Data_Sheet_2_Estimation of the Thermal Structure Beneath the Volcanic Arc of the Northern Andes by Coda Wave Attenuation Tomography.zip

In the Northern Andes, the magmatic arc rises from a broad area of active volcanism in the South, at the border between Ecuador and Colombia, to a linear north-directed trend of active and inactive volcanic cones. A ∼240-km-long west-east-striking slab tear (Caldas Tear) located approx. 5.5°N creates an offset in the volcanic arc. This tear looks like a significant controller of volcanic activity: its quasi-WE structure separates inactive magmatic bodies of late Miocene age or older in the North from Quaternary magmatic activity in the South. Coda wave attenuation tomography applied on seismic waveforms recorded between 1993 and 2018 illuminates the volcanic arc, which appears as a segmented structure derived from the complex process of subduction of the Nazca and Caribbean plates under the South America Continent. The attenuation measurements are transformed into thermal measurement using standard rock physics relationships, supported by thermal estimations and geothermal gradient observations measured in wells. Active and inactive magmatic belts are associated with a range of relatively low temperatures (∼640 to ∼810°C in the depth range of 25–100 km), which may be a consequence of the fluid content in hydrous minerals. Along the volcanic arc, the isotherms become shallower from South to North and are interrupted by a cold structure; this structure may reflect a lateral change of the mantle viscosity that prevents the continuity of the volcanic arc. Our estimations show an irregular depth-geometry of the isotherms, probably associated with recent slip events that have perturbed the thermal state along the study area. The isotherms also deepen to the West, probably due to the subduction process of the Nazca Plate (∼0–5°N). In some areas, the isotherms follow a trend similar to that of a thrust fault-related folding geometry, which suggests a recent process of regional perturbation. We hypothesize that the Panama Arc collision at the north and the effect that imprint the Carnegie Ridge against the continent at the South are responsible for these thermal effects. The northern anomaly suggests thickening of the lithospheric system that prevents the development of the volcanic arc at the north of the Caldas Tear.

在北安第斯地区，岩浆弧（magmatic arc）从南部厄瓜多尔与哥伦比亚交界的广泛活动火山作用区域向北抬升，最终形成呈线性展布的活动与休眠火山锥带。位于约5.5°N处、长约240公里的近东西向板片撕裂（slab tear）——卡尔达斯撕裂带（Caldas Tear）——造成了岩浆弧的偏移。该撕裂带对火山活动具有显著控制作用：其近乎东西向的构造将北部晚中新世及更古老的休眠岩浆体，与南部第四纪岩浆活动区域分隔开来。基于1993年至2018年记录的地震波形（seismic waveforms）开展的尾波衰减层析成像（coda wave attenuation tomography）研究，揭示了岩浆弧的结构特征：该弧段为纳斯卡板块（Nazca Plate）与加勒比板块（Caribbean Plate）在南美大陆下方复杂俯冲过程形成的分段式构造。借助标准岩石物理学关系式，并结合井中测得的热估算结果与地热梯度（geothermal gradient）观测数据，研究人员将衰减测量值转换为热测量值。活动与休眠岩浆带对应25–100 km深度范围内约640℃至810℃的相对低温环境，这一特征可能与含水矿物中的流体含量有关。沿岩浆弧方向，等温线（isotherms）从南向北逐渐变浅，并被一处冷构造截断；该冷构造可能反映了地幔黏度的横向变化，进而阻碍了岩浆弧的连续性。我们的估算结果显示，等温线的深度形态存在不规则性，这可能与近期的滑移事件有关——这些事件扰动了研究区域的热状态。在约0°至5°N区域，等温线还会向西加深，这可能与纳斯卡板块的俯冲过程有关。在部分区域，等温线的展布形态与逆冲断裂（thrust fault）相关的褶皱几何形态相似，这表明研究区域近期经历了区域性扰动过程。我们推测，北部的巴拿马弧（Panama Arc）碰撞事件，以及南部卡内基海岭（Carnegie Ridge）对大陆的挤压效应，是造成上述热异常现象的原因。北部的热异常现象表明，岩石圈系统的增厚阻碍了卡尔达斯撕裂带北侧岩浆弧的形成与发育。