SEASAT geoid anomalies and the Macquarie Ridge complex

The seismically active Macquarie Ridge complex forms the Pacific-India plate boundary between New Zealand and the Pacific-Antarctic spreading center. The Late Cenozoic deformation of New Zealand and focal mechanisms of recent large earthquakes in the Macquarie Ridge complex appear consistent with the current plate tectonic models. These models predict a combination of strike-slip and convergent motion in the northern Macquarie Ridge, and strike-slip motion in the southern part. The Hjort trench is the southernmost expression of the Macquarie Ridge complex. Regional considerations of the magnetic lineations imply that some oceanic crust may have been consumed at the Hjort trench. Although this arcuate trench seems inconsistent with the predicted strike-slip setting, a deep trough also occurs in the Romanche fracture zone.Geoid anomalies observed over spreading ridges, subduction zones, and fracture zones are different. Therefore, geoid anomalies may be diagnostic of plate boundary type. We use SEASAT data to examine the Macquarie Ridge complex and find that the geoid anomalies for the northern Hjort trench region are different from the geoid anomalies for the Romanche trough. The Hjort trench region is characterized by an oblique subduction zone geoid anomaly, e.g., the Aleutian-Komandorski region. Also, limited first-motion data for the large 1924 earthquake that occurred in the northern Hjort trench suggest a thrust focal mechanism. We conclude that subduction is occurring at the Hjort trench. The existence of active subduction in this area implies that young oceanic lithosphere can subduct beneath older oceanic lithosphere.     

Vertical tectonism in oceanic fracture zones

Anomalous topographic highs are found along many large oceanic fracture zones, frequently in the form of transverse ridges elongated parallel to the fractures both within and outside of the transform zone. These crustal highs are one or more kilometers shallower than the adjacent “normal” crust of equivalent age. Their elevation is frequently higher than the axial zone of lithospheric accretion. Geophysical and petrological data from the Vema, St. Paul, Romanche (Atlantic Ocean), Owen (Indian Ocean), Alula (Gulf of Aden), and other fracture zones suggest that the anomalous transverse ridges are not the result of excess volcanism, but rather of tectonic uplift of upper mantle and crustal blocks.Factors which may determine vertical tectonism along fracture zones include: (1) horizontal thermal conduction across a fracture zone from a lithospheric accreting segment; (2) viscodynamic forces operating in a fracture zone close to its intersection with an accreting segment; and (3) compressional and tensional horizontal stresses operating along a fracture zone, and created by several causes, the major one being small changes in the direction of spreading. Among these various factors, compressional and tensional horizontal stresses are probably the main cause of vertical tectonism in fracture zones. During the vertical motion of upper mantle/crustal blocks, ultramafic rocks, due to their physical properties, are uplifted preferentially.One of the implications of fracture zone vertical tectonism is that the age/depth of the crust relationship may not apply in regions with large fracture zones. In one well-documented case, a transverse crustal block at the Romanche fracture zone subsided during the last 5 m.y. at an average rate more than one order of magnitude faster than crust of equivalent age.     

Neogene crustal emersion and subsidence at the Romanche fracture zone,equatorial Atlantic

Partly phosphatized, oolitic-biogenic limestones were recovered 950–1300 m below sea level from two sites near the crest of a transverse ridge running parallel and adjacent to the Romanche fracture valley (equatorial Atlantic). Some of the biogenic contituents of the limestones (the benthonic foraminiferal genusAmphistegina; corals of the genusStylophora sp., scaphopods, etc.); their paleofacies assemblage (including echinoderms, gastropods, calcareous algae, etc.); and the presence of a well-developed oolitic facies, indicate that the limestones formed in very shallow water close to sea level. In addition, several features of the limestones (including the presence of stromatolite-like laminae, and dissolution features typical of subaerial diagenesis) suggest that the summit of the transverse ridge might have undergone episodes of emersion. The limestones were formed on a shallow carbonate bank at around the Miocene-Pliocene boundary, i.e., 5 ± 1 Myr ago, as determined by the age of fossil planktonic foraminifera and corals. The transverse ridge must have subsided since that time at an average rate of 0.2 mm/yr. It is unlikely that the vertical motions of the Romanche transverse crustal block were caused solely by accreting plate margin- or mantle plume-related volcanic and/or tectonic processes. It is suggested instead that such motions are related to vertical tectonism typical of large oceanic fracture zones.     

中国海-西太平洋地区典型剖面的重-磁-震联合反演研究

重-磁-震联合反演是获取地壳结构的重要方法.此次研究,我们主要基于全球最新的水深、重磁异常、沉积物厚度等数据,结合实测地震数据和前人研究成果,分析了中国海-西太平洋地区的莫霍面展布特征,并利用重磁震联合反演方法获得了跨越中国海-西太平洋典型剖面的地壳结构和异常体分布,揭示了陆壳到洋壳的典型变化规律.结果表明,从浙江地区到马里亚纳俯冲带,地壳结构大致呈现由厚到薄、由老到新、由复杂到简单的特征.浙江地区（扬子块体和华夏块体）地壳结构复杂,三层结构明显,地壳内断裂带发育,并伴有广泛的岩浆侵入;东海地区莫霍面起伏剧烈,地壳厚度变化较大,冲绳海槽地壳明显减薄,是其过渡壳性质的体现;西菲律宾海盆、九州-帕劳海脊、帕里西维拉海盆、马里亚纳俯冲带等构造单元地壳结构相对简单,二层结构明显.其中,西菲律宾海盆和帕里西维拉海盆地壳内部磁异常变化较为剧烈,海盆扩张过程中形成的磁异常体分布广泛,地壳厚度（5~8 km）明显小于陆壳;九州-帕劳海脊地壳厚度可达~20 km,缺失中地壳,表现为岛弧地壳结构;同源的西马里亚纳岛弧和东马里亚纳火山弧地壳结构相似,浅层磁异常体分布广泛,西马里亚纳岛弧地壳厚度（~17 km）略小于东马里亚纳火山弧（~20 km）,体现了裂离的不对称性;马里亚纳海槽具有正常的洋壳结构（~7 km）,但扩张中心未发生明显破裂.对比各构造单元地壳结构的异同点,我们进一步认识到,陆壳与洋壳之间不是孤立的,陆壳可能会演化出洋壳的结构或组分,板块的演化总是处于动态循环过程中.此研究加深了我们对中国海-西太平洋深部构造特征的整体理解,促进了我们对大陆边缘演化与板块相互作用的认识,深化了我国管辖海域及邻近地区的基础地质调查.     

Mesozoic magnetic lineations and the magnetic quiet zone off northwest Africa

An extensive compilation of recently acquired geophysical reconnaissance data has allowed the Mesozoic magnetic lineations (The Eastern Keathley sequence) to be identified and mapped in detail for the area off northwest Africa lying between Madeira and the Cape Verde Islands. These anomalies were generated as one limb of a symmetric spreading center (Paleo Mid-Atlantic Ridge) from about 107 to 153 m.y.B.P. Offsets in the lineation pattern serve to identify fracture zone traces whose trends are approximately east-west. The seaward boundary of the marginal quiet zone does not precisely define an isochron due to the presence of a variable width transition zone of intermediate amplitude magnetic anomalies. Crust underlying the marginal quiet zone was generated, at least in part, during the Jurassic, Graham normal polarity epoch. The quiet zone boundary is not offset significantly on opposite sides of the Canaries lineament as previously suggested. A possible counterpart of the U.S. east coast magnetic anomaly is observed in some areas near the shelf/slope break of Spanish Sahara and Mauritania. The presence of relatively high-amplitude (but not-correlatable) magnetic anomalies seaward of the Mesozoic sequence and presumably generated during the Cretaceous, Mercanton normal polarity epoch remains a paradox.     

Geoid anomalies across fracture zones and the thickness of the lithosphere

Recent advances in the measurement and interpretation of geoid height anomalies provide a new way to estimate the thickness of the oceanic lithosphere as a function of crustal age. GEOS-III satellite altimetry measurements show abrupt changes in sea level across fracture zones which separate areas of lithosphere with different ages. These changes have the correct location, amplitude, and wavelength to be caused by the combined gravitational attraction of the relief across the fracture zone and the isostatic support of this relief. Eight profiles of geoid height and bathymetry across the Mendocino fracture zone are inverted to determine the depth of the isostatic compensation, assuming that the compensation occurs in a single layer. These depths are then interpreted with a thermal boundary layer model of lithospheric growth. To explain satisfactorily the geoid measurements, the thermal diffusivity of the upper mantle must be 3.3 × 10^?3 cm² s^?1 and the thickness of the lithosphere, defined as the depth at which the geotherm reaches 95% of its maximum value, must be9.1km m.y.^?1/2 × t^1/2, where t is lithospheric age.     

南海海盆三维重力约束反演莫霍面深度及其特征

利用南海海盆及周边最新的重力,经过海底地形、沉积层的重力效应改正,并采用岩石圈减薄模型的温度场公式,校正了从张裂边缘到扩张海盆的热扰动重力效应.通过研究区的地震剖面和少量声呐数据得到的莫霍面深度点作为约束,采用基于"起伏界面初始模型"的深度修正量反演迭代公式,反演、计算了研究区的莫霍面深度及地壳厚度.结果表明,海盆区莫霍面深度在8~14 km之间,地壳厚度在3~9 km之间;东部海盆和西南海盆残留扩张中心沿NNE向展布向西南延伸至112°E,莫霍面深度超过12 km,地壳厚度在6 km以上,而西北海盆没有明显的增厚扩张中心;在西南海盆北缘的中沙地块南侧,存在一个近EW向地壳减薄带,地壳厚度在9~10 km;莫霍面深度14 km的等深线和地壳厚度9 km的等值线可指示洋陆边界位置.     

南海重力异常的沉积层密度改正及其对区域构造特征分析的意义

南海位于太平洋板块、印澳板块和欧亚板块交汇处,自晚中生代以来历经张裂作用、海底扩张以及印藏碰撞、菲律宾海板块西向运动等构造事件的叠加改造,不仅形成了复杂多样的构造格局,而且堆积了厚薄不均的沉积层.为了考察沉积层密度改正对利用重力资料分析南海不同尺度构造特征的影响,本文利用南海各区域不同深度沉积层的地震波速度及钻孔密度等数据,建立了沉积层与沉积基底密度差随深度变化的二次函数关系式,并基于该关系式,计算了南海沉积层相对基底密度低而产生的重力异常值.结果显示,南海沉积层的重力异常值在海盆区介于-40~-60 mGal,而在堆积巨厚沉积物的莺歌海盆地可达到-135 mGal;相对于空间重力异常、布格重力异常,经沉积层重力异常改正后的地壳布格重力异常更能突出深部不同尺度的密度结构和莫霍面的起伏特征,其总水平导数模更突显了南海西北部红河断裂带的海上延伸;利用谱分析技术估算岩石圈强度时,经沉积层重力异常改正的地壳布格重力异常数据获得的岩石圈有效弹性厚度值更为符合地质实际,特别是在长条形的巨厚沉积区如莺歌海盆地和马来盆地.分析表明,重力异常的沉积层密度改正对揭示南海构造特征具有重要的意义.     

南海岩石层及边界构造的地球物理特征

南海经历了中生代主动大陆边缘到新生代被动大陆边缘的转换,其岩石层地球物理场具有明显的块、带特征.本文通过综合分析南海地区深地震探测、面波层析成像、重磁异常以及地热与岩石层流变学等各种地质地球物理资料,对南海地壳及岩石层的综合地球物理特征进行了深入总结,发现深地震探测剖面所确定的洋、陆壳转换位置与空间重力异常梯级带位置较为一致,据此拟定了南海洋、陆壳的转换边界;依据多条地壳结构剖面中拉张减薄的程度确定了正常减薄陆壳、洋陆壳过渡带及洋壳等属性特征,并初步圈定了南海下地壳高速层的分布范围.对比分析了南、北陆缘地壳结构及其拉张减薄的变化特征,从综合地球物理特征的相似性上推测了北部陆缘的中西沙陆块与南部陆缘的南沙礼乐滩陆块具有共轭对称性.依据S波速度梯度变化确定了南海岩石层厚度分布情况,揭示出南海北部陆缘存在一条岩石层厚度的减薄带,且该减薄带与高热流带具有较好的一致性.在综合分析的基础上,以深地震探测剖面与重、磁异常变化的对应性为基础,划定了南海边界构造的位置.     

南海北部磁异常特征及对前新生代构造的指示

为了研究南海北部前新生代构造,利用新近的船载磁力测量数据,对磁异常进行变纬度化极,并反演计算视磁化强度和磁源重力异常,以及对三条OBS剖面进行重磁拟合.结果认为东沙隆起高磁异常带是浙闽沿海火山岩带向西的延续,其间被NW向古老的转换边界断裂F10错断;NE向的F2断裂是高磁异常带的南界,并限制了底侵活动的北界;F3断裂在...     

The odyssey of the Cache Creek terrane,Canadian Cordillera: Implications for accretionary orogens,tectonic setting of Panthalassa,the Pacific superwell,and break-up of Pangea

The Cache Creek terrane (CCT) of the Canadian Cordillera consists of accreted seamounts that originated adjacent to the Tethys Ocean in the Permian. We utilize Potential Translation Path plots to place quantitative constraints on the location of the CCT seamounts through time, including limiting the regions within which accretion events occurred. We assume a starting point for the CCT seamounts in the easternmost Tethys at 280 Ma. Using reasonable translation rates (11 cm/a), accretion to the Stikinia–Quesnellia oceanic arc, which occurred at about 230 Ma, took place in western Panthalassa, consistent with the mixed Tethyan fauna of the arc. Subsequent collision with a continental terrane, which occurred at about 180 Ma, took place in central Panthalassa, > 4000 km west of North America yielding a composite ribbon continent. Westward subduction of oceanic lithosphere continuous with the North American continent from 180 to 150 Ma facilitated docking of the ribbon continent with the North American plate.The paleogeographic constraints provided by the CCT indicate that much of the Canadian Cordilleran accretionary orogen is exotic. The accreting crustal block, a composite ribbon continent, grew through repeated collisional events within Panthalassa prior to docking with the North American plate. CCT's odyssey requires the presence of subduction zones within Panthalassa and indicates that the tectonic setting of the Panthalassa superocean differed substantially from the current Pacific basin, with its central spreading ridge and marginal outward dipping subduction zones. A substantial volume of oceanic lithosphere was subducted during CCT's transit of Panthalassa. Blanketing of the core by these cold oceanic slabs enhanced heat transfer out of the core into the lowermost mantle, and may have been responsible for the Cretaceous Normal Superchron, the coeval Pacific-centred mid-Cretaceous superplume event, and its lingering progeny, the Pacific Superswell. Far field tensile stress attributable to the pull of the slab subducting beneath the ribbon continent from 180 to 150 Ma instigated the opening of the Atlantic, initiating the dispersal phase of the supercontinent cycle by breaking apart Pangea. Docking of the ribbon continent with the North American plate at 150 Ma terminated the slab pull induced stress, resulting in a drastic reduction in the rate of spreading within the growing Atlantic Ocean.     

Magnetic anomalies in the Shikoku Basin: a new interpretation

Kaiko surveys over the Nankai Trough made available new magnetic and structural data for the northern Shikoku Basin. A survey of the oceanic lithosphere subducting below Southwest Japan along the central Nankai Trough revealed the existence of several north-south basement troughs. They are probably transform faults related to a north-south spreading system. We examine the possibility of a late phase of north-south spreading limited to the axial northernmost Shikoku Basin, active between 14 and 12 Ma. If this system was already active before that time, i.e. during the N55° opening of the southeastern basin, then a triple junction should be found between both areas.Based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin. From early east-west rifting to late north-south spreading, opening of the Shikoku Basin proceeded through multiple episodes of spreading. The analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space. Two successive counter-clockwise rotations of the spreading direction are postulated, at anomaly 6 (19 Ma) and at anomaly 5B (14 Ma), involving segmentation and rotation of the spreading ridge.     

南海新生代海底扩张的构造演化模式:来自高分辨率地球物理数据的新认识

根据高分辨率重、磁测网数据的分析,结合多波束海底地貌的构造解释,南海海盆新生代经历了两期不同动力特征的海底扩张,25 Ma的沉积-构造事件是其重要分界.早期扩张从约33.5 Ma开始至25 Ma停止,在东部海盆南、北两侧和西北海盆形成了具有近E-W向或NEE向磁条带的老洋壳,是近NNW-SSE向扩张的产物;晚期扩张从2...     

Dynamics of the mid-ocean ridge plate boundary: Recent observations and theory

The global mid-ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics. In this paper I present recent results of several aspects of mid-ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries. I show that the observed rift valley to no-rift valley transition (globally due to the increase of spreading rate or locally due to the crustal thickness variations and/or thermal anomalies) can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere, and most importantly, by the difference in the rheological behavior of the oceanic crust from the underlying mantle. The effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations. The accumulated strain pattern from a recently developed lens model, based on recent seismic observations, was proposed as an appealing mechanism for the observed gabbro layering sequence in the Oman Ophiolite. It is now known that the mid-ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities. The tectonics of ridge segmentation are also spreading-rate dependent: the slow-spreading Mid-Atlantic Ridge is characterized by distinct bulls-eye shaped gravity lows, suggesting large along-axis variations in melt production and crustal thickness, whereas the fast-spreading East-Pacific Rise is associated with much smaller along-axis variations. These spreading-rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid-ocean ridges: the mantle upwelling may be intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge.     

南海北部区域构造和陆壳向洋壳的转化

1.海底扩张和陆壳大洋化,均能形成洋壳。陆壳转化为过渡壳是大洋化的必经阶段。 2.航磁测区内的中央海盆,具有类似于大洋中海底扩张形成的对称磁异常条带。 3.在西沙北裂谷、莺歌海裂谷型拗陷,以及属于断陷盆地性质的珠一、珠二拗陷等地,有沿着断裂上升的类似于洋壳成分的地幔物质喷溢或在地壳上层侵位,那里的地壳都有不同程度的减薄,属于过渡型地壳。西沙北裂谷的“莫霍面”,比相邻的南北两侧高出约10公里。 4.在南海北部发生多中心微型扩张和大洋化。     

Modelling the composition of melts formed during continental breakup of the Southeast Greenland margin

We have developed a generic dynamic model of extension of the lithosphere, which predicts major element composition and volume of melt generated from initial extension to steady state seafloor spreading. Stokes equations for non-Newtonian flow are solved and the mantle melts by decompression. Strengthening of the mantle due to dehydration as melting progresses is included. The composition is then empirically related to depletion. Using a crystallisation algorithm, the predicted primary melt composition was compared with mean North Atlantic mid-ocean ridge basalt (MORB). At steady state, using half spreading rates from 10 to 20 mm yr^− 1 and mantle potential temperatures of 1300 to 1325 °C we predict a major element composition that is within the variation in the mean of North Atlantic MORB.

This model is applied to the Southeast Greenland margin, which has extensive coverage of seismic and ODP core data. These data have been interpreted to indicate an initial pulse of magmatism on rifting that rapidly decayed to leave oceanic crustal thickness of 8 to 11 km. This pattern of melt production can be recreated by introducing an initial hot layer of asthenosphere beneath the continental lithosphere and by having a period of fast spreading during early opening. The hot layer was convected through the melt region giving a pulse of high magnesian and low silica melt during the early rifting process. The predicted major element composition of primary melts generated are in close agreement with primary melts from the Southeast Greenland margin. The observed variations in major element composition are reproduced without a mantle source composition anomaly.     

Lithospheric model with thick oceanic crust at the continental boundary: A mechanism for shallow spreading ridges in young oceans

In a general lithospheric model of a simple divergent ocean and continental margin that satisfies the constraints of isostasy and gravity anomalies, the free-air gravity anomaly at the margin is modelled by an oceanic crust that thickens exponentially toward the margin from its common value of 6.4 km about 600 km from the margin to 17.7 km at the margin; this postulated thickening is supported empirically by seismic refraction measurements made near continental margins. The thickness of the oceanic crust matches that of the continental lithosphere at breakup, as observed today in Afar and East Africa, and is interpreted as the initial oceanic surface layer chilled against the continental lithosphere. With continued plate accretion, the chilled oceanic crust thins exponentially to a steadystate thickness, which is achieved about 40 m.y. after breakup. These findings contrast with the generally held view that the oceanic crust has a uniform thickness.During the first 40 m.y. of spreading, the thicker oceanic crust, of density 2.86 g/cm³, displaces the denser (3.32 g/cm³) subjacent material; by isostasy, the spreading ridge and the rest of the seafloor thus stand higher in younger( <40m.y.) oceans than they do in older(>40m.y.) oceans. This is postulated to be the cause of the empirical relationship between the crestal depth of spreading ridges and the age (or half-width) of ocean basins.     

Gravity anomaly, lithospheric structure and seismicity of Western Himalayan Syntaxis

A compiled gravity anomaly map of the Western Himalayan Syntaxis is analysed to understand the tectonics of the region around the epicentre of Kashmir earthquake of October 8, 2005 (Mw = 7.6). Isostatic gravity anomalies and effective elastic thickness (EET) of lithosphere are assessed from coherence analysis between Bouguer anomaly and topography. The isostatic residual gravity high and gravity low correspond to the two main seismic zones in this region, viz. Indus–Kohistan Seismic Zone (IKSZ) and Hindu Kush Seismic Zones (HKSZ), respectively, suggesting a connection between siesmicity and gravity anomalies. The gravity high originates from the high-density thrusted rocks along the syntaxial bend of the Main Boundary Thrust and coincides with the region of the crustal thrust earthquakes, including the Kashmir earthquake of 2005. The gravity low of HKSZ coincides with the region of intermediate–deep-focus earthquakes, where crustal rocks are underthrusting with a higher speed to create low density cold mantle. Comparable EET (∼55 km) to the focal depth of crustal earthquakes suggests that whole crust is seismogenic and brittle. An integrated lithospheric model along a profile provides the crustal structure of the boundary zones with crustal thickness of about 60 km under the Karakoram–Pamir regions and suggests continental subduction from either sides (Indian and Eurasian) leading to a complex compressional environment for large earthquakes.     

Morpho-tectonic study on late-stage spreading of the Eastern Subbasin of South China Sea

Three NE-trending linear structural zones with different strikes are present in the Eastern Subbasin of the South China Sea. They are distributed in the 350-km-wide central region of both sides of the Scarborough seamount chain, representing a morphological indication of the basement faulting. These three zones correspond respectively to three spreading episodes: the magnetic anomalies 6c -6a (24-21 Ma), 6a - 5e (21 - 19 Ma) and 5e - 5d (5c) (19 - 16 Ma). Instability, subsection and asymmetry characterize the seafloor spreading of the subbasin. The spreading directions change in a continuous way in each of the zones, but abrupt changes by 3°-5° occur when crossing the boundary between the zones, reflecting that the spreading direction has evolutionary characteristics of both gradual and sudden changes. NW-trending transform faults of the spreading become progressively densely distributed from the east to the west, cutting the NE-trending zones into several segments, between which the strikes of the NE-trending zones have marked changes. Such features indicate that the spreading axis is associated with subsection along the strike. Around 21 Ma (magnetic anomaly 6a), there was an important event of spreading acceleration, with the full rate rapidly increasing from 30.54 km/Ma to 42.88 km/Ma. This rate increment event corresponds to the sudden changes in the spreading characteristics of basement faulting, sedimentation, volcano activities, etc. The asymmetry of spreading over the eastern part of the Eastern Subbasin is generally larger than that over the western part, and the spreading rate is markedly larger on the southern side than on the northern side. As a result, the oceanic basin is wide in the east and narrow in the west, forming a significantly asymmetric pattern.     

Thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones

Brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model, with an emphasis on the spacing of fracture zones. Brittle deformation is represented by localized plastic strain within a material having an elasto-visco-plastic rheology with strain softening. We show that crustal thickness, creep strength, and the rule governing plastic flow control the formation of cracks. The spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value. Creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses, while only a thin crust can have primary cracks if its creep strength is low. For a thin crust, the spacing of primary cracks is inversely proportional to the creep strength, suggesting that creep strength might independently contribute to the degree of brittle deformation. Through finite versus zero dilatation in plastic strain, associated and non-associated flow rule results in nearly vertical and V-shaped cracks, respectively. Changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness, and thus related to brittle deformation. The fracture zone-free Reykjanes ridge is known to have a uniformly thick crust. The Australian-Antarctic Discordance has multiple fracture zones and thin crust. These syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa.