ed., The Tectonic and Geologic Evolution of Southeast Asian Seas and Is-lands. American Geophysical Union Geophysical Mono-graph, 23: 89–104

Tong, D., Ren, J., Lei, C., et al., 2009. Lithosphere Stretching


653

Model of Deep Water in Qiongdongnan Basin, Northern Continental Margin of South China Sea, and Controlling of the Post-Rift Subsidence. Journal of Earth Science, 34(6): 963–974

Vendeville, B. C., Jackson, M. P. A., 1992. The Rise of Diapirs during Thin-Skinned Extension. Marine and Petroleum Ge-ology, 9(4): 331–354. doi:10.1016/0264-8172(92)90047-i

Wang, J., Pang, X., Wang, C., et al., 2006. Discovery and Recognition of the Central Diapiric Zone in Baiyun De-pression, Pearl River Mouth Basin. Journal of Earth Sci-ence, 31(2): 209–213

White, R., McKenzie, D., 1989. Magmatism at Rift Zones: The Generation of Volcanic Continental Margins and Flood Basalts. Journal of Geophysical Research, 94(B6): 7685–7729. doi:10.1029/jb094ib06p07685

Whitmarsh, R., Manatschal, G., Minshull, T., 2001. Evolution of Magma-Poor Continental Margins from Rifting to Sea-floor Spreading. Nature, 413(6852): 150–154. doi:10.1038/35093085

Wu, S., Qin, Y., 2009. The Research of Deepwater Deposi-tional System in the Northern South China Sea. Sedimen-tology, 27(5): 922–930 (in Chinese with English Abstract)

Xie, W., Wang, T., Zhang, Y., Jiang, J., 2009. Characteristics and Dynamic Analysis of Cenozoic Rifting and Magmatism in Southwest Qiongdongnan Basin. Geotectonica et Metallogenia, 33: 199–205 (in Chinese with English Abstract)

Xie, X. N., Chen, Z. H., Sun, Z. P., et al., 2012. Depositional Architecture Characteristics of Deepwater Depositional Systems on the Continental Margins of Northwestern South China Sea. Earth Science—Journal of China Uni-versity of Geosciences, 37(4): 627–634 (in Chinese with English Abstract)

Xie, X. N., Jiang, T., Wang, H., et al. 2006. Expulsion of Overpressured Fluid Revealed by Geochemistry of For-mation Water in the Diapiric Structures of Yinggehai Ba-sin. Acta Petrologica Sinica, 22(8): 2243–2248 (in Chi-nese with English Abstract)

Xie, X. N., Ren, J. Y., Wang, Z. F., et al., 2015. Difference of Tectonic Evolution of Continental Marginal Basins of South China Sea and Relationship with SCS Spreading.

Earth Science Frontiers. 22(1): 77–87 (in Chinese with English Abstract)

Xie, Y. H., Li, X. S., Tong, C. X., 2015. High Temperature and High Pressure Gas Enrichment Condition, Distribution Law and Accumulation Model in Central Diapir Zone of Yinggehai Basin. China Offshore Oil and Gas, 27(4): 1–12 (in Chinese with English Abstract)

Yan, P., Deng, H., Liu, H. L., et al., 2006. The Temporal and Spatial Distribution of Volcanism in the South China Sea Region. Journal of Asian Earth Sciences, 27(5): 647–659. doi:10.1016/j.jseaes.2005.06.005

Yan, P., Zhou, D., Liu, Z. S., 2001. A Crustal Structure Profile Across the Northern Continental Margin of the South China Sea. Tectonophysics, 338(1): 1–21. doi:10.1016/s0040-1951(01)00062-2

Yilmaz, Ö., 2001. Seismic Data Analysis: Processing, Inver-sion, and Interpretation of Seismic Data. Society of Ex-ploration Geophysicists, Tulsa

Yuan, S. Q., Wu, S. G., Thomas, L., et al., 2009. Fine-Grained Pleistocene Deepwater Turbidite Channel System on the Slope of Qiongdongnan Basin, Northern South China Sea. Marine and Petroleum Geology, 26(8): 1441–1451. doi:10.1016/j.marpetgeo.2009.03.007

Zhao, F., Wu, S. G., Sun, Q. L., et al., 2014. Submarine Vol-canic Mounds in the Pearl River Mouth Basin, Northern South China Sea. Marine Geology, 355: 162–172. doi:10.1016/j.margeo.2014.05.018

Zhou, X. M., Li, W. X., 2000. Origin of Late Mesozoic Igneous Rocks in Southeastern China: Implications for Lithosphere Subduction and Underplating of Mafic Magmas. Tectonophysics, 326(3/4): 269–287. doi:10.1016/s0040-1951(00)00120-7

Zhu, W. L., Huang, B. J., Mi, L. J., et al., 2009. Geochemistry, Origin, and Deep-Water Exploration Potential of Natural Gases in the Pearl River Mouth and Qiongdongnan Basins, South China Sea. AAPG Bulletin, 93(6): 741–761. doi:10.1306/02170908099

Zou, H., Li, P., Rao, C., 1995. Geochemistry of Cenozoic Volcanic Rocks in Zhujiangkou Basin and Its Geody-namic Significance. Geochemica, 24(Suppl.): 33–45 (in Chinese with English Abstract)

Seismic Reflection Characteristics and Evolution of ...

Documents